Carbon dioxide absorption by plants
What tools can be used to define influence of plants on global warming?
One of the tools is a computer model that takes into account how much carbon dioxide plants take up, which can be calculated from satellite measurements. The other is a supercomputer, which can run simulations of plant growth, which is done in real time. The team hopes the combined tools will be able to give an accurate picture of how much carbon dioxide is absorbed by the world’s forests, lakes and grasslands.
“Our model is really powerful, and the next step is to integrate it with the satellite measurements to get a really high-resolution picture of how much carbon dioxide is being absorbed,” says Peter Macdiarmid, a senior scientist at the Pacific Northwest National Laboratory in Richland, Wash. “That’s a much more detailed picture than we have now.”
The team will present its results in the Dec. 4 issue of the journal Science.
The amount of carbon dioxide in the atmosphere is measured by monitoring the ratio of two gases in the air, carbon dioxide and water vapor. When the ratio of carbon dioxide to water vapor is higher than a certain threshold, the climate is said to be “carbonated.” Carbon dioxide is a greenhouse gas that traps heat in the atmosphere, causing the planet to warm up.
Macdiarmid and his colleagues, who include computer scientists at the Pacific Northwest National Laboratory, have developed a computer model that simulates how much carbon dioxide plants take up from the atmosphere.
The model is based on measurements of the amount of carbon dioxide in the atmosphere, which are collected by NASA’s Aura satellite. The satellite launched in 2002, and has been collecting data continuously since 2006.
“The problem is that satellite measurements only go back a few years, so we don’t have a long-term picture of how much carbon dioxide is in the atmosphere,” says Macdiarmid. “We need to use the most accurate measurements we have, which are taken by the Aura satellite.”
The team also needed to model the growth of the world’s forests, lakes and grasslands.
“We need to know how much carbon dioxide is in the atmosphere before we can model the uptake of carbon dioxide by the plants,” says Macdiarmid.
Macdiarmid’s team used a supercomputer to simulate the growth of plants and the absorption of carbon dioxide over the course of one year.
The scientists used the supercomputer to model how much carbon dioxide is absorbed by the plants in each part of the world.
“We found that forests in the United States and Europe absorb about 60 percent of the carbon dioxide in the atmosphere, and grasslands in the U.S. and Europe absorb about 20 percent of the carbon dioxide in the atmosphere,” says Macdiarmid. “But the ocean absorbs about 90 percent of the carbon dioxide in the atmosphere.”
The model also shows that plants are more efficient at absorbing carbon dioxide than most other terrestrial ecosystems.
“Plants absorb about 25 percent of the carbon dioxide in the atmosphere, but they can use up to 80 percent of the carbon dioxide they take up,” says Macdiarmid. “So they are a more efficient sink than forests, for example.”
While carbon dioxide in the atmosphere is measured by satellite measurements, carbon dioxide that has been absorbed by plants and returned to the atmosphere is not measured. The model that Macdiarmid and his colleagues have developed is the first to be able to simulate this process.
The scientists are now trying to understand how much carbon dioxide plants absorb in the future. They are also trying to improve the computer model.
“Our model is a great starting point,” says Macdiarmid. “But we want to improve it so we can model more accurately how much carbon dioxide is being absorbed by the world’s forests, lakes and grasslands.”
Some regions of the world, including parts of South America, North America, and the eastern Pacific Ocean, have lost carbon dioxide from land-based ecosystems in recent decades. Other regions have absorbed more. The rate of land-based carbon uptake varies widely from year to year.
Scientists have used land-based measurements of carbon dioxide to measure global carbon uptake since 1992, but in the current study, they analyzed how much carbon dioxide has been absorbed by terrestrial ecosystems from 1992 to 2012, using data from a network of more than 1,000 field sites around the world. The researchers combined these observations with the global carbon cycle, which is the global exchange of carbon between the atmosphere, the land, and the oceans.
“We found that the amount of carbon dioxide absorbed by terrestrial ecosystems has been increasing over time,” said lead author Mary Droser, a research associate at the University of Michigan, Ann Arbor. “The carbon uptake by terrestrial ecosystems has been rising faster than emissions of carbon dioxide, and is now the largest human-caused source of carbon dioxide in the global carbon cycle.”
Droser said the rate of carbon uptake is currently about 30 percent higher than emissions of carbon dioxide from land.
“The implication is that if we don’t reduce emissions, terrestrial ecosystems will continue to absorb a large amount of carbon dioxide, and we will not meet our carbon dioxide emissions targets,” Droser said.
Droser and her colleagues used an atmospheric carbon dioxide emissions-to-CO2 uptake model to calculate the carbon dioxide uptake by terrestrial ecosystems in the northern hemisphere. They also calculated how much carbon dioxide was absorbed by the ocean and the atmosphere, based on the amount of carbon dioxide that was emitted and the amounts of carbon dioxide that were absorbed.
The researchers found that the land-based uptake of carbon dioxide increased by about 15 percent per decade over the past decade, and the carbon dioxide uptake by the ocean and atmosphere has increased by about 3 percent per decade over the past decade.
Droser said that land-based carbon dioxide uptake is likely to increase further in the future as the world warms and soils and forests are exposed to higher levels of carbon dioxide.