Unprecedented rates of change in the global average temperature and in the GHG concentrations that drive this change are particularly challenging for the climate modeling enterprise because of the well-known fundamental properties of complex systems, of which Earth’s climate is a paradigmatic example. Because of the state and rate of change of Earth’s climate over recent decades, confident projections of extreme events are especially difficult to produce. This does not mean that climate science has nothing to say about the future of extreme events that can be useful to the intelligence community. What it means is that there are multiple scenarios of the future of climate events that are each likely enough that they deserve consideration by the intelligence community. They should not be treated as predictions but rather as possibilities for evaluation in terms of the social and political scenarios they might set in motion, the security issues that might ensue, and the preparedness of the U.S. government to deal with the consequences.
Fundamental climate science provides some useful concepts for thinking about the future of climate events despite the limits of predictability of particular events. Consider, for example, the implications of the fact that although Earth’s temperature remains well above the long-term average, the decade beginning in 1998 represented a hiatus in the longer overall global warming trend (National Aeronautics and Space Administration, 2012). A fundamental understanding of Earth’s climate system makes it clear that global warming has not stopped and that the hiatus will be brief. The past two years suggest that it may already have ended (Foster and Rahmstorf, 2011; National Aeronautics and Space Administration, 2012).
The past 130 years or so include periods with strong warming and periods with little or no warming (e.g., Easterling and Wehner, 2009). As we discuss further in Chapter 3, even under continuing climate change decades with no warming can be expected in the future—along with decades with above average rates of warming. The coupled climate system has naturally occurring decadal signals, such as the Pacific Decadal Oscillation and the Atlantic Multi-Decadal Oscillation, which can serve to mask or accelerate the average rate of warming on decadal time-scales. The underlying trends can be understood in relation to fundamental processes of energy balance: If the climate system is continuing to absorb more energy from the sun than it is emitting, as must happen when greenhouse gas concentrations are increasing, then that energy remains in the Earth system and must show itself sooner or later through increased temperatures along with other changes in the climate system. Recent research (Meehl et al., 2011) suggests that during the hiatus in recent years most of this excess energy has gone into the deep oceans, where it may show itself through the enhancement of coupled oceanic-atmospheric phenomena, such as a strong El Niño and warming ocean temperatures.