Water vapor is the most abundant greenhouse gas on Earth, significantly contributing
to global climate. Since the beginning of greenhouse gas climate modeling, global
climate models have shown that when atmospheric and surface temperatures increase,
so too does the atmospheres ability to hold more water vapor. This additional
water vapor absorbs heat, instead of allowing the energy to escape into space.
Earths surface then warms once again increasing the amount of water
vapor and warming the planet.
Climate modelers estimate that this never-ending feedback doubles how sensitive surface temperature is to an increase in human-produced greenhouse gases. But some climatologists think the climate system is less sensitive to greenhouse gases than the models suggest creating an area of contention in climate science. Understanding the water vapor feedback is crucial to correctly modeling future changes in Earths climate.
In the past, climate scientists have looked at heating variations for specific regions in order to study the role of water vapor in climate change. But reporting in the April 26 Science, Brian Soden of Princeton University and colleagues take a more global approach. They observed how the 1991 eruption of Mount Pinatubo in the Philippines created a global cooling pattern. Co-author Alan Robock of Rutgers University says Pinatubo provided a natural climate experiment to test model predictions of how water vapor behaves in the atmosphere.
Sodens team observed a decrease in water vapor as the upper atmosphere cooled after Pinatubos eruption. Their model simulated the temperature change correctly for the water vapor feedback. This means, Robock says, that the sensitivity of global climate models to greenhouse warming has also been correct.
A volcanic eruption has more in common with the type of climate perturbation well get from an increase in greenhouse gases than other previous phenomena looked at. This is a good piece of work that is drawing on a current climate perturbation that we can observe, says Tony Del Genio, a physical scientist at NASA Goddard Institute for Space Studies.
Sodens team, he says, looked at the response of water vapor to global cooling in order to look at the reverse scenario warming from increases in greenhouse gases. It is perhaps as close as were going to get to an example in the current climate that we might use to learn something about how well our models can predict the future climate.
Lisa M. Pinsker
NASA Astronaut Photography