This is actually a good segway 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, but 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.
This is a project in Central Texas located near Temple Texas, at the Agricultural Research Service Station 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 forests 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.
We established the experiment on a native grassland system, so we didn’t a certain mix in, or picked 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.
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 your’re a farmer, you can apply nitrogen to a field as a fertilizer. But in the actual forest, a 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 comming 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.
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 prevalant in the area, and that have been around for many, many years.
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.
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.
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.
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.
The following person was interviewed for today’s article.
Robert B. Jackson
Department of Biology and
Nicholas School of the Environment
Duke University
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.
And that might have a large effect on the balance of carbon in the atmosphere — and ultimately how much this greenhouse gas warms our planet.
