Confounding all projections of future climate is the possibility that abrupt changes or other climate "surprises" may occur. Abrupt changes in the climate system can occur when (1) there is a rapid change in forcing, such as a rapid increase in atmospheric GHG concentrations or reduction in aerosol forcing, or (2) thresholds for stability (or "tipping points") are crossed, such that small changes in the climate state are reinforced, leading to rapid shifts until the climate enters another stable state and stability is restored. Paleoclimate records indicate that the climate can go through abrupt changes in as little as a single decade (NRC, 2002a). For example, Greenland ice cores indicate that about 13,000 years ago, during the recovery from the last Ice Age, local temperatures fell more than 10°F (6°C) within a few decades and remained low for more than a millennium before jumping up more than 16°F (10°C) in about a decade (CCSP, 2007b). Since the Earth's temperature is now demonstrably higher than it has been for at least 400 years and possibly more than 1,000 years (NRC, 2006b), and GHG
concentrations are now higher than they have been in many hundreds of thousands of years, it is possible that we may be nearing other stability thresholds. However, we have only a limited understanding of what those thresholds might be or when the climate system might be approaching them.
One example of a potential abrupt change mechanism is the possibility that GHGs stored in permafrost (frozen soils) across the Arctic could be released in large quantities as high-latitude warming continues. Permafrost contains huge amounts of carbon that have been locked away from the active carbon cycle for millennia, and it has been demonstrated that thawing permafrost releases some of this carbon to the atmosphere in the form of CH4 and CO2 (Shakova et al., 2010). If the release of these GHGs accelerates as the Arctic continues to warm, this could potentially accelerate the warming, leading to a positive feedback on the warming associated with GHGs released through human activities (Lawrence and Slater, 2005; Schuur et al., 2009; Zimov et al., 2006). In a related example, high-latitude warming can also alter the types of ecosystems covering the land (for instance, a shift from tundra to forest), which in turn changes the reflective characteristics of the land surface and thus potentially exerts a further positive feedback on warming (Field et al., 2007a).
Other potential abrupt changes include rapid disintegration of the major ice sheets (see Chapter 7), irreversible drying and desertification in the subtropics as a result of shifts in circulation patterns (see Chapter 8), changes in the meridional overturning circulation in the ocean (Broecker, 1997, 2002; Stocker, 2000; Stocker and Schmittner, 1997), or the rapid release of CH4 from destabilized methane hydrates in the oceans (Archer and Buffet, 2005; Overpeck and Cole, 2006), all of which could dramatically alter the rate of both regional and global climate change. Other surprises that may be associated with future climate change include so-called "low-probability, high-impact" events, such as an unprecedented heat wave or drought, or when multiple climate changes interact with each other or with other environmental stresses to yield an unexpectedly severe impact on a human or environmental system. Some of these potential—or in some cases already observed—surprises are discussed in later chapters.
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