The carbon uptake of rainforests will be crucial to how the climate responds to human emissions.

The carbon uptake of rainforests will be crucial to how the climate responds to human emissions.

William R. Wieder/NCAR

Plants may not protect us against climate change

Plants are one of the last bulwarks against climate change. They feed on carbon dioxide, growing faster and absorbing more of the greenhouse gas as humans produce it. But a new study finds that limited nutrients may keep plants from growing as fast as scientists thought, leading to more global warming than some climate models had predicted by 2100.

Plants need different nutrients to thrive, such as nitrogen for making the light-absorbing pigment chlorophyll and phosphorus for building proteins. Farmers supply these in fertilizer, but in nature, plants have to find their own sources. New nitrogen comes from the air, which is 78% nitrogen by volume, but it is almost all in the form of nitrogen gas. Plants can't break this down, so they rely on soil bacteria to do it for them. Some plants, mainly legumes, have evolved nodules on their roots that harbor these bacteria. New phosphorus comes from weathering rocks or sometimes from sands blown on the wind from deserts.

Yet these two key nutrients are not particularly well accounted for in climate models. Only two of the 11 models used to project future warming in the most recent report from the Intergovernmental Panel on Climate Change (IPCC) considered the effects of limited nitrogen on plant growth; none considered phosphorus, although one paper from 2014 subsequently pointed out this omission.

So biogeochemist William Wieder of the National Center for Atmospheric Research in Boulder, Colorado, and colleagues looked at the projections for new plant growth in the various models and considered how much new nitrogen and phosphorus would be required for these projections to be met. They then estimated how much extra nitrogen and phosphorus would actually be available from natural sources and found that there would not be enough, revising the models accordingly.

Taking nitrogen and phosphorus into account brought down the researchers' average prediction of annual global carbon storage by 25% compared with the IPCC figures, the team reports online today in Nature Geoscience. Such a dramatic decline could turn the land from taking up carbon overall to pumping it out by 2100, as the rate of respiration by soil microbes, which exhale carbon dioxide, is predicted to increase in a warmer world. This could mean that Earth gets even hotter as the land starts to amplify human-induced warming rather than slowing it down.

There are various unknowns, however. For example, bacteria in soil release nitrogen and phosphorus as they break down dead plants, and so these microbes could increase the amount of available nitrogen and phosphorus. The paper is “solid, exciting research,” says ecologist Chris Field of Carnegie Institution for Science in Palo Alto, California, who notes that various models have looked at ways different factors might affect future plant growth. Biogeochemist Eric Davidson of the University of Maryland Center for Environmental Science in Frostburg says researchers need to do more fieldwork to see how nutrient conditions affect the growth of forests. “It's difficult to do, it's expensive, but it's the only way we can get better parameters for these models,” he says.