The Heat Is Online

Higher Temperatures Turn Plants From Sinks to Sources

No plant CO2 relief in warm world, Sept. 18, 2008


Plants are unlikely to soak up more carbon dioxide from the air as the planet warms, research suggests.


US scientists found that grassland took up less CO2 than usual for two years following temperatures that are now unusually hot, but may become common.


The conclusion parallels a real-world finding from Europe's 2003 heatwave, when the continent's plant life became a net producer, not absorber, of CO2.


The latest study is published in the scientific journal Nature.

Researchers extracted four intact segments of grassland, about 3 sq m in area and weighing about 12 tonnes each, from the prairies of Oklahoma, and placed them in special chambers at the Desert Research Institute (DRI) in Reno, Nevada.


Conditions in the chambers, such as temperature, moisture and sunlight, could be precisely controlled.


Drying out


Two of the four chambers were given a set of conditions mimicking what actually happens, on average, on the wild prairies.


Temperatures rose and fell with days and nights and seasons, and "rainfall" was injected in a realistic pattern.


The other two chambers received the same prescription with the exception that for a whole year, temperatures were always 4C higher.


The warmer plots saw a shortfall in carbon dioxide uptake of about 30% during the warm year and the one following.


DRI's Jay Arnone, who led the study, said two different mechanisms appeared to be responsible.


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


"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 understood for some time. But what happened in the following year, when temperatures returned to "normal", was not so familiar.


Even during the warm year with its meagre amount of photosynthesis, plants had put carbon in the soil.


So during the normal year following, soil microbes had extra carbon to process, which they did, emitting more carbon dioxide into the air.


Mixed forest


By complete coincidence, the study mimicked fairly closely events on the other side of the Atlantic.


As DRI researchers were turning up the heat in 2003 in their experimental plots, in Europe it was happening for real, with temperatures in some places reaching 6C above normal.


An analysis led by French researchers, published in 2005, showed that as the continent became hotter, Europe's plants changed from being net overall absorbers of CO2 to net producers.


A lot of faith is being placed in some circles in the capacity of trees and plants to maintain absorption of CO2 as concentrations of the gas rise, or even to use the extra CO2 to grow faster and absorb more of it.


It is one of the reasons behind the recent upsurge of interest in having western governments pay to protect tropical forests.


But the DRI research is one of a number of pieces of evidence suggesting it will not always work. Some ecosystems might continue to absorb carbon dioxide, and perhaps increase the rate of absorption; others may react to warming 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," said Professor Arnone.


"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."


Sinking feeling: Hot year damages carbon uptake by plants


Agence France Presse, Sept. 18, 2008


Plant and soil can take up to two years to recover from an exceptionally hot year, a finding that has implications for the combat against global warming, according to research published on Wednesday.


The recovery lag could cause a rethink about the ability of grasslands and soil to act as a sponge, also known as a "sink," that removes carbon dioxide (CO2) from the air, its authors said.


In an unusual experiment, scientists in Nevada carved out plots of grassland in central Oklahoma, each measuring more than 2.4 metres (7.8 feet) long, 1.2 metres (3.9 feet) wide and 1.8 metres (5.85 feet) deep, with their plant communities and soil left intact.


These miniature ecosystems were then installed in four container-sized labs in which light, darkness, temperature and rainfall could be carefully replicated and levels of CO2 monitored.


Over the next four years, two of the four chambers were programmed to reproduce the weather conditions of the original site, previously determined by a seven-year monitoring of that location.

The other two chambers, though, were exposed to a sudden rise in temperature during the second year of the experiment -- a hike of four degrees Celsius (7.2 degrees Fahrenheit).


"We wanted to create an extreme year and look at how the ecosystem recovered from it," said Jay Arnone, a professor at the Desert Research Institute (DRI) in Reno, Nevada, told AFP.


"Basically, we dosed it with heat one year, then we removed the heat and asked, 'how long do the effects that occurred in that year persist, and which processes are affected?'"


Arnone's team found that during this anomalously warm year and the year that followed, the two plots sucked up two-thirds less carbon than the plots that had been exposed to normal temperatures.

Carbon capture comes from plant growth, through synthesis, and from micro-organisms that break down carbon in the soil.


The hike of 4 C (7.2 C) is at the far end of core estimates made last year by the UN expert group, the Intergovernmental Panel on Climate Change (IPCC), for the average worldwide warming this century.


But such an increase is most likely to be gradual or in smaller hikes, not the dramatic year-on-year rise increase replicated by Arnone's team.


Arnone said the 4 C increase was based on between eight and 11 exceptionally warm years in the weather records, spanning from 1873 to 1999, where the plots had been dug up. These warm years were between 1.0 and 3.8 C (1.8 and 6.9 F) higher than the region's long-term average, but were not accompanied by drought.


The findings are important because they point to the impacts of extensive heatwaves on grasslands and forests that together account for 20 percent of Earth's land surface, said Arnone.


They also throw in a new factor for calculating the value of natural sinks, which play a critical role in offsetting man-made greenhouse gases.


Hot years or prolonged heatwaves, according to the IPCC, are likelier to become more frequent under climate change.


In such a scenario, sinks will be less able to soak up CO2. In turn, the CO2 that is not absorbed will remain in the atmosphere rather than be sequestered in the ground, thus accelerating the greenhouse effect.


In scientists' parlance, this is a "positive feedback," or a vicious circle likely to amplify climate damage.