Study Finds Global Warming as Threat to 1 in 6 Species
By Carl Zimmer

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 Thursday 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 Dr. Urban’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.

In 2003, Camille Parmesan of the University of Texas and Gary Yohe of Wesleyan University analyzed 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 hamsterlike 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’ 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 percent 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, Dr. Urban 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. (Scientists often pick out individual species to study because they already suspect they are vulnerable to climate change.)

Dr. Urban ended up with 131 studies examining plants, amphibians, fish, mammals, reptiles and invertebrates spread out across the planet. He reanalyzed all the data in those reports.

Overall, he found that 7.9 percent 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 percent of species would become extinct. At 7.7 degrees, 16 percent would.

Dr. Urban 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 Dr. Urban was likely underestimating the scale of extinctions. The latest generation of climate extinction models are more accurate, Dr. Pearson said: sadly, they also produce more dire estimates.

Dr. Wiens also noted that the tropics have been underrepresented in climate extinction studies. In Dr. Urban’s meta-analysis, 78 studies focused on species in North America and Europe, while only seven came from South America. Yet when Dr. Urban combined all the data from South American studies, he found that 23 percent of species were at risk of extinction. In North America, by contrast, only 5 percent faced extinction.

What makes this imbalance all the more glaring, Dr. Wiens 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.

Dr. Urban 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, Dr. Urban said, they will allow conservation biologists to pinpoint the species at greatest risk of extinction and help plan strategies to save them.

The scientists building these new models may be able to draw from data not just on living species, but extinct ones as well.

In this week’s issue of Science, an international team of scientists reported a new finding on ocean extinctions over the past 23 million years.

They found that some groups, such as marine mammals, have been far more prone to extinctions than others, such as mollusks. Biology can put certain species at extra risk: they may produce few offspring, for instance, or have a limited range.

Dr. Pearson said that climate extinction models will need to take other factors into account, as well. “What happens to other species in an ecosystem when a species goes extinct?” he asked. Its partners in that habitat might risk extinction as well.

Dr. Urban saw many other ways in which climate extinction models should improve. For one thing, they should take into account the cities, farms and other barriers that humans have put in the way of species seeking new habitats.

Given the stark results of research so far, Dr. Urban said, these new prediction models can’t come too soon. “We need to elevate our game,” he said.

全球暖化 最嚴重六分之一生物將滅絕

一項新的分析研究顯示，在最嚴重情況下，全球暖化將導致六分之一生物滅絕。

康乃狄克大學生態學家厄班整合和分析131份有關物種的研究報告，使用多種電腦模擬方法，得出全球物種平均的滅絕率將為7.9%；這一數據是所有物種、所有地區的平均值，而且建立在對未來人類溫室氣體排出量的預測假設之上。

這並不意味著所有生物都將滅絕，而是一些生物的數量將會產生不可逆轉的減少，直到逐漸消失。

在所有地區中，歐洲和北美的物種滅絕速率最慢，分別為5%和6%，而南美洲的情況最為惡劣，物種的滅絕率將達23%。

物種的滅絕率會隨著時間以及溫室氣體量而變化，目前的滅絕率處於只有2.8%。

如果氣溫上升攝氏二度，5.2%物種可能消失；如果上升攝氏三度，滅絕比率可能增至8.5%；最壞情況下，本世紀結束時上升攝氏4.3度，多達六分之一物種可能滅絕。

全球變暖將會導致很多生物將棲息地轉移到離南北極更近的地方。但是有些生物，尤其是如美洲鼠兔（pika）這樣的山地生物將會無處可躲，最後由於無法逃離熱浪而滅絕。

然而，厄班還表示，由人類所引起的動物棲息地流逝所導致物種滅絕比全球氣溫變暖導致的滅絕更為嚴重。

物種大滅絕正在進行當中，在所有因為自然原因而滅絕的物種當中，有1000個物種的滅絕是由於人類行為導致的。厄班說：「我不知道能否把現在的時代稱為大滅絕，但是我們的確在朝那個方向邁進。」

原文參照：
http://www.nytimes.com/2015/05/05/science/new-estimates-for-extinctions-global-warming-could-cause.html

2015-05-01．聯合晚報．A4．焦點．國際新聞組