"We wanted to reduce the uncertainty in one of the key unknowns in future climate-carbon cycle projections, which is the extent to which increases in carbon dioxide will increase land photosynthesis," Peter Cox of the University of Exeter, UK, told environmentalresearchweb. "So called 'CO2-fertilization' is hotly debated by ecologists, and the latest Earth system models also disagree on the role that CO2-fertilization plays in the land carbon sink. This is important because the land carbon sink currently absorbs about 20% of our emissions, and because the causes of the sink largely determine how it will change in the future."

Cox and colleagues Sabrina Wenzel and Veronika Eyring from the Deutsches Zentrum für Luft- und Raumfahrt (DLR), and Pierre Friedlingstein from the University of Exeter used atmospheric carbon data from Point Barrow in Alaska and Cape Kumukahi in Hawaii along with seven Earth system models.

"We use the full-range of Earth system model projections to tell us how to interpret an observed change in the current climate system – in this case the observation that the cycle of carbon dioxide concentrations in the atmosphere, which arises from the uptake of carbon dioxide by plants during their growing season, is increasing," said Cox.

At Point Barrow the amplitude of the seasonal carbon dioxide cycle increased from about 13 ppmv to 18 ppmv from 1974 to 2013. At Cape Kumukahi, the amplitude increased from about 8 ppmv to 9 ppmv between 1979 and the present.

From their results, the team estimated that if the carbon concentration in the atmosphere doubles, CO2-fertilization would increase land photosynthesis outside the tropics by about one third. This is a bigger effect than suggested by current Earth system models that include nitrogen limitations on plants, but smaller than most Earth system models that ignore such limits, according to the researchers.

"Increases in land photosynthesis with carbon dioxide are generally good news for ecosystem health, as land photosynthesis provides the primary resource for life on land," said Cox. "However, our results also imply that the land carbon sink is in a large part due to increases in carbon dioxide, which in turn means that the land carbon sink will decline rapidly when we stabilize carbon dioxide – as we must do to minimize the risks of climate change."

Now the team is exploring other possible "emergent constraints" on climate change using Earth system models, as part of the EU-funded CRESCENDO project. "Our next target will be to provide a constraint on the carbon budget consistent with stabilization of the climate at 1.5 and 2 degrees," said Cox.

Cox and colleagues reported their findings in Nature.

Related links

Related stories