Easterling’s finding that natural variability can cause short-term cooling, or no-trend periods within longer-term warming, correlates with other predictions about future temperatures.
In May 2008, a group of climate modelers in Germany published projections that incorporated current understanding of natural cycles to make a climate forecast for the next decade. The group forecast that global surface temperatures might not increase much over the next decade, as cooling driven by natural variability offsets human-caused warming. After a decade or so of stability, however, the model indicates that temperatures would begin to rise. In contrast, the UK Met Office predicted warming beginning in a few years from 2009. Both groups, however, agreed that after a short period of negative or no trend in the early 2000s, global temperatures would begin to rise, perhaps quickly.
In other words, the no-trend period in the Hadley Centre’s data set not only doesn’t surprise these climatologists, it’s consistent with what they have predicted. “You’re going to see these episodes where you don’t get warming in the surface air temperature and then it starts back warming very strongly,” Easterling says.
“If it kept cool for the next 25 years, I’d think maybe I was wrong,” Easterling continues, “but you can see periods as long as 16 years in our model run where temperatures stay the same without changing the long-term trend.”
Each year, the area covered by Arctic sea ice melts to its smallest extent by the end of summer in September. This map shows the median extent of sea ice during September from 1979-2000 (in yellow), and the minimum extent observed in 2009. Image courtesy of NASA Earth Observatory and National Snow and Ice Data Center.
The National Snow and Ice Data Center also offers an animation of images showing ice extent each September from 1979 to 2009.
Besides global temperatures, other lines of evidence point strongly to a warming planet. Since 1998, the extent of Arctic sea ice reached new record lows in 2002, 2005, and 2007. Arctic sea ice retreated so dramatically in 2007 that it broke all previous records a month before the end of melt season. And dramatic changes haven’t been unique to the Arctic. In 2002, some 3,250 square kilometers of the Larsen B Ice Shelf on the Antarctic Peninsula shattered over a period of just five weeks. In the wake of the ice shelf disintegration, glaciers feeding the shelf accelerated on their route into the Weddell Sea. “The reason people believe strongly that Earth has warmed is not just the temperature record, but all these other things,” Easterling explains.
Maximum Arctic sea ice extent for 2009 occurred in March, and minimum Arctic sea ice extent occurred in September 2009. In each image, the magenta line indicates the median extent of ice cover for that month for the period 1979–2000. View larger version in a new window. Images courtesy of National Snow and Ice Data Center.
Both minimum and maximum sea ice extents have decreased over the period of satellite observation. This graph shows the trends in minimum extent (red) and maximum extent (black) from 1979–2009 as the percentage difference in ice extent compared to the mean from 1979-2000. View larger version in a new window. Graph courtesy of Arctic Report Card.
The colors on these maps show Arctic sea ice distribution in March 2007, 2008, and 2009. Multiyear ice is shown as white, mixed ice is aqua, first-year ice is teal, and ice with melting on the surface is red. Dark blue represents open water and gold is land. These images were prepared using a combination of satellite observations and drifting ice buoys. Courtesy of Son Nghiem and Arctic Report Card.
“When you’re in a court of law, you have to swear to tell the truth, the whole truth, and nothing but the truth. The people who have been focusing on the ‘cooling’ have not been telling the whole truth,” Schlesinger remarks. “Natural variability may mask the effects of greenhouse gas emissions in the short term, but once an oscillation changes back to a mode that increases global temperatures, it will reinforce the effects of burning fossil fuels, and we could face a double whammy. So the worst mistake we could make is to confuse short-term cooling for a reversal of global warming.”
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