It's the nature of the future to sneak up on us faster than we expect. We know this. It's part of why we love doing this show. But when the Intergovernmental Panel on Climate Change (IPCC) released a new assessment on global warming a scant 10 days after we recorded a podcast episode on climate science, it took your otherwise unflappable hosts by surprise.
Several of the numbers we quoted during the episode - degrees of temperature change, atmospheric concentrations of greenhouse gasses, and average rate of ice loss from glaciers, for example - are a bit off from the new estimates. However, the contents of the IPCC's September 2013 report are largely reaffirmations of previous findings: That the Earth is warming faster than it has in the past, and that the warming is extremely likely to be anthropogenic - in other words, that it's humankind's fault.
The IPCC is even surer of that last part now than it was previously - its certainty rate has gone up from 90-100 percent to 95-100 percent.
So what else is different? The IPCC is now projecting a global rise in surface temperature for this century of 0.3-4.8 degrees Celsius (0.5-8.6 degrees Fahrenheit) - slightly lower than their previous projection of 1.1-6.4 degrees Celsius (2.0-11.5 degrees Fahrenheit) given in 2007.
But these are still very significant numbers. The 2007 report estimated that a rise of more than 3.5 degrees Celsius (6.3 degrees Fahrenheit) above the mean global temperature for 1999 (which was 15.6 degrees Celsius/60.1 degrees Fahrenheit) would likely be enough to drive 40-70 percent of all known animal and plant species to extinction.
The 2013 assessment also estimates that global sea levels will rise 0.26-0.98 meters (0.85-3.22 feet) over the 1986 levels during this century. That's up from the 2007 estimate of 0.18-0.59 meters (0.59-1.94 feet).
The third indicator of warming that climate scientists frequently refer to is greenhouse gas emissions/concentrations. The important takeaway from the new report is this:
"Concentrations of CO2, CH4, and N2O now substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years. The mean rates of increase in atmospheric concentrations over the past century are, with very high confidence, unprecedented in the last 22,000 years."
You may have noticed that I've used 'likely' and 'estimates' a lot in this post. That's because the IPCC itself speaks in likelihoods. Two things that haven't changed with this new report: Climate science is incredibly complex and estimates about the future are tricky to make. Anyone who tells you otherwise is probably trying to sell you something.
Adding to the confusion, even the most well-meaning journalistic publications have been scrambling Celsius/Fahrenheit conversions and conflating disparate parts of the report. And some other publications have been downright misleading - the UK Telegraph's headline "Top climate scientists admit global warming forecasts were wrong," for example, has really stuck in my science craw.
By which I mean to say what may be obvious to any skeptics in the audience: Trust no one. Or, OK, that's bad advice. Better would be: Read carefully, think critically, and make your own investigations of the source material. All of the IPCC's information about the report is collected here. The Summary for Policymakers is an excellent place to start.