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The Habitable Planet: A Systems Approach to Environmental Science 

Earth’s Changing Climate Interview with Chris Field

Interviewer: I’d like to start by asking you to tell us about this unique place, the Jasper Ridge Biological Reserve.

CHRIS: Jasper Ridge is special for several reasons. It’s a twelve hundred acre biological reserve, dedicated to research that’s part of the main campus of a major American university. It’s incredibly diverse. Basically all of the ecosystem types that occur in central California are represented here. And that goes all the way from very moist redwood forest to very dry Chaparral habitats to very species rich serpentine grasslands. It’s really unique in the sense that, in a single hour, you can bring a class out here, show them an ecosystem or an experiment, really get them engaged with how experiments are done, and get them back in time for their next class. It means that we can envision research experiences that involve, essentially, every undergraduate in the institution. That’s one of our goals, to make sure that every person who graduates from Stanford University has a Jasper Ridge experience that’s got a research component to it. We’re not there yet, but that’s where we’re headed.

Interviewer: How does it help your work being so close to the main campus?

CHRIS: The thing that is wonderfully important for us about the proximity to the campus is that we can do measurements with a level of intensity that wouldn’t really be practical anywhere else. We can have instruments that are fully protected that run for years on end. If something breaks, you can go fix it the next day rather than having to wait until the next expedition goes out to the site. It means it’s practical for us to have year-round experiments. And it’s also practical to engage a wide range of scientists who have other things to do. The standard way that ecologists do field research is they teach their classes during the winter and then they take two months during the summer and go to the field site. That works okay for the Artic, for example, where there’s a two-month growing season. But lots of ecosystems do interesting and important things year-round. And Jasper Ridge is wonderful in allowing the full range of scientists, all the way from the senior investigators to the first year graduate students, to be involved on a year-round basis.

Interviewer: Tell us a little bit about yourself. How did you first get interested in ecology?

CHRIS: I liked outdoors stuff from the time I was a kid. I was really into mountaineering as a high school student. My family always lived on the edge of the forest. My father was in the sawmill business. And from the time that I was old enough to have a job, I either worked cutting down trees in the woods or sawing them up in the sawmill. One of the reasons that I decided to become an ecologist is that, having grown up in an environment in which the sustainability of forest resources was really a key issue, something that was the topic around my family’s dinner table, I really felt that the future ability of our society to maintain the sustainability, was more dependant on getting the science right than it was on my participating in the family business. And that really led me to see what academics offered and then to focus in academics on the issues that I felt were most important to understanding the scientific foundations of sustainability.

Interviewer: How did you first get involved in global change research?

CHRIS: The history of global change research is really very rich and very interesting. Beginning in the 1950s, people realized that atmospheric CO2 concentration was increasing. Shortly after that, people realized that, first of all, plants are sensitive to CO2 concentration directly and the climate is sensitive to CO2 concentration directly. By the 1970s, people were talking about experimental studies to assess the impacts on ecosystems. The motivation for these studies got to be even larger once we realized that terrestrial habitats are currently a large sink for carbon. Approximately twenty-five percent of the carbon that’s emitted from fossil fuel combustion is stored, at least temporarily, in ecosystems on land. So the sensitivity, combined with the fact that there’s this relatively large amount of storage, really motivated a series of efforts to get to the bottom of how it worked.

The experiment started in a really modest way, where people grew plants of mainly agronomic species, in small pots, in growth cabinets, under a range of elevated CO2 concentrations or under a range of temperatures. We realized, over a period of years, that there were lots of artifacts that were introduced by growing plants in isolation or in these controlled growth cabinets. We’ve moved progressively toward more and more sophisticated designs to get it closer to the way that natural ecosystems work.

I would say that at the time we started the Jasper Ridge Global Change Experiment, there were essentially two themes that were relatively mature. One was a theme of focusing on elevated CO2 as an important global change and the other was the theme of focusing on warming as an important global change that’s a consequence of the elevated CO2 concentration. But there had been essentially no work that had combined the two.

We knew, as a consequence of other studies at Jasper Ridge, that the grasslands were a very attractive model system in the sense that the individual plants are small. You can take them through several generations and you can apply a wide range of treatments with a modest amount of infrastructure. That created a perfect context for looking at the interaction between elevated CO2 concentration warming and some other important global changes as well. In the experiment here, we decided to add two more important global changes, nitrogen pollution and changes in precipitation. Essentially, the global change research community had started, by the middle of the 1980s, calling out for these integrated experiments. It took a long time to figure out what was a conceptual context and a technical infrastructure that was compatible with doing it. By the middle of the 1990s, we were pretty clear on how we could do it and then by the late 1990s, we’d assembled the resources in order to be able to launch the experiment.

Interviewer: That’s when you actually got started? What was the first year that you got underway?

CHRIS: This experiment was officially started in the fall of 1997. The first thing we did was study all the plots with no manipulations whatsoever so we were sure we understood what the background conditions were, that there weren’t important patterns across the site. Once we had one year of background studies, then we put the treatments in place in the summer of 1998 and turned the treatments on in the fall of 1998. We have been following the plots continuously since then.

Interviewer: I want to go back to that beginning when you were first designing the experiment. What were some of the preliminary findings from other research that might have given you a working hypothesis?

CHRIS: I would say there were three trends in the literature and in the quantitative models about ecosystem responses to global changes that we were especially interested in testing. The first is that by the time we started this experiment, there was a lot of controversy about the magnitude of growth responses to elevated atmosphere of CO2. Most plants increase the instantaneous rate of CO2 uptake because they increase their instantaneous rate of growth, when they’re exposed to elevated atmospheric CO2. But there were a range of different [long-term] results about plant growth. Individual plants in pots or individual plants separated in the ground tended to grow much faster under elevated atmospheric CO2. And the data from ecosystems were quite mixed. The other trend that was important is that in contrast to the general idea that increased atmospheric CO2should increase plant growth, there was evidence that warmer temperatures tended to accelerate decomposition, the release of organic matter stored in plants and in soils, basically creating a positive feedback to warming by increasing atmospheric CO2, in contrast to the negative feedback caused by increased plant growth. The third trend that we were really interested in exploring concerns the fact that real global changes are emerging over a period of decades, a long enough time period that, in most ecosystems, the dominant organisms go through one or a few complete generations. The challenge with most of the global change experiments that had been done prior to that is that people were interested in the ecosystems that stored the most carbon in forest ecosystems. You couldn’t take an experiment of a reasonable length through several generations. You basically ended up with an experiment at the end where you had the same exact individual plants that you had when you started. They were just a little bigger and a little older. We wanted to know what was the relative importance of the ecological dynamic, the replacement of one set of plants by another one, in contrast to the changes in the physiology of the plants that were there at the start. So these three themes, the magnitude of the CO2 response, the magnitude of the warming response, and the role of ecological dynamics were the key ideas that led us to the design.

Interviewer: Specifically, what were the unanswered questions that were driving this research? And are they still unanswered?

CHRIS: Probably the two questions that more than any other drive all of the research concerning ecosystem responses to global change concern feedbacks to the carbon cycle and what we might call ecosystem services. The feedbacks to the carbon cycle concern the point that ecosystems on land are taking up a relatively large amount of the carbon that’s emitted by human actions, essentially providing a subsidy. What we’d like to know is: as we move into the future, will that carbon uptake, that subsidy, increase? Will it decrease? Will it go away and turn from a subsidy into an extra burden on the atmospheric carbon dioxide? The reason this is really important is that on an annual basis the flux of carbon dioxide into terrestrial ecosystems and then out again is about ten times the flux of carbon dioxide from fossil fuel combustion to the atmosphere. Now the ecosystem flux tends, in general, to be very close to balanced. What that means, however, because it’s so large, is that a small imbalance represents a large number relative to this fossil fuel flux.

We have good mathematical models that address the process of photosynthesis, the process of CO2 uptake. But we don’t have very good models that address the full set of ecosystem processes that includes decomposition, species replacements, insect effects, disturbance by fire, the full set of things that occur in real ecosystems. The Jasper Ridge Experiment allows us the opportunity to get at that in a very precise way that lets us use it as a model system for understanding the controls on carbon balance in other ecosystems.

A preliminary calculation tells us that the amount of leverage that ecosystems have on atmospheric CO2 is really quite large. Most of the models that have been used over the past decade are projecting continued uptake of the carbon dioxide by terrestrial ecosystems with an amount that might be on the order of half to two-thirds the total amount that’s been emitted by fossil fuel combustion over the last century. A number of newer studies, however, indicate that, especially with global warming, ecosystems could convert from CO2 sinks to CO2 sources, and they could release as much as half or three-quarters of the amount of CO2 that’s been released by fossil fuel combustion over the last century or so. The reason this is so important is that if terrestrial ecosystems continue to absorb atmospheric CO2, the growth rate driven by fossil fuel combustion is less than it would otherwise be. If terrestrial ecosystems begin to release carbon dioxide, the growth rate becomes faster than that driven by fossil fuel combustion alone.

The other major motivator for the Jasper Ridge Global Change Experiment concerns ecosystems services. These are the benefits that ecosystems provide to people, and they range from products like food and fiber, to maintenance of important background features like pollinator services, provision of clean water and clean air, to what we might call cultural or spiritual services, like the opportunities for recreation or spiritual renewal. We’re trying to figure out how the changes in ecosystems that are driven by the full set of global changes that we’re manipulating modify the ability of important ecosystems to provide these services.

An example of the kind of changes we’ve seen is that several of the realistic global changes lead to a loss in wildflower species at Jasper Ridge. Our most realistic treatment combination results in about a thirty percent loss of wildflower species over a three year time period. Maybe there’s no tremendous economic value to the wildflower diversity, but the maintenance of biological diversity moving into the future is a really critical issue that humanity needs to deal with. On a more practical level, we have seen that some of the global changes increase the susceptibility of these grassland ecosystems to invasion by important weeds. These are weeds that seriously degrade the value of grasslands for grazers and other economically important uses.

Interviewer: I’d like to know the extent to which the average person should care about these kinds of issues.

CHRIS: Global change imposes a wide range of stresses on the future. Everything we know at this point indicates that those stresses are likely to be of major importance. They are likely to fundamentally alter the range of opportunities that our children and their children encounter. We can talk about separate impacts of global climate change from global changes in ecosystems. Current estimates for global warming, resulting from continued increase in fossil fuel use, depend a lot on what the trajectory of humanity’s use of fossil fuels moving into the future is. If the range is a warming of something like a three to eleven degrees Fahrenheit increase in average temperatures over the coming century, we’re looking at changes in ecosystems that could really be quite profound, changes in ecosystems, transportation, industry, human health that could really be quite profound.

A good way to think about that is the difference in global average temperatures between the interglacial that we’re now in and the last full glacial about twenty-five thousand years ago, is around eleven degrees Fahrenheit. That was enough difference in global average temperature that the sea level was five hundred feet lower than it is now. There was more than half a mile of ice sitting on Wisconsin. Another way to frame the magnitude of potential warming that we’re talking about is that currently the temperature difference between Death Valley and Los Angeles is about ten degrees Fahrenheit. That’s not to say that Los Angeles will become Death Valley, but that’s the magnitude of the temperature difference.

When we look at the possible impacts of the possible outcomes of global warming, we see them in a wide range of areas that potentially impact future opportunities. These include agriculture, human health… especially stress from heat waves. I don’t think it’s generally appreciated that in the summer of 2003 in Europe, when average temperatures were about ten degrees Fahrenheit warmer than normal, forty thousand people died from heat related stresses in a single summer…a clear example that warming can have profound impacts on human health.

We’re learning more and more about the way that warming influences a potential range of vector-borne diseases, about the way that warming impacts the availability of fresh water, which is needed for agriculture and for urban areas, and for end-stream uses; also about the way that global warming impacts infrastructure, whether cities are habitable, whether transportation routes are usable. It’s clear already, for example, that over the past several decades, temperatures have risen in the high latitudes in the Artic and Canada and Alaska, have increased dramatically. There are already villages that are being moved because the very land that they’re sitting on is no longer compatible with the maintenance of stable buildings.

I should talk a little bit more about impacts of other kinds of global changes because while global warming is perhaps the most challenging of the global changes to deal with, it’s far from the only one. Humans have exerted a wide range of influences over the earth’s surface in the last century. These range from damming major rivers to adding large amounts of nutrients to ecosystems to converting large amounts of forests to agricultural ecosystems to transporting a wide range of non-native plants and animals. There are several important outcomes of these manipulations that we’ve already seen. They range from the dead zone at the mouth of the Mississippi River to the exaggerated flood damage that’s caused by keeping rivers in very strictly defined channels to the pervasive influence of non-native plants and animals, which have disrupted biological diversity and opportunities for agriculture in almost every continent. I think that when you add these two kinds of stresses together, the climate related stresses and the non-climate related stresses, you create a very difficult set of challenges that requires all of our creativity and all of our understanding to deal with effectively.

Interviewer: Let’s go from the big picture down to the narrow focus. Tell us a little bit about your methodology in the global change experiment.

CHRIS: In order to create a realistic possible future environment for these grasslands, we’re manipulating four environmental factors. We’re doubling the concentration of atmospheric CO2. It’s currently about four hundred parts per million, or 0.04 percent in the atmosphere. Our targeted level for the experiments is 0.07 percent, seven hundred parts per million, which is a level that, depending on CO2 emissions, we might reach anywhere in the second half of the century. We’re increasing the average temperature using heat lamps by about three degrees Fahrenheit. This is at the low end of the projections for the second half of the century. But it’s a practical level for us to achieve without a really massive infrastructure. We’re also adding nitrogen pollution. It’s better known as acid rain. But it’s biologically available nitrogen that comes also from fossil fuel combustion and other industrial processes. We’re adding it at a level that’s typically experienced now in areas in Northern Europe, in the Northeastern U.S., which are relatively polluted. The fourth environmental factor that we’re manipulating is rainfall. In general, at the worldwide scale, we know that as it gets warmer, the amount of precipitation basically has to increase because more water is evaporating from the ocean. We don’t know whether a given spot will be wetter or dryer. But by having a precipitation increase, we can untangle the interaction between the other treatments and the precipitation, and more or less create a framework where we can evaluate the impact of precipitation in each of the other factors. The way our experiments are designed is that we have two levels of each of these four major factors, CO2, warming, Nitrogen pollution, and extra precipitation. And we have all of the different combinations of the two levels. So we have all the two treatment combinations, the three treatment combinations, and the four treatment combinations. Each of those is replicated eight times. Because we’re imposing these treatments on a natural ecosystem, we have lots and lots of variability from place to place and time to time. And the only way that we can be confident that we’re seeing the true effects of our treatments and not just environmental variability, is if we have enough replicates of each treatment that we can extract out the signal from the noise.

Interviewer: How is the experiment physically laid out?

CHRIS: The way our experiment is laid out is that over about three acres of landscape, we have a hundred and twenty-eight small plots. Each of these plots is a little less than one square yard and it has anywhere from two to five thousand individuals of the dominant organisms, which are these annual plants. The hundred and twenty-eight plots are clustered into circles of four plots. Within each circle, there are plots that receive nitrogen pollution, precipitation, both nitrogen pollution and precipitation, and then neither. Each of the circles is exposed to a larger scale treatment that may be elevated carbon dioxide, it may be warming, it may be both, or it may be neither. As I said, these four environmental factors create a total of sixteen different treatments. The overall infrastructure is really quite simple. The warming comes from heat lamps. The carbon dioxide that we get is basically a waste product from fossil fuel refining. It’s the same carbon dioxide that’s used in a carbonated beverage factory. It’s delivered in a tanker truck and we basically release a cloud of it over the ecosystems with a computer controlled system that’s measuring what the background concentration is and what the wind speed is. Precipitation is added with a home style misting system. And the nitrogen pollution is added, more or less as you would fertilize your lawn.

Let me say that our goal is to project these grasslands into about 2075. We would hope that as you walk into the plots, you basically are seeing visions of California for 2075. Part of the reason we have so many different treatment combinations is that we’re not sure exactly what the future is going to look like. But there are really two other motivations for the range of treatments. One motivation for the wide range of treatments is that we want to understand how the factors interact. We can understand a lot about the underlying mechanisms of the CO2 response by looking at the way that it interacts with the nitrogen or the warming response. We’re using this experiment as much to learn basic things about ecosystems as we are to learn applied things about ecosystem responses to global change. Then the third motivation for the wide range of treatments we have is that it’s very difficult to apply some of these global change treatments without having artifacts. A good example is warming. It’s very difficult to design any kind of a warming treatment that doesn’t also produce a drying. When you look at the results of a warming treatment, you don’t know whether you’re looking at the warming response or the drying response. But if you have a warming and a watering treatment, you can be sure that you can separate the warming response from the response of the change in available moisture. Those three things, the projecting ecosystems in the future, the ability to look at the interactions, and the ability to be sure that we’re not studying artifacts, all combine to make this array of treatments a relatively compelling one.

Interviewer: How do you measure the results? How do you get the data back from the plots?

CHRIS: There are a few core measurements that we’re emphasizing and a large number of ancillary measurements we take to let us understand the core measurements. The three things that are of greatest interest to us are whether or not carbon is being stored or lost from the plants or the soils. In the plants, that means are the plants growing bigger — are they not growing bigger? For the soils that means is more organic matter incorporated into the soils? The second question that’s really important for us is how are the ecosystems changing in terms of their biological composition? Are there more species? Are there fewer species? Are the species weedy or less weedy? Are we gaining or losing in biological diversity?

The third kind of question that we’re really, really interested in concerns the provision of ecosystem services. Is the water that’s being lost from the ecosystems clean or less clean? Are the ecosystems leaking nitrogen pollution or are they storing nitrogen? Are nutrient cycles being disrupted? Are the sites becoming more attractive for weeds or less attractive for weeds? A wide range of human-related factors are part of our core measurement set.

In order to understand these responses, we look at lots of basic plant physiology, ecosystem ecology, and biogeochemistry. That involves things like looking at the instantaneous rate of growth of plants by measuring the rate at which they remove CO2 from the atmosphere or release CO2 to the atmosphere. We’re interested in the way the different treatments modify soil moisture. We use an advanced technique that measures a speed at which radio waves moves through the soil to measure soil moisture. We’re really interested in knowing the availability of the nutrients that tend to control growth in most terrestrial ecosystems; and we can use a variety of techniques to do that all the way from looking at the abundance of non-radioactive isotopes of key nutrients, to looking at instantaneous rates of uptake and loss of the nutrients, to looking at the expression of genes that the plants use in order to produce the proteins that help them acquire these nutrients.

Interviewer: Could you give a basic description of the gas exchange experiment?

CHRIS: We know that plants grow by removing carbon dioxide from the atmosphere. They use the energy from sunlight to convert carbon dioxide into carbohydrates into plant. We also know that on an annual basis plant growth and plant decomposition are approximately balanced. Under the conditions that we impose, there are lots of reasons to think that this balance – between uptake through photosynthesis and loss through decomposition – could be disrupted. We’d like to know whether it’s disrupted, how much it’s disrupted, and what the mechanism of disruption is.

In order to do that, it’s really useful to measure the instantaneous rate of CO2 uptake and loss. The basic technique we use for doing that is that we pass a defined column of air across a plot of ecosystem and we measure the CO2 concentration in the air before it reaches the plot and after it reaches the plot. If the plants are growing and removing CO2 from the air, the concentration after the plot will be lower than what it was before. If they’re decomposing more and they’re adding CO2 to the air, the concentration in the air will be higher following the plot than it is before. By measuring the rate of air movement over these plots and the CO2 concentration before and after, we can get an instantaneous measurement of the plant growth rate or the plant decomposition rate.

Interviewer: How do these other experiments fit into the larger experiment?

CHRIS: We think of the Jasper Ridge Global Change Experiment as something more like a laboratory than something like an experiment. It basically is an environment in which we’re encouraging investigators to bring a wide range of their own questions that are stimulated by our results, but may come from a kind of expertise and a kind of perspective that we wouldn’t necessarily have. Our collaborators on this project, of whom they’re a large number, include specialists in plant gene expression, in plant microbe interactions, in soil chemistry, in plant physiology, plant biochemistry, nutrient cycling and in global biogeochemistry. The project of measuring the instantaneous growth rates that Claire Lunch is doing is one that we knew from the outset of the experiment was likely to have a strong ability to help us interpret. We didn’t really have the capability to do it in a scale that matched the scale of the experiment. Claire’s real innovation was to come up with the idea of a relatively simple infrastructure. I know it doesn’t look simple; but the pieces are all simple enough that they can replicated to a scale that we can now launch it at the scale where it lets us address the key questions. Claire is completing her PhD research. There are other researchers on the experiment who are collaborating who are high school students, some who are faculty members at major universities, and more or less everywhere in between. We have collaborators from several continents and lots of universities around the U.S.

Interviewer: The other major experiment that you spent some time with is the phosphorous experiment. Tell us a little bit about where that came from, what you’re trying to find out, and what your methodology is.

CHRIS: One of the most unexpected results we’ve had in the Jasper Ridge Global Change Experiment is the realization that under a wide range of conditions, increased atmospheric CO2 does not lead to increased plant growth. In fact, under many conditions, elevated atmospheric CO2 actually prevents plants from taking full advantage of other resources that are available in the environment. This has quite profound implications for our understanding of ecosystem response to global change and for future climate change. If plants, in fact, don’t grow more under elevated carbon dioxide, it means that atmospheric CO2 is likely to grow faster in the future than we have been anticipating. It basically puts a lot more pressure on societies to figure out how to control emissions of carbon dioxide rather than stepping back and hoping that ecosystems will help us solve the fossil fuel problem.

Once we realized this, it was really important to figure out what the mechanism was; because we know from lots of laboratory studies that the instantaneous rate of plant growth or photosynthesis almost always increases under elevated atmospheric CO2 . We did a variety of experiments that tried to infer the mechanisms from the observations that we were able to take in the existing experiment; these lines of evidence pointed to the fact that there was another mineral nutrient, another kind of fertilizer, that was required for plant growth that was preventing them from taking advantage of the elevated atmospheric CO2 and it might even be becoming less available under elevated CO2. But we couldn’t be sure unless we did a separate experiment. The evidence suggested that this limiting resource was probably phosphorus.

So we set up an experiment to specifically test the hypothesis that limited phosphorus availability, in this and many other ecosystems, might be limiting the ability of terrestrial ecosystems to take advantage of increased atmospheric CO2 in a way that allowed increased plant growth and increased carbon storage. To do the experiment, we extracted intact monoliths of soil. We basically drove PBC pipes into the soil and pried them out so that the soil column was essentially undisturbed. And then we established plant communities of known composition on these soil monoliths; and we exposed them to a three-way factorial experiment with two levels of atmospheric carbon dioxide, two levels of nitrogen inputs and ambient increased phosphorus availability. The hypothesis is that with increased phosphorus availability, we would see a large increase in the response to elevated CO2 and nitrogen, and a very large increase in the response to both elevated CO2 and nitrogen concentration together.

We have two ways that we’re testing this. One approach is to just to say, well, does increased phosphorus availability relieve the limitation that we’ve seen in the other experiments? The second approach is to say, well, is there evidence that a second or third generation of plant growth is essentially glomming onto the phosphorus that we add in a way that makes it less available? And so we have a series of first measurements and a series of extended measurements where we look at the feedbacks that come from exposure to the global change treatments from more than one year. We don’t have any results yet from this experiment, but the preliminary indication is that there does appear to be a big phosphorus effect and a big phosphorus/nitrogen interaction. We’re really excited to see what the outcome is.

Interviewer: What happens during harvest time?

CHRIS: For lots of the questions that we’re trying to address in the Jasper Ridge Global Change Experiment, we need an accurate indication of how much the plants are growing. We have both direct and indirect methods for quantifying this. The indirect methods basically take a lesson from the satellite technologies; and they measure plant growth non-destructively by looking at the color of the light that’s reflected from them. Basically greener plants reflect more green light and less red light. The direct method we use is basically the same that a farmer or a forester would use. If we want to know how much the plant grew, we go in there and we cut some of the plants down and we dry them and weigh them. That’s our highest accuracy method and it’s one that involves a big input of effort during the spring every year as the plants are reaching their full size. What we’ll do next week is we’ll come in with a large number of people. We’ll harvest part of every plot and we will separate the harvested plants into different species and we will measure the nutrient concentrations in the plant tissue and we will quantify changes in species composition, overall carbon storage, and plant growth — the ability of the plants to support grazing and other kinds of activities.

The measurement of total plant growth is one of the key factors that we’re trying to measure in the ecosystem, and it’s one of the key things that connects ecosystem performance in the experiment with what we expect the critical feedbacks to be in the future.

Interviewer: Can you define the term “net primary production”?

CHRIS: When plants grow, they remove carbon dioxide from the atmosphere, producing more plant. The technical term for more plant is biomass. Over longer periods, we typically measure biomass or plant accumulation by measuring differences in the standing crop of the stuff. That’s usually referred to with a different term — the term of “primary production” or “net primary production,” which is the total amount of plant growth over some defined time period. Net primary production at the global scale is essentially the plant process that removes carbon dioxide from the atmosphere and, therefore, is probably the biological variable that connects ecosystems most closely with the atmosphere.

Interviewer: What results have you seen in terms of net primary production?

CHRIS: Probably the most important core message is that under a wide range of conditions, terrestrial ecosystems do not store more carbon under elevated atmospheric CO2, which has profound implications for the future concentrations of atmospheric carbon dioxide. Basically what it means is that we can’t continue to depend on ecosystems to subsidize our emissions of CO2 from fossil fuel combustion. The second result that’s really important is that under a wide range of global changes we see a degradation in the ability of these grassland ecosystems to provide the goods and services that people count on from ecosystems.

Interviewer: What are the implications for humanity?

CHRIS: The real implication of both conclusions, the conclusion concerning carbon storage and the conclusion concerning ecosystem services, is that we can’t count on ecosystems providing a continuing subsidy for humanity. We need to take responsibility for the way we’re manipulating the Earth’s system; and in particular, I think we need to be extremely careful that we avoid pushing the ability of ecosystems to provide goods and services towards a tipping point where they can no long provide things that we really fundamentally depend on. My sense is that what that means is taking a more responsible approach toward environmental stewardship. It means being really aggressive about finding ways to limit CO2 emissions to the environment. It means being really aggressive about thinking about environmental conservation through plans to control the way that we influence ecosystems through the atmosphere, through land use, and through changes in the air and water.

As I think about the implications of the research we’re doing in Jasper Ridge for the future of the world, I can’t really escape the conclusion that there’s nothing more important than getting a handle on fossil fuel emissions. The difference in impacts between a trajectory in which humanity aggressively looks for alternatives to fossil fuel and a “business as usual” strategy where we aggressively burn all of the resources is just profound in terms of human health, transportation, ecosystems, economic opportunities, and I guess this is somewhat leaving the science realm. But my sense is that we’re really almost out of time in this business and that real leadership could make a difference in both creating economic opportunities and in preventing long-term damages.

I think of climate change as essentially building a big pile of rocks on the highway in front of us and we are careening at full speed toward this pile of rocks. What we need to do is to figure out a way to move the rocks off of the road. If we go on the strategy we’re going now, we’re going to smash into this rock pile. There are some things that allow us to maybe build a four-wheel drive vehicle and, at least for the wealthy countries, to bounce over the rocks; that’s the strategy that’s called adaptation. We could do carbon sequestration; that’s essentially moving the rocks from the front of the pile to the back. We don’t have less of a problem, but we don’t get to it quite as soon. Or we can figure out a way to move the rocks off of the road. That’s mitigating the climate problem. My sense is that it’s so serious that we need to take advantage of all those strategies. But none of them are going to be successful unless we make a real national global focus in cutting the emissions of carbon dioxide to the atmosphere.

Aggressive efforts to limit the emissions of carbon dioxide and other heat-trapping gases to the atmosphere will lead to a world that is hopefully not too different from the future, one that still provides essential ecosystem goods and services. With the business as usual approach, where there’s aggressive exploitation of fossil fuel resources and uncontrolled emission of other heat-trapping gases, I think we’re looking at a future that’s much bleaker, where we’re looking at temperatures that are warmer than the planet has seen for the last several million years, where there are critical shortages of water and other essential services that are provided by ecosystems, and where stresses on people and ecosystems and the global habitat are unlike the stresses that we’ve encountered ever as a species.

Interviewer: Are the results you’re getting consistent with the results of other researchers?

CHRIS: The Jasper Ridge Project connects with the global effort to understand ecosystem responses. When we think of the Jasper Ridge Global Change Experiment as a model system, it’s an especially tractable set of ecosystems that we can use to approach our sophisticated questions that are as relevant to tropical rain forest or Arctic tundra as they are to California grasslands. But we couldn’t be confident in our answers unless we had collaborators in many other parts of the world who are doing experiments that are similar in some respects. It’s not practical to do a four-factor experiment on many replicates on tropical rain forests. But carefully selected rain forest measurements can really help us tell whether the mechanisms that we’re exploring here apply directly or in some other way to ecosystems in different parts of the world. We’re really part of a global community of researchers who are trying to understand the underlying mechanisms and the implications of those mechanisms. We work through a variety of global coordinating mechanisms to make sure that the findings are distilled down and we can pull out the really general points from the pieces of information that are specific to this site or this other site. I think that the research enterprise needs to be viewed in many senses much broader than Jasper Ridge, but the Jasper Ridge experiments provide a key component of really projecting an important ecosystem in a realistic way with a realistic suite of global changes into the future.

Interviewer: Could you be a bit more specific about the other parallel research programs?

CHRIS: The results from Jasper Ridge make a lot more sense in the context of research that’s occurring in many other parts of the country and the world. The Jasper Ridge Experiment is a complicated design because it is so tractable that we can do so many things. But we’re seeing a lot of experiments with quite similar results. We have collaborators at the University of Minnesota who just published an important paper a few weeks ago showing that in their grassland ecosystem they found the same result as us, that you didn’t see a big growth response to elevated atmospheric CO2 unless you also provided nitrogen. A group working on a pine forest in North Carolina recently published almost the same result. A group of collaborators working in the desert in Nevada found that even though elevated CO2 increased plant growth, it increased the growth of an invasion species that tended to bring wildfire into desert habitats that weren’t flammable otherwise. I think that across the community what we’re seeing is that the responses to global changes tend to be complicated; they tend to be pushing the ecosystems in the direction that tends to make them more disturbance prone; and they tend to not be providing the additional carbon storage that we had hoped for when we took the most simple-minded approach to this kind of experiment.

Interviewer: How do all the various on-going investigations at Jasper Ridge fit together?

CHRIS: The community of researchers who are working on the Jasper Ridge Global Change Experiment is coordinated in working on a series of big questions. But we, the leaders of this experiment, recognize that we don’t necessarily have the perspective or the insight to ask the most sophisticated or exciting questions from all of the different disciplinary perspectives. And so we try and find a middle ground where we all work together to say what are the big goals we’d like to accomplish. But then we encourage individual initiative in saying the best approach to do that or from my disciplinary perspective — what I’d really like to know — is something slightly different that will support your big question in this other way. And so we have something of a “thousand flowers blooming” approach to integrating the different experiments, and we use a variety of mathematical modeling tools, of conceptual modeling tools, and just flat out argument in order to figure out how the pieces fit together.

Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
  • Closed Captioning
  • ISBN: 1-57680-883-1