University Place

Biogeochemical Surprises in a Changing Climate

Cassie Immel: Thank you for watching Wednesday Nite at the Lab tonight. Tonight we will welcome Dr. Desai of the Department of Atmospheric Sciences and Oceanic Sciences at UW Madison. He has a BA from the environmental studies and computer science at Oberlin College in Ohio. He has his masters in geography with the University of Minnesota in Minneapolis. And he got his PhD in meteorology with the Pennsylvania State University. And tonight, he’s going to be talking about some surprises that his lab has found out and how those recent observations have begun to challenge some of our dominate views from the climate ecosystem and their relationships. So now I’d like to welcome Dr. Desai. [APPLAUSE]

Ankur Desai: Thanks, Cassie, for a nice introduction. I hope everyone can hear me well. I’m going to talk about something that is maybe a big word I first need to define and something about something you probably know about which is climate change at least from what you know in the media and maybe what you’ve learned elsewhere. And what I want to talk about are some surprises in biogeochemistry, and I’ll define that word in a moment, in a changing climate. And as Cassie said I’m in the Atmospheric and Oceanic Sciences Department here, I’m also affiliated with the Nelson Institute on the Environment, the Center for Climatic Research where we do a lot of this research.

So, the very first question I should answer is biogeo what? Biogeochemistry sounds like a made-up word for the indecisive scientist. But it has a specific meaning, and it relates to the fact that when we’re talking about ecosystems and we’re talking about how the earth system works with the atmosphere, theres both biology, geology and chemistry and there’s even physics involved. But we couldn’t get all of those terms to fit together in one word, so we have two words.

So, I say land and ocean ecosystems which I’m going to be talking about today, primarily land systems, have biophysical and biogeochemical dependence on the atmosphere. So what I mean by that, by biophysical I say things that grow on land or in the ocean need water, they need heat, they need solar radiation and so all of those things, of course, are provided by the atmosphere so the exchanges of those quantities between the two are going to be our biophysical interactions.

And then biogeochemical is the next step. It’s about all the chemical interactions that occur, the biochemical interactions. Photosynthesis is the stuff of life, as well as respiration that we do as we’re breathing here digesting our dinner. And it’s the cycling of nutrients, and the ones I want to focus on here is going to be carbon, but carbon and nitrogen are probably our two most important ones especially when were thinking about plants.

And what I’ll be talking today is primarily about plants and plant photosynthesis. So as the atmosphere is changing, both changes in CO2 and changes in the climate, these are changing the ecosystems. Ecosystems respond to climate change and vice versa. And of course, some of these we can understand, but a lot of these lead to surprises.

So here’s my little jack-in-the-box. This one I pulled off a website of the Consumer Product Safety Commission, it was recalled. So that is a surprising jack-in-the-box. [LAUGHTER] But surprises can be pleasant and pleasing like that, but sometimes they can be downright disturbing. Here’s Bill Nye the Science Guy. They can lead to curiosity, or they can lead to je ne sais quoi. This is for the closing ceremonies at the Vancouver Winter Olympics where they had giant inflatable canoes and beavers and I don’t get it. [LAUGHTER] Some surprises just leave you puzzled.

But ecosystems, in general, are evolutionarily adapted to their climate. There’s a certain climate and plants, in general, are going to adapt to maximize the resources they can take out of that climate. So they’re evolutionarily adapted to the climate in short-term variability. So obviously year to year the weather changes, you have a warm winter, you have a cold winter, and most of the small-scale variability plants are going to generally survive. There’s going to be some competitive pressures but not too much. So expectations of how these ecosystems respond to climate variation by making measurements of ecosystems, putting them in the lab, seeing how they change with temperature, moisture and light. It’s the bedrock and science of ecosystem ecology.

This is how we study how plants interact with each other and the atmosphere that they expect. But surprises are likely given that there’s a complex interplay between ecosystems and climate. Ecosystems are biologically active systems that are in competition for resources with other systems. And then the climate, of course, has variability that may be beyond what they’re adapted to. But, of course, surprises are no fun from the perspective of managing because if we want to manage an ecosystem, say, to maximize productivity, say, for a forest, a surprise, where Im defining surprise as an unexpected response to variation and climate, is no fun. But it’s also, from my perspective, it’s a lot of fun because it’s how we learn about how ecosystems respond to climate. So it’s how science progresses as well. And the point, the reason Im giving this talk now is I think, I believe, we are likely to enter an era where surprises will become more common.

We’re entering an era of climate destabilization. So why? Well, these are maybe not too surprising figures but its always fun it looks at them and just boggle at how much the world has changed over the last century. And this is just from 1990. And I use this slide a lot with my undergraduates or versions of this, and that’s always amusing because most of them were born in like 1989. So I get to say, since you were kids, but I don’t think I can do that here with maybe one or two exceptions. So since you were younger, since 1990 populations increased 22% to six and a half billion people. Population has doubled since 1975 or since the 1960s, basically. Oil consumption grew about 25%. So we’re consuming more oil, actually at a rate faster than population is glowing. 85 million barrels, a barrel is 42 gallons. 85 million of them every day. 22 million of those just in the US. And gross world product, how rich we are, grew 40%, it shrank a little last year. But $59 trillion.

At the same time, the number of threatened species, species that are threatened by habitat destruction, that may be listed as endangered but not necessarily, has increased 40%. The best estimate is 16,000 but its almost certainly a major underestimate because we actually don’t even know most of the species on the planet.

So this is kind of the documented species that we keep track of, it’s actually from the UN, about 16,000. And we can see a population doubling from 1850 to 1930, it took 80 years to go from one to two billion. But from 1930 to 1975, it took 45 years, and the expectation is in about 2015 we’ll be at eight billion. There’s lots of discussion among UN population experts about fertility rates and whether theyll decline or whether theyll level off at something like eight or nine billion. I’m not an expert in that, Im going to go with this and say theres a lot of change in the last hundred years. It’s pretty incredible if you think about it. A lot of it is good things. Probably the largest causes of population increases have primarily been things that have increased, improved the quality of life. We have green revolution which significantly improved the amount of agricultural productivity we can get on a single hectare of land. Basically cured malnutrition for some time until populations kind of caught up with that. Public health, vaccination, hand washing, all of these things are huge. Major advances in the 20th century.

At the same time, though, its put a lot of pressure on our ecosystems and climate. And one perspective of this is looking at the amount of carbon dioxide in the atmosphere. We’ll get to what exactly carbon dioxide does, but here’s a figure that the National Oceanic and Atmospheric Administration makes measurements of CO2 all around the world including with partnering with my lab. And here’s a global average carbon dioxide, and its measured in parts per million. So if there’s a million molecules of air then how many of those are carbon dioxide. And here we can see from 1980 to 2009, I believe is when this end, the blue line is the monthly average and the red line is a smooth curve that goes between them, that interpolates on the annual scale. And you can see that we’ve gone from something like 340 to 380 parts per million just since 1980. And you can see since 1990. Currently we’re at something like 386 parts per million. Now, at one level that sounds like a really small number. But at another level I hope to convince you that ecosystems respond strongly to the climate changes that are interacted with that change in CO2.

Now I don’t necessarily want to get into a treatise on the relationship between carbon and climate, that’s a whole topic in and of itself. Some of you have heard various things in the news about it. We can discuss it and I’m happy to take questions on it. Here, let’s focus on the fact that we know CO2 is rising. This is a really solid measurement. Here’s a picture of one of my sites. I have a 400-meter tower in Northern Wisconsin outside of Park Falls, and I work with NOAA to measure CO2 continuously. We have this little trailer thats filled with lots of fun toys. And right here, for example, is this system that’s measuring continuously CO2 using infrared spectroscopy, and then we also take flask samples where we take glass tubes and we fill it with air and we mail it to Boulder, Colorado, where a lab also analyzes it for other greenhouse gases. They also measure carbon dioxide and a number of other variables, and I know how solid these measurements are. We measure this down to .2 parts per million precision.

So we have a really good, solid feeling for CO2 change in the atmosphere. And we know this global average pretty well. And we have now hundreds of sites around the world making this measurement. And if you go back in time and we take ice core measurements in Antarctica and in Greenland and we look at the bubbles trapped in ice core. So here’s a picture of an ice core stolen from Wikipedia, the font of all sorts of fun pictures. And these are annual layers that you can see in the ice, so this is something like maybe a century of ice core. And within each layer you can date the layer based on radioisotopes, and you can extract bubbles of air and measure the amount of CO2 in the air, and then you can make a record of CO2 through time. And this has been done here, 800,000 years into the past. So the axis on the bottom is years before present, and the axis on the Y axis is carbon dioxide in parts per million. And a couple things are really striking.

So this is a mixture of three ice cores, and then the instrumental record is also tagged on to the end right here. And the long-term average before 1850 is 232 parts per million. And you can see over 800,000 years we’ve kind of gone between something like 180 parts per million and maybe we’ve hit 300 parts per million. Of course the uncertainty on these measurements are much larger than .2 PPM because of all the issues that go with measuring bubbles in ice that you dug out of a hole in Antarctica. But we can see that the instrumental record shows a significant and dramatic rise since the start of the Industrial Revolution up to 385 parts per million. And we know that CO2 is linked to climate from our long-term understanding of how earths orbital changes occur and how carbon dioxide and temperature are linked which I’ll describe in a minute. But one thing we know, for example, is all of these periods here, CO2 is roughly low, are well-connected to previous ice ages.

The last ice age ended 11,000 years ago. And of course we’ve been since then coming out of that ice age. If you talk to geologists theres some suggestion that we, theoretically, should be coming into an ice age, but most likely weve prevented that. What geologists talk about is on a scale of thousands of years. I’m talking on a scale of a hundred years. In a hundred years the long-term changes like that don’t matter. What matters is how weve changed the atmosphere through our activities. And we know it’s our activities because if you look at this, this is a best estimate of global energy production from 1850 onward, and so you have year on the X and a unit of “x” joules per year, just think of it as an amount of energy, and you can see we’re using nuclear, hydro, gas, oil, coal and wood. And you go back to 1850 and our best estimate is we used mostly wood as our energy source.

Coal kind of comes in especially mining of much of the anthracite in Pennsylvania and then into other parts of the world and primarily in England as well. And you bring in oil as it struck in the late 19th century starting in Titusville, Pennsylvania, and moving onward. And it’s a whole bunch of carbon. All of these energy sources except for nuclear and hydroelectric emit carbon dioxide when they’re burned. These are all fuels that are made from long-term deposition of dead plants and dead seashells and dead animals building up over tens of millions of years that we’ve now mined and burned.

So what we know is that since 1990, in addition to those population and production changes, that CO2 emissions which pretty much track economic growth also grew 25% just like gross product to 27 billion tons of CO2 per year. And CO2 in the atmosphere grew 10%. Hmm, well, that’s a puzzle. If I said emissions grew 25%, shouldnt the atmosphere also grow 25%? We see here that CO2 grew 10% to 385 parts per million. At current rates, we know that CO2 is likely to exceed 500 parts per million sometime this century. And if we go back to this slide, 500 parts per million is not even on this graph. We’re way, way up there. So there’s bound to be surprises in how ecosystems respond to the significant change in CO2 in the atmosphere, first of all, and second of all the climate response to that.
But this puzzle here, why is the rate of atmospheric CO2 half the rate of emissions, roughly? Every year we pump out 27 billion tons, and the atmosphere only grows about half that much. Well, here’s a figure showing, in the red curve here, total fossil fuel emissions from 1950 onward, and this ends about 2000. And that’s a mix of both fossil fuel emissions and then land use emissions which is primarily deforestation primarily in the tropics leading to a significant quantity of carbon being added to the atmosphere. And you look at the red curve and focus on the red curve compared to the bar graph. The bar graph is how much the atmosphere grew. And two things become obvious. One, this curve here is very smooth, this bar graph is very messy. It’s not because it’s noise, its because the atmosphere really has lots of variability year to year in how fast it grows. And if you look at the difference between the two, thats going to be something related to things that use carbon and how much they use it.

And so I call it the ecosystem carbon sink. If we emit this much carbon and the atmosphere only grows this much, then the difference between the two must have gone somewhere. And where it goes is into the land and into the ocean. And one thing that’s very interesting about the land and the ocean is unlike our fossil fuel emissions which is very smooth, and this curve we know pretty well because we pay for fossil fuels, therefore we keep track of things like that. Unless you’re lucky and you get them for free. And the ecosystems respond, theyre very dynamic. Year to year they take a different amount of carbon. What I’m trying to get to here is that the oceans in general are nowhere near as dynamic as the land. And partly it’s because the oceans have a lot of heat reservoir, they respond slowly to changes, there’s lots of mixing and so while oceans are continually taking up carbon year to year, they do some amount. So we can say we can take out some of this to oceans.

But really this year to year variability is strongly driven by small water bodies and by land systems. So we need to understand how ecosystems take up carbon if were going to understand how atmospheric carbon dioxide grows which is going to be our primary driver of climate change. This is a complex figure so no need for me to really walk through this other than to say we know to some extent the global scale where carbon goes. About half of it goes into the ocean which is all of the boxes on the right side. And this is kind of a best guess estimate of all of those numbers. And on the right about half of it goes into the land, into the vegetation and into the soil, eventually. And you can see fossil fuels on the bottom. I should be pointing; I should use this. Fossil fuels on the bottom, and this is what we’re emitting, and this is where the stuff is going in, eventually.

The problem is figuring out how much goes into each one of these. And then figuring out how if you change the climate and if you change the CO2 how do those things change. And that turns out to be an interesting puzzle. And the big deal, of course from the perspective of climate, is for all debates about climate aside, the science between carbon dioxide and how-to radiation budget the amount of energy that’s used by the atmosphere is very solid. I can write a climate model in two equations. This is what I do in my class. And you can exactly pretty much track the temperature change in the 20th century and get a pretty good estimate of the temperature change in the 21 centuries just by making simple assumptions. Because what we know is that carbon dioxide, this is a figure from the Intergovernmental Panel on Climate Change for assessment and reported in 2007, I encourage everyone to read the summary for policymakers, very accessible to the public, available online PDF.

And one of the figures that’s a classic figure is this figure here that shows the amount of forcing and radiation. So the amount of energy thats stored in the atmospheric system in watts per square meter due to changes in the earths atmosphere from carbon dioxide, methane, nitrous oxide, halocarbons, everything else. So you see up in front I circled these because these are ones that land ecosystems respond to. The amount of CO2 in the atmosphere is strongly going to be driven by the fossil fuel rate on the land and the same thing with methane. And then there’s these ones on the bottom. Because the other way to change earths climate is not by changing the atmospheric composition but by changing the surface of the earth. Clearing land changes the amount, say, how bright land is which affects how much energy goes back into the atmosphere.

And you can see here these numbers here are positive which means increased CO2 has generally been a net warming on the atmosphere. But land use, and these are aerosols, so essentially when plants emit and factories and fossil fuels emit particles that make it dimmer so that blocks the sun out, basically, and then it also changes how clouds grow. So those generally, we suspect, have mostly a cooling effect. But the net effect total, this plus this, is still in that warming effect. And this is what we know for the 20th century, about one and a half watts per square meter leading to roughly a half degree Celsius or a one-degree Fahrenheit change in global average temperature. And this works because we have a pretty good handle on how energy flows through the system, through the atmosphere, through the ocean and the land.

This is another classic figure from Kevin Trenberth. This is an updated version that he recently published about the amount of energy flowing in from the sun, through the atmosphere, through the surface, back into the atmosphere and then back out to space. This is also once again a complex figure that has a lot of numbers. We can simplify this by just putting a bunch of circles on it, thanks to the magic of PowerPoint, and we can think of this as basically sunlight, infrared radiation that’s emitted by the earth’s surface and interacts with the atmosphere, weather which kind of works in between everything, moves stuff around, weather moves heat and moisture around, it brings cold air from Canada and warm air from Texas up here, and land and oceans.

What’s interesting about this figure, the atmosphere doesnt do all that much with solar energy, it reflects some of it, it absorbs some of it, but most of it gets absorbed by the surface. The surface then heats up and emits infrared radiation which is absorbed by greenhouse gases in the atmosphere, primarily water vapor and carbon dioxide. Now I don’t mention water vapor in climate change discussions, but water vapor is a feedback. It’s very hard to control the amount of water vapor in the atmosphere because water is the active gas that’s responding primarily to temperature. It’s by changing to CO2 that water vapor then changes to temperature changes that then lead to feedbacks.

But what we say here is basically for all of these pieces, the solar, the infrared and the weather they’re all connected to the land and ocean ecosystems. So feedbacks between how the land and ocean ecosystems absorb heat, moisture, carbon and nutrients, biophysical and biogeochemical, strongly are going to impact atmospheric climate. And the big deal is this is now an estimate, once again from the IPCC, of the last thousand years of global average temperature. And this is from a mix of tree rings, plus ice cores, plus a few other data sets. What you can see is obviously theres a lot of uncertainty to this as you go further back in time, and some of you might have followed some of the debate with the recent email scandal at the Climatic Research Unit. A lot of that was about tree rings and how to derive temperatures from that.

I’m not an expert in that so Im going to go with this and say roughly we’ve had, over the last thousand years, a relatively stable climate. So over the period of, essentially, the flourishing and progress of human civilization, weve generally had a stable climate, and though we have had climate fluctuations, theyre generally related to large scale disruptions in human civilization. And most of these have generally been related to things like volcanic explosions or large-scale climate variability. Starting around the mid-19th century we start seeing a warming effect, some of this is natural, some of this is related to fossil fuel combustion, and into the 20th century.

Now what’s interesting is now if we go to a future of 500 to maybe a thousand parts per million of CO2, what do we expect? And really, it’s all about this figure. What we do when we do climate modeling is trying to quantify every single piece of this, and then try to understand how changing to CO2 impacts that. Now what you’ll see is this really large spread. That large spread is not from our lack of understanding of how things work. That large spread is down here in this so-called scenario. It’s about people, people are unpredictable. How many are there going to be and what kind of energy are they going to use and where are they going to emit it? And so we have to make assumptions. We call these scenarios and theres many scenarios based on what expect our future to look like. And that’s what’s driving a lot of this uncertainty of these one to six degrees Celsius, which is roughly two to 10 degrees Fahrenheit, change in temperature over the next hundred years. The climate uncertainty from the modeling and from our work of understanding all of those pieces are mostly in here.

Now I’m going to say some of the surprises are likely outside of that envelope. Now you’ll see two other lines on here. This red line that says historical high temperature level for the last 400,000 years. I should say over the four and a half billion-year history of the earth there’s been plenty of temperatures that have been far above this and certainly ones that maybe have been lower than this. But we’re not interested in that because there were no ecosystems necessarily back then. We’re interested in recent change.

And you can see here proposed temperature thresholds for so-called dangerous climate change. And dangerous is maybe a loaded word. The point here is most people agree that ecosystem surprises and the way ecosystems respond to climate are likely to be more dramatic when you get above about two degrees above where we are now. So a lot of negotiation has been based on how we keep climate to warming beyond two degrees because that’s when we believe a lot of things like agricultural productivity, which under a warming climate should increase, after two degrees it starts to decrease in most areas of the world. Is focused on here in this idea of how we stay under that. It turns out to be a really, really hard target if we dont put a lot of investment, say, into moving away from fossil fuels.

And we can make projections then of climate change here from the decade of 2020 and the decade of 2090, and you can see in 2020, a mere decade or so away from now, we would expect at least some moderate amount of warming maybe on the order of a degree. It might be even hard to detect in lots of parts of the world, maybe even a few spots that are still cooling. But by 2090, assuming business as usual in fossil fuel emissions, we’d expect significant warming, especially at high latitudes. Also in the tropics this is here to like signal in the tropical Pacific about warmer El Nino-like state. Maybe some cooling out here in Greenland or less warming related to ice melt coming off of Greenland and into the oceans. Lots and lots of interesting little pieces to that.

What I’m interested in is can we use measurements today to understand how ecosystems respond to changes in the temperature to help us improve our understanding of how these larger scale climate uncertainties and climate changes are going feedback. And we can see this already in Wisconsin in terms of climate change. This is for 1950 to 2006, a very careful work by Chris Kucharik here at UW and a grad student, working on changes in daily maximum and minimum temperature. And climate change, when you get down to this scale that ecosystems care about, the regional scale, so here’s a map of Wisconsin, reds are warming, blues are cooling, is very complex at that little scale because all sorts of little pieces occur. As you can see here, changes in winter maximum and minimum temperature are the strongest trend over the last 50 years. Something like several degrees Fahrenheit over the last 50 years. And I think most of us have an intuitive sense for that. Winters have generally been warmer, last winter or so as an exception to that, but actually globally it’s been one of the warmest winters. It’s the second warmest January in a hundred years. There was, obviously, not so great winter in Vancouver with cherry blossoms going on everywhere. Not so great for skiing anyway. In the summer maximum temperatures have actually been cooling in Wisconsin. So we have all of these interesting feedbacks that we need to understand, and fall is even more puzzling.

But we have a problem which is that if we try to model how the land uptake, the amount of carbon dioxide being taken up by the land, changes with time and we take our best models and our best guesses of how that does, and we look here from 1850 to 2100 and here’s land uptake in billions of ton of carbon per year, over the 20th century all of these models, each one of these lines is a different model, they all kind of agree. But then you start adding climate change on it, and you start getting out of the envelope where these ecosystems are adapted to and we have lots and lots of uncertainty about which direction they’ll go in because of competitive pressure, because of climate stress, because of CO2 fertilization so increasing CO2, feeding the plants basically. And some of these models go nuts in the future. They start really emitting large quantities of carbon. The land system is just basically giving up. Essentially what you’re seeing is the soil decomposing at rapid rates, permafrost melting. Other models are like this is the most beautiful world Ive ever lived in because were going to have a lush tropical garden. Which one is right? I don’t know, we don’t know. No surprises here.

What I’m going to tell is what Im going do. The better we can reduce the uncertainty of how carbon responds to climate and ecosystems, the better we can model future climate change. That’s really our goal here is to understand climate change in response to ecosystems. Now I’m going to present three stories, with however much time I have, to illustrate some surprises that our lab has worked on that are helping us understand how to improve and make this picture better so that instead of all of this spaghetti out here, we can have kind of some sort of understanding predictability. Right now we lack a predictability. And this is going to add a lot of uncertainty to that projection of future climate change.

So my first story. This one’s mostly a biophysical feedback. So this is about ecosystems responding to heat, moisture, and this one is about heat. We know roughly that is takes longer to boil a pot of water than to heat up a room. Water takes longer to warm up than air. And we know this in a climate system the oceans have a lagged response to the atmosphere. The oceans are receiving heat from the atmosphere and they take a long time to heat up. So climate change warms air faster than water should seem like a really straightforward response. And here’s my little figure of that if you’re a visual thinker. Some amount of forcing, and I put it in watts per square meter, change in temperature. Here’s land. You add forcing, it gets warmer. Here’s water, it should take less.

So my hypothesis should be that large lakes, like the Great Lakes, the ecosystems are generally buffered from the impact of global warming. They shouldn’t necessarily be all that impacted by changes in temperature. And here’s some big lakes. This is an air photo of the Great Lakes, and here we are down here in the lesser Great Lake. And here’s Lake Superior. World’s largest lake by surface area, second or third largest by volume depending on which seas you count as lakes. And here we’ve taken buoy data from the lake and we’ve taken air temperature data from a number of other stations and interpolated them over the lake, and we have good measurements from 1985 to about the present. And we look at the land temperature and we can see a pretty robust warming trend in land over here. This is summer average. And we see a pretty nice trend in water too. And, in fact, if you subtract water minus land, and so this is delta temperature, that looks good. But this trend is decreasing closer to zero. What that implies is that water in Lake Superior in the summer is warming faster than land. That’s a surprise. That’s how we are in the lab when we find something exciting, generally. Not with the red hair, though.

What’s going on? So let’s take a look at one buoy. So here’s a single buoy where weve made temperature measurements, my colleague Jay Austin at University of Minnesota Duluth has worked most on this. And you can see, of course, that they put the buoy in water in March after you’re out of the ice-free season. April actually here in northern Wisconsin. And it warms very slowly because the lake is really mixed down. As you get warmer, warm water is less dense above four degrees Celsius. That water is going to kind of stratify. So you’re going to have warm water on top and cold water on the bottom. How many people here like to swim in Lake Superior? Not many. It’s pretty cold, right? Maybe a few of you do, very brave. But it’s cold most of the year. Except for that shallow surface water, the first tens of meters.

So usually sometime in the middle of the summer you can actually get into the water and its okay. But can see as soon as that water stratifies, and heres picture here based on some measurements we made under water and we’ve interpolated it using a model, of temperature with depth. So you can see from surface down to a hundred meters, this isnt exactly the same location but its similar. And you can see 20 degrees Celsius to near zero. And as soon as that lake stratifies and you have this warm layer on top of this really cold layer, the lowest, deepest depths of Lake Superior stay cold year-round, it starts to warm rapidly. And you have storms occasionally that mix some of that water down and it gets really cold right after a water storm, a storm the water gets cold. But for the most part it says very warm. And then fall comes, storms come through, it gets really windy, it mixes everything up again and that cold water comes near the surface and ice starts to form in the winter.

Well, it turns out, based on the research being done here, that if you look at the amount of ice on the lake, so the total amount of fractional coverage, Lake Superior rarely freezes over, but the amount of ice is going to strongly control when this lake starts to mix and stratify, which is then going to affect how much heat we can get in a surface part of the lake, where most of the ecosystems in the lake are photosynthesizing. So what we find is that from 1973 to 2005, through the work of Jay Austin and John Magnuson who are experts on lake ice, lake ice coverage, average coverage is in blue, or average is about 25%, has declined significantly. And it’s gone up in the last year or two, but in the last year it was a pretty record ice cover. And you can see maximum ice coverage, of course, every once in a while, freezes over, and minimum ice coverage, of course, in the winter is just going to be near zero. This decline means warmer winters equals less ice equals more heat into the lake. More time to build this temperature trend into the lake. Which then means warmer summers. So warm summer lakes. And this is the main reason that this lake is warming faster than the atmosphere above it in the summer is because it’s a memory effect of what’s going on in the winter.

Now what’s the ecosystem impact to this? And that’s a little bit more complicated because lake ecosystems are not just dependent on temperature, theyre dependent on exchange between the atmosphere of carbon dioxide in the water which have primarily a chemical reaction. What we see if we actually take a first order model of Lake Superior circulation, we couple it to a model of lakes ecosystem, and I’m going to focus not on the top one, well come back to this one with time, is the length of the stratified season. So the number of days in the stratified, the stratified season is when ecosystems can grow because it’s warm, is slowly increasing with time. And the depth of the mix layer, so how deep down the layer of warm water is, has gotten maybe slightly deeper.

What’s the impact of this? Hard to tell. What we’ve done so far is weve developed this 3-D model, primarily my colleague in our department, Galen McKinley and her grad student, theyve run the 3-D model of lake circulation and productivity. And what you see in the blue line is the model’s estimate of lake productivity in amount of partial pressure of carbon dioxide in the water. And what you see in red is what the atmosphere has, how much CO2 is in the atmosphere. And it’s the difference between the two that’s going to determine how much CO2 is going to come in or out of the lake. If there’s more CO2 in the water, then the air is going to come out and vice versa.

Impacts? We don’t know. Our best guess, if we look at models and we look at the seasonal cycle of our PCO2, our partial pressure of carbon dioxide in the water, CO2 is dissolved in gas phase in water, theres a lot of variability and it seems to be related to that stratified season length. So you compare, for example, 1997, a cool year; 1998, an El Nino year, warm. And you can see there’s this really big difference. 1997 is in red, 1998 is in green. Check out how different that cycle of CO2 is in the lake.

We’re still at a stage of trying to figure out how important is that impact. Our estimate so far is that it probably does impact the productivity of the lake to a significant quantity, but on a basin-wide scale the productivity at that lake is so low that the variability maybe isnt significant for the perspective of ecosystem change. It is a surprise to the lake. And of course the other thing thats happening in the lake with warmer lakes more evaporation is dropping lake levels. And that, of course, affects infrastructure on the lake which has been a big issue up there for the last several years.

So the moral so far of this story, this is a short story, I like short stories, large lake heat budgets are sensitive to the dynamics of ice and lake heat budgets drive lake stratification and, consequently, lake productivity. So now I’m going to fix my little figure here, and if I’m going to add large lakes and if I’m going to understand how ecosystems and climate interact, I need to be sensitive that lakes with ice are going to act differently than my assumption of how water normally is.
So I can draw this new line.

Let’s go to something maybe a little bit more familiar, which is a forest. And maybe we can do better in a forest than a lake. Lakes are messy. They’re wet and it’s hard to get measurements in them. There are very few measurements actually in Lake Superior productivity. So story two. A warmer spring. In general, we’ve seen globally climate-wise, spring has been warming relatively fast, we saw that in the Wisconsin slides too, so if spring warms then plants can start growing earlier. We can put stuff in the ground. Probably many of you who are gardeners have this kind of sense that you can actually get stuff in the ground earlier than you used to. The USDA, a few years ago, just updated all of their maps so that we have now moved zones from the little zone maps they put on the seeds. They did that really quietly, actually, without really making much publicity about it. A warmer spring leads to longer growing seasons for plants.

And we’re going to look at high elevation forests where we know that they’re very temperature limited. So I say forest productivity in the Rocky Mountains should benefit from global warming. So here’s my little map, productivity, amount of carbon taken in, and start a spring early versus late or long growing season with an early spring or short growing with a late spring. So if you have a short growing season, you have low productivity; if you have a long growing season you have lots of productivity. That makes sense. A lot of these climate models expect warmer temperatures leading more productivity in most forests. And we know this from measurements.

Here’s some measurements in two north eastern forests, a deciduous forest and a spruce forest in Massachusetts and in Maine, respectively. And we know that the onset of spring, late springs lead to lower productivity, this is gross primary production anomaly, so don’t worry about what that is in detail but its productivity. And early springs lead to higher productivity. It’s a very strong correlation in this forest. And this is where we have a lot of information here at Harvard Forest. And we know in high elevations that spring has been rapidly warming in many mountain areas because if you have changes in temperatures, you’re less buffered from other landscapes in the mountains.

So this is from work from my graduate student who’s been looking at temperature trends in mountainous areas. This is elevation on the bottom, its kind of cut off here, sorry about that, and temperature per year, degrees per year over the last 30 years. And you can see there’s a lot of scatter at low elevations and you can see it’s roughly near zero, maybe positive. Once you get to high elevations, youre almost always certainly positive. You’re seeing it’s getting warmer in degrees per year. And this is over the entire western US. But here are some measurements made of productivity. And this is a different measure, but it’s related, positive is more uptake, so more positive, more uptake. And you can see here growing season length. Short. Long. Short growing seasons, lots of productivity. Long growing seasons, less productivity.

Hmm, that’s not right. That’s a surprise. That’s a cute baby. I don’t know who it is. A little bit on measurement since I’m a measurement person, I like to talk about how these things are measured. Those measurements made in those are based on something known as eddy covariance. And the idea here is that the atmosphere is continually turbulent and if you stick measurements that measure that turbulence above a forest and you look at the CO2 coming up and down and you measure how fast it’s going up and down by measuring wind speeds and carbon dioxide with these two instruments here and you do this 10 times a second for an entire year and then you average these things up at 30 minute intervals and then you look at annual values, you can estimate annual productivity. It’s a lot of work.

And you have to build these towers in the woods, which is fun to do, and it’s a lot of data coming from essentially very high-tech sensor, but it turns out to be a really good way to look at how ecosystems respond to weather and climate. So this tower here, the measurements I showed you are from sub-alpine spruce and lodge pole pine dominated forest. And here’s the tower and heres the annual measurements. So this is the average seasonal cycle over the last decade at this site. And you can see here this is daily uptake. In the winter it’s negative because basically all thats happening is respiration so youre not taking up any carbon, youre just decomposing. And then suddenly spring comes in. We come right up. Very, very fast things green up. Even though this is an evergreen forest, the productivity still works like a deciduous forest. It really comes up fast as soon as the snow melts and as soon as the temperatures warm.

But then something funny happens in the summer. It’s starts to ramp down. And if anyone’s been out in the southwest US, you know that in late July, early August, things really start looking brown out there. And what’s happening is moisture stress. It just doesn’t rain a lot in the summer. Until later in August. If you go back out there in late August, it seems like there’s a thunderstorm every two days. And especially up there in the mountains. Almost every day there’s a thunderstorm. It’s incredible. We’re actually kind of scared to work on this tower in that time of the year because youre always keeping your shoulder out looking for lightning. That’s why I hire grad students to go out there instead. [LAUGHTER] But you can see it comes back up. And then winter comes in and it shuts down.

So what’s happening? So here’s that figure again about production and growing season. It turns out, and this is work from a grad student at the University of Colorado, who showed that if you look at snow water equivalent, which is a measure of the amount of moisture in snow, in late winter and you compare it to growing season length, it’s really strongly correlated, almost in the same exact pattern as NEP, as production. And what that implies and what you can see here in this figure on the right, is the first day of the growing season is directly, linearly correlated with how much snow is available. So if you have more snow, it takes longer to melt and then even if you have a warm spring, youre going to have some of that snow stick around.

So what’s going on? Well, it turns out that snow, that the forests in the Rocky Mountains and other sub-alpine mountainous areas are incredibly dependent on snow for water. Because they don’t get a lot of rain in the summer, their productivity is driven almost entirely by snow. And so here’s a model where weve taken an ecosystem model and we’ve kept track of the amount of snow and water where the water is coming from thats used by a plant. And we can actually do this by looking at deuterium isotopes in snow and water and they have different signals. So if you go and take sap out of a tree and you measure the amount of deuterium in it, you can tell whether that water was from old snow or from fresh precipitation. And you can see from this the amount of productivity driven by snow is huge. It’s a majority of this forest. So the amount of snow strongly drives how productive this forest is going to be in the summer. That’s interesting. We didn’t really know that.

So the moral here now is that warming temperature is not necessarily a net positive for productivity. Because changes in moisture in snow really matter in western forests. So while this may be true in eastern temperate forests, its almost certainly the case that this is more likely the case for mountain sub-alpine forests. And it tells us that we need to do a better job modeling the water cycle in our understandings of carbon cycle. And that’s where a lot of the science is.

Last story and then some thoughts. Wet places like temperate, lets get away from the Rocky Mountains where we know its really dried all summer. Let’s go somewhere where its wet. Wet places like temperate regions like here, especially north here, northern Wisconsin, are not sensitive to variations in precipitation. So year to year it might be a little bit — or it might be a little bit dryer and, yeah, your garden might respond to it, but a forest as a whole they have a lot of mass, biomass, they should generally be well adapted to taking advantage of the fact that there’s ample moisture all summer long.

And especially wetlands. Wetlands are inundated with water. North temperate wetlands shouldnt vary with the groundwater elevation thats fluctuating with the amount of precipitation sending in the water. So here’s my little plus or minus 10% precip and productivity. Flat line. Some work here by my graduate student who’s in the audience now. Which is good. Beautiful slides. A bunch of wetlands that hes been working at, and so here we are in northern Wisconsin which is a wet place and here’s the boreal forest in Canada, also pretty wet, lots and lots of wetlands. The Peatland in Alberta and then heres another bog in Ontario and a few sites right here that our lab maintains and runs. And here’s picture of a few of those.

And these are all, once again, the same technology, eddy covariance. Now we don’t need a tall tower because most of the vegetation is pretty short. So we can go and build these little tripods and platforms, maybe get a little bit taller, and get the same kind of measurement. Continuous, 10 times a second measurement of exchange between the atmosphere and the surface of carbon dioxide, moisture, heat, momentum and then infer annual productivity from that. And if we look at all of these wetlands, some of them are fens which means they’re primarily driven by groundwater input and they’re not very acidic so you have a wider variety of vegetation on them versus bogs where you have kind of low peatlands and kind of more mossy type vegetation.

And if you look at the fen and you look at productivity across that gradient and you look at each site has multiple years of data, there’s a really strong relationship between water table height, which is how high the water is either below ground or above ground, and productivity. And dry conditions lead to higher productivity in the fens. Wet conditions lead to low productivity. That’s weird. That shouldn’t be the case. So once again a surprise. Now a baby monkey sort of devolving here.

Water table elevation, so heres some data from one of our sites where we measured water table with a pressure transducer. You can kind of use atmospheric pressure to infer fluctuations in water. Is driven by precipitation and evaporation. That makes sense. So here is one wetland site, a shrub wetland, where water table has been declining continuously for seven years as this area has been drying out. And we know that basically annual precipitation values are strongly related to that water table depth. So we know that precipitation is driving this wetland. Precipitation is affecting water table is affecting productivity.

And for Wisconsin it worked really nice where a lot of these wetlands are in an area that weve seen over the last 50 years a strong drying trend. Precipitation per year. Most of Wisconsin like most of the Midwest is getting wetter. Which you might expect. A warmer climate means you have a faster rate of evaporation which means you have faster rates of precipitation. For reasons that we don’t quite understand, this region of Wisconsin is getting dryer. Some of this might be lake effect, this is Michigan, Upper Peninsula. Some of it might be feedbacks between vegetation. It’s very interesting.

And of course this is messy because we don’t have, it’s hard to get good measurements of precipitation. But because we have this trend, we have a natural experiment to see how an ecosystem responds to drying. And we know this is more than just a trend of a few wetlands. This is a paper published a couple years ago showing that if you look at all the Great Lakes, Superior, Michigan, St. Clair, Erie, Ontario, and you look at all these small little lakes in northern Wisconsin, Crystal, Sparkling, and big old Muskie, they’re all dropping and they’re all very coherent at how quickly they’re dropping their levels. This is water level in meters standardized. So we know there’s something about water going on here thats more than just simple variations in precipitation.

But it still doesn’t explain whats going on in, say, Canada where we don’t necessarily have the same robust drying trend. What we know is to some extent this is being driven by fens being a little bit more plastic. What that means is the adaptation of plants to changes in precipitation is going to be a function of what that species can do, what is this range of variability. And what we see is that as it dries out, if we look at the water use efficiency, so the amount of water use, molecules of water per amount of molecules of carbon taken in, as it gets dryer it actually goes up in fens. So fens are actually defending against drying water. In fact they’re doing a really good job such that their productivity actually increases over what you might expect. Bogs, not so much. Maybe going more in a direction you might expect. It’s actually struggling.

So what we think is that, what we suspect, and we don’t have a strong verification of this is that bog plants are generally less well adapted to variations in precipitation. Whereas, fens generally can do pretty well. In fact, they can actually increase their productivity as you have taller vegetation coming in that’s able to actually thrive. Obviously imagine if you totally dry out a wetland, it will eventually become a forest which is, on average, more productive than a wetland. So to some extent this is not as surprising as we thought. But the moral is ecosystems in wet regions are not immune to shifts in precipitation like we kind of suspect when we do lots of simple estimations of how precip and productivity respond.

Plants adapt to change, and the time scale depends on the kind of ecosystem. And so maybe temperate fens are like this: I don’t have time, but some of my other research Ive done shows that maybe forests actually respond like this, maybe what you might expect. Although it’s kind of surprising that even a forest up here is going to respond to a 10% change in precipitation. So if we were to review the morals, we say nothing is simple. Which is good. It’s job security for me. But not so fun for managing ecosystem. Rising temperature can add beneficial and deleterious effects on ecosystems. Depends on the season, depends on lags. So winter snow melt and spring temperatures impacting lakes and forest.

We have to think about how climate change in one season affects something in the future. Moisture stress tends to covariate with precipitation and temperature, and we’re not doing a good job of really thinking about how moisture impacts ecosystems at the scale of understanding climate change. We know it at the small scale. You can go in the lab and you can dry out a plant and see it wilt. And you do all of that and thats basic science. But once you get out of the ecosystem in the field, theres so many interactions playing at once we don’t necessarily have as good a feel for how things should respond. There are many kinds of stresses on ecosystems. It’s these interactions where we find our surprises.

Now to wrap up I’m going to talk about what about feedbacks. So everything we talked about so far is about climate impacting ecosystems. The other direction also happens. If ecosystems change, they also can then change to climate. Because if you change how much carbon you’re taking in, youre affecting the atmospheric CO2 budget. Maybe that’s not as important than other things. If you cut down a forest or you change the reflection and the amount of energy budget, recall that first picture about how the land system and the ocean systems were connected to the solar radiation, the infrared radiation and the weather, it can influence all of those. You can change some of those by changing the reflectivity of the surface. If you go from, say, a dark forest to a snow-covered field, you have a very different amount of solar radiation that’s being absorbed and a very different temperature that will respond in the atmosphere.

And here’s a classic figure from Jon Foley, who’s at University of Minnesota, on vegetative case and you deforest that, and you would expect many changes in higher surface temperature, less evaporation, higher reflection of sunlight, maybe less humanity leading to reduced precipitation. We have some feel for how these things work. And of course for each story, theres a feedback. So if you go back to the lakes, when we talked about the lakes getting warmer faster than the land, well first we know about simple feedbacks of the lake, most of us are familiar with things like lake effect snow. Of course people in Milwaukee have just recently been bombarded with that six-inch rapid snow fall in two hours that caused hundreds of accidents, and so lake effect is a major part of our life of the atmosphere being modified by warm lakes, evaporating water and then dumping it over cold land.

And here’s a nice satellite image of all of these little streaks of clouds being generated by the lakes. And so you can see as the wind blows from west to east that you generate lake effect clouds. And this is beautiful. But there’s other things that are happening. So some of the work I’ve done recently showed that if you look at the wind speeds over the lake, and we know it’s getting warmer, the lake is warming faster than the land, if you look at the wind speed, over land, yeah, there’s a slight increase in wind speed. And this is probably related to large scale changes in atmospheric circulation due to climate change. So if you look at the lower atmosphere, we can take atmospheric models and instead of looking at the surface we can look higher up which is more driven by large scale circulation, and we can see that the increase in wind speeds over land is related to a lower atmosphere. But if you look at the lake, and we have measurements both from buoys and from satellite scatterometry where we actually kind of track sun glints off of water as it’s being pushed by the wind off of NASA satellites, we see that there’s a strong trend in wind speeds over the lake that’s faster than the trend over land. Almost twice as fast. And it turns out that this is a feedback.

So as the lake is warming due to what we talked about earlier, less ice equals more heat, thats actually destabilizing the atmosphere above the lake which is causing more momentum to be mixed down to the lakes surface which is increasing wind speeds. And that goes back to that figure that we showed earlier that showed current speeds were also increasing. So we’re seeing an atmospheric feedback that’s then leading to increases in current speed which we don’t have a good understanding at all how its impacting the lake. And we can see here there’s a very tight coupling between that temperature gradient between land and water and the wind speed gradient between over the lake versus over the land.

Story two is interesting, too. So we talked about productivity being impacted by temperature primarily by snow. There’s a whole other story about biotic competition going on in western US that’s huge. What we’re seeing in the western US is most forests generally have various insects that they generally defoliate or — or do something. Insects and plants have long co-evolved. And we have outbreaks of insects even up here. Things like tent caterpillars every X number of years and it causes lots of damage to trees and so forth. But for the most part trees recover and they do their thing. Occasionally there have been evasive species that are much more damaging of course, like emerald ash borer which is probably going to wipe out significant quantity of our ash trees in Wisconsin in the next decade.

But beyond that, there’s also climate impacts to this. So the pine beetles which bore into pine trees and which lead to direct mortality by cutting off their nutrient flow, have been on the march in lots of the country and they haven’t been dying off like the way they used to. So normally you get a pine beetle outbreak, trees get killed, pine beetles die in the winter because it’s cold. Well it’s turning out its getting warmer in the winter. These pine beetles need temperatures near negative 30 or so to be killed off. In winters past, you would expect at least one or two cold outbreaks at 10,000 feet elevation that are going to be that cold. Not surprising. We haven’t had those now. So what’s happening is the range of these pine beetles, the mountain pine beetle in British Columbia and Colorado, also the spruce beetle and the pinyon ips beetle throughout the inner mountain west, have been expanding in range.

If anyone’s been to Colorado recently or British Columbia recently and you’re driving around the forest, its incredible how many dead trees there are. Virtually most of the ski resorts in Colorado, I’m a big downhill skier, and you just spend your time staring at all these dead trees. There’s going to be a huge change in what skiing, skiing is going to look more like the alps because there’s not going to be these nice tree runs anymore. Which is sad for me. But what we’re seeing is an interaction between climate, warming climate leading to more beetles that are sticking around year after year going after a larger quantity of trees. They usually go after healthy trees because they like the sap of those. Now they’re going after stressed trees, which are then increasing the potential for forest fires because a dead forest burns easily because there’s no moisture in the trees. Which can then lead to a carbon cycle feedback.

This is something that’s really emerging in the last few years. And it’s now become so evident, especially across Wyoming and Colorado, there’s a few towns in Wyoming that I was reading about recently decided to cut down all the trees in the town because they wanted to stop the spread of this. The public is really attuned to this and they’re really attuned to the climate impacts of this throughout the southwest US. And especially in British Columbia. The economy of British Columbia right now, which is primarily forestry-driven, is expected to be devastated.

Very interesting feedbacks. And we can see this is in our data. My lab, in collaboration with the colleague in the Natural Center for Atmospheric Research, measures CO2 in the Rocky Mountains on the tops of mountain peaks. Fun work. So here’s one in a valley. This valley has gone from something about in 2006, 90% alive to 2009, 90% dead. This is a huge valley. This is near Winter Park, Colorado, Fraser Valley. And if we look at the nocturnal CO2 in the valley, which builds up at night due to decomposition, it’s actually been going down. So what we’re seeing is as these plants dry, and here’s a nice aircraft photo out the wing into Fraser Valley, and this might be hard to see the colors here but these trees are all brown. This is in early summer. These are all dead lodge pole pines. And what we’re seeing is CO2 is going down which is suggesting to me that there’s less respiration, autotrophic respiration, so plants that respire because they take in carbon, is actually going down. It’s very interesting. We don’t know what the impacts of this is yet. This is emerging research.

And finally with the third story we can look at deforestation. We don’t have large scale deforestation going on around here. So this is work from another one of my grad students whos looking at simple models of land atmosphere feedbacks, and where hes deforested a small region of the tropics here in the Amazon, and you can see this large-scale feedback in precipitation. It isn’t large, we’re talking a few percent decline in precipitation immediately after this deforestation. But you can imagine as you start playing with this model you can really ask some interesting questions. How is this relevant to story three which was about northern wet areas?

Well, while we’re not deforesting land, were definitely changing the composition of the land. Most of this area was clear-cut in the late 1800s, early 1900s, and most of what we see today is forest that’s growing back. And as we go into an era where we have lots of forests being replaced, early aspen being replaced by later hardwoods, lots of management decisions being made about which kinds of forests are going to thrive and which are going to have to be replanted, most of the land around here is managed. We’re going to have a very different impact on how much carbon uptake there’s going to be and what kind of feedbacks theres going to be. We don’t know at all what kind of feedbacks are likely. This is fun to look at and this is something that we’re starting to look into. This goes back to this initial figure that just says the only thing to get out of this is that there’s lots of feedbacks between ecosystem, climate, carbon, moisture, and that we have a lot of to learn about these by doing these simple experiments in the field where we go out, make measurements, look at natural climate variability and try to make some understanding that can then help us improve those models so that our uncertainties decline.

So to conclude, different ecosystems respond differently to climate change and temperature and precip. So a simple moral here is dont treat all ecosystems with the same paint brush. And complex interactions between the physical environment and all biological systems, including plant/insect relationships, should not be underestimated. Virtually none of these carbon cycle models that we’re looking at future CO2 have anything about insects involved. Models to incorporate tests and verify these interactions will help us anticipate surprises.

And so from a management perspective, the best we can do is anticipate. If I can give you a probability that a surprise is likely over some period, we can then do a better job of managing land for being resilient. Ecosystem resilience is kind of what we’re looking at for the buzz word for sustainable development. Itself a buzz word. Slow, steady rates have changed. So slow, steady change, ecosystems are well adapted to slow change. You can argue the climate has always been changing, why should we care? Well, slow, steady rates of change allow us to anticipate and react to surprises, they allow ecosystems to anticipate and react to surprises through evolutionary mechanisms.

Rapid climate change, the kind that we’re seeing over the next century, is something that maybe most ecosystems have not been adapted to, and certainly we know that in previous geological rapid climate changes, to the extent that we have some records of such, there has been large scale changes in ecosystems by looking at pollens. And so rapid climate surprises are likely to exacerbate ecosystem surprises. So from a policy perspective, there is a strong motivation to slow climate change in terms of minimizing ecosystem disruption because we know, based on, I hope I’ve convinced you that to some extent there are many surprises that come from climate variability.

I’d like to acknowledge my lab. Here’s a picture from last year of many of the folks who work in my lab and most of the work could not be done without them. Of course our funding agencies, the graduate school here, as well as the National Science Foundation, National Oceanographic and Atmospheric Administration, US Department of Ag, NASA, Department of Energy. Dozens of collaborators, as you might imagine all of this research involves people like me, who is a meteorologist, working with ecologists, working with hydrologists, working with all sorts of fun people who have very different ways of looking at a problem, and when we sit down and work together, that’s when we learn. And, of course, the University of Wisconsin Alumni Association for this opportunity to present at Wednesday Nite at the Lab. And here’s the cutest one. That’s mine. Thanks. [APPLAUSE]