Extreme weather events are becoming more common as the world warms 鈥� and any one could deliver a knockout blow to an endangered species
JUSTIN WELBERGEN will never forget the day he watched hundreds of animals die. It was January 2002 and he was observing a colony of flying foxes in northern New South Wales, Australia. The temperature had just peaked at 43 掳C when the bats started behaving oddly. They usually just dozed or squabbled noisily in the treetops, but this day was different.
鈥淭hey were really distressed,鈥� says Welbergen, who was studying the social behaviour of flying foxes for his PhD at the University of Cambridge. 鈥淭hey were fanning themselves and panting frantically. Some were licking their wrists to cool down. And then suddenly they began falling from the trees 鈥� it was raining flying foxes. If they weren鈥檛 dead when they hit the ground they died soon after. Others expired in the trees. It was gruesome.鈥�
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On that single afternoon, some 2000 bats died in the colony Welbergen was studying, and thousands more perished in colonies nearby. All told, around 30,000 flying foxes have died in Australia during heatwaves since 1994, with juveniles and adult females the hardest hit (). Welbergen called his experience 鈥渁n awful epiphany. I thought the bats could cope with the heat. I was wrong.鈥�
What Welbergen witnessed could be a harbinger of an increasingly dangerous world in which rare weather events such as heatwaves, deluges, droughts and storms become much more common. These extreme events could have a severe impact on wildlife and ecosystems, possibly even driving some species to extinction.
Unheard-of extremes
If there is one near-certainty about our changing climate, it is that it is getting warmer. As this happens, climate models suggest that once-rare heatwaves will occur more often and unheard-of extremes will strike on occasion.
For example, Claudia Tebaldi, a statistician working with climate scientists at the National Center for Atmospheric Research in Boulder, Colorado, and her colleagues have used computer simulations to model the likelihood of future climatic extremes. They found that heatwaves are likely to become far more frequent ().
鈥淭he causes are pretty simple,鈥� says Tebaldi. 鈥淚magine that global temperature has a bell-shaped distribution. Most times are normal but a few periods are unusually warm or cold. Warming the climate system makes the upper tail of the temperature distribution fatter. That creates more chances for extremely hot events to happen.鈥�
Extreme temperatures could affect ecosystems across the planet, but biologists are increasingly worried about one in particular: tropical rainforests, which are full of species that are adapted to near-constant temperatures. Among the most vulnerable are the myriad birds, mammals, lizards and frogs specialised for cool, cloudy mountaintop forests.
鈥淲hen things hot up, these mountain-dwelling species literally have nowhere to go,鈥� says Stephen Williams at James Cook University in Townsville, Queensland, who studies the effects of global warming on Australian rainforest wildlife. 鈥淎ccording to our models, if average temperatures rise by more than 2 or 3 掳C, many of these species will vanish鈥� ().
In all likelihood it will be heatwaves rather than an incrementally rising thermometer that sound the death knell for many species. This is the likely explanation for the catastrophic decline of the white lemuroid ringtail possum (Hemibelideus lemuroides, see photo) in northern Queensland. Adapted for cool rainforests above 1100 metres elevation, this attractive marsupial was not seen for nearly four years following an intense heatwave in 2005. After searching every nook and cranny of its former geographic range, Williams and his team eventually found just four individuals still alive. 鈥淭he white possum is just one more heatwave away from oblivion,鈥� says Williams.
Species elsewhere in the tropics are vulnerable too. Earlier this year, a team led by Barry Sinervo of the University of California, Santa Cruz, reported that in Mexico, warming is driving lizards out of areas they used to inhabit. Around 12 per cent of local populations of one type of lizard have disappeared since 1975 ().
In the hot, steamy lowlands, meanwhile, many animal and plant species are already living near their maximum thermal limits. A spike in temperature could be particularly devastating in the vast Amazon and Congo basins, says Robert Colwell at the University of Connecticut in Storrs. With the nearest mountains thousands of kilometres away, heat-stressed species in these regions would have nowhere to flee ().
Heatwaves are not the only worry. Climate models also predict that heavy precipitation events such as rainfall deluges and major floods are likely to increase. 鈥淎 warmer atmosphere holds more moisture,鈥� says Tebaldi. 鈥淭hat provides more fuel for heavy precipitation.鈥�
Torrential rain is not in itself a great threat to wildlife. But a warmer atmosphere could also promote strong droughts, which are.
In 2005, a completely new kind of drought struck the Amazon. Droughts in this region are normally driven by El Ni帽o events in the Pacific Ocean, which starve the drier eastern and southern parts of the Amazon basin of rainfall. But the 2005 drought was different. Its cause was exceptionally warm sea-surface temperatures in the Atlantic, which, remarkably, hit the western Amazon hardest, especially in Peru and western Brazil. This is the wettest and most species-rich part of the basin, and contains few drought-tolerant species. Rainforest trees died in droves. There were no recorded extinctions, but the dying trees emitted large quantities of carbon to the atmosphere ().鈥漈he 2005 drought stunned even the climate experts,鈥� says Amazon forest specialist Thomas Lovejoy of the Heinz Center, an environmental think tank based in Washington DC.
The same unprecedented conditions that spawned this bizarre drought also created hurricane Katrina, which devastated the Gulf of Mexico. Hurricanes, cyclones and typhoons batter large swathes of the Earth each year, and many climate scientists believe these storms will intensify in a hotter world.
鈥淐yclones are basically giant heat-dissipating machines,鈥� says Jonathan Nott, a storm researcher also at James Cook University. In the Atlantic Ocean, megastorms 鈥� those rated at 4 or 5 on the five-point 鈥� have increased in frequency over the past century, seemingly in concert with rising sea-surface temperatures (Eos, vol 87, p 233). Such megastorms can flatten coastal and island forests and severely damage other habitats, such as coral reefs (). Enduring storm damage is nothing new for ecosystems, but the worry is that they will have less time to recover between severe batterings. Disturbed habitats are also more vulnerable to invasions of weeds and feral animals, and more prone to wildfires.
How will the planet鈥檚 flora and fauna respond as weather extremes become more frequent? That is difficult to say, though the latest research suggests such events already play a key role in determining which species live where. For example, the geographic ranges of many bird species reflect their capacity to handle extreme high temperatures (). The same is true for trees ().
Individuals living near the margins of their geographic ranges are particularly vulnerable, often dying en masse during unusual droughts, heatwaves or cold snaps. More frequent extremes could shrink their geographic ranges or eliminate them altogether.
Perhaps the scariest thing about weather extremes is their inherent unpredictability. Just because we are almost certain they will become more frequent doesn鈥檛 mean we can predict exactly when and where they will occur. Our inability to draw local conclusions from global-scale computer models means we are flying half-blind. If you want to predict the date of the next killer drought in the Sahel, the next destructive flood in Pakistan or the next devastating fire in Australia, you might as well go down to the local casino and roll the dice.
鈥淲e are almost certain that weather extremes will become more frequent, but we cannot predict where and when they will occur鈥�
So is there anything we can do to stop these ecological heatshocks? First and foremost, we must continue trying to reduce greenhouse-gas emissions. 鈥淐urbing emissions is the all-important ingredient in buying time and options to manage change in natural communities,鈥� says Luke Shoo, a climate modeller also at James Cook University. 鈥淚f we let emissions run away we鈥檒l be swamped by the speed and magnitude of change.鈥�
Second, we need to create nature reserves that are as big and topographically diverse as possible to encompass a wide range of microclimates. Mountainous regions are especially good at providing refuges that help to buffer extremes (). Such buffering might explain how a handful of lemuroid possums have clung to survival in a fraction of their former range. Nature reserves also need to contain reliable water sources to sustain wildlife during droughts.
Limiting disturbances in reserves, such as wildfires, habitat disruption and hunting, is also essential, as the combined effects of multiple environmental stresses appear to be a leading cause of species extinctions ().
Such efforts to beef up our nature reserves will have an important added benefit: they will slow the rapid rise of greenhouse gases. This is especially true in the tropics, where the razing of forests spews some 5 billion tonnes of carbon dioxide into the atmosphere each year 鈥� around 17 per cent of global emissions. Preserving natural habitats will not only make ecosystems more resilient, it will also directly combat climate change.
The bottom line is that a warming world is a more dangerous and less predictable one for wildlife. If we are going to keep heating our planet, we need to be prepared for the consequences. The dead flying foxes are just the start.
