The Physics of Climate Change – How Earth’s “greening” affects temperature

Many people hear the term “greener earth” and envision flourishing rainforests. But former Junior Faculty Fellow Dan Li hears this phrase and pictures mathematical equations. Li and colleagues use computational models to understand the role that physics plays in climate change so that they can propose effective mitigation strategies.

Their article, recently published in Science Advances, suggests that the rise in vegetation or “greening” of the earth over the past few decades alleviates some of the effects of climate change, thanks to air turbulence. Turbulence, or the movement of air over land, isn’t often discussed in relation to the changing climate. But it actually seems to cool the earth’s surface by dissipating heat. Without it, the temperature of the earth’s surface would rise even faster.

Li was selected as a Junior Faculty Fellow by the Hariri Institute for Computing in the fall of 2016. His research usually focuses on understanding how urbanization affects the environment, from a local to a global scale. The mathematical models he uses to understand urban environments provided him with a clear computational lens to investigate how lusher landscapes might affect the temperature of the land.

Li and colleagues investigated how the recent increase in vegetation across the globe due to climate change, fertilization, and land management impacts local land temperatures. The team had two ideas: either there are more trees evaporating water in the area and the surface is cooled, or there are more trees darkening the land below and the surface is warmed because darker areas absorb more radiation from the sun. The researchers carefully built a mathematical model to tease apart how different physical interactions between the land and the air independently affect the land’s temperature.

The team discovered that Li’s mathematical model didn’t support either of their ideas. Rather, the model helped the researchers realize a third possibility:  when there are more trees, there is more turbulence and the land is cooled because the movement of air over the land dissipates heat on the land’s surface.

Though unexpected, the effects of turbulence made sense to Li. “There are more people studying the biological side of evaporation rather than the physical side. But evaporation is a process controlled by both physiology and physics so it makes sense that turbulence could affect land surface temperature.”

It is important to note that even if the temperature of the land is cooler due to this greening effect alone, the overall surface and air temperatures are still warming. The local cooling effect due to Earth’s “greening” is much smaller than the warming of the atmosphere due to climate change. “The cooling effect does not negate the larger warming trend. If you didn’t have this cooling effect the surface would actually be warming much faster,” said Li. Though climate change is increasing vegetation and cooling the land locally in some places, it is also warming the atmosphere at a dangerous pace globally.

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