September 19, 2008

Plants May Not Absorb Additional CO2 from Warming

New research finds that plants are not likely to absorb any additional carbon dioxide (CO2) from the atmosphere as the planet heats up.

Researchers base their conclusion on a recent study that found grassland took up less CO2 than usual following two years of historically warm temperatures.  The study also supports findings during Europe's 2003 heatwave, when the continent's plant life became a net producer, not absorber, of carbon dioxide.

Researchers extracted four segments of grassland about 3 sq m in area from Oklahoma prairies.   The segments, weighing about 12 tons each, were then placed in special chambers, in which conditions could be tightly controlled, at the Desert Research Institute (DRI) in Reno, Nevada. 

Two of the four chambers were put in conditions simulating what actually occurs on the wild prairies, with temperatures rising and falling according to days, nights and seasons, and "rainfall" injected in a real-world pattern.  The other two chambers were set in the same conditions but with temperatures 4C degrees higher.

The researchers found that the warmer plots observed a 30 percent shortfall in the carbon dioxide absorption during the warm year and the one following.

DRI's Jay Arnone, who led the study, told BBC News two different mechanisms appeared to be responsible for the reduction in CO2 absorption.

"So in the warm year, the temperature goes up and causes more evapotranspiration from the plants," he said.

"But plants have evolved to 'know' that when it gets dry they should curb their water loss, so they reduce the apertures of their stomata (pores) to conserve water, and that constrains the amount of CO2 they can take up (by photosynthesis)."

This response has been known for some time.  However, what occurred in the following year, when temperatures returned to "normal", was not previously known.

Even during the warm year, with its small amount of photosynthesis, plants had deposited carbon in the soil.  So during the normal year following, soil microbes had extra carbon to process, which they then emitted, resulting in more carbon dioxide in the air.

By sheer coincidence, the study simulated events in Europe.

As the DRI researchers were heating up their 2003 experimental plots, in reality this was happening in Europe, where temperatures in some places reached 6C above normal.

An analysis by French researchers, published in 2005, found that as Europe became hotter, the continent's plants went from being net CO2 absorbers to being net producers.

The results are troubling, as a great deal of faith is being put into the ability of plants and trees to maintain CO2 absorption as atmospheric concentrations rise.

Indeed, this faith is partially responsible for the increasing interest in having western governments fund efforts to protect tropical forests.

However, the DRI research among many recent studies suggesting this may not work.   While some ecosystems might continue to absorb carbon dioxide, and perhaps even increase their rate of absorption, others may actually react by releasing the greenhouse gas.

"We conducted this study under current ambient levels of CO2 so we don't know for sure what'll happen in the future," Professor Arnone said.

"But we don't anticipate a huge effect of [elevated] CO2 on these systems. As high temperatures become more commonplace, you might expect a persistent reduction in the uptake of CO2 by natural ecosystems, and that may mean that the net rate of CO2 elevation may increase."

The study is published in the scientific journal Nature.