The UN's Intergovernmental Panel on Climate Change ("IPCC") has more bad news for us. Its long range forecast still looks hot, and the IPCC is more confident than ever that humans are largely the cause. On Friday, September 27, the IPCC issued a Summary for Policymakers on the "physical science basis" of climate change. This is the first part of the IPCC's Fifth Assessment Report to be published. The summary report contains numerous findings, but you may want to begin by thinking about five aspects of them.
1. It is "extremely likely" that we're the culprit. The IPCC observes that warming in the climate system is unequivocal. But there has been debate about its cause. Based on growing evidence, the report finds it is "extremely likely" that human influence has been the dominant cause of observed global warming since the 1950s. In the IPCC's previous report, issued in 2007, the IPCC was 90% certain of this conclusion. Now it is 95% certain.
2. We need a carbon budget. For the first time, the IPCC takes a stab at calculating essentially a global limit on anthropogenic CO2 emissions. Science has long estimated that a temperature rise of 2 degrees Celsius above the temperature of preindustrial times is the point after which the most damaging effects of global warming would happen. The report estimates the level of total CO2 emissions since the industrial revolution that would trigger a temperature rise of this magnitude. That number is subject to variation of course, but the report projects it is likely that no more than about one trillion tons of CO2 could be released without triggering this rise in temperatures. We have released about one half of that amount so far, and projections are that at current rates, the other half trillion tons could be released from anthropogenic sources in the next several decades.
3. Temperatures of the last fifteen years are not that comforting. Climate change skeptics have focused on the fact that the rise of global surface temperatures leveled out in the last fifteen years. The IPCC report explains that this recent trend may be due to natural variability. It observes that trends based on records of short duration are very sensitive to beginning and end dates and may not reflect long term climate trends. Nonetheless, in identifying possible explanations for the fifteen year hiatus in warming, IPCC recognizes that the possible explanations for it are not proven. It also recognizes the possibility that in some models, there may be an overestimate of the response to increasing greenhouse gas.
4. There is much we do not know. We don't know the cause of the fifteen year leveling of global warming. We don't know how quickly the oceans will rise. We don't know the likelihood and rate of extinctions. We cannot accurately predict the localized effects of warming temperatures. Much of the report is a detailed exercise in characterizing probabilities and confidence levels of predicted global climate trends over time. The report characterizes the likelihoods of trends it identifies, and they range from the virtually certain to low confidence levels, depending on the trend and timeframe.
5. We will hear more from the UN. The Summary Report for Policymakers focuses on the physical science basis of climate change, and the full version of this part is expected soon. This physical science part is only the first of three that will together comprise the IPCC Fifth Assessment Report. The Fifth Assessment Report follows the Fourth Assessment Report which was published in 2007. In 2014, the two additional parts of this Fifth Assessment Report will be issued concerning (1) likely impacts and (2) steps to limit climate change. As the report is issued, it likely will prompt renewed efforts for a global climate treaty. The UN Secretary General, Ban Ki-moon, urged world leaders to work toward a new global agreement to cap greenhouse gas emissions and declared his intention to call a meeting of world leaders next year.