Today, the United Nations' Intergovernmental Panel on Climate Change (IPCC) published a new Summary for Policymakers (SPM), cataloguing what is known about the physical basis for man-made global warming. The IPCC states that there is now 95 percent certainty that more than half of the warming over the past 60 years can be attributed to human activity. Since 1951, global average temperature has increased by approximately 0.6°C to 0.7°C. The dominant cause for increasing average global temperatures, the panel finds, is the rising level of carbon dioxide in the atmosphere emitted by burning fossil fuels. In addition, "Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years."

Beyond the certainty that humanity is causing the planet to warm more than it otherwise would, what does the SPM have to tell us? One crucial issue is climate sensitivity, conventionally defined as the amount of warming that doubling carbon dioxide in the atmosphere would eventually produce. In its 2007 report, the IPCC estimated that climate sensitivity was between 2 degrees Celsius and 4.5 degrees Celsius, with the best estimate being 3 degrees Celsius. The new report finds that "climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)." In IPCC parlance, likely means that the authors believe that there is more than a 66 percent chance that they’ve gotten the right estimate for climate sensitivity, whereas extremely unlikely means that they think there is less than 5 percent chance that they are wrong.

However, several recent studies have reported that climate sensitivity could be somewhat lower than the SPM suggests. For example, an article in the May Nature Geoscience concluded that the "most likely value of equilibrium climate sensitivity based on the energy budget of the most recent decade is 2.0 °C, with a 5–95% confidence interval of 1.2–3.9 °C." This is a whole degree lower than the best estimate calculated by the IPCC in 2007. Interestingly, the new SPM, unlike its predecessor reports, provides no best estimate of climate sensitivity. In any case, lower climate sensitivity would mean that future warming will be slower, giving humanity more time to adapt and to decarbonize its energy production technologies.

A big issue bedeviling the IPCC is how to take into account the 15-year "hiatus" in which average global temperatures have not significantly increased. Such a long pause was not predicted by any of the climate computer models relied upon by the IPCC. The new SPM deals with this awkward situation basically by waving it away, asserting that a 15-year pause is too short a time from which to draw any conclusions regarding future warming trends.

"Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends," the SPM states. Nevertheless, the report asserts this: "The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence)." What is the observed long-term trend? An increase of 0.12 °C per decade. To illustrate how sensitive temperature trends are starting and ending dates, the SPM notes that for 15-year periods starting in 1995, 1996, and 1997 the global average temperature trends are 0.13, 0.14, and 0.07 °C per decade, respectively.

So are the models relied upon by the IPCC really all that good at simulating trends in global average temperature? An August 28 article in the journal Nature Climate Change suggests that they are not. The Canadian climate researchers pointed out that while global average temperatures rose over the past 20 years at a rate of 0.14 ± 0.06 °C per decade, 37 of the models relied upon by the IPCC simulated an average rise of 0.30 ± 0.02 °C per decade. "The observed rate of warming given above is less than half of this simulated rate, and only a few simulations provide warming trends within the range of observational uncertainty," conclude the authors. It gets worse. For the period after 1998 until 2013, the researchers note, "The observed trend of 0.05 ± 0.08 °C per decade is more than four times smaller than the average simulated trend of 0.21 ± 0.03 °C per decade." Given this disparity, how can the IPCC have "very high confidence" that model projections agree with observed temperature trends when they clearly don’t?

The new SPM elsewhere does acknowledge with "medium confidence" that internal decadal variability is the cause of much of the difference between observations and the simulations. The SPM adds that the models cannot be expected to simulate the timing of the sort of natural climate variability that has produced the current 15-year pause. "If the IPCC attributes to the pause to natural internal variability, then this begs the question as to what extent the warming between 1975 and 2000 can also be explained by natural internal variability," the Georgia Tech climatologist Judith Curry observes. "Not to mention raising questions about the confidence that we should place in the IPCC's projections of future climate change."

Another significant issue addressed by the SPM is projected sea level rise. As warming seas expand and as icecaps and mountain glaciers melt in Greenland and Antarctica, the extra water drains into the ocean, increasing sea level. In its 2007 report, the IPCC crudely estimated that sea level could rise by 2090 by between 7 to 24 inches. For context, sea level rose about 8 inches between 1870 and 2004. Taking into account various scenarios about how much future warming the world might expect, the new SPM has boosted projected sea level rise from a low of 10 to a high of 38 inches by 2100. If warming turns out to be lower than the models project, then so too will future sea level rise.

"Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions," declared Thomas Stocker, the co-chair of the physical science group, in a press release. The Obama administration is all on board: In a statement hailing the new report, Secretary of State John Kerry warned, "Boil down the IPCC report and here’s what you find: Climate change is real, it's happening now, human beings are the cause of this transformation, and only action by human beings can save the world from its worst impacts." The action will resume when the 19th Conference of the Parties to the United Nations Framework Convention on Climate Change convenes in Warsaw this November. Stay tuned.