The percentage of the earth’s land surface covered by extreme heat in the summer has soared in recent decades, from less than 1 percent in the years before 1980 to as much as 13 percent in recent years, according to a new scientific paper.
The change is so drastic, the paper says, that scientists can claim with near certainty that events like the Texas heat wave last year, the Russian heat wave of 2010 and the European heat wave of 2003 would not have happened without the planetary warming caused by the human release of greenhouse gases.
Those claims, which go beyond the established scientific consensus about the role of climate change in causing weather extremes, were advanced by James E. Hansen, a prominent NASA climate scientist, and two co-authors in a scientific paper published online on Monday in the Proceedings of the National Academy of Sciences.
这种说法超出了科学界对于气候变化在极端天气事件中的角色的现有共识。美国国家航空航天局(NASA)知名气候科学家詹姆斯·E·汉森(James E. Hansen)和另外两人合著的这篇科学论文，周一发表在《美国国家科学院院刊》(Proceedings of the National Academy of Sciences)的网站上。
“The main thing is just to look at the statistics and see that the change is too large to be natural,” Dr. Hansen said in an interview. The findings provoked an immediate split among his scientific colleagues, however.
Some experts said he had come up with a smart new way of understanding the magnitude of the heat extremes that people around the world are noticing. Others suggested that he had presented a weak statistical case for his boldest claims and that the rest of the paper contained little new.
The divide is characteristic of the strong reactions that Dr. Hansen has elicited in the debate over climate change. As the head of the Goddard Institute for Space Studies in Manhattan, he is one of NASA’s principal climate scientists and the primary custodian of its records of the earth’s temperature. Yet he has also become an activist who marches in protests to demand new government policies on energy and climate.
就汉森在气候变化辩论中引发的强烈反响而言，这种分歧是典型的。作为曼哈顿戈达德太空研究所(Goddard Institute for Space Studies)的负责人，他是NASA主要的气候科学家之一，也是NASA地球温度记录的主要保管人。但他也已成为一名活动人士，参与街头抗议，要求政府出台能源和气候方面的新政策。
The latter role — he has been arrested four times at demonstrations, always while on leave from his government job — has made him a hero to the political left But it has discomfited some of his fellow researchers, who fear that he may be sowing unnecessary doubts about his scientific findings and climate science in general.
Climate-change skeptics routinely accuse Dr. Hansen of manipulating the temperature record to make global warming seem more serious, although there is no proof that he has done so and the warming trend has repeatedly been confirmed by other researchers.
Scientists have long believed that the warming — roughly 2.5 degrees Fahrenheit over land in the past century, with most of that occurring since 1980 — was caused largely by the human release of greenhouse gases from burning fossil fuels. Such emissions have increased the likelihood of heat waves and some other types of weather extremes, like heavy rains and snowstorms, they say.
But researchers have struggled with the question of whether any particular heat wave or storm can be definitively linked to human-induced climate change.
In the new paper, titled “Perception of Climate Change,” Dr. Hansen and his co-authors compared the global climate of 1951 to 1980, before the bulk of global warming had occurred, with the climate of the years 1981 to 2011.
在这篇题为《气候变化的感知》("Perception of Climate Change")的新论文中，汉森博士及合著者将全球变暖主要阶段到来之前的1951年至1980年间的全球气候，与1981年到2011年间的气候做了对比。
They computed how much of the earth’s land surface in each period was subjected in June, July and August to heat that would have been considered particularly extreme in the period from 1951 to 1980. In that era, they found, only 0.2 percent of the land surface was subjected to extreme summer heat. But from 2006 to 2011, extreme heat covered from 4 to 13 percent of the world, they found.
“It confirms people’s suspicions that things are happening” to the climate, Dr. Hansen said in the interview. “It’s just going to get worse.”
The findings led his team to assert that the big heat waves and droughts of recent years were a direct consequence of climate change. The authors did not offer formal proof of the sort favored by many climate scientists, instead presenting what amounted to a circumstantial case that the background warming was the only plausible cause of those individual heat extremes.
Some experts said they found the arguments persuasive. Andrew J. Weaver, a climate scientist at the University of Victoria in British Columbia who reviewed the paper before publication, compared the warming of recent years to a measles outbreak popping up in different places. As with a measles epidemic, he said, it makes sense to suspect a common cause.
有些专家说，他们认为这些论证有说服力。加拿大不列颠哥伦比亚省维多利亚大学(University of Victoria)的气候科学家安德鲁·J·韦弗(Andrew J. Weaver)在论文发表之前对其进行了评审。他把近年来的变暖比作在不同地区爆发麻疹。他说，就像在麻疹疫情中一样，怀疑有同一来源是合理的。
But some other scientists described the Hansen paper as a muddle. Claudia Tebaldi, a scientist with an organization called Climate Central that seeks to make climate research accessible to the public, said she felt that the paper was on solid ground in asserting a greater overall likelihood of heat waves as a consequence of global warming, but that the finding was not new. The paper’s attribution of specific heat waves to climate change was not backed by persuasive evidence, she said.
但是，其他一些科学家形容汉森的论文是一本糊涂账。克劳迪娅·泰巴尔迪(Claudia Tebaldi)是寻求让公众了解气候研究的组织——气候中心(Climate Central)的科学家。她表示，她觉得在断言热浪总体发生几率更高是全球变暖的一个后果方面，这篇论文是站得住脚的，但这一研究发现并不新鲜。她说，这篇论文将一些特定的热浪归因于气候变化，这是缺乏有说服力的证据支持的。