At least to the frontline climate scientists, it is becoming increasingly evident that the process of global warming presents a threat to human societies of essentially unprecedented proportions. They know beyond any reasonable doubt that the thermal impulse now being imparted to the earth’s ecological system by aggregate human activity is occurring at a rate greater than any that has been documented in the entire 65 million year paleoclimate record. The current rate of CO2 accumulation in the atmosphere is ten times higher than at any time over the past 400,000 years for which annual estimates have been made based upon ice core data. For earlier periods estimates are made for longer time periods, but the natural rate of CO2 accumulation that 50 million years ago drove atmospheric concentrations and deep ocean temperatures to the highest estimated levels on record was a factor of 20,000 less than the current rate. That distant process occurred over millions of years. At the higher rates currently prevailing, the inexorable process of reestablishing energy equilibrium will occur over a time span that will certainly be much shorter and will certainly affect the operating conditions of human societies, but the exact character, magnitude, timing, or location of the consequences cannot yet be determined.
The paleoclimate record that is the source of these observations is summarized in figure 1, which displays deep ocean temperatures over the 65 million year period for which empirical estimates have been made. As noted by Hansen and Sato, current deep ocean temperatures are roughly comparable to those that prevailed during the Eemian interglacial period 130,000 years ago but current sea levels are 4 – 6 meters lower than they were during the Eemian period. Moreover, under the impulse of global warming current deep ocean temperatures are virtually certain to match by mid century those of the earlier Pliocene period when sea levels were 25 meters higher than they currently are. Those comparisons indicate that much of the energy balance adjustment necessitated by the current global warming trend has yet to occur, and they suggest either that sea levels will rise more rapidly than currently expected or that some other dynamic not yet identified is at work. And indeed we are beginning to see changes in sea ice formations more rapid that prevailing climate models can account for, as figure 2 indicates. These observations provide compelling and indisputable strategic warning but the immediate implications cannot be specified with any degree of consensus.
Those who reject the warning categorically or attribute the implications to some safely distant time period are intent on imposing the burden of proof on those who credit it, and the skeptics are advantaged by established scientific rules that require a high level of statistical confidence for any assertion of causal effect. Climate scientists are aware that unqualified adherence to that rule probably means that a consequence of global warming disastrous to human societies would be unavoidable by the time it could be documented, but they are handicapped in issuing credible warning that extends beyond the scope of their current evidence. As a practical matter the skeptics have prevailed up to this point, and reaction to the global warming process is best described as marginal and indecisive. Public awareness is developing in reaction to experience with local effects, however, most recently in the United States to hurricane Sandy. The burden of proof question is becoming a battle line. Given the evidence that is emerging, it is reasonable to expect that at some point a sense of serious danger will override the impulse for denial, and it is not too early to anticipate the implications.
Those implications fall into two main categories. In principle it would be possible to contain the process of global warming by transforming the basis for energy generation and use, an option widely known as mitigation. The degree of mitigation required to preserve the current operating conditions of human societies is a matter of disputable judgment, but those who currently take the danger seriously are suggesting that atmospheric CO2 concentrations would have to be held to 450 ppm or less to give reasonable chance of assuring that outcome. That means that that energy generation would have to shift from approximately 20% reliance on sources that do not emit carbon to something like 80% by mid century as overall energy use is increased by a factor or 2 to 3 to accommodate the projected increase in the human population. Such a transformation is technically conceivable but would require a massive redirection of current market circumstances that is not being attempted and not even seriously discussed. Without effective mitigation, the burden of response to global warming will fall on the process of adaptation as communities throughout the world are forced to respond to the environmental circumstances they encounter. Mitigation would require explicitly organized global coordination. Adaptation is a more distributed process strongly affected by local circumstance. Since effective mitigation is a distant prospect, it is evident that adaptation will carry the main burden for several decades, and it appears that the ability to organize effective mitigation will substantially depend on failures of adaptation establishing global warming as a serious threat in public consciousness.
There is in addition a third option that is more appropriately considered an ominous temptation than a legitimate alternative. As demonstrated by large volcano eruptions that have periodically occurred, sulfate particles injected into the stratosphere can reduce average surface temperatures by as much as a half of a degree Celsius over the course of a single year by reflecting some portion of incident sunlight back into space. That and other measures that would have the same effect have been termed geoengineering or, more recently and more circumspectly, solar radiation management (SRM). Since a single sulfate particle can offset the thermal effect of 200,000 or more CO2 molecules, it is reasonably apparent that particular technique would not be expensive or technically demanding. Most countries and even wealthy entrepreneurs could do it. SRM techniques are being advocated as emergency measures that may be urgently required if a disastrous global warming effect appears on a schedule more rapid than any conceivable mitigation effort, but the compelling question is how that judgment is to be made. Actions with major global consequence will have to be globally vetted in some accepted way, and there is no procedure currently in place that is adequate to deal with the consequences that an effective SRM technique would have.