Vegetation holds key to climate control

New studies shine a light on the intricate relationship in which climate affects vegetation, which in turn impacts on the global climate.

LONDON, 23 March, 2020 − Here’s an easy way to warm the tropics even further: just fell some rainforest, and the local temperatures will soar by at least a degree Celsius, showing the role played by vegetation.

There is also a good way to temper the summer heat of temperate Europe: just abandon some farmland, leave it to go wild and leafy, and the thermometer will drop by perhaps as much as 1°C.

And, paradoxically, there is even a leafy way to warm the Arctic: burn lots of fossil fuels, precipitate a climate crisis, advance the growth of spring foliage by three weeks or so, and check the thermometer. The region will be even warmer, just because the Arctic has become greener.

These apparently contradictory findings are, more than anything else, a reminder that the pas de deux of vegetation and atmosphere is complex, intricate and finely balanced. Nor are they inconsistent, as each study simply takes the measure of vegetation change on local or regional climate.

Reducing heating

In sum, and for the time being, the big picture remains that forests absorb carbon, and more vigorous growth absorbs more carbon to significantly reduce the average rates of global heating across the entire planet.

In effect, all three studies demonstrate that vegetation moderates extremes of temperature in three climate zones.

Brazilian scientists report in the Public Library of Science journal
PLOS One that they subdivided a tract of the Atlantic rainforest in the southeast of the nation into 120-metre squares, measured those segments that had been part-felled or clear-felled, and read the local land surface temperatures.

If even one fourth of a hectare had been cleared, the local temperature went up by 1°C. If the entire hectare had been razed, the rise could be as high as 4°C.

Risk to trees

The Atlantic rainforest is one of the world’s richest ecosystems: it covers 15% of Brazil, but 72% of the population lives there. It holds seven of Brazil’s nine largest drainage basins, delivers water to 130 million people and its dams provide 60% of the nation’s hydroelectric power.

Between 2017 and 2018, around 113 square kilometres of this forest was cleared. As temperatures continue to rise, some tree species could be at risk.

“We don’t have enough data to predict how long it will take, but in the long run, rising temperatures in Atlantic rainforest fragments could certainly influence the survival of tree species in the forest, albeit some species more than others,” says one of the report’s authors, Carlos Joly, professor of plant biology at the University of Campinas in Brazil.

“The forest is extremely important to maintaining milder temperatures on the local and regional scale. Changes in its function could disrupt this type of ecosystem service.

“Abandoned cropland – or land cover change more generally – and its role in regional climate can help us adapt to and mitigate the effects of climate change”

“The Atlantic rainforest doesn’t produce water but it protects the springs and permits the storage of water in reservoirs for consumption, power generation, agricultural irrigation and fishing, among other activities.”

By contrast, Europeans have achieved a local 1°C cooling simply by abandoning farmland that was no longer sufficiently productive.

Between 1992 and 2014, the European Space Agency satellites compiled detailed maps of the continents, measuring the extents of evergreen needle-leaf forest, deciduous broadleaf woodland, open shrubland, crop fields, urban and built-up areas, wetlands, peatlands, grassland and mosaic areas of crops and wilderness.

In those 24 years – partly because of dramatic political changes that followed the collapse of the Soviet Union – around 25 million hectares of farmland was abandoned.

Drying wetlands

Although farmland was colonised elsewhere, the continent was left with 5 million hectares – an area the size of Switzerland – to be colonised by trees and other natural foliage, European scientists report in the journal Nature Communications.

Overall, the loss of cropland in Western Europe was associated with a drop of 1° in spring and summer. In eastern and northeastern Europe, however, temperatures rose by as much as 1°C, partly because what had once been wetlands began to dry.

“We are already at a mean warming of about 1.8°C on the land, and we will be about 3°C on the land even if we are successful at stabilising the average global temperature at 1.5°C,” says one of the report’s authors, Francesco Cherubini, director of the Industrial Ecology Programme at the Norwegian University of Science and Technology.

“That means we take action to adapt to a warming climate, and land use planning is one action that can bring local cooling benefits.”

The Arctic greens

“The message is quite clear. Abandoned cropland – or land cover change more generally – and its role in regional climate can help us adapt to and mitigate the effects of climate change. And by improving agricultural systems, we can free up land for multiple uses.”

But while Europe is changing, and forest in the tropics is being lost, the Arctic is becoming greener: as temperatures rise, vegetation has moved northwards and spring has arrived ever earlier, and growing seasons have lasted longer.

The science of measurement of seasonal change in plant and animal behaviour is called phenology. Chinese and US scientists report in Nature Climate Change that they looked at computer models of vegetation change and factored in the numbers: on average, in the last four decades, leaf-out has advanced by an average of more than four days a decade, and in some cases up to 12 days a decade.

That means snow-covered ground has retreated, and green leaves have moved northwards, and become denser.

Climate feedback

Snow reflects solar radiation, and darker colours absorb it. That means that local landscapes in the north have tended to become even warmer with each decade.

In the Canadian archipelago, the air has been measured at 0.7°C warmer, and parts of Siberia and the Tibetan plateau − far from any leafy canopy − have warmed by 0.4°C and 0.3°C respectively because advanced leaf-out further south means more water vapour, which moves north to change patterns of cloud cover and snowfall.

Climate scientists see this as positive feedback: climate change begets even faster climate change. Global heating tends to accelerate. Climate change affects vegetation, which in turn affects climate yet further.

“Positive feedback loops between climate and spring leaf phenology is likely to amplify in the northern high latitudes,” says Gensuo Jia, one of the researchers from the Chinese Academy of Sciences. “The impact of vegetation change on climate is profound in spring.” − Climate News Network