Friday, December 25, 2015

We exceeded 1C warming more than a decade ago.

How Close Are We to 'Dangerous' Planetary Warming?

Michael E. Mann
Director of Penn State Earth System Science Center;
Dec. 23, 2015

It has been widely reported that 2015 will be the first year where temperatures climbed to 1C above the pre-industrial. That might make it seem like we've got quite a ways to go until we breach the 2C limit. But the claim is wrong. We exceeded 1C warming more than a decade ago. The problem is that here, and elsewhere, an inappropriate baseline has been invoked for defining the "pre-industrial." The warming was measured relative to the average over the latter half of the 19th century (1850-1900). In other words, the base year implicitly used to define "pre-industrial" conditions is 1875, the mid-point of that interval. Yet the industrial revolution and the rise in atmospheric CO2 concentrations associated with it, began more than a century earlier.

Unfortunately, even the Intergovernmental Panel on Climate Change (IPCC) has fallen victim to this problematic convention in their latest (5th) assessment report. The key graphic (Fig. 1 below) in the Summary for Policy Makers ("SPM") of the report measures net anthropogenic (i.e. human-generated) carbon emissions and the resulting warming that can be expected. Both the emissions and warming and measured relative to an 1870 baseline.



The graph shows the warming of the Northern Hemisphere (in degrees C) due to human-generated greenhouse gases ("GHG") alone, as estimated by the various climate models used in the IPCC 5th assessment report (the black curve -- the "multimodel mean" is the average over all of the climate model simulations that were done). The graph has been annotated to indicate the warming observed by 1800 and 1900. It is evident that roughly 0.3C greenhouse warming had already taken place by 1900, and roughly 0.2C warming by 1870. While that might seem like a minor amount of warming, it has significant implications for the challenge we face in stabilizing warming below 2C, let alone 1.5C, as we shall see below.


using the more appropriate 1750-1850 pre-industrial baseline, we see that the Northern Hemisphere average temperature (gray squiggly curve) has already warmed nearly 1.2C. Temperatures have exceeded 1C above pre-industrial levels for most of the past decade. So 2015 obviously won't be the first time this has happened, despite press reports to the contrary.


limiting CO2 concentrations to 450 ppm (orange dashed curve in Fig. 3) would indeed limit warming to about 2C relative to pre-industrial. Problem solved? Not quite...

While greenhouse warming would abate, the cessation of coal burning (if we were truly to go cold-turkey on all fossil fuel burning) would mean a disappearance of the reflective sulphate pollutants ("aerosols") produced from the dirty burning of coal. These pollutants have a regional cooling effect that has offset a substantial fraction of greenhouse warming, particularly in the Northern Hemisphere. That cooling would soon disappear, adding about 0.5C to the net warming. When we take this factor into account (orange dotted curve), the warming for 450 ppm stabilization is now is seen to approach 2.5C, well about the "dangerous" limit. Indeed, CO2 concentrations now have to be kept below 405 ppm (where we'll be in under three years at current rates of emissions) to avoid 2C warming (blue dotted curve).

So evidently, we don't have 1/3 of our total carbon budget left to expend, as implied by the IPCC analysis. We've already expended the vast majority of the budget for remaining under 2C. And what about 1.5C stabilization? We're already overdrawn.


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