By Holly Evarts
A new analysis of global satellite observations shows that vegetation can powerfully alter atmospheric patterns that influence climate and weather. Using a new approach, researchers found that feedbacks between the atmosphere and vegetation can explain up to 30 percent of variability in precipitation and surface radiation. The paper, published today in Nature Geoscience, is the first to look at such interactions using purely observational data. The findings could improve predictions of water supply, drought and heat waves that are critical to crop and water management, the authors say.
“While we can currently make fairly reliable weather predictions, as, for example, five-day forecasts, we do not have good predictive power on sub-seasonal to seasonal time scales,” said study leader Pierre Gentine, a professor at Columbia Engineering and associate of the Columbia Water Center. “By more accurately observing and modeling the feedbacks between photosynthesis and the atmosphere, we should be able to improve climate forecasts on longer scales.”
Vegetation can affect climate and weather by the release of water vapor into the air during photosynthesis. The vapor alters surface energy flows and potentially leads to cloud formation. Clouds alter the amount of sunlight that can reach the earth, affecting the energy balance, and in some areas can lead to precipitation.
“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,” says Julia Green, Gentine’s PhD. student and the paper’s lead author.
Recent advances in satellite observations of solar-induced fluorescence, a proxy for photosynthesis, enabled the team to infer vegetation activity. They used remote sensing data for precipitation, radiation and temperature to represent the atmosphere. They then applied a statistical technique to understand feedbacks between the biosphere and the atmosphere. Theirs is the first study investigating land-atmosphere interactions to determine both the strength of the predictive mechanism between variables, and the time scale over which these links occur.
The researchers found that substantial vegetation-precipitation feedback loops often occur in semi-arid or monsoonal regions. In addition, strong biosphere-radiation feedbacks are often present in moderately wet regions, for instance in the Eastern United States and in the Mediterranean, where precipitation and radiation increase vegetation growth. Vegetation growth enhances heat transfer and increases the height of the earth’s boundary layer, the lowest part of the atmosphere that is highly responsive to surface radiation. This increase in turn affects cloudiness and surface radiation.
Current models underestimate feedbacks “mainly because they underestimate the biosphere response to radiation and water stress response,” Green says. “We found that biosphere-atmosphere feedbacks cluster in hotspots, in specific climatic regions that also coincide with areas that are major continental CO2 sources and sinks. Our research demonstrates that those feedbacks are also essential for the global carbon cycle—they help determine the net CO2 balance of the biosphere.”
Gentine and his team are now exploring ways to model how biosphere-atmosphere interactions may change with a shifting climate, as well as learning more about the drivers of photosynthesis, in order to better understand atmospheric variability.
Paul Dirmeyer, a professor at George Mason University who was not involved in the study, said the researchers had “put forward an intriguing and exciting new idea, expanding our measures of land-atmospheric feedbacks from mainly a phenomenon of the water and energy cycles to include the biosphere.”