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The New Climate Data: So What?

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After much anticipation, the Intergovernmental Panel on Climate Change on Friday revealed it’s new assessment of climate change, after two years of deliberation. The bottom line: global temperatures and sea levels will rise even faster than everyone thought. At a long press conference held after IPCC released its report, Climate Change 2013: The Physical Science Basis, U.S. climate experts explained what the new data boil down to for the U.S. and the world. Here are the facts they emphasized, and the comments they made on what the findings mean for policy and the planet.

Temperature. If the world aggressively cuts back on CO2 emissions, the global mean surface temperature by 2100 could be kept below a 2 degree Celsius rise (compared with 1986-2005), considered the limit beyond which severe consequences will arise. But if nations continue on the present course, temperature could rise by up to 4.8 degrees C. “We do have a choice,” said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research and a lead author of the IPCC report, during the press conference. “We can choose to mitigate, which will keep us on the lower curve. If we don’t do anything, we’ll end up on the higher curve.” Even a 3 degree C rise, Meehl said, “will have serious impacts on agriculture and water resources—major disruptions to systems that are important to human welfare.”

Sea level. Under the same two scenarios, sea levels would rise by a minimum of 0.26 meter, or a maximum of 0.98 meter. The rate of sea level rise from 1993 to 2010 was 3.2 millimeters per year, higher than the IPCC’s last assessment in 2007. The panel also predicts that under the “business as usual” scenario, sea level rise would soar to 8 to 16 mm/yr by 2100. These estimates are the most dramatic change from 2007.

U.S. temperature and precipitation. Starting now and continuing into the future, there will be fewer cold days and more extremely warm days all across the U.S. Winters will be warmer. Extreme rains will increase. General precipitation will rise in the northern half of the country. In prior years the IPCC thought precipitation in the Southwest would decrease but now it concludes that precipitation may remain somewhat similar. However, because rising temperatures mean more evaporation and because less snow will fall at high elevations, the net effect will be less water in the region.

Atlantic hurricanes. Scientists have been saying for some time that Atlantic hurricanes will increase in intensity but not necessarily in number—although very recent research, which the IPCC did not consider, indicates that the number may increase too. Regardless, any increase in intensity will not be driven by overall global temperatures but by rising water temperature in certain regions of the oceans. The IPCC says more work is needed to identify those hot spots, which could improve forecasting if tropical storms cross those regions.

Arctic sea ice. “The “summer minimum ice cover”—the amount off ice remaining on the water at the height of summer warming—has dropped by 9 to 14 percent each of the past three decades. By 2100 the minimum could be 43 to 94 percent of what it is now—meaning that we could see an ice-free Arctic in summer by that year.

“The pause.” Climate deniers and skeptics in recent months have been claiming that global warming is over because the rise in atmospheric temperatures has slowed from 1998 to 2012. But the so-called pause or hiatus means the rate of increase has slowed—temperatures are still rising. The IPCC confirmed what other scientists have been maintaining: that a decade-long spell of climate conditions in the tropical Pacific Ocean caused more heat than normal to be stored in the ocean, helping to lessen heat rise in the atmosphere. This kind of spell, they also maintain, is a common part of the natural variability in global atmospheric and ocean patterns. The pattern, in which so-called La Nina conditions predominate more so than El Nino conditions, happens somewhat regularly over time; the most recent spell ended in the 1970s. Soon enough, the condition will end again. But even during this time, global temperatures have continued to rise. The decade 2000-2010 was the hottest ever recorded.

Policy. So will world leaders be more likely to act on the news that climate change will continue to worsen? “In 1990 the IPCC had already released compelling evidence,” Meehl said. “Since then I would have thought that more would have happened by policymakers in addressing this problem. The hope now is that with more information—and more detailed information—policymakers will think now is the time to do something with it.” Tad Pfeffer, from the University of Colorado, also warned politicians and the public to not misinterpret predictions. “It is important to understand what really constitutes a threat,” said Pfeffer, also a lead IPCC author. “A half-meter rise in sea level will be very disruptive, but people may discount it because it’s not the dramatic worst-case scenario. The smaller predictions tend to get ignored.” And that, Pfeffer said, would be a costly mistake.

You can hear an audio recording of the press conference, including the Q&A with journalists, here.

Graph courtesy of Intergovernmental Panel on Climate Change

The views expressed are those of the author and are not necessarily those of Scientific American.

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