December 3, 2012

Scientists question key carbon allies: plants, soil

New studies raise doubts about carbon storage, which could mean more immediate action is needed.

Robert Krier / McClatchy News

When climate scientists try to estimate how much the Earth will warm due to increasing levels of greenhouse gases in the atmosphere, a key consideration is the role of plants and soils. The more carbon they absorb, the more they reduce the global warming potential.

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Water from melted ice rushes along the surface of the Greenland ice sheet through a supra-glacial stream channel in this July 4 photo. A new report says heat-trapping pollution from all nations increased by about 1 billion tons last year.

The Associated Press

Greenhouse gas emissions grow

WASHINGTON — The amount of heat-trapping pollution the world spewed rose again last year by 3 percent. So scientists say it's now unlikely that global warming can be limited to a couple of degrees, which is an international goal.

The overwhelming majority of the increase was from China, the world's biggest carbon dioxide polluter.

Of the planet's top 10 polluters, the United States and Germany were the only countries that reduced their carbon dioxide emissions.

Last year, all nations combined pumped nearly 38.2 billion tons of carbon dioxide into the air from the burning of fossil fuels such as coal and oil, according to new international calculations on global emissions published Sunday in the journal Nature Climate Change.

That's about a billion tons more than the previous year.

The total amounts to more than 2.4 million pounds of carbon dioxide released into the air every second.

Because emissions of the key greenhouse gas have been rising steadily and most carbon stays in the air for a century, it is not just unlikely but "rather optimistic" to think that the world can limit future temperature increases to 2 degrees Celsius (3.6 degrees Fahrenheit), said the study's lead author, Glen Peters at the Center for International Climate and Environmental Research in Oslo, Norway.

Three years ago, nearly 200 nations set the 2-degree C temperature goal in a nonbinding agreement. Negotiators at a conference under way in Doha, Qatar, are trying to find ways to reach that target.

The only way, Peters said, is to start reducing world emissions now and "throw everything we have at the problem."

Andrew Weaver, a climate scientist at the University of Victoria in Canada who was not part of the study, said: "We are losing control of our ability to get a handle on the global warming problem."

In 1997, most of the world agreed to an international treaty, known as the Kyoto Protocol, that required developed countries such as the United States to reduce greenhouse gas emissions by about 5 percent when compared with the baseline year of 1990.

But countries that are still developing, including China and India, were not limited by how much carbon dioxide they expelled. The United States never ratified the treaty.

The latest pollution numbers, calculated by the Global Carbon Project, a joint venture of the Energy Department and the Norwegian Research Council, show that worldwide carbon dioxide levels are 54 percent higher than the 1990 baseline.

– Seth Borenstein/The Associated Press


The 2011 figures for the biggest polluters:

1. China, up 10 percent to 10 billion tons.

2. United States, down 2 percent to 5.9 billion tons

3. India, up 7 percent to 2.5 billion tons.

4. Russia, up 3 percent to 1.8 billion tons.

5. Japan, up 0.4 percent to 1.3 billion tons.

6. Germany, down 4 percent to 0.8 billion tons.

7. Iran, up 2 percent to 0.7 billion tons.

8. South Korea, up 4 percent to 0.6 billion tons.

9. Canada, up 2 percent to 0.6 billion tons.

10. South Africa, up 2 percent to 0.6 billion tons.

But recent studies indicate assumptions about plants' and soils' capacity in the so-called "carbon cycle" may be overly optimistic. If these studies are correct, even bigger cuts in greenhouse gas emissions will be needed to prevent drastic, irreparable climate shifts.

Not only is it possible plants won't be able to absorb as much carbon as climate models project, but plants' response to the carbon cycle could actually amplify global warming, Paul Higgins and John Harte write in the November edition of the Journal of Climate.

It all comes down to mobility.

Carbon dioxide is recognized as critical for photosynthesis, so the more there is in the atmosphere, the more there is available for plant growth. As Earth's climate warms, the theory has been that trees and other plant communities would treat the added CO2 as fertilizer and grow bigger and faster.

But because climate conditions will be changing, to take advantage of the added CO2 some plant communities will have to migrate to neighboring areas that provide the necessary growing conditions. The speed at which plants can make these moves is the question.

Higgins, associate director of the American Meteorological Society's Policy Program, said it has been very difficult to build global ecosystem models that are sensitive to the limitations of plant migration.

"If you look at the models, they had no real constraints on plant mobility," he said. "They basically assume that any type of plant can grow in any location where the climate is the same."

But it isn't quite that simple.

Part of the problem is that the scientists who study plant migration and the scientists who build carbon-cycle models have tended to work separately, Higgins said.

"What we did (in the study) wasn't something that people tended to do before," he said. "We had to break the model."

Scientists who have examined the fossil record by studying pollen deposits found in bogs have found that in past periods of climate change, plants had limited capacity to migrate.

The paper by Higgins and Harte, an environmental science professor at the University of California, Berkeley, describes a range of possible impacts this slower migration might have on the carbon cycle.

"Constrained plant movement would exacerbate the losses of carbon due to changes in climate, because plants wouldn't be able to move to those locations where climate becomes favorable for them," Higgins said. "In some locations (or for some plant types) there would be increases in carbon storage, but in other locations carbon losses from plants and soils could be large. Thus, total carbon storage could go down.

"Our results suggest that the overall effect of climate change would be to reduce carbon storage in plants and soils. CO2 fertilization could counteract the consequences of climate change. However, CO2 fertilization might or might not actually happen. If it doesn't, then the losses of carbon due to climate change could be large."

The United Nations' Intergovernmental Panel on Climate Change has estimated the amount of warming by 2100 due to greenhouse gas emissions ranges from 1 to 6 degrees Celsius. Many argue that even a 1-degree rise will have devastating impacts.

Another recent study concluded that global climate models have been overestimating plants' ability to absorb carbon by 3.4 percent. That study found that trees can't take advantage of extended warm periods, as previously assumed, because photosynthesis basically shuts down in late summer and early fall, even when the days continue to be warm.

Another study released this fall concluded that plants won't be the major carbon sinks many had hoped, for a different reason: Soils often don't have enough nitrogen and phosphorus to take advantage of the added CO2 in the atmosphere.


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