The above lines of evidence (supported by well over 100 recent scientific papers), while not definitive and, in some cases controversial, suggest that the balance of evidence may be swinging toward more extreme global warming and sea-level rise outcomes. While some of the observations may be due merely to natural fluctuations, their conjunction and, in some cases, positive feedbacks (from permafrost melting, biomass changes, arctic sea ice retreat and melting of Greenland) are causes for concern. Some of the links between major elements of the climate system are shown in Figure 1.1. Several of these links indicate positive feedbacks. Overall, they illustrate the need to consider the whole system, not just its individual parts in isolation.

Figure 1.1 Links between parts of the climate system, including feedbacks that may accelerate climate change and its impacts

Global dimming changes /

Ocean & atmospheric circulations [annular modes, gyres, ENSO}

Extreme events incl. floods, droughts fire, hurricanes, storm surges

Figure 1.1 Links between parts of the climate system, including feedbacks that may accelerate climate change and its impacts

The observations and linkages suggest that critical levels of global warming may occur at even lower greenhouse gas concentrations and/or anthropogenic emissions than was considered justified in the IPCC (2001) report. The observed changes in Greenland and Antarctica suggest that a more rapid rise in sea level may be imminent, as has been observed in recent years (Church and White, 2006; Rahmstorf, 2007). Indeed, Rahmstorf et al (2007) find that emissions, global surface temperature and sea level rise are all increasing at rates at the very highest end of the IPCC range. Several of the points above suggest rapidly occurring regional impacts are imminent. Taken together, these recent developments increase the urgency of further improving climate models, and of taking action to reduce emissions in order to avoid the risk of unacceptable levels of climate change (see also National Research Council, 2002; Pittock, 2006; Schellnhuber et al, 2006; Steffen, 2006; Time Magazine, 2006).

A responsible risk management approach demands that scientists describe and warn about seemingly extreme or alarming possibilities, for any given scenario of human behavior (such as greenhouse gas emissions), if they have even a small probability of occurring. This is recognized in engineering design (for instance, for the safety of dams and bridges) and in military planning (where large resources are devoted to guarding against, and deterring, hopefully unlikely threats) and this practice is also commonplace in the insurance sector. The objective of policy-relevant advice must be to avoid unacceptable outcomes, not to determine the most likely outcome.

The recent developments discussed above might simply mean that the science is progressing. However, it also may suggest that up until now many scientists have consciously or unconsciously downplayed the more extreme possibilities at the high end of the uncertainty range in an attempt to appear moderate and 'responsible' (that is, to avoid scaring people). However, true responsibility requires providing evidence of what must be avoided: to define, quantify and warn against possible dangerous or unacceptable outcomes.

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