Blowing hot and cold

 作者:师姬     |      日期:2019-02-26 11:01:07
By Fred Pearce THE Arctic has warmed by twice as much as the rest of the planet over the past 150 years—but half the warming took place more than 60 years ago and was probably entirely due to natural influences. This is the conclusion of the most comprehensive analysis yet of past temperatures in the far north. Jonathan Overpeck of the US government’s National Oceanic and Atmospheric Administration at Boulder, Colorado, and colleagues looked at the widths of tree rings, rates of ice core melting and the isotopic composition and thickness of lake and marine sediments, as well as thermometer readings. Since about 1840, the Arctic has warmed by an average of 1.5 °C, compared to a global average of around 0.6 °C, they report in this week’s Science (vol 278, p 1251). The study, which covers the past 400 years, reveals strong natural temperature cycles. The 17th century was cold and the 18th century warm. The 19th century started off very cold, which the authors say explains the repeated failures of European explorers to find the icebound Northwest Passage. Then began a sustained period of warming that has continued, with a brief interruption in the 1960s, to the present day. Overpeck says that between 1840 and 1920, the warming “is likely to have resulted from natural rather than human influences”. The build-up of greenhouse gases in the atmosphere during this period was less than a third of the build-up since 1920. But he believes that since then, human influence has been “probably the dominant cause”. The researchers attribute the rising temperatures in the 19th century partly to changes in solar heating caused by cyclical “wobbles” in the Earth’s orbit around the Sun, known as the Milankovitch effect. At the same time the Arctic was recovering from a dramatic cooling caused by a series of major volcanic eruptions in the early 1800s. These eruptions released sulphate particles into the atmosphere which scatter solar radiation and cool the planet. Such temperature changes are amplified in the Arctic through the “ice albedo effect”. Any cooling increases Arctic ice cover, which reflects solar heat back into space, so amplifying the cooling. But warming melts the ice, exposing the land and open water, which absorb more heat. Overpeck warns that the huge natural variability found in Arctic temperatures should make researchers careful about ascribing all current warming to human influences. On the other hand, says coauthor Ray Bradley, a geoscientist at the University of Massachusetts in Amherst, the more researchers match fluctuations in past centuries to natural influences, the clearer it becomes that there is “an excess warming this century”. This warming is probably down to humans,