The Global Warming Statistical Meltdown

Judith Curry,  former chairwoman of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, writes at WSJ:

According to the U.N. Framework Convention on Climate Change, preventing “dangerous human interference” with the climate is defined, rather arbitrarily, as limiting warming to no more than 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial temperatures. The Earth’s surface temperatures have already warmed about 0.8 degrees Celsius since 1850-1900. This leaves 1.2 degrees Celsius (about 2.2 degrees Fahrenheit) to go.

In its most optimistic projections, which assume a substantial decline in emissions, the Intergovernmental Panel on Climate Change (IPCC) projects that the “dangerous” level might never be reached. In its most extreme, pessimistic projections, which assume heavy use of coal and rapid population growth, the threshold could be exceeded as early as 2040. But these projections reflect the effects of rising emissions on temperatures simulated by climate models, which are being challenged by recent observations.

Human-caused warming depends not only on increases in greenhouse gases but also on how “sensitive” the climate is to these increases. Climate sensitivity is defined as the global surface warming that occurs when the concentration of carbon dioxide in the atmosphere doubles. If climate sensitivity is high, then we can expect substantial warming in the coming century as emissions continue to increase. If climate sensitivity is low, then future warming will be substantially lower, and it may be several generations before we reach what the U.N. considers a dangerous level, even with high emissions.

The IPCC’s latest report (published in 2013) concluded that the actual change in 70 years if carbon-dioxide concentrations double, called the transient climate response, is likely in the range of 1 to 2.5 degrees Celsius. Most climate models have transient climate response values exceeding 1.8 degrees Celsius. But the IPCC report notes the substantial discrepancy between recent observation-based estimates of climate sensitivity and estimates from climate models.

Nicholas Lewis and I have just published a study in Climate Dynamics that shows the best estimate for transient climate response is 1.33 degrees Celsius with a likely range of 1.05-1.80 degrees Celsius.

Read the full article here.