Mounting evidence suggests that basic assumptions about climate change are mistaken: The numbers don’t add up.
At the recent United Nations Climate Summit, Secretary-General Ban Ki-moon warned that “Without significant cuts in emissions by all countries, and in key sectors, the window of opportunity to stay within less than 2 degrees [of warming] will soon close forever.” Actually, this window of opportunity may remain open for quite some time. A growing body of evidence suggests that the climate is less sensitive to increases in carbon-dioxide emissions than policy makers generally assume—and that the need for reductions in such emissions is less urgent.
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.