Carbon dioxide, or CO2, makes up a tiny fraction of Earth’s atmosphere, and yet CO2 can have a big effect on long-term warming of our planet. A scientist talks about how Earth deals with CO2.
And in the U.S., even though we have approximately 5% of the world’s population, we give off about a quarter of fossil fuel emissions. So we’re far and away the dominant player, globally . . .
Fossil fuel emissions release carbon dioxide, or CO2 — a greenhouse gas.
About half goes into the atmosphere. The other half goes into the oceans and land. Rob Jackson is with Duke University’s Biology Department and Nicholas School of the Environment.
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 that 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. An interesting scientific question is whether that service will continue.
He recently led a study of areas where grasslands are being taken over by trees and shrubs. He found that the land doesn’t store carbon as well there.
Source for numbers used in today’s article:
The following is from the Intergovernmental Panel on Climate Change 2001 “Climate Change 2001: The Scientific Basis” Cambridge University Press:
These are the exact numbers in gigatons carbon (10×15 g C) per year globally for the decade of the 1990s:
- Emissions (fossil fuels and cement): 6.3
- Atmospheric increase: 3.2
- Oceans: -1.7 (net uptake)
- Land: -1.4 (net uptake)
So about one half of all emissions to the atmosphere stay there (3.2 out of 6.3 total), and the other half is taken back up by the oceans and plants/land (roughly one quarter each).
The following is a transcript of a interview with Dr. Jackson:
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 intersting 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.
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.
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 occuring as people thought before. There wasn’t as much carbon storage as people had though previously.
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.
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.
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.
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 shrub-lands 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.
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, Diana Wall 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, a long-term USDA site, the Sevilleta, 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 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.
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.
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.
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, 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.
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.
So those two processes, the introduction of grazing, and fire suppression, is what’s 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.
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.
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 eucolypts, 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 a 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.
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.
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 a grassland, it has a large potential to store carbon in the soil. And that’s why the restoration of grasslands could work.
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.