Autumn and spring temperatures in the northern hemisphere have risen by about 1.1°C and 0.8°C respectively over the last 20 years. The carbon balance of terrestrial ecosystems is highly sensitive to these climatic changes. The temperature increases have also been accompanied by a "greening" trend, characterized by a longer growing season for plants and increased photosynthesis.

All this led many scientists to believe that spring and autumn warming would increase carbon sequestration via photosynthesis and that the period of net carbon uptake would rise in the future. Researchers at the Laboratoire des Sciences du Climate de l'Environnement in Paris, France, and colleagues in Europe, China, Canada and the US are now saying that this may not be true. They have found that the length of the net carbon uptake period has actually decreased at nearly all northern hemisphere atmospheric carbon dioxide stations during the last two decades because of rising autumnal temperatures.

"This finding stands in apparent contradiction with autumn greening and longer-lasting vegetation activity detected at mid-to-high northern latitudes by remote sensing and phenomological data," leading author Shilong Piao told environmentalresearchweb.

Using atmospheric records from the last 20 years, Piao and colleagues observed a trend towards an earlier autumn-to-winter carbon dioxide build-up. This suggests that there is a shorter net carbon-uptake period and increasing carbon losses in autumn. These losses occur as ecosystems respire.

The scientists found that, although both plant respiration and photosynthesis increase as autumns warm up, respiration increases more than photosynthesis, yielding a net loss of carbon to the atmosphere. They say that these losses may offset as much as 90% of the increased carbon uptake that occurs during springtime. "The findings also suggest that if future [autumn] warming occurs at a faster rate than in spring, the overall ability of the northern ecosystems to sequester carbon may diminish," says Piao. The only way to stop this trend is to reduce carbon emissions, he adds.

The researchers came to their conclusions using satellite observations of vegetation greenery and modelling of the biosphere. For a more complete picture, they also used large-scale atmospheric carbon dioxide concentration data from the NOAA–ESRL air-sampling network and site-level "eddy covariance" flux measurements. This technique can directly measure the net carbon exchange between ecosystems and the atmosphere. "Atmospheric data offer the unique advantage of long and well calibrated time series, while the shorter eddy-covariance data point more directly to how climate impacts ecosystem functioning," explained Piao.

The scientists will now look in more detail at the interaction between temperature and the terrestrial carbon cycle for the other seasons.

The work was reported in Nature.