Climate change could drive to extinction as many as one in six animal and plant species, according to a new analysis. In a study published recently in the journal Science, Mark Urban, an ecologist at the University of Connecticut, also found that as the planet warms in the future, species will disappear at an accelerating rate. “We have the choice,” he said in an interview. “The world can decide where on that curve they want the future Earth to be.”
As dire as Mark’s conclusions are, other experts said the real toll may turn out to be even worse. The number of extinctions “may well be two to three times higher,” said John J Wiens, an evolutionary biologist at the University of Arizona. Global warming has raised the planet’s average surface temperature about 1.5 degrees Fahrenheit since the Industrial Revolution. Species are responding by shifting their ranges.
Under the scanner
In 2003, Camille Parmesan of the University of Texas and Gary Yohe of Wesleyan University analysed studies of more than 1,700 plant and animal species. They found that, on average, their ranges shifted 3.8 miles per decade toward the planet’s poles. If emissions of carbon dioxide and other greenhouse gases continue to grow, climate researchers project the world could warm by as much as 8 degrees Fahrenheit.
As the climate continues to change, scientists fear, some species won’t be able to find suitable habitats. For example, the American pika, a hamster-like mammal that lives on mountains in the West, has been retreating to higher elevations in recent decades. Since the 1990s, some pika populations along the species’s southernmost ranges have vanished.
Hundreds of studies published over the past two decades have yielded a wide range of predictions regarding the number of extinctions that will be caused by global warming. Some have predicted few extinctions while others have predicted that 50 per cent of species face oblivion. There are many reasons for the wide variation.
Some scientists looked only at plants in the Amazon while others focused on
butterflies in Canada. In some cases, researchers assumed just a couple of degrees of warming while in others they looked at much hotter scenarios. Because scientists rarely were able to say just how quickly a given species might shift ranges, they sometimes produced a range of estimates.
To get a clearer picture, Mark decided to revisit every climate extinction model ever published. He threw out all the studies that examined just a single species, such as the American pika, on the grounds that these might artificially inflate the result of his meta-analysis. Mark ended up with 131 studies examining plants, amphibians, fish, mammals, reptiles and invertebrates spread out across the planet. He reanalysed all the data in those reports. Overall, he found that 7.9 per cent of species were predicted to become extinct from climate change.
Estimates based on low levels of warming yielded much fewer extinctions than hotter scenarios. By his calculation, with an increase of 3.6 degrees Fahrenheit in surface temperature, 5.2 per cent of species would become extinct. At 7.7 degrees, 16 per cent would. Mark found that the rate of extinctions would not increase steadily, but would accelerate if temperatures rose.
Richard Pearson, a biogeographer at University College London, called the new meta-analysis “an important line in the sand that tells us we know enough to see climate change as a major threat to biodiversity and ecosystems.” But he said that Mark was likely underestimating the scale of extinctions. The latest generation of climate extinction models are more accurate, Richard said: sadly, they also
produce more dire estimates.
Who’s at risk?
John also noted that the tropics have been underrepresented in climate extinction studies. In Mark’s meta-analysis, 78 studies focused on species in North America and Europe while only seven came from South America. Yet when Mark combined all the data from South American studies, he found that 23 per cent of species were at risk of
extinction. In North America, by contrast, only 5 per cent faced extinction. What makes this imbalance all the more glaring, John said, is the fact that most of the planet’s species live in tropical forests such as the Amazon. If climate extinction research took tropical diversity into account, the planet’s overall risk would be much higher.
Mark acknowledged that his meta-analysis is far from the final word. “This is a summary of the best information we have right now,” he said. As predictions improve, Mark said, they will allow conservation biologists to pinpoint the species at greatest risk of extinction and help plan strategies to save them.
