Why Table 12.4 of IPCC AR5 should not be trusted

At the end of September, Working Group 1 (WG1) of the International Panel on Climate Change (IPCC) published the first part of its Fifth Assessment Report (AR5). This report, entitled ‘Climate Change 2013: Physical Science Basis’, now has its own website, from where the Summary for Policy Makers (SPM) and Full Report (chapter-by –chapter) can be downloaded (as PDF files). Observers of contrarian cyberspace will have noticed that a certain Table in the full report (Table 12.4) has been widely touted as proof that climate change is no longer the problem “alarmists” think it is. Even high profile professional people (who ought to know better) have demeaned themselves by publishing guest posts on unscientific websites: e.g. ‘Understanding the IPCC AR5 Climate Assessment’ by Professor Richard Lindzen on WattsUpWithThat. The problem with such a mis-reading of the speculative data in Table 12.4 (summarising ‘Components in the Earth system… susceptible to abrupt or irreversible change’ on page 12-78) is that it completely contradicts the historical data in Table SPM.1 (summarising ‘Observed Changes in the Climate System’ on page SPM-23). Whereas Table 12.4 suggests climate scientists consider it improbable that a variety of catastrophic tipping points will be passed in this Century, Table SPM.1 indicates that they have generally high confidence that we are already witnessing increased frequencies of various unusual weather events. Thus, trying to use Table 12.4 to suggest that anthropogenic climate disruption (ACD) is not the massive problem the IPCC has confirmed it is (with 95% confidence), is one of the most shameful pieces of cherry-picking I think I have ever seen. As with his mis-use of graphs in his presentations to the already-sceptical, this suggests that Lindzen knows what he is doing and is deliberately trying to mislead and or misdirect people. As I would not just want anyone to take my word for it, I have reproduced within this post both tables and the text that accompanies them in their respective reports. However, rather than start with Lindzen’s shameless cherry-picking of Table 12.4 and attempted inversion of the IPCC’s position, which would only confuse people (as I suspect is his intention), perhaps I should have started with the facts as presented in the SPM. If so, I can only apologise and now attempt to redress the situation. As is the way with everything else, the SPM has been reduced to a series of Headline Statements (PDF available here). However, such sound bytes are very useful and, in the current context, the most relevant are as follows:

• Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence).
• The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). Over the period 1901–2010, global mean sea level rose by 0.19 [0.17 to 0.21] m.

I should also wish to draw your attention, noting that a gigatonne (GT) is roughly equivalent to a cubic kilometre of ice, to the following sound bytes from section B.3 of the SPM regarding the Cryosphere (page SPM-7):

• The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of the ice sheets, was very likely 226 [91 to 361] GT/yr over the period 1971−2009, and very likely 275 [140 to 410] GT/yr over the period 1993−2009.
• The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [–6 to 74] GT/yr over the period 1992–2001 to 215 [157 to 274] GT/yr over the period 2002–2011.
• The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [–37 to 97] GT/yr over the period 1992–2001 to 147 [72 to 221] GT/yr over the period 2002-2011. There is very high confidence that these losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica.

As noted on page SPM-7, 100 GT/yr of ice loss is equivalent to about 0.3 mm/yr of global mean sea level rise (SLR). It is perhaps also worth noting, here, that SLR is currently 3.0 mm/yr (and accelerating all the time). If your head is already spinning with all this data, just try to focus on one fact: The SPM contains two numbers for decadal average melting rates in Greenland, namely 34 GT/yr in 1990s and 215 GT/yr in 2000s. As we shall see, these numbers not only bring into question the cherry-picking of Table 12.4; this six-fold increase in ice loss in just 10 years suggests that the conclusions stated in Table 12.4 are incompatible with what is already happening. As every teenager knows, you cannot confidently draw a straight line through anything less than three points on a graph. However, we are not dealing with a linear relationship here and, if we were, the above numbers (i.e. 34 and 215) would imply Greenland had been gaining ice prior to the 1990s (which it wasn’t). This therefore gives us even more confidence that the annual rate of ice mass loss in Greenland has accelerated. Indeed, perhaps the person who compiled Table 12.4 should have consulted Wikipedia:

• 1961 to 2003: +25 to –60 GT/yr => 1982, –18 GT/yr
• 1993 to 2003: –50 to –100 GT/yr => 1998, –75 GT/yr
• 4/2002 to 11/2005: mean 215 GT/yr => 2004, –215 GT/yr
• 3/2002 to 9/2012: total ~2900 GT => 2007, –276 GT/yr
• 2008 to 2012: mean 376 GT/yr => 2010, –367 GT/yr

Table 12.4 expresses high confidence that the disintegration of Greenland ice sheet is exceptionally unlikely in this Century. This may be because the total volume of ice (i.e. If 2,850,000 km³) is so large. However, at what point does the accelerating ice loss become catastrophic (especially if it is already irreversible in any timescale relevant to human civilisation)? It must be accepted that the accelerating ice mass loss will eventually reach a maximum rate (due to the finite amount of solar radiation (etc) and then decline as surface area declines. However, it would seem unwise to assume we have reached that point already. That being the case, the disintegration of the Greenland ice sheet is already happening. In my opinion, this is a catastrophe – we just haven’t had to deal with the consequences yet. Either way, how can the IPCC ignore the fact that the melting of the Greenland lcesheet is already accelerating? As it is almost certainly already beyond our capacity to stop it, this makes the relevant entry in Table 12.4 seem counter-factual. Given what Joe Romm has called the ‘planned obsolescence’ of the AR5 – as a result of systematically ignoring the effects of positive feedback mechanisms that are already observable – I suspect that the same is true for all the other entries in Table 12.4. Given all of the above, it is even more ludicrous to try – as Lindzen and others have done – to use Table 12.4 to falsify the basic position of the IPCC, which is that ACD is already happening and already accelerating; and the longer we wait the harder it will get to stop it.