I took a rather shallow look at the heavily nuanced “what about RCP8.5” debate a couple of days ago, based on a Chris Mooney article. Not until I read a wonderfully coherent analysis from Michael Mann on his website did it become clear why I felt dissatisfied. Attacks on RCP8.5 aren’t just attacks on a particular scenario. They open a window for politicking, for cavilling from the required urgency.
As Mann puts it, this latest kinda-optimistic burst “doesn’t account for non-linearities and, most importantly of all, doesn’t include so-called ‘carbon cycle feedbacks’, that is to say, the feedback mechanism by which global warming can actually release more CO2 (or e.g. methane), adding further to the warming. Indeed, this deficiency applies to all studies that are based on specifying CO2 concentrations rather than emissions, and it applies to the current commentary by Hausfather & Peters.”
Putting it more bluntly:
There is some good news here. The numbers show that escalating efforts around the world to decarbonize our economy are starting to pay dividends. We’re starting to bend that emissions curve downward. But we need to reduce emissions by a factor of two over the next decade and bring them down to zero in a matter of a few decades if we are to avert catastrophic climate change impacts. We have to get off fossil fuels far more quickly than we’re on track to do under current policies. This latest commentary doesn’t change that at all.