Trying to reflect sunlight artificially could backfire
FRISCO — When it comes to global warming nothing is simple — and that includes proposals to mitigate the effects of heat-trapping greenhouse gases with planetary scale geo-engineering schemes. One of the ideas that’s been floated is to seed the atmosphere with material in order to reflect some of the sun’s incoming heat.
But a recent study by German researchers suggests that it probably won’t work — and could have unexpected consequences for the global water cycle. In their model, based on an energy balance analysis, the researchers showed that the water cycle responds differently to heating by sunlight than it does to warming due to a stronger atmospheric greenhouse effect. Upsetting that balance could shift the Earth’s rainfall patterns, the researchers said. “These different responses to surface heating are easy to explain,” said Axel Kleidon, of the Max Planck Institute for Biogeochemistry in Jena, Germany. Using a stovetop scenario as an analoy, Kleidon said, “The temperature in the pot is increased by putting on a lid or by turning up the heat … but these two cases differ by how much energy flows through the pot,” he said.
A stronger greenhouse effect puts a thicker ‘lid’ over the Earth’s surface but, if there is no additional sunlight (if we don’t turn up the heat on the stove), extra evaporation takes place solely due to the increase in temperature. Turning up the heat by increasing solar radiation, on the other hand, enhances the energy flow through the Earth’s surface because of the need to balance the greater energy input with stronger cooling fluxes from the surface. As a result, there is more evaporation and a stronger effect on the water cycle.
In the new Earth System Dynamics study the authors also show how these findings can have profound consequences for geoengineering. Many geoengineering approaches aim to reduce global warming by reducing the amount of sunlight reaching the Earth’s surface (or, in the pot analogy, reduce the heat from the stove). But when Kleidon and Renner applied their results to such a geoengineering scenario, they found out that simultaneous changes in the water cycle and the atmosphere cannot be compensated for at the same time. Therefore, reflecting sunlight by geoengineering is unlikely to restore the planet’s original climate.
“It’s like putting a lid on the pot and turning down the heat at the same time,” explains Kleidon. “While in the kitchen you can reduce your energy bill by doing so, in the Earth system this slows down the water cycle with wide-ranging potential consequences,” he says.
Kleidon and Renner’s insight comes from looking at the processes that heat and cool the Earth’s surface and how they change when the surface warms. Evaporation from the surface plays a key role, but the researchers also took into account how the evaporated water is transported into the atmosphere.
They combined simple energy balance considerations with a physical assumption for the way water vapor is transported, and separated the contributions of surface heating from solar radiation and from increased greenhouse gases in the atmosphere to obtain the two sensitivities. One of the referees for the paper commented: “it is a stunning result that such a simple analysis yields the same results as the climate models.”