Two major papers this week suggest that global warming is likely to accelerate at a slower pace than had previously been thought. In the first, Susan Solomon of the US National Oceanic and Atmospheric Administration has identified a possible cause of lower temperature increases in the noughties compared to the nineties. In the second, David Frank of the University of Bern in Switzerland suggests that the effect of a natural process that worsens man-made climate change has been overstated.
Together with a different set of Bern researchers, Solomon and colleagues in Boulder, Colorado, have shed some light on the poorly understood impact of water in the stratosphere on the world’s temperature. Water vapour in the stratosphere – the layer of the atmosphere above that which we live in – is known to cool that part of the atmosphere down but warm the troposphere underneath. However, it’s taken having satellites floating around the planet to measure accurately how much water is actually there.
The period in which measurements have been possible includes the rapid warming of the 1990s, and slower warming since. Solomon and her colleagues’ measurements show that water concentrations were higher during the warm period, and lower during the cool period. Creating computer models from these data suggest that the drop in stratospheric water vapour after 2000 decreased the rate of warming compared to what would have been otherwise expected by about 25%. The 1990s increase in water vapour could “have steepened the rate of global warming from 1990–2000 by about 30%,” the January 29 Science paper says. The researchers point out that it’s not yet known whether this is a feedback through which the earth tries to cool itself, or something that changes periodically.
The previous day, Frank’s team’s Nature paper tackled the question of just how much CO2 is released from biological sources when the planet warms up, adding to what man is making by burning fossil fuels. “Approximately 40% of the uncertainty related to projected warming of the twenty-first century stems from the unknown behaviour of the carbon cycle,” they write. They evaluated all available large-scale temperature reconstructions and estimates of over the past thousand years. Their results suggest that the likely range of how much CO2 is put into the atmosphere for each degree of warming is likely to be in the lower half of current estimates.
Although neither Frank’s or Solomon’s research team suggests that global warming is about to reverse, taken together they might mean that humanity has longer to fight it than previously thought.
Another process currently helping to cool the globe is the formation of clouds under the ozone hole in the Southern hemisphere, researchers revealed in Geophysical Research Letters on Wednesday. Higher winds that are linked to ozone loss whip up more sea spray, which creates more clouds. These clouds reflect heat from the sun back out into space, helping counteract greenhouse-gas based global warming. The link between the winds that create clouds and ozone loss could lead to the strange situation where a fully-repaired hole could lead to faster worldwide temperature rises.