Climate: Cloud Mixing Means Extra Global Warming

Doubling of atmospheric carbon dioxide points to the higher end of warming estimates.

The mixing of clouds—like these cumulus clouds—is setting the globe on a path to warmer temperatures, scientists say.

A decline in ocean cloud cover projected in climate models points to more than 5.6°F (3°C) of global warming coming in this century, on the high end of past global warming estimates, warn climate scientists in a new study. (See also: "Global Warming Effects Map.")

"This degree of warming would make large swaths of the tropics uninhabitable by humans and cause most forests at low and middle latitudes to change to something else," says Steven Sherwood of Australia's University of New South Wales, who led the study.

The changes, Sherwood says, would take Earth "back to the climate of the dinosaurs or worse, and in a geologically minuscule period of time—less than the lifetime of a single tree."

Atmospheric scientists have long asked how high atmospheric temperatures will rise if greenhouse gases double. This "climate sensitivity" estimate has emerged as a key climate question, with estimates ranging from a low of about 2.7°F (1.5°C) to highs of more than 8°F (4.5°C) of warming in this century.

While estimates have gone up and down, carbon dioxide concentrations have increased to a level about 40 percent higher than the preindustrial average in the last century, largely due to the burning of fossil fuels.

In a new analysis released by the journal Nature, an international research team led by Sherwood reports that better cloud-cover physics narrow the spread of disagreement among the climate sensitivity estimates.

Cloud Cover Cook-Off

In the study, the researchers looked at ocean clouds, which at low altitudes reflect sunlight and lead to cooler global temperatures. The Nature study suggests that global warming will mix growing amounts of higher, drier air with ocean clouds over the course of the century, thinning out the clouds and reducing their cooling effect.

"It is an elegant and important paper," says Penn State climate scientist Michael Mann.

The finding matters, he adds, because a 2013 Intergovernmental Panel on Climate Change (IPCC) report had widened its range of climate sensitivity estimates to embrace the low estimates for how high temperatures will rise by the year 2100. (See also: "Global Warming Report: 5 Big Takeaways.")

"I argued that the IPCC had erred," Mann says, based on historical climate patterns. Sherwood and his colleagues, he says, "provide a rigorous physical explanation of just why."

Climate Sensitivity Settled?

"So can we declare the long-running debate about climate sensitivity to be over?" say climate scientists Hideo Shiogama and Tomoo Ogura of Japan's National Institute for Environmental Studies, in a commentary accompanying the study.

"Unfortunately not," they conclude. "Sherwood and colleagues' study represents a big advance, but questions persist."

For one thing, better estimates of ocean cloud cover explain only about half of the variation in climate sensitivity estimates. Uncertainty over the cooling effects of ice cover and clouds over the continents remain.

But Mann argues that the paper adds to growing concerns about the "uncertainty" in climate change science being more bad than good for humanity: "We should be taking into account worst-case scenarios when we attempt to gauge the risks posed by climate change."

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