Climate models underestimate global warming by exaggerating cloud brightening

Many clouds surround the Southern Ocean. Lawrence Livermore scientists have found that climate models don't accurately portray clouds and in turn underestimate global warming.
Credit: Photo courtesy of NASA
Researchers at Lawrence Livermore National Laboratory and Yale University have found that climate models are aggressively making clouds "brighter" as the planet warms. This may be causing models to underestimate how much global warming will occur due to increasing carbon dioxide. The research appears in the April 8 edition of Science.
As the atmosphere warms, clouds become increasingly composed of liquid rather than ice, making them brighter. Because liquid clouds reflect more sunlight back to space than ice clouds, this "cloud phase feedback" acts as a brake on global warming in climate models.
But most models' clouds contain too much ice that is susceptible to becoming liquid with warming, which makes their stabilizing cloud phase feedback unrealistically strong. Using a state-of-the-art climate model, the researchers modified parameters to bring the relative amounts of liquid and ice in clouds into agreement with clouds observed in nature. Correcting the bias led to a weaker cloud phase feedback and greater warming in response to carbon dioxide.
"We found that the climate sensitivity increased from 4 degrees C in the default model to 5-5.3 degrees C in versions that were modified to bring liquid and ice amounts into closer agreement with observations," said Yale researcher Ivy Tan, lead author of the paper.
Climate sensitivity refers to the change in global mean surface temperature due to a doubling of carbon dioxide. Climate models predict between 2.1 and 4.7 degrees C (3.75 to 8.5 degrees F) of warming in response to a doubling of carbon dioxide.
"We saw a systematic weakening of the cloud phase feedback and increase in climate sensitivity as we transitioned from model versions that readily convert liquid to ice below freezing to model versions that can maintain liquid down to colder temperatures, as observed in nature," Tan explained.
In nature, clouds containing both ice crystals and liquid droplets are common at temperatures well below freezing. As the atmosphere warms due to carbon dioxide emissions, the relative amount of liquid in these so-called mixed phase clouds will increase. Since liquid clouds tend to reflect more sunlight back to space than ice clouds, this phase feedback acts to reduce global warming. The icier the clouds to begin with, the more liquid is gained as the planet warms; this stabilizing feedback is stronger in models containing less liquid relative to ice at sub-freezing temperatures.
"Most climate models are a little too eager to glaciate below freezing, so they are likely exaggerating the increase in cloud reflectivity as the atmosphere warms," said LLNL coauthor Mark Zelinka. "This means they may be systematically underestimating how much warming will occur in response to carbon dioxide."
These results add to a growing body of evidence that the stabilizing cloud feedback at mid- to high latitudes in climate models is overstated. Moreover, several recent studies have concluded that other important cloud feedbacks also are likely to exacerbate warming rather than dampen it. These include amplifying feedbacks from increases in cloud top altitude and from decreases in the coverage of subtropical low clouds.
"The evidence is piling up against an overall stabilizing cloud feedback," concluded Zelinka. "Clouds do not seem to want to do us any favors when it comes to limiting global warming."

Comments

Post a Comment

Popular posts from this blog

Climate Change Skeptics Lash Out At New Global Warming ‘Hiatus’ Study That Questions Ocean Temperature Measurements

Image
Share on facebook 227 Share on twitter 22 Share on linkedin Share on google_plusone_share More Sharing Services 128 An estimated 35,000 walruses are pictured crowding on a beach near Point Lay, Alaska, because of a lack of sea ice in this September 2014 photo. A study published June 4, 2015, refuting the notion that the Earth’s temperature rise has been interrupted since 1998 was lambasted by the usual skeptics and their supporters.  Corey Accardo/NOAA/NMFS/AFSC/NMML/Via Reuters A new climate study was released Thursday that refutes the claim that there’s been a slowdown, or “hiatus” in the rate the Earth is warming, but some conservative-leaning media sites skipped covering the study itself and chose instead to promote the views of the minority of climate change deniers, such as  Christopher Monckton . For example, the Drudge Report, an immensely popular headline aggregator operated by conservative Matthew Drudge, promi...
Read more

Global warming could drive world temperatures up 7 degrees by 2100

Another piece of the  global warming  puzzle has been revealed. According to new research published on Jan. 1 in the journal  Nature , increased levels of carbon dioxide in the atmosphere will reduce the number of clouds that form. This, in turn, will cause temperatures to rise much higher than previous climate-change models have predicted. According to this new research, global temperatures could rise 4 degrees Celsius or 7.2 degrees Fahrenheit by the year 2100, and twice that by 2200. Lead researcher Steven Sherwood at the Climate Change Research Center at Australia's University of New South Wales, told  The Guardian  that this level of climate change would be "catastrophic rather than simply dangerous. 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."   In a  news release , Sherwood said this new...
Read more

Why Is Critical Technology to Stop Global Warming Stalled?

Image
Pumping CO2 underground can help fight climate change but remains untested for the most part March 12, 2015  | By  Howard J. Herzog  and  The Conversation US Injection of CO2 and other gases into geological formations has been practiced for many years.  Credit:  SaskPower There are many uncertainties with respect to global climate change, but there is one thing about which I have no doubts: we will not solve climate change by running out of fossil fuels. Understanding this leads to three posible pathways we can follow to lower greenhouse gas concentrations, and explains why I’ve chosen to focus  my research  on carbon capture and storage. We can: continue to burn fossil fuels with little or no restrictions. We will blow by atmospheric CO2 concentrations of 450 parts per million (ppm), the level many scientists think is dangerous to exceed, and we will keep on going. We have enough fossil fuels to easily go above 1,000 ppm. The impac...
Read more