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Cracking abrupt climate change

Large, abrupt climate changes punctuate the paleoclimate record. Because such changes are not well reproduced by the present generation of climate models, similarly abrupt changes could surprise us in the future, says a new report from the National Academies’ National Research Council (NRC).
 


This SIR-C/X-SAR image shows part of the vast Namib Sand Sea on the west coast of southern Africa. The magenta areas are fields of sand dunes, and the orange area along the bottom is the surface of the South Atlantic Ocean. Steep dunes organize the sands, causing bright radar reflections off the dune faces. Bright green signifies rocky hills poking through the sand sea. Because the region receives only a few centimeters of rain per year, radar images of this hyper-arid region often image sub-surface features. The NRC report on abrupt climate change stresses the importance of studying unique hydrologicl areas to better understand and predict future changes in the water cycle, including droughts and floods. NASA/JPL
 

Whereas most climate change research has focused on slow, gradual changes, the NRC report says future research should aim to understand and plan for sudden changes. “Our research into global change should involve a real target on abruptness,” says Richard Alley, chair of the NRC’s Committee on Abrupt Climate Change, which authored the report.

The NRC report, Abrupt Climate Change: Inevitable Surprises is a collaborative study among the Board on Atmospheric Sciences and Climate, the Polar Research Board and the Ocean Studies Board. It aims to fill a gap in the information provided by the climate community to the scientific and policy communities about abrupt climate change.

The report defines an “abrupt” change as one where the climate response is faster than the changes causing it. On a human time scale abrupt is “something that is fast enough that ecosystems or economies have trouble dealing with it,” Alley says. The changes could affect temperature, precipitation, evaporation and extreme weather events.

The warming at the end of the Younger Dryas cold interval 11,600 years ago, for example, triggered an abrupt cooling period followed by an abrupt warming. These changes occurred over 10 years but accounted for one-third to one-half of the climate difference from the ice age to present-day conditions. “So you’ve got 100,000 years to end an ice age but you get almost half of it in 10 years. That’s an abrupt change,” Alley says.

Alley adds that climate models have failed to represent such abruptness, increasing the chances for surprise in the future. “There’s a general impression that the big, inclusive global models don’t behave quite as abruptly as the real world does,” Alley says.

“There are no surprises in this report for anyone who has followed debates within the climate community over the past decade,” says Matt Huber, a paleoclimate researcher at the Danish Center for Earth System Science. “The report is a good snapshot of the current scientific consensus.”

While the nature of the report’s data and methods are excellent, Huber says, it has a narrow focus that may limit the scope of its voice. Drawing on instrumental and social science data as well as high-resolution paleoclimate records, such as ice core data, the report largely focuses on more recent climate events. These include thorough treatments of the events before the Younger Dryas cold interval, the Younger Dryas itself, the Eemian, the Holocene, and encompass climate changes, trends and modes of variability in the instrumental period. “Thus, the entire history of climate has been telescoped down so that the most recent periods receive the vast majority of the attention, and the view is mostly through the lens of ice cores.”

Documentation of past abrupt climate changes is much better in colder regions such as Greenland and Antarctica than in warmer regions such as the Pacific and tropics. The report acknowledges these gaps, and therefore does not include much discussion of abrupt changes in past warm climate periods. The committee does recommend improving the geographic and temporal breadth of paleoclimate data, and including these proxies in models.

Kevin Trenberth, head of the National Center for Atmospheric Research’s Climate Analysis Section, says the NRC analysis represents the step forward climate models need to take to incorporate feedback processes that drive abrupt climate changes. At the end of the Younger Dryas, for example, melting of the large ice sheet over North America and the North Atlantic caused freshwater to flow into the ocean and change the seawater’s density. Changing this thermohaline circulation “can completely change ocean circulation behavior, presumably on time scales of a few years, and so this is one of the things you have to have in a model,” Trenberth says.

Models that omit these processes cannot predict future changes that could result from a similar flushing of the North Atlantic, Trenberth adds. Global warming could melt glaciers and increase overall evaporation resulting in this flushing process. Such a change in the ocean’s circulation could deliver a cold spell to places such as Western Europe and increase heat elsewhere on Earth. Only by understanding these climate feedbacks throughout the geologic record, Trenberth says, can models accurately simulate global changes.

“In general, most of the recommendations of the panel are good and should lead to a focus on fruitful avenues: monitoring of the thermohaline circulation and the hydrological cycle, improved computer models, improved paleoclimate proxy data sets,” Huber says.

Lisa M. Pinsker
 

Read about abrupt warming at the Late Paleocene Thermal Maximum


Related Links:

Read the prepublication copy of Abrupt Climate Change: Inevitable Surprises

Visit the National Center for Atmsopheric Research Web site


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