Circular global warming

All it takes is one hot year, and a major greenhouse-gas absorber goes on a two-year vacation.

A team of US scientists has conducted what it says is the first study to measure the effect that an unusually warm year can have on an ecosystem’s ability to soak up atmospheric carbon dioxide. Plants provide this valuable service, and questions remain about how effective they will continue to be if global warming reaches into the range the Intergovernmental Panel on Climate Change (IPCC) has projected for the end of this century. Scientists have long known how plants act as net sources and absorbers from one season to another. But it’s been tougher to capture the details of what happens from year to year, or over the course of several years, says Jay Arnone, a researcher at the Desert Research Institute in Reno, Nev., who led the study.

The team set up 12 tall-grass prairie ecosystems in specially designed, climate-controlled containers, each with a volume of roughly 241 cubic yards. Each ecosystem grew in soil layers six feet deep.

In the second year of the four-year experiment, the team turned up the heat on six of the ecosystems by 7.2 degrees F. for the year – consistent with IPCC projections as well as one-year changes observed in the historical record for Oklahoma, whose prairie ecosystem the team was replicating. In the third year, the team returned the temperatures to “normal” and compared the results with the control plots.

During the hot year, the affected ecosystems responded by taking up far less CO2 than during the normal year. And while the plants recovered their ability to soak up CO2 the following year, the ecosystem as a whole still fell short of normal activity, largely because soil microorganisms continued to release CO2. The results imply that as climate warms and unusually warm years become more common, terrestrial ecosystems are likely to become increasingly less efficient at taking in CO2.

The results appear in today’s issue of the journal Nature.