The recently released Intergovernmental Panel on Climate Change’s Fifth Assessment Report on the physical science of climate change contains such a wealth of information, but at times it can be overwhelming.
Yes, there’s clearly a lot of evidence to support the idea that increased emissions of greenhouse gases will warm the planet. But what does it all mean? The IPCC in their anxiousness to present all the evidence can at times leave us stranded without a clear guidepost as to what we should or need to do about global warming.
Well, the Climate Institute put out a chart earlier this week that does a great job of summing it all up and forms one of our two charts of the week.
It shows three alternative paths for global emissions, what they would mean for global temperature rise, and then the major implications from that temperature rise. All within one chart. It really speaks for itself.
Source: The Climate Institute (2013)
Now, to take it up to one further level of complexity are the two charts below, taken from the International Energy Agency’s World Energy Outlook. On the left hand side, they show what the concentration of greenhouse gases (the key metric that will drive temperature rise) is likely to reach if we:
1) In red, just keep with current global energy and climate policies as at mid-2012;
2) In blue, what would happen if the politicians delivered on their promised future policies; and
3) In green, a scenario where we really got serious about reducing greenhouse gas emissions and aimed to keep the atmospheric concentration of CO2-equivalent below 450 parts per million.
Then on the right hand side, they show what kind of temperature rise would result from each scenario. Now the thing is, there is no single precise level of temperature rise that we can be certain will result from a given concentration of greenhouse gases, rather it is a spread of probabilities, and this is shown in the chart.
Projections of greenhouse gas concentrations and temperature rise probabilities for different policy paths
Source: International Energy Agency – World Energy Outlook 2012
You can see in the right hand chart that the current policies scenario (the red line) gives a small chance that all will be relatively OK and temperature rise will remain below 2 degrees. But it’s most likely temperature will be above 4 degrees and you’ve even got a reasonable probability of incredibly catastrophic temperature rise of 6 degrees or more. The New Policies scenario still means we’re most likely to exceed a 3 degrees rise but at least the probability of 6 degrees or more is very unlikely, but could not be ruled out. Then there’s 450 ppm - what makes this reassuring is not just that the median rise is a manageable 2 degrees but the tail becomes extremely thin beyond 4 degrees, meaning the chances of absolutely catastrophic outcomes are kept very low.
As explained in the article The ‘average Joes’ flirting with climate risk (October 2), in life we don’t just worry about the most likely middle-of-the-road outcome, we take extra precautions to make sure we keep the chances of irreparably bad things occurring very low. The above IEA chart helps to illustrate how the choices we make about policy affect the chances of catastrophic temperature rises, not just the average outcome.