Climate sensitivity constrained by temperature reconstructions over the past seven centuries

Abstract

The magnitude and impact of future global warming depends on the sensitivity of the climate system to changes in greenhouse gas concentrations. The commonly accepted range for the equilibrium global mean temperature change in response to a doubling of the atmospheric carbon dioxide concentration, termed climate sensitivity, is 1.5-4.5 K (ref. 2). A number of observational studies, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 K to above 9 K (refs 3-8). Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. We use large-ensemble energy balance modelling and simulate the temperature response to past solar, volcanic and greenhouse gas forcing to determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5-95 per cent range shrinks to 1.5-6.2 K, thus substantially reducing the probability of very high climate sensitivity.

Support for global climate reorganization during the “Medieval Climate Anomaly”

“…It is plausible that this cooling increased the sensitivity of regional (non-local) Nordic Sea ice cover, allowing abrupt transitions towards much reduced (increased) ice extent near the onset (termination of the MCA) producing the sharp changes in North Iceland SSTs. Possible triggers for such transitions include (i) changes in surface irradiance, with relatively high irradiance during the MCA (high solar and low volcanic forcing during the MCA) and sharply lower irradiance (low solar, high volcanic forcing) at the onset of the LIA (Crowley 2000;Hegerl et al 2006;Ammann et al 2007; see also Anderson et al 2008), (ii) changes in oceanic heat transport and surface fluxes related to the varying strength of the southwesterly flow across the high latitude North Atlantic (stronger NAO during the MCA, weaker NAO during the LIA; e.g., Lamb 1965;Trouet et al 2009), (iii) AMOC variability (oceanic heat transport; Sicre et al 2008a), and (iv) the long term downward trend in summer SST noted earlier (Sicre et al 2008b). Such an inverse relation between the NAO and ice extent in the Nordic Sea is apparent the modern record (Deser et al 2000;Visbeck et al 2003).…”

Global warming in Amazonia: impacts and Mitigation

“…The problem is that this "most probable" value for climate sensitivity implies that there is a 50% chance that the real value is higher than this, and it could be a lot higher. The point at which there is 95% assurance that the real value is included is now believed to be about 6.2 o C according to a reassessment in 2006, down from a previous assessment that was substantially higher (Hegerl et al, 2006). There needs to be much more scientific work on constraining the estimates of climate sensitivity, and much more policy work on the implications of changes at the high end of the range.…”

Desmatamento na Amazônia: dinâmica, impactos e controle

“…Os resultados de Stainforth et al (2005) indicando aumento de 14 o C na Amazônia até aproximadamente 2070 sob alta sensitividade climática está defasada devido a revisões para baixo das probabilidades de valores muito altos para a sensitividade climática (Hegerl et al, 2006). Presumindo proporcionalidade, o aumento de 14 o C seria alcançada 30 anos mais tarde em 2100.…”