Is a warmer Earth a greener Earth? Remote sensing says no

As the vast tall jungles of the Amazon evince, plants thrive in warm wet environments. Global warming, by promoting evaporation, should make some parts of the world wetter as well as warmer. Could our climate-changed world become greener? Would those extra plants, by sequestering carbon dioxide, help mitigate global warming?

Maybe. Rising temperatures also promote desertification. And if it gets too hot, the enzymes that catalyze photosynthesis stop working. Earth could become browner, not greener. In a paper published today in Science, Maosheng Zhao and Steven Running of the University of Montana in Missoula have tackled the green-versus-brown question in what I think is the only way possible: They analyzed remotely sensed data.

The Moderate Resolution Imaging Spectrometer has been mapping Earth’s vegetation from space since 1999, when NASA launched Terra, the spacecraft that carries MODIS and four other environment sensors. The picture above is a MODIS image of the Amazon basin.

Unless you work in the field of remote sensing or, like me, have friends who do, you might not appreciate the science behind Terra and other Earth monitoring missions. MODIS is just as sophisticated as any imaging spectrometer built for astronomy. Its 36 wavelength bands span the range from 400 nm to 14.4 μm. Every day, Terra beams down a terabyte of data, a rate comparable to that generated by Higgs-hunting particle accelerators. And to extract useful data, you need to solve and apply radiation transfer formulas that Subrahmanyan Chandrasekhar and others developed for stellar atmospheres.

Zhao and Running used a decade’s worth of MODIS data to evaluate Earth’s net primary production (NPP) of carbon—that is, the mass of carbon from atmospheric carbon dioxide to terrestrial biomass. The decade 2000–09 was the warmest ever recorded. During that time, NPP rose in the Northern Hemisphere but it fell in the Southern Hemisphere in response to extensive droughts.

Unfortunately, the global balance turned out to be negative. Zhao and Running found that NPP decreased by 0.55 petagrams. Earth is getting browner, not greener.