The Intergovernmental Panel on Climate Change (IPCC) released its Summary for Policymakers of its 4th Scientific Assessment of Climate Change (4AR) in Paris earlier today. (The whole report will be published in May.) The Summary declares that "warming in the climate system is unequivocal" and that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." In the lexicon of the Summary's authors, very likely means that they believe that there is more than a 90 percent chance that the last half century of warming is humanity's fault. Temperatures are increasing largely because humanity is pumping greenhouse gases like carbon dioxide from burning fossil fuels into the atmosphere. The 4AR's conclusion about humanity's responsibility for higher temperatures is the strongest yet from the IPCC.

It's taken more than a decade and a half of climatological research to reach this conclusion. Here's how the previous three IPCC assessment reports described humanity's influence on the climate. In 1990, the First Assessment Report (FAR) declared, "The size of the warming is broadly consistent with predictions of climate models, . . . but the unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more." In 1996, the Second Assessment Report (SAR) asserted, "The balance of evidence suggests a discernible human influence on climate." And in 2001, the Third Assessment Report (TAR) more confidently stated, "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

There is good news of a sort in the 4AR Summary. Researchers believe in most scenarios that average global temperatures and sea level rise are likely to be somewhat lower than previously projected. Let's look at earlier IPCC projections to get a sense of how climate change findings have evolved since 1990. Although each report stated its projections in ways that make it somewhat difficult to make direct comparisons, here's the gist of them. In 1990, the FAR found that computer climate models projected that global mean surface temperature could increase by about 1 degree Celsius above the present value by 2025 and 3 degrees Celsius before the end of the next century. The "best estimate" for sea level rise due to melting glaciers and thermal expansion was about 60 centimeters (25 inches) by 2100.

In 1996, the SAR lowered the projected increase in average global temperatures by 2100 of about 1.0 to 3.5 degrees Celsius (1.8 to 6.3 degrees Fahrenheit) by 2100 with a best estimate of 2 degrees Celsius ((3.6 degrees Fahrenheit. The SAR forecasted that sea-level could rise between 15 to 94 centimeters (6 to 37) inches by 2100 with a best estimate of 50 centimeters (20 inches). In 2001, the TAR widened the projected range of projected temperature increases by 2100 to 1.4 to 5.8 degrees Celsius (2.5 to 10.4 degrees Fahrenheit). On the other hand, the TAR dropped its estimates of sea level rise by 2100 to 9 to 88 centimeters (4 to 35 inches) with a mean estimate of 45 centimeters (18 inches).


So what does the latest report foresee? The Summary offers six scenarios  for possible temperature increases by the end of this century. In the low scenario the likely range of temperature increase between 1.1 to 2.9 degrees Celsius (2 to 5.2 degrees Fahrenheit) with a best estimate of 1.8 Celsius (3.2 degrees Fahrenheit). In the worst case scenario, average temperature rises to between 2.4 Celsius and 6.4 Celsius (4.3 to 11.5 degrees Fahrenheit) with a best estimate of 4 degrees Celsius (7.2 degrees Fahrenheit). Except for the worst case scenario, the top temperatures are lower than the maximum projected by the TAR in 2001.

However, the IPCC's new Summary continues the trend of lowering sea level increases that is found in its previous scientific assessments. Again depending on the scenario chosen, projected sea level rise by 2100 could be between 18 to 59 centimeters (7 to 23 inches). The report notes that sea level rose about 7 inches during the 20th century.

The new estimate for sea level rise has proved to be one of the more controversial aspects of the IPCC Summary. For example, Stefan Rahmstorf from the University of Potsdam in Germany, is a co-author of a brief report in Science this week that suggests the previous IPCC projections "may in some respects even have underestimated the change, in particular for sea level." He thinks sea level could rise as much as 55 inches over the next century.

Interestingly, a new study published in Geophysical Research Letters earlier this month reported that the "rate of sea level change was found to be larger in the early part of last century." In the first half of the 20th century sea level rose by about 2 millimeters per year, while averaging about 1.5 milimeters per year in the second half. This is pretty good agreement with the IPCC assessment that reports that sea level rose by about 1.8 milimeters per year between 1961 and 2003. However, the IPCC finds that sea level rose fastest between 1993 and 2003 at about 3 millimeters annually, while the GRL report finds the fastest increase occurred between 1975 and 1985 at about 5 millimeters per year.


Details like sea level rise will continue to be debated by researchers, but if the debate over whether or not humanity is contributing to global warming wasn't over before, it is now. The question of what to do about it will be front and center in policy debates for the next couple of decades. How strongly humanity may want to mitigate future climate change and at what cost depends on how likely the worst case projections turn out to be. The European Union wants to set a goal of avoiding an increase of more than 2 degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial levels to prevent "dangerous anthropogenic interference with the climate." In the most optimistic case, humanity could basically sit on its collective hands because an increase of this magnitude is close to the lower bounds of the new IPCC estimates in some scenarios. However, as the new IPCC Summary makes clear, climate change Pollyannaism is no longer looking very tenable.

Disclosure: As I have disclosed ad nauseam I still own 50 shares of ExxonMobil (which just reported record profits) and my family owns some pretty poor land in McDowell County West Virginia that might have some coal under it. I detail my climate change skepticism saga here. The folks at Exxonsecrets have this to say about me. I wish they would get around to updating it some day. And oh yeah, I generally prefer warmer temperatures, so I vacation in the tropics when I can afford it.

Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.