Earth's climate cannot be replicated in a lab. So to understand how this critical component of the planet's heat regulation works, scientists must rely on "natural experiments." Such natural experiments take apocalyptic form, such as the eruption of Mount Pinatubo in the Philippines in June 1991 that sent 10 cubic kilometers of ash, gas and other materials sky high. By tracking how this eruption affected the global climate--and determining how to trace its footprint in other records--scientists have turned the catastrophe into a tool for comprehension. "The big problem with climate--and trying to study it--is you can't play with it in the lab," says atmospheric scientist Joanna Futyan of Columbia University. "We were trying to use this abrupt event as a natural experiment: something dramatic happened and you can look at how the atmosphere responds to it."

Futyan and physicist John Harries of Imperial College London analyzed how the atmosphere's humidity and temperature responded to the eruption as well as the overall radiative balance of the planet--in other words, the difference between the energy in sunlight absorbed by Earth versus the amount radiated back to space. The spectrum of this energy sent back into space from the surface (measured via satellite) has changed in the past 30 years as part of global warming, but the rate and magnitude of this change remain difficult to measure and rely on a variety of atmospheric processes, such as the amount of water vapor.

The atmospheric response to the Pinatubo eruption reveals that this system reacts rapidly, with sunlight-blocking sulfate aerosols ejected by the volcano cooling the planet within four months. By six months, the planet radiated 2.6 watts per square meter less heat to space than before the eruption. Humidity dropped as a result, but slowly, and by the end of 1992 the climate had once again reached equilibrium, the researchers write in the January 2 Geophysical Research Letters. "From the observations of Pinatubo, the net flux [of energy] brings itself back into balance quickly," Futyan says.

Pinatubo also left its mark on the weather. When the volcano erupted, it sent sulfur dioxide shooting into the atmosphere, where a wavelength of ultraviolet light transformed some of the sulfur molecules into a lighter isotope--a unique chemical sign of such stratospheric eruptions. Falling back to the surface, the sulfate bearing this specific isotopic ratio collected in undisturbed areas, such as the snow pack on Antarctica. Isotope chemist Mark Thiemens of the University of California, San Diego, and his team dug through 30 tons of snow in search of such an isotopic record, which has already been observed in the geologic strata of ancient Earth.

Both Pinatubo and its predecessor--the eruption of Mount Agung in 1963--left such traces in the snow, while lesser eruptions that did not blow as sky high left different marks, Thiemens and his team reveal in the January 5 Science. By understanding this chemistry, it may be possible to extend the volcanic record--and its influence on climate--back in time.

The effects of catastrophic eruptions like Pinatubo may be transitory, but their record both in the climate and its residue present a picture of how the climate may respond to other so-called forcings, such as human emissions of greenhouse gases. It also helps assess how this complex system might react to human attempts to tinker with it in order to avoid the potentially catastrophic effects of such climate change--such as injecting sulfate aerosols into the sky as proposed by atmospheric chemist and Nobel laureate Paul Crutzen. "It is a quantitative way to see how sensitive the stratosphere is to perturbations," Thiemens notes. "It gives you a feel for the chemistry because nature has run some of the experiments for you."

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