Happy New Year. It’s Worse Than We Thought.
December 31, 2013
New research from the University of New South Wales discussed by study author Steven Sherwood, above.
Sherwood’s paper shoots more holes into lingering hopes that climate sensitivity, the amount of warming we expect for a given rise in CO2, might be lower than we thought – that maybe temp rises could be more moderate in the future.
One of the last, lingering, tattered bastions of climate denial has been that, somehow, there might be some kind of moderating feedback in the system, that, as climate warmed and brought more moisture into the atmosphere, more clouds might form, reflecting heat and moderating the changes. This has been the hobby horse for the Richard Lindzens and Roy Spencers of the world.
That hope is being steadily crushed as we learn more.
“Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from preindustrial times are not reproducing the correct processes that lead to cloud formation,” said lead author from the University of New South Wales’ Centre of Excellence for Climate System Science Prof Steven Sherwood.
“When the processes are correct in the climate models the level of climate sensitivity is far higher. Previously, estimates of the sensitivity of global temperature to a doubling of carbon dioxide ranged from 1.5°C to 5°C. This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3°C to 5°C with a doubling of carbon dioxide.”
The research indicates that fewer clouds form as the planet warms, meaning less sunlight is reflected back into space, driving temperatures up further still. The way clouds affect global warming has been the biggest mystery surrounding future climate change.
Professor Steven Sherwood, at the University of New South Wales, in Australia, who led the new work, said: “This study breaks new ground twice: first by identifying what is controlling the cloud changes and second by strongly discounting the lowest estimates of future global warming in favour of the higher and more damaging estimates.”
“4C would likely be catastrophic rather than simply dangerous,” Sherwood told the Guardian. “For example, it would make life difficult, if not impossible, in much of the tropics, and would guarantee the eventual melting of the Greenland ice sheet and some of the Antarctic ice sheet“, with sea levels rising by many metres as a result.
The research is a “big advance” that halves the uncertainty about how much warming is caused by rises in carbon emissions, according to scientists commenting on the study, published in the journal Nature. Hideo Shiogama and Tomoo Ogura, at Japan’s National Institute for Environmental Studies, said the explanation of how fewer clouds form as the world warms was “convincing”, and agreed this indicated future climate would be greater than expected. But they said more challenges lay ahead to narrow down further the projections of future temperatures.
Below, Andrew Dessler describes earlier research on the climate sensitivity question.
“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.”
“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.”)
Climate Sensitivity Settled?
“So can we declare the long-running debate about climate sensitivity to be over?” say climate scientists Hideo Shiogama and Tomoo Oguraof 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.”
Below, Kevin Trenberth on climate sensitivity.