A September paper by the world's leading body of scientists studying the effects of human activity on the world's climate suggested there was a slim chance that greenhouse gas emissions would force global warming to a smaller degree than previously suspected. But a new study yanks the rug out from under that slight bit of optimism.
The new study, published Sunday in the journal Nature Climate Change, suggests that the amount of increase in global temperature for each ton of carbon dioxide we release into the atmosphere may be higher than had been hoped. Climate scientists refer to this relationship as "climate sensitivity."
A report put out in September by the Intergovernmental Panel on Climate Change suggested that it was possible that the actual amount of warming for each ton of CO2 emitted might be very low. If that were true, it would give global society a bit more time to reduce its emissions of greenhouse gases before catastrophe ensued. But according to Sunday's paper, the September IPCC report failed to account for the effect of "aerosols" such as smog and fine dust, which reflect sunlight and can cause temporarily lower temperatures in parts of the world with polluted air.
Aerosols cool the surface of the earth beneath them by reflecting sunlight back into space. If less sunlight hits the surface, less solar energy turns into heat at the surface. In other words, aerosols work like parasols, shading the earth below and keeping it cooler. Aerosols are concentrated over the cities of the Northern Hemisphere, and that's where their cooling effect is the most pronounced. Land has less thermal mass than ocean, which means that land areas cool more rapidly when shaded by aerosols.
According to NASA climate scientist Drew Shindell, the author of Sunday's paper, the September IPCC study assumed aerosols were distributed uniformly over the Earth's surface rather than concentrated over Northern cities. That assumption biased the IPCC's results, says Shindell, causing them to conclude that the observed warming so far implied the possibility of low sensitivity.
Instead, says Shindell, when you account for the actual behavior of aerosols and other atmospheric pollutants such as ground-level ozone, the resulting conclusions about the Earth's climate sensitivity are significantly more pessimistic than those in the IPCC's study. In a measurement called Transient Climate Response (TCR), which assumes that the amount of CO2 in the atmosphere doubles over the next 70 years, Shindell and his colleagues determined that the increase in greenhouse gases will most likely force a global temperature increase of 1.7° Celsius (a bit more than 3° Fahrenheit), with a minimum increase of 1.3ºC. In September, the IPCC suggested that the TCR could be as low as 1.0°C.
Three degrees Fahrenheit may not seem like all that big an increase, but it's worth remembering two things: First, even seemingly small increases in global temperature can dramatically increase the strength of weather anomalies such as storms and droughts. Second, of the range of scenarios for how human society responds to the threat of climate change, the one that most closely resembles the path we seem to be on -- paying lip service to the issue while increasing exploitation of fossil fuels -- has us more than tripling the CO2 in the atmosphere by 2100.
For the record: An earlier version of this article described Drew Shindell as the "lead author" of the Nature Climate Change paper. He's actually the sole author. ReWire regrets the error.