Rainforests make rain. Shade trees cool microclimates. Despite knowing these things for decades or even centuries, science has done a poor job of measuring these effects. Now, satellite data has been used to rectify this, showing that in some regions local vegetation changes rainfall and sunlight by 30 percent. The findings could improve seasonal weather forecasting and add urgency to forest protection.
Plants transpire water vapor during photosynthesis, often releasing more than that which evaporates from a lake of the same size. Within a few days, the vapor falls as rain. This changes the precipitation and cloud cover at the site and downwind.
“Until our study, researchers have not been able to exactly quantify in observations how much photosynthesis, and the biosphere more generally, can affect weather and climate," said Julia Green, a Columbia University PhD student, in a statement. Green is first author of the paper published in Nature Geoscience.
Green and her co-authors found that models of global atmosphere underestimate these feedback loops, largely because they fail to fully take into account the extent to which vegetation responds to changes in the amount of sunlight and water.
Green measured the solar-induced fluorescence plants produce as they grow to estimate monthly photosynthesis by area, which in turn indicated vegetation growth. This was compared with data on rainfall, radiation, and temperature.
The strongest feedback loops were found in places where both water and sunlight constrain growth. In these places, extra rain leads to more growth, as does exposure to greater sunlight. Elsewhere, one of these is so abundant that increases have no effect. Such areas also tend to be rich in C4 plants, whose growth tends to be more responsive to changes in water availability.
This seems like common sense once observed, but models of the Earth's atmosphere have processed it poorly. To be fair, the patterns are complex and vary greatly. For example, in the Eastern United States, increased photosynthesis results from reductions in the low- and mid-level clouds that are widespread in summer. In contrast, in the eastern Mediterranean region, mid- and high-level clouds are more important. Other strong feedback areas include monsoonal Asia and southern Brazil.
These feedback hotspots also have an oversized influence on the global carbon cycle, representing either particularly strong sinks where carbon dioxide is drawn from the atmosphere or natural sources where it is released.
The authors are now exploring how such feedback is likely to change in a warming world. They also propose that weather forecasting models incorporate this information to more accurately predict wet and dry seasons in time to be useful for farmers and water supply managers.