The following person was interviewed for today’s show. Our thanks to:

Robert B. Jackson
Department of Biology and
Nicholas School of the Environment
Duke University

ES:

RJ: Let’s start with the carbon cycle. In the U.S., we give off about 6 billion tons of carbon dioxide, greenhouse gasses, each year. And in the U.S., even though we have approximately 5% of the world’s population, we give off about 1/4 of fossil fuel emissions. So we’re far and away the dominant player, globally, in the carbon cycle in terms of fossil fuel emissions. Why people should care, or why the land and oceans matter in the carbon cycle, is that many people don’t know CO2 concentration should actually be going up twice as fast as it is based on the rate of carbon dioxide given off in industry and by cars and other transportation. And the reason it’s not going up as fast as it should be, based on the emissions, is because the oceans and the land are taking up about half of that carbon dioxide back up. So this is a natural service that the oceans and the land are providing for people. And an interesting scientific question is whether that service will continue. There are many scientific reasons, and many experiments suggesting that the rate of that terrestrial sink, the ability of natural or at least land-based systems and plants to store extra carbon will start to decrease in the future.

ES:

RJ: So, globally, half the carbon dioxide is given off in industry and in cars goes back naturally into the oceans and land. And that’s also true in the U.S., where about 1/3 to 1/2 of the CO2 that’s emitted here in this country goes back into land. And the two primary places where that carbon goes on land is into plants, primarily trees, and then also into soil, or the soil organic matter, the dirt in the ground. And the two places where it goes most often in the U.S. is firstly re-growing forests in the Eastern U.S. And these were lands that were cropped, they were planted a century or two ago and they’ve been abandoned as agriculture has shifted. As they were abandoned, the forests have come back and have started to grow on those sites again. And as the trees grow, they’re storing carbon in the trunks of the trees. That may not sound like a lot of carbon, relative to what’s in the atmosphere, but it is. The amount of carbon stored in plants is about comparable, around the world, to the total amount of carbon in the earth’s atmosphere. But even more surprisingly, the amount of organic carbon in the soil is about twice as much as what’s in the atmosphere. So what happens on land, what happens to the plants and soil, really does make a difference. It can have a huge potential effect on the ultimate balance of carbon in the atmosphere. Here in the U.S., 1/3 to 1/2 of the carbon that we emit is taken back up on land. I’ve already mentioned the re-grown Eastern forests as the first place where that goes. But the next largest sink for carbon on land is something we call woody plants encroachment, or woody plant invasion. And it’s a different phenomenon. Rather than forests re-growing where they used to be, this is trees and shrubs, essentially invading or taking over grasslands where they didn’t used to be in the past. So this is happening primarily in the south and western U.S. And it’s a phenomenon related, at least in part, to fire suppression and to grazing. So areas that have historically burned, where fire kept seedlings and the woody plants from becoming established. When people move into an area, we suppress fires for personal safety and to save our property. And this gives a chance for trees and shrubs to overtake the grass. And there’re many, many places here in the U.S. that are now much woodier than they used to be 100 or 150 years ago. And that phenomenon of woody plant encroachment was the phenomenon that we studied in the Southwestern U.S. across a gradient of sites – the experiment that you mentioned.

ES:

RJ: First of all, woody plant encroachment isn’t just happening in the U.S. It’s happening all over the world. And that includes places in China, in South America, in Australia, South Africa. So it really is a global phenomenon. And it’s happened primarily as Europeans have settled areas and introduced grazers such as cattle and have also changed the natural fire cycle, so that grasslands and savanna systems burn less frequently now in many places around the world than they used to. And in doing so, it’s allowed these woody species to become established. When those trees and shrubs grow into and area that was formerly grasslands, you have a new place that stores carbon. In other words, in grassland, there’s no wood in the system. But when a shrub or a tree especially at high densities, starts to take over an area, you’re storing atmospheric carbon in the trunks of those shrubs and trees. And that’s the first place where carbon storage occurs with woody plant encroachment. But the other place where you have to look, that’s also equally important, in many grassland systems more important, is below ground in the soil. And, as I’ve already mentioned, the soil often holds twice the carbon that’s in trees and in vegetation – much more than twice in the case of grasslands. And so we looked at six paired sites across the southwestern U.S., and each pair had a native grassland system and across the fence-line, or a road, a parallel, or a paired site that was invaded by shrubs and trees. And what we found is that, especially at the wetter sites, as the trees moved in and stored carbon in the trunks, we lost carbon below ground, so that there wasn’t as much of a gain occurring as people thought before. There wasn’t as much carbon storage as people had though previously.

ES:

RJ: So the primary place where the carbon is what’s called soil organic matter – or the soil organic carbon. And this isn’t living roots – although living roots do store carbon. But a far bigger place where the carbon is stored in the soil is in decomposing plant tissue – so decomposing leaves, roots that break off from the plant, organisms that die in the soil – it’s basically all the living material eventually dies, and as it dies, it becomes this organic pool. And by organic, we really mean life – the molecules of life — carbon, and hydrogen and oxygen. When you pick up a handful of soil in your garden, and you can tell a rich soil because of all the organic matter that it holds. It’s kind of the difference between – compare two extreme soils. Compare one that’s pure sand, and one that’s more like a peat moss that has a lot of vegetable material in it. And the latter case, the material with more organic material, the more peat moss-like soil, it’s more fertile. It holds on to water better, it holds on to nutrients better, in addition to storing carbon. So it has a lot of extra benefits. And really, soil fertility – or soil organic matter – to a farmer, forms for the basis for soil fertility and nitrogen supply and stuff like that, so farmers work very hard to build up soil organic matter in their farmlands.

ES:

RJ: So fire is one way, but fire only affects the ver shallow soil layer. Rarely do fires burn hot enough to affect deeper layers – you know, more than the tops, 5 cm of soil. The way that carbon is recycled back into the atmosphere is through microbial activity. So microbes – bacteria and fungi, and other bugs – basically chew up the soil, they chew up the organic matter, and they use it for food – just like us eating a salad at the dinner table. This is what they’re getting out of the soil. They’re chewing up that material and giving off carbon dioxide into the atmosphere. Another way to think about soil organic matter, or soil carbon, is to think about putting manure on a field. You know when you manure your garden, you’re essentially adding organic matter back into the soil to make it richer so it can hold onto water and nutrients better.

ES:

RJ: So in that experiment we worked at a range of sites beginning with the driest grasslands in the U.S. – those in the deserts of a New Mexico and Colorado, and then we moved eastward through the south and southwestern U.S., towards the wet edge of where grasslands occur in America, and that’s sort of the Kansas and East Texas range – the old tall grass prairie sites that are so fertile. And as you move from west to east in the U.S., you generally move from drier to wetter, and the systems themselves become more productive, they grow more as water is more available in the eastern half of our country as compared to the western half. So we worked at a range of sites to look at the interaction with precipitation on how much the plants were able to grow, and how much carbon they were able to store. And then, really the key component to our experiment was this paired comparison. At each of these six sites throughout the U.S., we compared native grassland to one adjacent to that native grassland but that had been invaded by shrubs and trees. So we were trying to control the soil and all the other things that might vary in field, and just isolate the effect of the plants on carbon storage. And what we found is that at the drier sites, when shrubs for example had moved into a desert system, you got a little bit of carbon stored in the plants themselves, and in some places a little bit stored below ground. But really not very much on a complete basis, or on a mass balance basis. But at the wetter sites, we had a lot of carbon stored at the trees, because, again you move from small shrubs to large trees like mesquite and juniper, but while this carbon was being stored above ground, in the decades after invasion by the trees, this organic matter pool decreased below ground, so soil carbon was lost. And the reason this is important from a policy perspective is that what people had done previously to look, or to evaluate how much carbon was being stored at such sites was just to come in and cut the trees down, and weigh the carbon in the trees themselves. And they were not taking into account potential losses that might occur below ground.

ES:

RJ: Well there were a couple of reasons that made us want to look at it. And one of them was that there’s twice as much carbon in the soil as there is in the atmosphere or the plants. So, it’s a large potential pool. And the second reason is that we did some work, really though looking at global databases, so we went and looked at records that the USDA had kept – our Department of Agriculture – and that other agencies had kept around the world. And we looked at the amount of carbon that’s stored in the soil under grasslands, and compared that with the amount of carbon stored under shrublands and woodlands for about 250 places around the world. And that analysis suggested that the grasslands stored more carbon in soil and organic matter than the shrubs or trees did. And that’s really the reason why we went to this more detailed, careful paired comparison. Because we had good evidence that such a change might be occurring and that rainfall might play a role in that relationship. So this study was a combination of fieldwork throughout the U.S., and kind of computer work, and using records that have been taken over the last 50 years or so.

ES:

RJ: The study is ongoing, and it’s been occurring now for about five years. In addition to myself, there are other collaborators on the project too, like “Will Pockman”:http://www.unm.edu/~pockman/, “Diana Wall”:http://www.geo.utexas.edu/faculty/banner.htm of Colorado State University, Jay Banner, who’s a geologist at the University of Texas at Austin, and one of my former graduate students Stephen Jobb?gy. So the study has been going on for five years, and at each of these sites we have made detailed measurements of the plants themselves, so what species occur there, we cut the plants, the shrubs or trees down to weigh or to account for the carbon in their biomass. We have taken soil cores down to 30 feet at each site, so we use the drilling rig to extract cores from the soil so we can analyze how much carbon there is, not just in the top layers, but throughout the profile that might be affected by the plants. That was also something that was unique to our study. Now the sites ranged from dessert grasslands in New Mexico, and these include the “Jornada Desert Range”:http://usda-ars.nmsu.edu/, a long-term USDA site, the “Sevilleta”:http://sevilleta.unm.edu/, which is a long-term ecological research site run by, among other organizations, the University of New Mexico. We worked at the “Shortgrass Steppe Long-term Ecological Reserve”:http://photoscience.la.asu.edu/photosyn/education/photointro.html in Colorado. And then in Texas, we woke at a series of state sites, or state run facilities. And when you think about those sites, you’re again moving from desert grasslands, quite dry, the driest sites got about eight inches of rain a year. And the wettest sites had 30-40 inches of rain a year. It’s quite a broad range.

ES:

RJ: You asked me what was unique about the study, and really, the most unique thing about it was to combine both above ground and below ground changes. And the reason that people hadn’t done that as often before is the ease of studying the site. And that is it’s much easier to go into a site and simply cut the trees down and weigh them to see how much carbon is in them, or cut the shrubs down and weigh them, then it is to take soil cores and analyze how much soil organic matter is in those cores, which has to be done in the laboratory – it’s just a lot of work. And in addition, you have to think about how deeply in the soil you want to measure. So we went about 30 feet, and there were changes down, not for the whole 30 feet, but there were changes to 6, 8 or even 10 feet in the amount of carbon that the soil held. So it was just a much more difficult set of measurements to make, then it is simply to weigh just how much carbon is in the trees or shrubs themselves.

ES:

RJ: So the sites that we studied range from about 30-100 years after the shrubs had moved in. So we looked at sites where there were aircraft photos, historical records, where we had a good history of the management of these sites and when the woody species first appeared. So we wanted to know just how quickly the changes occurred. And what we found is that, especially at the wetter sites, where the plants are more productive, and you really go – instead of shrubs in the deserts – to full grown trees, say in east Central Texas. So there was more carbon stored in the trees. But the more rainfall that a site got, the more carbon was lost below ground. And we don’t exactly know the mechanism for this, but my best guess is that grasses, when you think of an old prairie system, grasses put almost all the carbon, or a lot of the carbon they fix in photosynthesis goes to their root, or it goes directly into the soil. And trees are not as efficient at doing this, and they put a higher proportion of their biomass above ground. So, we believe this is one of the reasons why these losses are occurring – that you’re not getting the high input from the grasses into the soil to maintain those soil carbon pools. So at the wetter sites, the gains in carbon that was stored in the trunks of the trees were generally offset, or counter balanced, basically, by losses below ground. So that the sites were not acting as sinks for carbon from the atmosphere. Or if they were, they were acting as relatively small sinks – much smaller than people previously thought. So we found some other interesting things too. The community of organisms of soil fauna that lives underground was very different under the woody vegetation than under the native grasses. And under the native grasslands, the soil fauna communities – such as nematodes and other organisms – were more diverse. There were more tropic levels, more kinds of the organisms under the native grasslands, than under the invaded woody sites. So there were changes both in the makeup of the soil, and there were also important changes in the organisms that live in that soil and depend on that soil for their food source.

ES:

RJ: This is happening in many places around the world, in almost every continent – the amount of area that remains as grassland is decreasing. Now a lot of grasslands have already been converted to crop, to farmer’s fields and such. But the native grasslands that remain are increasingly being taken over by shrubs and trees. And there are really diverse sets of factors that come into play here. Two of the most important are grazing – the introduction of cattle – and fire. Now when we introduce cattle into an area, they don’t all plants evenly. They favor herbs, relatively soft material, and they don’t eat spiny, thick-leafed shrubs. And so what happens is that grazers move into a system, and they preferentially select the herbs, and leave the woody species behind, or allow those woody species to grow. Furthermore, if in the case of a common species like mesquite, the seed pods are attractive to a cow, and the cow, or a deer or some other animal eats those seed pods, spreads the seeds around as it walks across the land, and essentially propagates the woody species that way. So grazing is one way that shrubs and trees have increased in abundance into grasslands. And the second way that is equally or more important is this interaction with fire. I spent many of my years growing up in central Texas, and the last Indians to control the hill country of Texas were “Comanches”:http://earthobservatory.nasa.gov:81/Library/glossary.php3?xref=global%20carbon%20budget, they were horse-riding people. And they rode horses across what was essentially an open, park-like savanna, entirely covered with grasses, and then an occasional tree. And we know this to be the case, both from historical records of settlers, as they rode wagons and moved into areas, they wrote about what the land was like. And we also know this from historical and biological records. Well as European settlers moved into central Texas and other areas of the south, and to other areas in the South and West of the U.S., and kept fires from burning, those areas filled in with trees and shrubs. And now you could no more ride a horse across the hill country in Texas than fly to the moon – it’s just entirely filled in with species like junipers and mesquite.

ES: Hold that though one minute please while I change tapes.

RJ: To finish that thought one final difference between the woody species and the grasses and herbs are that the tissue is different. The wood is tougher to decompose, and has lower quality than the grasses and the herbs. So this also affects the animal and the microbial communities that live in the soil and rely on the plant.

ES:

RJ: So those two processes, the introduction of grazing, and fire suppression, are what are driving the change, a similar change all around the world. And there may be some other factors that come into play, and some differences locally, but those two things together are what are transforming grasslands many places globally.

ES:

RJ: One of the things that our study does is to suggest that in the U.S. at least, the amount of carbon that we thought was being taken up by the plants and stored in the ecosystem as a whole, is lower than we previously believed. And this is because of the offsetting changes in the carbon pool, gains in the trees above ground, but then losses in the soil pool below ground. And what that basically means is that the U.S. as a country is even farther out of balance – in a sense – from our fossil fuel emissions, than what’s going back into the land. So policy makers want to know – how much carbon dioxide they have to keep from going into the atmosphere to stop, or to slow the increase in CO2, to slow the rise in greenhouse gasses. What this suggests is that reductions in greenhouse gasses from the tailpipes of our cars, smokestacks of industry, need to be scaled back slightly more than what we though previously. And thinking about the policy implications some more, I mentioned that CO2 concentrations, carbon dioxide concentrations in the atmosphere should actually be going up twice as fast as they currently are. And a lot of diverse set of scientific evidence all points to a similar thing – that thing is that we need to act quickly, forcefully, to reduce greenhouse gas emissions that the natural service that we’re getting from the oceans and land – which is a good thing – is not likely to continue at the same rate, especially on land. And that really, meaningful cuts in fossil fuel emissions are going to be needed very quickly.

ES: Let me give you a moment to pause.

RJ: So let me think about plantations for a little bit. Plantations, or growing trees where they weren’t previously, is one mechanism being discussed as a way to store carbon from the atmosphere. And, that will certainly work. There’s no question that you can take a grassland, in a very productive part of the world, grow trees on it, and store carbon in the biomass of those trees. Before we do that on a large scale, and by large scale to make a difference in the global carbon budget. Let’s say that there are six units of carbon that we admit from fossil fuels. Let’s say that we wanted to try and tackle one unit per year through storage and plantations. You might have to plant an area the size of Alaska, or twice the size of Texas to do that. We’re talking about really large areas to make a dent in the fossil fuel emissions. Because those fossil fuel emissions are extremely large. So there are a couple of things that we need to check carefully before we advocate using plantations as a way to store carbon. One of them, we need to check and see that the carbon is maintained in the soil, so that we’re not losing carbon. We need to realistically take into account all of the carbon costs in growing those trees, so how much fossil fuels and carbon we use to plant them, whether the soil is tilled or prepared in any way. So to do a full accounting of the carbon and the economic costs of such an exercise. And then the other issue that we might think about before we plant twice the area of Texas into pines or eucalypts is to ask what other changes might occur if we were to do that. Some things that come to mind include changes in the water balance, so there’s good evidence in Australia, for example, that large-scale plantations of eucalypts can reduce the amount of water that’s in streams and watersheds. There is typically acidification of the soil that happens when you grow trees like pines and eucalypts in grassland. So there are many, many powerful changes that will occur in a system in addition to storing carbon that we should probably look at before we implement plantations in a large-scale.

ES: What are some other options being considered by scientists? I imagine cutting emissions is being considered…

RJ: Obviously cuts in emissions is one. Let me get back to you about, what can the science do. Well I think that scientists, and the people who studied this sort of thing, can do a couple of things. We provide better data for the policy makers to understand – how much carbon is going into the oceans and into the land — so really understanding the carbon cycle and the biology and the physical processes that are important. And then the next thing that we can do is to ask, will the rates of sequestration of storage on land, and in the oceans, continue for the next hundred years, or the next fifty years, or two hundred years? That makes a huge difference for policy makers, because imagine trying to address the current rise in CO2 in the atmosphere in a process like the Kyoto Protocol, only to find out that in 50 years that rise will be two, or four-fold higher? So it might be twice as high as I’d mentioned, or at least significantly higher if this land based sequestration stopped occurring. And there are some reasons to suggest that it might stop, or at least slow. Imagine a dinner plate where you keep piling steak on someone’s dinner plate. Steak is a good food, it’s high in nitrogen, but eventually, you keep piling steak on someone’s plate, they need something else to grow, so they need some vegetables or some grain or some other food source. Well plants are just the same way. We keep adding CO2 into the atmosphere, and CO2, carbon dioxide, is a fertilizer for plants. But they also need water, and they need nitrogen, and they need phosphorus, and so as CO2 continues to rise, the ability of the plant to use that CO2 is likely to start to slow. And so, through a number of studies from around the world, we’re trying to understand whether this sink on land will continue in the coming decade.

You asked about what else can we do. Well, in addition to reducing fossil fuel emissions, we can promote renewable energy sources such as solar power, wind power. There are a lot of mechanisms being discussed to store carbon not just on land in trees, but to say pump it into the deep ocean, which isn’t really getting rid of the carbon permanently, it means that it takes hundreds of years of thousands of years to come back to the surface. But it’s essentially buying people time. And that’s how I view plantations. Even if plantations only help us on say a 10 to 20 year time scale, if they allow us more time to transform our technology, to build new plants and things like that, then that’s still a meaningful adjustment. But the amount of carbon going into the atmosphere is so large, that plantations probably won’t do much more than buy us a little time. They won’t make the problem go away.

ES: **********

RJ: Another mechanism besides plantations for how we might store carbon is instead of putting trees into grasslands, is to restore native grasslands. So imagine taking a degraded grassland, or abandoned agricultural soil, and restoring a prairie on that system. You can store a lot of carbon in the soil from that prairie. In doing so, you can have a lot of other benefits. It’s good for native diversity. The natural organisms can come back and use that system. It’s productive for a certain amount of grazing. And so I think that we could look at habitat restoration, and storing carbon in the soil of grasslands and wetlands, as another mechanism, in addition to plantations – and one that uses the native vegetation to do it rather than placing a whole brand new community into a system that completely changes the way a system looks, the animals that use the system, and everything else about it. So I guess promoting the restoration of grasslands in the West, and the restoration of wetlands, I think is a great way to store some carbon and to have a lot of other benefits as well.

I’ll add one thing to that. In agriculture, when farmers plow a field, a lot of soil carbon, or soil organic matter, goes into the atmosphere quickly. So the plow literally chews the soil up. And as it chews and turns that soil up, it makes the organic material available to microbes. They use it as a food, and the CO2 goes up into the air. And when you take such an abandoned agricultural field, and you restore it into grassland, it has a large potential to store carbon in the soil. And that’s why the restoration of grasslands could work.

ES:

RJ: For example, Ducks Unlimited has a project now in the Prairie Pothole region of the U.S. and Canada, where they are brokering habitat restoration in grasslands, where power companies are paying landowners to restore habitat, to restore grassland habitat, and to preserve that habitat. So, in a sense it might be cheaper for a power company to pay a farmer or a landowner to restore their land and to store carbon in the soil, than is to actually pay to reduce the carbon at the smokestack. And that’s a really novel idea. That brings up, or opens a lot of possibilities to mitigate greenhouse gas emissions. You know, we want to give economic incentives to companies to reduce their emissions directly. That’s a key part to any greenhouse gas plan. But we also want to be flexible, to allow companies to find other meaningful ways to store carbon. And if they can pay farmers to preserve grassland habitat, to restore abandoned agricultural fields to prairies, and document the amount of carbon that goes into the soil, and that is stored as a result, then that’s a valid way to reduce the carbon going into the atmosphere. Those kind of creative mechanisms are increasingly becoming part of the discussions in how to reduce greenhouse gas buildup.

project 2

ES:

RJ: This is actually a good segue into the gradient project, because, you have to look at the history of carbon dioxide to look at what the change means. So, before the industrial revolution, the concentration of carbon dioxide in the earth’s atmosphere was roughly 275 parts per million (PPM). And today it’s about 370 PPM, so it’s about 1/3 higher than it was a couple of hundred years ago. And if you look back over the last half a million years or so, it was never more than about 300 PPM at any time during that 1/2 million years. By any credible scenario, if you look at the United Nation’s projections for where we’d be with atmospheric CO2 emissions in the coming century, those projections range from about 500 to about 900 PPM. So we’re talking about 2 or 3 or even 4 times of historical levels. It’s very likely, almost certain, in this century, that the earth’s CO2 concentration will be over 500 PPM. And it may approach 1000 PPM. When it reaches those kinds of concentrations, those are concentrations that the Earth hasn’t seen in tens of millions of years. And for me, that’s a really sobering thought, because, one thing that’s not so well known about carbon dioxide in the atmosphere is that once it’s there, it lasts a long time. So the half-life of CO2 in our air is over 100 years. What that means is, if a problem becomes severe, with global warming and climate change, it’s with us for many generations. It’s with us for hundreds of years, and there’s not a lot we can do about it. And for me, I think that’s the greatest motivation for change, is that, if we wait too long, when problems crop up they’re going to be very expensive to try and fix. And they’re going to be with us for a long time. Not just us – they’ll be with our grandchildren, and our grandchildren’s grandchildren. And for me, that’s the best motivation to change of all.

ES:

RJ: This is a project in Central Texas located near Temple Texas, at the “Agricultural Research Service Station”:http://twri.tamu.edu/twripubs/NewWaves/v11n3/project-3.html there, of the Department of Agriculture. Our colleagues there are Wayne Pauly and Hiram Johnson at the ARS. This project in the grassland is about the length of a football field. And it’s the only experiment in the world to provide a continuous gradient of carbon dioxide from previous concentrations, where we were at the end of the last Ice Age to where we’ll be in the coming century. So it applies a range of CO2 concentrations, from 200 PPM through 275, where we were a couple of centuries ago, with the start of the industrial revolution, through 370 PPM – where we are today – and then all the way up to about 550 PPM. So it’s the only project in the world to provide a continuous gradient of carbon dioxide and see how the grassland responds to that system. And it’s also unique in looking not just at what may happen in the future, but at what changes have already occurred in the past since the start of the industrial revolution. It’s a native prairie system, a diverse mix of grasses and forbes and herbs, and the goal of the project is to understand how the ecosystem responds to the extra CO2, and also to understand which things change incrementally or what we call linearly, in a straight line fashion, and where the important thresholds are. The policy makers are desperate to know where the uncertainties, where are the surprises. They want to know, are the concentrations, for which certain processes change dramatically, where the system changes irreversibly. And those are the kinds of things we can look at with a gradient approach that you can’t do just by taking the current concentration of CO2 and comparing it to some future concentration. We want to know how various processes change all the way along there. Now, one of the most interesting take-home messages from that project is that the changes that have already occurred since the start of the industrial revolution, and since the end of the last ice-age, are actually bigger than the changes that are projected to occur in the coming century. And that project provides some good evidence for a positive effect of CO2 on plant growth, grass growth and forage for cattle, for example, but it also suggests that we’re reaching a point now in the atmospheric CO2 concentration where the plants in the ecosystem are not as good at using the extra CO2 and not as good at storing it. So it suggests that we’re at an important threshold, that as CO2 continues to rise, the ability of the grassland to take up that CO2 is diminished. And for reasons that we’re already discussed, that has important policy implications, because it suggest that land based systems, at least, will not be able to continue to take up as much CO2 as they have in the past.

ES:

RJ: We established the experiment on a native grassland system, so we didn’t mix in, or pick the species that we used. We took natural grassland in the old prairie region there in Central Texas, and we built the system over the top of that. And the system looks like a giant caterpiller, essentially. And imagine taking air from the atmosphere into one end of the tunnel, and blowing it down that tunnel, that tunnel’s made of clear plastic so the light can get through. And as the air moves along that tunnel, the plants take the carbon dioxide out of the atmosphere, through photosynthesis, and they lower the CO2 concentration. So the one end of one tunnel, the CO2 concentration is just the normal air concentration. And at the other end of the tunnel, it’s much lower – the paleo concentration, 200 PPM. And at the other leg we took normal air and added extra CO2 into that air. So that as that air moved along, in the tunnel, it also was reduced, but when the air left that tunnel, it was back to the ambient level. So we essentially had half of the experiment that looked at paleo carbon dioxide concentrations, those from the past, and half of the experiment that looked at future, or high CO2 concentrations. And the experiment was controlled for temperature, humidity, and basically everything was kept the same in the system except the CO2 concentration. So we were isolating the effect of CO2 on the plants in the ecosystem, and examining how they would change to an increase in carbon dioxide concentrations.

ES:

RJ: So if we look at the end of the last ice age, say 10,000 years ago, the Earth’s carbon dioxide concentration in our air was roughly 200 PPM, and it fluctuated for the last 1/2 million years between about 180 and 280 PPM or so. But then at the start of the industrial revolution, the CO2 concentration really started to increase rapidly. It’s increased now 1/3 from what it was a couple of hundred years ago. And it’s projected easily to double more in this coming century. Now CO2, carbon dioxide, is a fertilizer for plants – it’s a food for plants. And they take carbon dioxide out of the atmosphere, and this is what they make sugars from, they convert it into leaves and stems and roots and plant material. But plants use water and other nutrients as well – they need water and other nutrients. So as CO2 has risen over the past couple of hundred years, there’s good evidence that plants have grown more because of that, because of that fertilizer effect. But as CO2 continues to rise, as concentrations get higher and higher, to 4 and 5 and 600 PPM, other factors in nature start to limit the ability of the plants to grow — such as water availability, or the availability of nitrogen. If you’re a farmer, you can apply nitrogen to a field as a fertilizer. But in the actual forest, natural grassland, or a natural system in the Western United States, isn’t fertilized. And so nutrient limitations and water limitations become increasingly important and can strain the ability of the plants and the ecosystem to use the extra CO2. So what our study shows, and what a number of other studies have shown, is that the ability of the plants in the soil to take up extra carbon is not as great in the coming century as the gains that have occurred in recent centuries, as CO has risen from 270 to say 370 PPM. As carbon dioxide continues to increase from today’s concentrations, to those in the future, plants will be less able to use that extra carbon dioxide, they’ll gain less from it. And this is one of the reasons why land-based carbon sinks will be likely to slow in the coming century – that they won’t take up as much carbon dioxide from fossil fuel emissions as they have, say in the last 50 years.

There’s a picture of this system on our website, if you want to see it visually, if that would help you.

ES:

RJ: The plants that we chose, the plants that were there naturally, have been there a long time. We wanted to look and see how, to examine how the natural ecosystem and the natural plants changed in response to carbon dioxide. So that was the reason why, for example, we didn’t start in an agricultural field, or we didn’t pick a particular species and used that in our experiment. We wanted to use nature’s diversity in place now in grasslands and examine how that changes with CO2. But these are species that are prevalent in the area, and that have been around for many, many years.

ES:

RJ: The thing that makes this project unique is the emphasis both on past and future of carbon dioxide concentrations. There are a number of experiments in the U.S. and around the world, that compare forests and grasslands and other systems at current ambient CO2 concentrations – you know, those around us right now – with some level from the future, sometimes twice that concentration. But logistically, there’s a good reason why people were unable to look at past concentrations in the field, or in nature, and that is because it’s a lot easier to add extra carbon dioxide into the air and around plants, than it is to take carbon dioxide out of the air. So scrubbing carbon dioxide from the air, and having experiments in the field is very difficult to do. And our experiment used unique technology that allowed us to do just that. To reduce the carbon dioxide concentrations that the plants were growing in, and to maintain them in a controlled fashion. And so that’s one of the aspects of our experiment that was unique, this emphasis on both past and future changes, and the fact the other people were really unable to look at those past changes in the field, to look at the paleo concentrations of carbon dioxide.

ES:

RJ: So our study found that many of the plant’s physiological processes, like photosynthesis, responded fairly linearly to increases in carbon dioxide all across the range of the experiment. But overall production of all the plants and soil carbon storage basically saturated at 400 PPM. Now that’s important because, that’s the CO2 concentrations essentially of today. So we’re at 370 PPM right now. Now our experiments said that, at least for some processes, including carbon storage in the soil, that there really weren’t any gains above 400 PPM. Now for me, that’s one of the most important parts of the study, because it suggests that, right now, we’re at a threshold where the benefits of the extra CO2 may not be all that great. That’s also relevant from a policy standpoint, because if natural land-based systems start to slow down in their ability to store extra carbon in the atmosphere, that means that the increase in extra CO2 concentration will be even faster now than it currently is. So the carbon dioxide from the tailpipes of our cars, and from industry, will be more likely to stay in the atmosphere rather than going into land systems. The key result of that experiment is that threshold effect, at 400 PPM. And that means that we’re almost there, basically. Because of these other nutrient and water limitations, the system becomes much less efficient at using the extra carbon dioxide than it was before.

ES:

RJ: That’s exactly right – we’re getting a free service from nature, and that’s a good thing, there’s no doubt about it. But there are a lot of people relying on the fact that that service is going to continue. And there’s an increasing set of evidence in the scientific literature that said it won’t continue, or that if it does continue, it won’t continue to the same extant. And that has dramatic policy implication for the rate at which carbon dioxide goes up into the atmosphere. So the natural systems don’t just keep taking and taking and taking. You know we talked about the plantations before, you know, they’re another example of this. You know, people look at how much carbon that’s stored in the trees and the plantations, and that can be a good thing. But that carbon is also more vulnerable to such things as hurricanes or fire, or there are a whole bunch of things that has to happen to keep that carbon from going back into the atmosphere.

ES: Is there anything else that you’d like to share with the listener’s of Earth and Sky?

RJ: Well, the key thing is to address fossil fuel emissions head on. And the issue of generational time is one that I stress in my new book, and that is identifying which problems that will be with us a long time if they’re severe. And global warming and climate change is one of those problems for me. Because carbon dioxide lasts hundreds of years in the atmosphere, if problems are severe, they’re going to be with us for a long time. And I would much rather see us address fossil fuel emissions now than leave those problems to future generations.