Abrupt Change

Much of the analysis of future global climate has assumed that changes will be incremental and relatively slow. Thus, while major effects could emerge, they will only arise over decades or even centuries. However, an emerging literature has been developing that suggests an alternative: the climate system may reach a threshold and tip quickly into a new state -- with massive changes arising in a few years rather than over a period of decades to centuries. Such abrupt shifts have been observed extensively in the geologic record -- indeed, they seem to be the norm rather than the exception.

While climate models often agree on mid-range estimates of a climatological response, it is more difficult to find agreement with regard to extreme scenarios. However, a new study by Tebaldi and colleagues illustrates the emerging consensus among nine of the world's leading climate models on future extreme climate events. Simulating conditions between 1980-1999 and 2080-2099 and adopting scenarios for various warming regimes, the models examined ten indicators of climate extremes and all agreed on several key conclusions. While the average growing season could lengthen in Europe, Asia, and North America and frost days could decrease in number, much of the news is negative. Dry events, which can lead to or exacerbate droughts, may be more characteristic of southern Europe, eastern Brazil, and the western U.S. Meanwhile, regions that lie in higher northern latitudes will experience heavy precipitation, and increased risk of heat waves along with considerably warmer nights, will affect all regions.

Implications: Extreme events are difficult to plan for and, the authors note, contribute to the majority of climatic damages. The events described in this study would have devastating economic, ecological, and social impacts and should be taken seriously in management plans, adaptation initiatives, and mitigation targets. The authors do suggest that the risk of such extreme changes is reduced under lower emissions scenarios.

Warming is accentuated at the poles in part because of changes in albedo, or surface reflectivity. This is due in part to melting ice exposing open water, which more readily absorbs incoming solar radiation (open water, with a dark surface, absorbs more energy than does the reflective ice). This positive feedback partly explains the dramatic changes in the Arctic over the last several decades. A new study by Marika Holland et al. attempts to determine whether such positive feedbacks, exacerbated by Atlantic Ocean warming, can lead to rapidly accelerating changes in Arctic sea ice cover. Employing the Community Climate System Model, Holland et al. studied trends of recent ice retreat and constructed future ice scenarios for the coming century. The results indicate that ice retreat will not be constant but, rather, that accelerating changes in ice cover will characterize future decades. Notably, the models suggest that by 2040, there may no longer be any summer sea ice coverage in the Arctic, an estimate that is decades earlier than previous estimates.

Implications: The authors suggest that the risk of complete Arctic summer sea ice loss after one year of accelerated ice retreat is higher under augmented greenhouse gas emissions, and it will be increasingly difficult to regain ice coverage during winters. Feedback loops such as these may be found in other parts of the climate system; virtually all point to greater change than might be expected under the more common incremental modest global warming scenarios usually presented in the literature.

Leifer et al. show that methane, a greenhouse gas that, on a mass basis, has 23 times the global warming potential of CO2, is being released from geologic sources on the sea floor. Studying the ocean bottom off the coast of Santa Barbara, California, Leifer et al. measured the seepage of methane with the use of video cameras, test dye, and flight photography of the bubbling methane once it had reached the surface. The results provide unprecedented documentation of a methane eruption from a marine seepage today.

Implications: Some climate scientists have hypothesized that if methane were to be released from natural geologic sources on the sea floor, it could rise and leak into the atmosphere, augmenting greenhouse gas levels. Blowout events, such as one documented in this study, can lead to rapid transfer of methane. This research enhances our understanding of potential methane budget increases and the possibility of abrupt climate change. If, as some scientists have asserted, warming ocean temperatures lead to a collapse of methane hydrates (crystalline structures that lock up methane), the gas could be rapidly transferred to the atmosphere from depths of hundreds of meters. This could be a significant problem, since some scientists (e.g. see Archer, David. "Fate of Fossil-fuel CO2 in Geologic Time." Journal of Geophysical Research 110(C09S05), 21 September 2005.) believe that hydrate deposits are as abundant as all fossil fuel resources combined (equating to several thousand gigatons of carbon)

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In addition to the world's oceans, soils and vegetation, permafrost -- or permanent frozen land -- is a substantial reservoir of carbon. Zimov, Schuur and Chapin quantify the carbon content of permafrost and find that 950-970 gigatons of carbon are stored in various permafrost types – a total that more than doubles estimates from previous (incomplete) studies. (For comparison, there is approximately 650 gigatons of carbon stored in vegetation globally.) Their study also shows that this stored carbon is quickly released when permafrost is thawed.

Implications: Thawing of permafrost occurs when there are prolonged periods of temperatures above freezing. We are already witnessing such a phenomenon today -- and expect more as global warming continues. In addition to the landscape degradation that occurs when permafrost thaws, the massive volumes of carbon stored in the permafrost could have devastating consequences for the future climate should it be released into the atmosphere.


    Sea level pressure is the atmospheric pressure caused by the weight of the total column of air on the sea. Zhang and Song have found that sea level pressure has been reduced between 2–13 percent over the past five decades. Using data collected by ships and marine stations, as well as two general circulation models, they attribute the change in sea level pressure to recent warming. They conclude that the overturning circulation of the atmosphere, which results from varying pressure gradients and is key in weather and climate regulation, is weakening. Zhang and Song expect such weakening to lead to a decline in the strength of the trade winds – and over time, sustained El Niño conditions.

    Implications: While the trade winds are no longer pivotal for intercontinental commerce, they are critical for migrating species as well as a significant influence on global weather patterns. Changes to these -- and perhaps, more seriously, expected sustained El Niño systems -- can have considerable negative social and ecological consequences.