“In a worst-case scenario, where no action is taken to check the rise in greenhouse gas emissions, temperatures would most likely rise by more than 5°C by the end of the century.”
—Dr. Vicky Pope, head of climate change predictions at the UK’s Hadley Centre, Dec. 2008

“Without a change in policy, the world is on a path for a rise in global temperature of up to 6°C.”
—International Energy Agency, World Energy Outlook, Nov. 2008 [traditionally a very conservative agency]

“We are on the precipice of climate system tipping points beyond which there is no redemption.”
—James Hansen, director, Goddard Institute for Space Studies (NASA), December 2005

One year ago in a previous post, I thought these types of projections were alarmist and unwarranted. Since then, I’ve found steadily more people suggesting similarly dramatic numbers. Many believe that an average global warming of 1°C is unavoidable, and some claim that—if we do nothing to stop it—we could be headed for 5°C or more. [Update: and that’s just the global average. Over inland North America, you can add roughly another 50%, for a total of 7.5°C. Coastal North America would see lower temperature changes, but would face the dire impacts of sea level rise of 0.8 – 2.0 metres.]

The IPCC Numbers

Is there a definitive scientific source for these projections, however? With a little digging, I’ve found similar estimates from the Intergovernmental Panel on Climate Change (IPCC) itself, a source that I consider authoritative. The figure below shows annual emission scenarios (left) and temperature effects (right) for six different scenarios. For reasons I’ll explain later, I believe A1FI and A2 are the two scenarios that correspond most closely with a “business-as-usual” approach to carbon emissions. A1B is also worth considering, as a scenario “business-as-usual” is not possible due to limited fossil fuel supplies. (While the peak of conventional oil production is almost certainly around the corner, this scenario could well require constrained supplies of all fossil fuels—including coal, tar sands oil, shale oil and natural gas.)