14.6 The fat tail of anthropogenic CO2 adjustment time

Fig. 13.11 Projected Arctic sea ice decline

Model simulations (continuous colored curves), and observations (black curve) of Arctic sea ice extent for September (1935–2090). Colored curves for RCP scenarios are model averages (CMIP5). After Walsh et al. (2014). Brown dashed curve is a model based on the known 60 and 20-year periodicities in Arctic sea ice. Black continuous curve is NSIDC September Arctic sea ice extent for the satellite window (1979–2021), while 1935–1978 September Arctic sea ice extent data is from Cea Pirón & Cano Pasalodos (2016) reconstruction. Dark red dashed curve is a sigmoid survival curve fitted to 1979–2012 data with ice-free conditions near 2030, following the Arctic seaice death spiral proposed by Mark Serreze (2008). The conservative projection (brown dashed curve), explains the pause in Arctic sea-ice melting since 2007 and suggests over 2 million km2 of Arctic sea ice remaining by summer 2100.

further reduce Arctic sea ice that could be down to c. 2.5 million km2 of summer sea ice (table 13.1) by 2100. With such low levels it cannot be ruled out that some summer might see an ice-free condition (< 1 million km2). This forecast is not too far from the IPCC RCP 4.5 projection (Fig. 13.11; table 13.1), probably because the cryosphere (except Antarctica) is showing a strong response to the increase in temperature and soot (light absorbing particles) levels.

The conservative forecast however is in stark contrast to the many alarmist projections from polar scientists that believe Arctic sea ice is past a tipping point and only accelerated rates of melting are possible now. Those projections that see an Arctic free of ice every summer before 2100 are very likely to be wrong. Lack of significant melting progress for the next decade and a half might clarify the issue.

13.8 Consequences for sea-level rise

In 2007 the IPCC made public its Fourth Assessment Report (AR4). Among AR4 emissions scenarios was B1, that contemplates slow growth in CO2 emissions to 2050 followed by moderate decrease in emissions afterwards. This scenario is the one that best agrees with the conservative projection outlined above, and projects a 300 mm increase in sea levels for 2000–2100 (central estimate; Fig. 13.12). Seven years later the IPCC published its Fifth Assessment Report (AR5), and among the new scenarios RCP 4.5 is the most similar to B1. However, the IPCC sealevel model is now a lot more aggressive and projects 525 mm for similar emissions (table 2). Such a strong upward revision responded to claims that models used in the 4th report substantially underestimated the observed past sea-level rise, although no acceleration has been observed since 1993. Despite the 60% increase due to a change of assumptions, the IPCC was severely criticized for producing estimates of sea-level rise that were too conservative. To provide a view that satisfied the consensus, Horton et al. (2014) conducted an expert elicitation (poll) on sealevel rise among authors of articles related to sea-level rise. Although they were only asked for a low and high scenarios, a mean projection can be obtained by averaging both (Fig. 13.12). This intermediate scenario derived from Horton et al. (2014) projects a rise of c. 800 mm for 2000–2100. In 2017 NOAA published their updated global sealevel rise scenarios where the intermediate scenario, that is most consistent with RCP 4.5, forecasts one meter of sealevel rise for 2000–2100 (Sweet et al. 2017; Fig. 13.12). Surprisingly, and despite lack of acceleration in sea-level rise since 1993 (28 years), projections are becoming significantly more pessimistic with time. It reminds of Mark Twain's famous quote: “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact” (Life in the Mississippi, 1883).

Past sea-level increases for the last 70 years have taken place under rapidly increasing emissions. However,