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The big heat hit India earlier than usual: temperatures of 44°C in April have almost certainly hammered hopes for a generous wheat harvest in the subcontinent. Even before the month was over, desperate citizens were yearning for dust storms to darken the skies and lower the temperature.
Welcome to an altered future. Four new studies—each quite separate—appear to confirm that heat, drought, and hunger on massive scales can only get worse with rising global temperatures. One of the most ominous is a prediction that ferocious summer temperatures and extended droughts are increasingly likely to happen simultaneously.
Heat and drought together multiply the risks of wildfire. A second study looked at the impact of forest fires—in 2021, these incinerated large reaches of Siberia and the western United States—to conclude that they are actually intensifying Arctic warming: “brown carbon” ash from the flaming forests was probably twice that from fossil fuel combustion. More global warming means more heat and drought, which means more fire hazard, which means yet more warming.
A third research team examined the combined impact of global heating and the stresses of modern agriculture to confirm that the toll upon the global insect population is accelerating: the two factors together have almost halved the numbers of insects over farmland, and reduced the number of species in the examined sites by more than a fourth.
Biodiversity is a measure of the health of the planet: the richer and more lusty the natural life, the better it is for everybody. The richest terrestrial habitats have always been the great forests. But when these are degraded—by drought, by wildfire, by the axe, the farmer, the miner, or the rancher—the consequences go far beyond the release of yet more greenhouse gas to raise temperatures even further. Forest loss, according to a fourth study, can seriously raise local temperatures, change wind patterns, alter the reflectivity of local land surfaces, interfere with the composition of clouds, and interrupt regional rainfall patterns.
Which brings us neatly back to the challenge of heat and drought. European scientists began by defining their double trouble as summers in which the average temperature was higher than in 90% of the years between 1950 and 1980, and in which rainfall was lower than in nine-tenths of the same three decades.
Then, having made a count of the times both heat extremes and drought were recorded concurrently, they simulated a future at least 2°C warmer than in pre-industrial times. They report in the journal Nature Climate Change that between 1950 and 1980, heat and drought happened together just 3% of the time. In tomorrow’s warmer world, such double events become four times more likely.
Quite how this plays out depends very much on local rainfall patterns: more heat can often also mean heavier rainfall. For central Europe, even with more precipitation, this could mean simultaneous heat and drought every 10 years, or—if conditions get more dry—every four years. Central North America could be heading for such simultaneous events every six years in the worst case, every nine years if rainfall increases.
“Climate change may shift the distribution of precipitation in certain regions,” said lead author Emanuele Bevacqua of the Helmholtz Centre for Environmental Research in Leipzig, Germany. “The pattern of precipitation depends on atmospheric circulation, which determines regional weather dynamics through numerous interactions over large parts of the globe.”
One of these interactions has been at work in the Arctic, which is warming three times faster than the rest of the planet. Research in the journal One Earth makes it clear that wildfires far to the south contribute powerfully to this effect by filling the atmosphere with particles that absorb radiation and raise atmospheric temperatures.
In 2017, researchers aboard a Chinese icebreaker examined aerosols in the polar atmosphere. They concluded that brown carbon was an important factor and that the impact of brown carbon from blazing biomass—forest timber, foliage, undergrowth, and peat—was responsible for at least twice as much warming as brown carbon from fossil fuel exhaust and chimneys. Over the Arctic, the impact is almost a third that of black carbon from fossil fuel burning. And, of course, more brown carbon means more warming, in a feedback loop.
“The increase in brown carbon aerosols will lead to global or regional warming, which increases the probability and frequency of wildfires,” said senior author Pingqing Fu of Tianjin University. “Increased wildfire events will emit more brown carbon aerosols, further heating the earth, thus making wildfires more frequent.”
Global warming, too, is now seen as making insects less frequent. Insects, according to thoughtful research in 2017, “create the biological foundation for all terrestrial ecosystems. They cycle nutrients, pollinate plants, disperse seeds, maintain soil structure and fertility, control populations of other organisms, and provide a major food source” for other creatures. Life can’t function without them.
British scientists report in the journal Nature that they looked at 20 years of data about insect diversity and abundance, assembled in more than 750,000 records of almost 18,000 insect species—beetles, butterflies and moths, wasps, bees, ants, flies, grasshoppers, dragonflies, damselflies, and so on — between 1992 and 2012. They read 264 published studies from more than 6,000 sampled locations, in almost all of the world’s habitats. And then they made comparisons.
In those places where agriculture was most intensive, and the climate had also warmed substantially, the sheer number of insects had fallen 49% and the number of different species was 29% lower than in those places still with natural plant cover and with no significant recorded warming. In those regions where farming was less intensive, but increasingly hot, surviving natural habitat buffered the losses. Where three-fourths of the land was more or less wilderness, insect abundance was down only 7%; in places where natural cover was only 25%, the fall was 63%.
“Our findings highlight the urgency of actions to preserve natural habitats, slow the expansion of high-intensity agriculture, and cut emissions to mitigate climate change,” said study lead Charlie Outhwaite, of University College London. “Losing insect populations could be harmful not only to the natural environment, where insects often play key roles in ecosystems, but it could also harm human health and food security, particularly with losses of pollinators.”
But forests do more than simply nourish animal life and store carbon: research by U.S. scientists in the journal Frontiers in Forests and Climate Change makes that now very clear. The foliage sends aromatic chemicals (scientists call them biogenic volatile organic compounds) up into the atmosphere, where they play a role in cloud formation. They increase the concentration of water droplets in the clouds, which makes them brighter so they reflect more energy back into space. If the forest is reduced, so are the cooling effects of the clouds above. And that’s not the only way in which forests help moderate local temperatures.
Rainforests help provide their own rainfall. “Once you cut the trees you remove the pump that transfers water from the surface to the atmosphere, which affects downwind rainfall,” said co-author Louis Verchot of the International Centre for Tropical Agriculture in Colombia.
Forests even control the wind., with high tree canopies pushing heat away from the surface and into the atmosphere. “Imagine a smooth surface, the wind just flows straight across and the heat from the sun comes straight down. But with the canopy and its surface like a crown of broccoli, those air parcels bounce around and the heat is dispersed,” said first author Deborah Lawrence of the University of Virginia.
“If we are looking for aggregate climate benefits, both local and global, then we should be working very hard to grow and maintain forests in the tropics and looking to sustainably manage the forests outside the tropics.”