鈥淭HE bright sun was extinguish鈥檇鈥� begins Byron鈥檚 poem Darkness. It was written in 1816, the 鈥測ear without a summer鈥� that followed the eruption of Mount Tambora in Indonesia.
Dust, gas and aerosols hurled into the upper atmosphere by volcanic eruptions can shroud the Earth and cool temperatures worldwide for a year or more. But it now appears that major equatorial eruptions have another impact on our climate. They may also make winters warmer in Europe and over most of the continental northern hemisphere.
Mick Kelly and colleagues at the University of East Anglia鈥檚 Climatic Research Unit have sifted through the past 120 years of world temperature records to measure the global impact of major volcanic eruptions in the tropics, from Krakatoa in Indonesia in 1883 to Pinatubo in the Philippines in 1991. Because of the nature of atmospheric circulation above the Earth, the effects of eruptions in these regions are transmitted globally.
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In the latest issue of the International Journal of Climatology (vol 16, p 537), they say that in every case, except one, the world cooled by around 0.2 掳C for up to two years after the eruption. The exception was the eruption of El Chichon in Mexico in 1982, which coincided with a countertrend of warming caused by one of the strongest El Ni帽o events in the Pacific Ocean this century.
But the cooling was not universal. Over the continental areas of the higher latitudes of the northern hemisphere, eruptions were followed by an unusually warm winter. When Europeans bemoaned 1816 as the year without a summer, they had probably failed to notice that it was also the year without a winter. As Kelly notes: 鈥淭he net effect is to suppress the seasonal cycle with colder summers and warmer winters.鈥�
Several researchers 鈥� including one of Kelly鈥檚 collaborators, Jia Pengquin of the Chinese Academy of Meteorological Sciences in Beijing 鈥� had predicted such a warming using computer simulations of global climate. They suggested that the volcanic pall would trigger changes in the heat gradient through the atmosphere that might strengthen the winter jet stream in polar regions. This in turn, 鈥渟trengthens surface westerly winds in the middle latitudes, particularly in the North Atlantic, resulting in a stronger maritime influence over land鈥�, says Kelly. In winter, winds blowing from the sea tend to keep the land warm. 鈥淥ur climate data for the first time provides strong evidence that the models were right,鈥� says Kelly.
But, he warns, even major eruptions such as Pinatubo cannot head off global warming. Their impact, at best, will cancel out less than a decade of global warming, and the effect fades within two years, leaving the world as warm as before.
However, analysis of volcanic eruptions could be vital in predicting the progress of global warming, says Kelly. 鈥淚n order to know how sensitive the climate system will be to the man-made greenhouse effect, we need to know more about how it reacts to natural disturbances.鈥�