Are we already past dangerous climate change?

26 02 2014

Are we already past dangerous climate change? asks a Mark Cochrane follower over at Peak Prosperity dot com….. This paper (PDF) – not peer reviewed – he ads, is a critique of the AR5 WG1 SPM. David Wasdell lays out a case that the current situation is far worse than the AR5 posits. He uses a much higher value of Earth System Sensitivity to show that we have no carbon budget left, even for 2C. He also uses other sensitivity estimates to show that there is, at best, very little or no budget left to avoid dangerous climate change.

Wasdell is one of the climate scientists that Guy McPherson leans heavily on for his NTE scenarios, so this question is quite pertinent to How Guy McPherson gets it wrong..

Mark Cochrane

Mark Cochrane


Mark replied: Yes we are………But

I read through the 21 page document and it is a good expose of how the politics of the IPCC process shades an already conservative (consensus) representation of what the science indicates is likely to occur as a function of a given amount of greenhouse forcing. For those who do not know, the IPCC process requires 100% consensus of all the authors nominated by all the countries to agree on their interpretation of the data. This means that in the end, after marathon sessions of back and forth, the final interpretation depends on just how far the most skeptical scientists are willing to be moved. These are not ‘alarmist’ interpretations as some would have you believe. Furthermore, the public does not really look at the IPCC report, they look at the Summary for Policy Makers (SPM) which is a short Cliff Notes version. The short version is wrangled over by political appointees (not scientists) who literally argue over every word and every figure. They produce a sanitized version of the report that all governments can support. Again, not conducive to ‘alarmist’ or even the most likely scenarios.

That said, the author (Wasdell) of the document you linked and the IPCC authors are talking about two different outcomes with regard to the made up 2C line in the sand for “dangerous” climate change. The SPM is focused on the so-called fast feedbacks (e.g. radiative forcing of greenhouse gases) and the likely temperature impacts within this century. Wasdell is making the case for including both fast and slow feedbacks and using a final equilibrium temperature as the metric of “dangerous” temperature changes. The slow feedbacks include things like changing albedo values from melting ice sheets and methane release from melting permafrost. These slow feedbacks stretch out for hundreds (maybe thousands) of years. So even if we manage to keep increases under 2C this century (unlikely), the planetary temperatures will certainly rise above that value over the following centuries. Though everything we see shows the 2050 and 2100 outcomes of the model projections the IPCC also does some analyses of “Long-term” changes out to 2300. Here is the figure from the AR4.

The unrealistic goal of 2C is pretty obvious even on the sanitized Figure 10 in the SPM of AR5 (AR5 SPM link; figure is on page 28). Every scenario but the fantasy RCP 2.6 blows past 2C  by 2100 and the trajectories are shooting even higher into the future.

The IPCC and especially the SPM make use of the Equilibrium Climate Sensitivity (ECS) for a doubling equivalent of carbon dioxide concentration, which basically accounts for fast feedbacks and is taken to probably be between 2 and 4.5C, with 3C used as the most likely value (note some models show it to be >5C). Wasdell is arguing to use what is termed the Earth System Sensitivity (ESS) which includes both long and short term feedbacks. It is poorly constrained but may be twice as large as the ECS values.

In the near term there is not a huge difference in using one over the other and what needs to be kept in perspective is that both approaches are postulating an outcome for a doubled CO2 equivalent that is held constant which is not actually a realistic model of what is likely to happen. It does give a measure of sensitivity though. If you are going to try to make mitigating attempts, going after the fast feedbacks is the most reasonable approach since doing things like reducing methane emissions from fossil fuels (or cows!) could make meaningful changes in the short term that have long term significance, while trying to shade glaciers to mitigate long term feedbacks would be ludicrous.

Ultimately though, the 2C threshold is just a chimera created to give policymakers and the public a figure to hang their concerns on. Barring a miracle or a global catastrophe, we are not likely to reduce emissions by the >80% necessary before 2020 in order to maybe squeak in under 2C this century.  In any case, the climate change (0.8C) that we’ve already experienced has led to numerous extinctions and many thousands of human deaths (e.g. 80,000 in 2003 Europe, 50,000 in 2010 Russia). Surely this has already reached “dangerous levels”.




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