by Judith Curry
Section 12.5.5 in the WG1 Report provides some important insights on what is most commonly regarded as the ‘dangerous’ aspects of AGW.
For background on this topic, see this previous Climate Etc. post Redefining dangerous climate change.
I was pointed to section 12.5.5 by a post at BishopHill. The relevant text is below:
12.5.5 Potentially Abrupt or Irreversible Changes
12.5.5.1 Introduction
This report adopts the definition of abrupt climate change used in Synthesis and Assessment Product 3.4 of the U.S. Climate Change Science Program CCSP (CCSP, 2008b). We define abrupt climate change as a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems (see Glossary). Other definitions of abrupt climate change exist. For example, in the AR4 climate change was defined as abrupt if it occurred faster than the typical time scale of the responsible forcing.
A number of components or phenomena within the Earth system have been proposed as potentially possessing critical thresholds (sometimes referred to as tipping points, (Lenton et al., 2008)), beyond which abrupt or non-linear transitions to a different state ensues. The term irreversibility is used in various ways in the literature. The AR5 report defines a perturbed state as irreversible on a given timescale if the recovery timescale from this state due to natural processes is significantly longer than the time it takes for the system to reach this perturbed state (see Glossary). In that context, most aspects of the climate change resulting from CO2 emissions are irreversible, due to the long residence time of the CO2 perturbation in the atmosphere and the resulting warming (Solomon et al., 2009). These results are discussed in Sections 12.5.2–12.5.4. Here, we also assess aspects of irreversibility in the context of abrupt change, multiple steady states and hysteresis, i.e., the question whether a change (abrupt or not) would be reversible if the forcing was reversed or removed (e.g., Boucher et al., 2012). Irreversibility of ice sheets and sea level rise are also assessed in Chapter 13.
In this section we examine the main components or phenomena within the Earth system that have been proposed in the literature as potentially being susceptible to abrupt or irreversible change (see Table 12.4).
Abrupt changes that arise from nonlinearities within the climate system are inherently difficult to assess and their timing, if any, of future occurrences is difficult to predict. Nevertheless, progress is being made exploring the potential existence of early warning signs for abrupt climate change (see e.g., Dakos et al., 2008; Scheffer et al., 2009).
Table 12.4: Components in the Earth system that have been proposed in the literature as potentially being susceptible to abrupt or irreversible change. Column 2 defines whether or not a potential change can be considered to be abrupt under the AR5 definition. Column 3 states whether or not the process is irreversible in the context of abrupt change, and also gives the typical recovery time scales. Column 4 provides an assessment, if possible, of the likelihood of occurrence of abrupt change in the 21st century for the respective components or phenomena within the Earth system, for the scenarios considered in this chapter.
(click image for full size).
The section then goes on to discuss each of these in detail (with most of the ice sheet material in ch 13).
But the bottom line is this. The only one of these changes likely to occur in the 21st century is disappearance of the summer sea ice, and by ‘disappearance’ I assume they mean what they usually do when they say this: < 1 M sq km left.
Bishop Hill summarizes it this way:
Abrupt changes that aren’t really abrupt and irreversible changes that aren’t really – er – irreversible.
Catastrophe? Er – we don’t know.
JC summary: For background on this topic, see this previous Climate Etc. post Redefining dangerous climate change. The classic reference on the topic of abrupt climate change is the NRC report Abrupt Climate Change: Inevitable Surprises.
Admittedly there are numerous definitions of ‘abrupt climate change’, but the IPCC chooses a fairly trivial one: seemingly, the climate shift circa 2001 would qualify as an ‘abrupt’ climate change.
But the real issue is this. The IPCC approach, using highly damped deterministic global climate models, is incapable of producing abrupt climate change (beyond the melting of Arctic sea ice, which is not irreversible even on timescales of a decade).
The most scientifically interesting, and societally relevant topic in climate change is the possibility of abrupt climate change, with genuinely massive societal consequences (the disappearance of Arctic sea ice and regional forest diebacks arguably don’t qualify here). The IPCC has high confidence that we don’t have to worry about any of the genuinely dangerous scenarios (e.g. ice sheet collapse, AMOC collapse) on timescales of a century. These collapses have happened in the past, without AGW, and they will inevitably happen sometime in the future, with or without AGW. Are the IPCC overconfident in their conclusions on these also?