Human cultures have faced climate changes many times before, sometimes responding with ingenuity, other times succumbing to ecological devastation. The current change in climate brings a special challenge. Research by Washington State University scientists shows that the high levels of carbon dioxide that are boosting the earth’s temperature are also changing the way ecosystems work. When it comes to carbon, plants and the soil organisms they depend on have their limits—and those limits may soon affect us.
Even for seasoned archaeologists, it was an unsettling find. At the site in Belize of a sizeable Maya town, in the layer of artifacts left just before the town was abandoned, John G. Jones and his colleagues from around the United States came upon a pit full of skulls.
“In this pit there were 29 healthy human heads,” recalls the Washington State University archaeologist. “Well, they were healthy up to the point when they were removed.”
The skulls had belonged to men, women, and children. Their teeth were in excellent condition, a good sign that they came from members of the ruling elite. During the decades before the heads were separated from their bodies, the Maya had been struggling with political degeneration and increasingly bloody wars. Then the climate changed. The rains stopped. Crops failed for several years running, and the soil of their barren fields lay open to erosion by wind and every short cloudburst.
“We’re having a very discontent population,” says Jones, who tends to speak of ancient people and events in the present tense. “One of the reasons they’re discontent is because of climate change. Their rulers’ job—their only job—is to keep the rains coming. It was a cushy job for a couple thousand years. But all of a sudden what we’re seeing is the rain stops coming, and the environment changes. People are starving.
“What could they do? They could yell at the rulers, saying, ‘You promised you’d give us rain.’ They could blame it on the gods, or ask the gods to intercede. They could move.”
In the end, he says, the people rose up, dispatched their failing rulers, and dispersed. The skull pit marks the end of habitation at that site. Similar events may have occurred in other Maya centers, as the great civilization collapsed under the stress of political unrest and ecological devastation.
“They had a good run,” says Jones. “They had 2,500 years or so. Most cultures don’t last that long.”
Jones traces changes in climate, and how human societies responded to those changes, by examining pollen grains trapped in lake and estuary sediments. Pollen is tiny but incredibly durable; as long as it stays completely dry or completely wet, he says, it lasts just about forever. He has easily identified pollen grains more than 300 million years old.
On field expeditions, Jones and his graduate students use a five-horsepower motor to drive a coring tube deep into sediment near village sites being investigated by other archaeologists. The team they work with includes experts on soils, cultural artifacts, radiocarbon dating, and tiny plant crystals called phytoliths, but Jones goes first, using pollen to sketch the major event horizons in the core. “We’ll say, ‘Hey look, something’s happening at 180 centimeters below the surface, and another one at 400 centimeters. Let’s get radiocarbon dates on these right now, because that takes a little time.'” After he lays out the basic framework, he and the others go back and fill in the details.
His three-inch-wide, 11- to 15-yard-deep cores record a vast span of environmental and human history. They go deep enough to reach material laid down before people settled the area. Jones and his colleagues recently showed that agriculture in the New World began around 5200 BC, significantly earlier than previously thought. It was driven in part by population growth and the need for a reliable food supply, and in part by a change in climate that made cultivation more likely to succeed.
Jones reads these events in the small but distinct tracings in the cores. A layer of charcoal from prolonged, large-scale burning shows when people first cleared the forests. Then tree pollen disappears from the cores, and pollen from maize and squash appears in large numbers. The wild ancestors of those plants grew in very different habitats, so finding them together in a place where neither would grow on its own means human cultivators brought them there. Weed pollen shows up along with the crops; even the first farmers had to contend with unwelcome visitors. By the time the Maya came into Belize between 1800 and 1500 B.C., the land was already shorn of trees and under cultivation.
Then, after centuries of continuous farming, come signs of the ecological disaster that precipitated the downfall of the Maya: a sharp decline in crop and weed pollen, and a layer of eroded soil so common throughout the region that it’s been labeled “Maya clay”.
“I can’t think of one core that doesn’t have it,” says Jones. “It’s not just in Belize, it’s in Guatemala, it’s in Mexico; everywhere we look, we see the same kind of thing.
“This is a change in climate on a horribly denuded landscape, and all of the soils are just washing away. . . It’s a sobering picture.”
Massive erosion is an obvious disaster in any agricultural system, but more subtle changes to the soil can be just as devastating. WSU ecologists Dave Evans (’90 Ph.D. Botany) and Rick Gill are finding that high levels of carbon dioxide (CO2), the major cause of our current climate change, are altering plants and soils in ways that could profoundly affect the health of ecosystems and our ability to feed ourselves.
Unlike previous “natural” warming trends, present-day warming is not the result of sunspot activity or cyclical fluctuations in temperature. This episode is being caused by a huge increase in the amount of CO2 and certain other gases in the earth’s atmosphere. Like panes of a greenhouse, these gases keep heat in; hence the name “greenhouse gases.”
Among the greenhouse gases, carbon dioxide merits special attention because, quite apart from its role in boosting temperature, it has profound effects on living things. Plants take in atmospheric CO2 through their leaves. If they have enough water and nutrients such as nitrogen, they use the CO2 to make sugars and other molecules. Plants then leak some of those sugars from their roots, which benefits the soil bacteria and fungi that, in turn, provide the plants with nitrogen that they process from sources in the soil. The reciprocal interactions among plants, fungi, and bacteria are the basis of all plant growth, all forests, all grasslands, all agriculture.
Rick Gill, along with colleagues from Duke University and the USDA’s Agricultural Research Service, has been studying how a grassland ecosystem copes with different levels of CO2. On their research site in north Texas, a 110-yard-long tube of greenhouse fabric encloses a narrow strip of native prairie in a sort of atmospheric time tunnel. Constrictions in the tube divide it into 5.5-yard-long segments, and pumps create a stepwise gradient of CO2 in the segments from one end of the tunnel to the other. At one end, representing the future, CO2 is at 550 parts per million (ppm), the level it’s predicted to reach by 2100. In the middle of the tunnel, CO2 is at today’s level of 375 ppm. At the other end,
CO2 is at 220 ppm, the level that prevailed in earth’s atmosphere for at least ten thousand years, until the Industrial Revolution and our large-scale burning of fossil fuels began nudging it upwards in the mid-1800s.
The researchers first asked whether plants would grow bigger and faster in the presence of a higher level of CO2. That possibility is the basis of one of the major strategies proposed for fighting climate change: using plants to take more CO2 out of the air and sequester it as part of the structure of the plant. Gill and his colleagues found that higher CO2 levels did lead to more plant growth—for a few seasons. Then growth stalled.
Gill says there’s a limit to how much carbon plants can take in and how much carbon-rich plant material, such as fallen leaves, soil microbes can recycle before they run into other constraints. Both plants and microbes need nitrogen and other nutrients as well as carbon. Some nitrogen sources are small molecules that microbes can process easily. Others are so large and complex, and require so much energy to break down, that most microbes don’t bother. They simply slow their growth rather than attempt to mine the nitrogen out of those sources.
Under elevated CO2, says Gill, “plants are much more efficient, they photosynthesize more, they grow more. But then that litter falls to the ground. Microbes have to break it down, and before long they run out of nitrogen. So there’s a natural negative feedback in the system that slows down carbon sequestration.”
The feedback is called “progressive nitrogen limitation.” It’s been observed in forests as well as grasslands, and it poses a big problem for carbon sequestration schemes.
“It doesn’t let the system respond to the rising CO2 in the way that we’re forecasting it will,” says Gill. “We spend all of our time focusing on carbon, carbon, carbon, with the hope being that, as we pump more and more CO2 into the atmosphere, native ecosystems will suck some of that out.”
Gill says the notion of carbon sequestration is so widespread, it’s become embedded in many of the models used to predict the extent of climate change.
“They assume that we’re only going to be at 550 parts per million 90 years from now, because a third of the CO2 we produce will end up in trees or in soils,” says Gill. “What we’re showing is, don’t count on it. Because the assumption that’s built into that is that you’ve got abundant water and abundant nutrients. And we’re saying, that doesn’t happen.”
At Dave Evans’s study site in the Mojave Desert, just north of Las Vegas, nine rings of PVC pipe supported by thin posts hang above the creosote bushes and burro-weed. Twenty-five yards across and about five feet off the ground, the white rings look like something out of a science-fiction show, a signal to alien landing craft, perhaps. In fact they are part of one of the longest-running experiments ever attempted to try to assess the effects of extra CO2 on natural ecosystems.
The experiment ran for 10 years. Six of the rings served as controls, three with ambient air blown over the plants they surrounded and three “silent,” with no blowers. In the three experimental rings, the pipes blew enough CO2 into the area enclosed by each ring to raise the level there to 550 parts per million.
While his colleagues from the University of Nevada assessed how the plants responded, Evans examined the soils and underground organisms. He found that Mojave soil reacted very differently than Gill’s grassland soil. In areas of high CO2, it had more nitrogen available for plants and soil microbes to use. But there were a couple of catches. In normal years, most of the extra nitrogen was scarfed up by soil organisms, not plants. And wet years were something else again.
As anyone who has visited a desert during a wet year knows, rain works wonders. Plants that may not have been seen for a decade or more flourish and bloom. Evans’s colleagues found that in wet years the high-CO2 rings showed a lot more plant growth than the rings without added CO2. But it wasn’t the creosote and other large native plants that grew more, or the small native annuals that nature buffs might go to the desert to see.
“The biggest increase was in invasive annuals,” says Evans. “It’s the invaders that were responding.” Areas within the high-CO2 rings were choked with red brome, a close relative of the cheatgrass that has spread through Washington in recent decades. Areas in control rings and outside the rings had very little of it. It was the combination of extra CO2 and extra water that allowed the red brome to flourish.
Evans says invasive species can rapidly exploit new nutrient supplies. The Mojave’s native plants can’t make the same kind of quick score. They specialize in endurance and the ability to make do with a little. “It’s in it for the long haul,” says Evans of creosote, which can live for a thousand years.
The main threat from invasive species in the Mojave may come at the end of the growing season, says Evans.
“Red brome is good fuel for wildfires. The grass dies and dries in late summer. So one consequence of elevated CO2 in arid ecosystems could be an increase in fire frequency or intensity, due to the presence of these invasive annuals. That could kill the system, because the native plants aren’t tolerant of fire at all.”
Evans sees the long-term effects of that whenever he visits the site on the Hanford reservation where he did his doctoral research. Twenty years ago, he says, the landscape was sagebrush interspersed with bunchgrasses that offered good grazing opportunities for cattle and wildlife.
“Then cheatgrass came in. It all burned 10 to 15 years ago, and now all we see is the cheatgrass. The sagebrush hasn’t come back.”
And the burning continues. This past August, fire charred more than 20,000 acres of cheatgrass-dominated land at the Hanford reservation.
When they took a closer look at what was happening in their soils, Gill and Evans found subtle but significant changes. In Gill’s tunnel, plants in the middle and in the high-CO2 segments of the tunnel grew more and deeper roots.
“They end up building fewer and fewer leaves and more and more roots, trying to tap into the nitrogen that’s available,” he says. That could pose a problem for grazing animals, both domestic and wild. The total size of plants will be the same, but more of each plant will be below ground, where grazers can’t reach it.
The search for more nitrogen affects the Community of soil microbes too, says Gill.
“There’s pretty good evidence that bacteria aren’t very good at it,” he says. “They’re small, they’re individual cells, they need their carbon source and their nitrogen source adjacent to one another. But if you’re a fungus, you’ve got long mycelia, and so you can tap in [to deep sources of nitrogen]. So what we see is that as CO2 increases, you get a shift from a bacterial-dominated community to a fungal-dominated community.”
The same thing happens in most forest systems that have been studied, and Evans also found it in desert soils. During the last three years of his Mojave experiment, he supplied the test rings with a different isotope mix of CO2 that allowed him to trace where the extra CO2 ended up.
“What we found is that mainly the fungi are seeing this new carbon,” says Evans. Bacteria took up very little of it.
Gill says the shift toward fungi, at the expense of bacteria, could send ripples through the whole biosphere.
“It changes the patterns in which nutrients are being made available,” he explains. “It changes the
nature of the organic material within the system.”
The details may be complicated, but the conclusion is simple: CO2 that we have put into the air is changing the way the soil works.
Then there’s the look back offered by the “old” end of Gill’s tunnel. It’s one of the few scientific studies of ecosystem response to climate change that doesn’t take today’s conditions as its starting point.
“People think that today is sort of the standard that we’re working off of,” says Gill. “I think we often fail to recognize that today is very different from prehistoric conditions. Plants lived [in an atmosphere that contained] from 220 to 260 parts [of CO2] per million for a long time, for 10,000 years, since the last interglacial, and they evolved to deal with that. In a hundred years, we’ve increased [the concentration of CO2] by 150 parts per million—a huge change.”
Exposing plants and soil to a preindustrial level of CO2 provided startling results. From the low-CO2 segments of the tunnel to the middle, where CO2 is at current levels, “we see lots of changes in the way the plants photosynthesize, the rate at which they lose water, how they use the nitrogen, and the microbial community [in the soil],” Gill says. “It’s very, very sensitive over that initial range. As you move from preindustrial levels up to where we are today, there’s pretty good indication that there’s a net accumulation of carbon—that these ecosystems did act as carbon sinks, slowing the rate of greenhouse gas accumulation.
“But then when we start [at today’s level] and move up to 550, what we find is that they aren’t nearly as sensitive, they aren’t as well-suited to absorbing elevated CO2.”
In other words, grassland ecosystems have already adapted to today’s higher CO2 levels—and may not be able to adapt further to the even higher levels they’ll be exposed to in coming years. With similar changes going on in other ecosystems, the implications for us are grave. We can no longer count on earth’s resilience to bail us out, any more than the Maya could reclaim their lost soil by offering prayers to the gods.
“The ability of native ecosystems to function on our behalf, at least from a carbon-balance standpoint, is not likely to continue as strongly as it has in the past,” says Gill. “As far as forecasts go, consider the future to be worse than you thought.”
So what do we do about climate change? What can we do?
Perhaps the clearest message from Gill’s and Evans’s work is that we have to stop putting so much CO2 into the atmosphere. As the saying goes, when you’ve gotten yourself into a hole and you’re trying to get out, the first step is to stop digging.
Gill says biodiesel and other new energy technologies could make a big dent in our CO2 output and give us a chance to address the damage already done. Carbon sequestration programs, which encourage people to plant trees to offset the CO2 they generate by driving a big car or running a business, are more problematic. They’ve gotten a lot of attention as a possible cure, but nitrogen limitation and other constraints make them a short-term solution at best.
“You may be able to tweak the system so that over the scale of 50 years, 100 years, you increase carbon storage, but that’s only if there’s abundant nitrogen and enough water, and we haven’t paid nearly enough attention to that,” says Gill. “We can’t just look at this in terms of carbon sequestration. We have to start looking at the interconnections.”