June 8, 2017, by Tim Radford
More rain falls in rainforests because the trees create the conditions it needs.
Image: By Michael Cory of Brooklyn, US, via Wikimedia Commons
A study of the interaction between sunlight, air, water and foliage should lead to better longer-term climate forecasts, scientists say.
LONDON, 8 June, 2017 – Plants make a world of a difference – and one group of climate scientists has now put a value on that difference. The traffic between atmosphere and vegetation is enough to explain up to 30% of the variations in rainfall and sunlight, they say.
In effect, they have put the figures into calculations of feedback: so much more rain falls because the trees create the conditions for more rain, according to a new study in Nature Geoscience.
The finding is not in principle surprising. Tropical biologists have known for decades that evapotranspiration from a tropical rainforest falls back as rain, so that the tree canopy has a role in managing its own climate.
What is new is that one team of US researchers has identified those regions where it happens, and then calculated the difference in raindrops and sunlight from above which can be attributed to the green things at work below.
And they claim that their paper is the first to pinpoint this traffic between the living world and the climate machine in terms of observational data, rather than simulation or theory.
“By more accurately observing and modelling the feedbacks between photosynthesis and the atmosphere, we should be able to improve climate forecasts on longer timescales”
So, by knowing a little more about the mechanics of sunlight and rainfall, researchers might be able to make weather and climate predictions that would pay off in terms of crop management, food security, water supplies, droughts and heat waves.
“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 scale, which is essential for food security,” said Pierre Gentine, an earth scientist at Columbia University in the US.
“By more accurately observing and modelling the feedbacks between photosynthesis and the atmosphere, as we did in our paper, we should be able to improve climate forecasts on longer timescales.”
The principles are simple enough: sunlight falls on plants which photosynthesise new tissue and at the same time release water vapour, which changes the levels of radiant energy at the surface and sooner or later forms a cloud, which then blocks sunlight, and in some places falls back as rain.
“But, 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 PhD student who led the research.
The researchers worked from US space agency records of solar-induced fluorescence, which is an indicator of photosynthesis, and therefore plants at work. They had data for precipitation, radiation and temperature, so they had a record of the atmosphere.
They then used statistical techniques to try to put a value on any feedback loop between living things and the gentle rain from heaven. And, at the end of the study, they could name the seasons and places at which this feedback was most likely to occur, and how much difference it made to levels of rainfall.
The effect was easiest to identify in the moderately wet regions of the eastern US and the Mediterranean, as well as semi-arid zones and monsoonal regions. Such findings are rarely conclusive, and need to be confirmed by separate studies. But as a consequence, climate scientists know just a bit more about the vital power play between sunlight, air, water and foliage.
Forests and grasslands are the agencies most likely to help moderate the accelerating climate change triggered by profligate human combustion of fossil fuels.
And researchers probably now know more about the planet’s forests than ever before. There is evidence that even the arid lands are “greening” in response to the extra carbon dioxide in the atmosphere, but scientists also know that this will not be enough to limit the global rises in temperature.
And they know – calculations were published only weeks ago – that no possible expansion of the forests could ever substitute for reductions in fossil fuel use.
But at least they have better data to feed into climate calculations, the Columbia scientists argue. “Current Earth system models underestimate these precipitation and radiation feedbacks mainly because they underestimate the biosphere response to radiation and water stress response,” said Green.
“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 and have implications for improving critical management decisions in agriculture, security, climate change, and so much more.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.