The Intergovernmental Panel on Climate Change (IPCC) is releasing over time its fifth assessment report (AR5), and chapter 12 of the Working Group I section The Physical Science Basis makes for some fascinating reading, of a kind rather underreported in the recent news accounts of the AR5. Consider for example Table 12.4 (p. 12-78), bearing the snappy title “Components in the Earth system that have been proposed in the literature as potentially being susceptible to abrupt or irreversible change.” Here is what IPCC has to say about nine such potential apocalypses (all italics in the original, percentage definitions added).
- Collapse of the Atlantic Meridional Overturning Circulation (a circulating current in the north Atlantic in which waters at high latitudes are cooled, sink and flow toward the equator, are warmed, rise to the surface, and then flow once more toward higher latitudes): Very unlikely (0-10%) that the AMOC will undergo a rapid transition (high confidence).
- Ice sheet collapse: Exceptionally unlikely (0-1%) that either Greenland or West Antarctic Ice sheets will suffer near-complete disintegration (high confidence).
- Permafrost carbon release: Possible that permafrost will become a net source of atmospheric greenhouse gases (low confidence).
- Clathrate methane release: Very unlikely (0-10%) that methane from clathrates will undergo catastrophic release (high confidence).
- Tropical forests dieback: Low confidence in projections of the collapse of large areas of tropical forest.
- Boreal forests dieback: Low confidence in projections of the collapse of large areas of boreal forest.
- Disappearance of summer Arctic sea ice: Likely (66-100%) that the Arctic Ocean becomes nearly ice-free in September before mid-century under high forcing scenarios such as RCP8.5 (medium confidence).
- Long-term droughts: Low confidence in projections of changes in the frequency and duration of megadroughts.
- Monsoonal circulation: Low confidence in projections of a collapse in monsoon circulations.
So: the worst of the potential horror stories is the possible disappearance of the summer arctic ice, an outcome that IPCC now views only as “likely” with “medium confidence,” and only under an RCP (representative concentration pathway) of 8.5 watts per square meter in 2100, the highest of four World Meteorological Organization radiative forcing assumptions about changes in the balance between incoming and outgoing radiation caused by increases in greenhouse gas concentrations. IPCC notes as well that this potential effect would be “reversible within years to decades,” as opposed to centuries or millennia for some of the other possible effects.
How much poorer ought we to make ourselves so as to avoid these possibilities, even under the assumption that US policies would make a measurable difference? (It would not.) I ask; you decide.