When plants ?sweat?, the planet gets cooler. But how much more hotter can it get when that sweating rate slows down because of rising carbon dioxide levels? New research from the Indian Institute of Science (IISc) suggests this warming will increase at a constant rate as carbon dioxide levels rise, even when photosynthesis actually reaches a saturation point.
In specific regions, up to a third of the warming from increased carbon dioxide levels can be due to its impact on plant physiology.
The sweating of plants, or transpiration, occurs when water escapes from pores of a leaf called stomata when they open to absorb carbon dioxide for photosynthesis. The stomata, typically, adjust to the carbon dioxide levels in the atmosphere by opening less widely when there is greater quantity of the gas in the air, which means the plant conserves water. So, when plants ?sweat? less, the cooling effect from evaporation is lost.
?Does this have a measurable effect on the planetary scale? That was the investigation we have been doing in the last four or five years,? says Govindswamy Bala, associate professor at IISc?s Centre for Atmospheric and Oceanic Studies, who directed the research. ?You can observe this in the laboratory, but how big is it compared to the greenhouse effect? Typically we found it to be in the order of 10-20%.?
In regions with thickly forested areas, the warming could be at least 30% of the greenhouse effect, he says.
Scientists have previously reported that evaporated water helps cool the Earth as a whole and not just the local area where it takes place. The transpiration from the Earth?s green canopy has been known to play a significant role in the water cycle, accounting for nearly half of the evaporation on land. It is seen as a key component to explain trends in rainfall and floods.
According to Bala, the greenhouse effect of increased carbon dioxide content in the Earth’s atmosphere has been a major concern, but its impact on plant physiology is being keenly researched only recently.
?In India, the research is at its infancy with little observational or experimental studies on the impact of climate change on the physiology and phenology of forests,? says NH Ravindranath, professor at the IISc?s Centre for Sustainable Technologies, who is a co-author of the paper. ?We need to do both modelling and observation.?
An expert in the area of climate change and forests, Ravindranath says studies in Europe and America have clearly shown how forests are responding to climate changes.
The research paper, by lead author Ranjith Gopalakrishnan published last week in Environmental Research Letters, is among the first to investigate the decreases in canopy transpiration as a function of carbon dioxide concentrations using a global model. Other previous global modelling studies have looked at such effects for only a doubling of carbon dioxide levels, says Bala. The researchers used a Community Land Model developed at the US-based National Center for Atmospheric Research to simulate the heat and water exchange between the Earth?s surface and atmosphere. The model mimics the characteristics of some 16 plant types, such as grass and shrubs.
Using observation-based precipitation and surface air temperature to generate data over a 50-year period between 1950 and 1999, the researchers studied models over a range of carbon dioxide levels increasing from 100 ppmv (parts per million, by volume) to 1,200 ppmv. The current concentration of carbon dioxide in the atmosphere is around 400 ppmv.
While photosynthesis saturates after 800 ppmv, the modelling showed that canopy transpiration continued to decline at a constant rate of about 5% for every 100 ppmv increase in carbon dioxide levels. Evapotranspiration, the sum of canopy transpiration and the loss of water from the soil, will continue to decline by 1,305.45 km of water per year globally for every 100 ppmv increase in carbon dioxide accompanied by an increase in sensible heat, it noted.
?The basic message is that this particular effect is not going to saturate as you go to higher levels of carbon dioxide concentration. So, in the future, the decline rate is constant,? says Bala.
In September, researchers from the Carnegie Institution for Science at Stanford University, Lawrence Berkeley National Laboratory and IISc had reported their findings on the global effect of cooling from evaporation. The cooling effect of evaporating water is typically offset by the release of heat when vapour condenses in the atmosphere. However, through modelling, the scientists found that enhanced surface evaporation increases the amount of low level clouds in the atmosphere which help scatter solar radiation and cool the planet.