Grasslands stripped of plant diversity absorb about 15% less CO2 by 2050 in a PIK-Kiel model

The model's case for diversity is substitution. "Climate change could reduce the productivity of plants that at the moment dominate pastures and meadows. But if plant diversity is maintained, other plants that cope better with climate impacts have the chance to become more prominent and stem the decline," said Stephen Wirth, the PIK researcher who led the study ^[8]. Local experiments had already shown that plant diversity supports grassland productivity. This study takes that finding to a global vegetation model ^[7]. The study was not designed to estimate how much diversity real grasslands will actually lose by 2050 ^[6].

The two global figures can be checked against each other. If the 15% and the 11 billion tonnes describe the same below-2-degree run, diverse grasslands would be taking up roughly 73 billion tonnes of CO2 a year in 2050, since 11 divided by 0.15 is about 73 ^[14]. That is a flow into plants through photosynthesis ^[4]. The thing this doesn't tell you is how much of the shortfall would otherwise have ended up stored. Storage would show up in the soil, and the account of the study reports only that soil carbon stocks are lower under reduced diversity ^[5]. It does not include figures for the 4 degree run ^[6].

The global average also combines regions that move in opposite directions. "In some regions, remaining plants can compensate for the loss of other plant types, and productivity may even increase. Globally, however, the benefits of maintaining diversity prevail in both climate scenarios," said Susanne Rolinski, a PIK co-author ^[10]. The authors caution that the model simplifies plant traits and interactions, so the exact magnitude of the effects is uncertain ^[11].

I think the sign of the result is sturdier than its size. The benefit of diversity holds globally under both warming scenarios ^[10]. The 15% and the 50% depend on how well a simplified model lets one plant type stand in for another ^[11].

The authors' recommendation is about grazing. "To maintain grassland productivity under climate change, we need management practices that preserve existing diversity over the long term and prevent overgrazing," said Friedhelm Taube, a Kiel University co-author ^[12].