How Close Are We to 'Dangerous' Planetary Warming? [View all]

[n2doc]

by Michael Mann

In the wake of the COP 21 UN Climate Summit in Paris (see this recent Huffington Post piece for my take on the agreement), a number of important questions still remain unanswered. Take for example the commitment reached by the 197 participating nations to limit warming below the "dangerous" level of 2C relative to pre-industrial time (neglecting for the time being the aspirational goal of a substantially lower 1.5C limit acknowledged in recognition of the danger posed to low-lying island nations). The question immediately arises: How much time do we have until we reach the danger zone? How close are we to the 2C warming limit?

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&quot 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 various future emissions scenarios are called "RCP"s (for "Representative Concentration Pathways&quot and they reflect varying assumptions regarding our future efforts to limit carbon emissions. The "RCP 2.6" scenario (dark blue), the most aggressive of the scenarios (from the standpoint of ramping down carbon emissions), corresponds to limiting net carbon emissions to about 3000 Gigatons (3 trillion tons) of CO2. We've already burned through about 2000 Gigatons, i.e. we have expended two thirds of our apparent "carbon budget".

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http://www.huffingtonpost.com/michael-e-mann/how-close-are-we-to-dangerous-planetary-warming_b_8841534.html