Hansen et al 2006 Global Temperature Changes

Fig 6 shows that the 21st century migration distance

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Unformatted text preview: ange. Some species are less mobile than others, and ecosystems involve interactions among species, so such rates of climate change, along with habitat loss and fragmentation, new invasive species, and other stresses are expected to have severe impact on species survival (37). The total distance of isotherm migration, as well as migration rate, affects species survival. Extinction is likely if the migration distance exceeds the size of the natural habitat or remaining habitat fragment. Fig. 6 shows that the 21st century migration distance for a BAU scenario ( 600 km) greatly exceeds the average migration distance for the AS ( 100 km). It has been estimated (38) that a BAU global warming of 3°C over the 21st century could eliminate a majority ( 60%) of species on the planet. That projection is not inconsistent with mid-century BAU effects in another study (37) or scenario sensitivity of stress effects (35). Moreover, in the Earth’s history several mass extinctions of 50–90% of species have accompanied global temperature changes of 5°C (39). We infer that even AS climate change, which would slow warming to 0.1°C per decade over the century, would contribute to species loss that is already occurring due to a variety of stresses. However, species loss under BAU has the potential to be truly disastrous, conceivably with a majority of today’s plants and animals headed toward extermination. Discussion The pattern of global warming (Fig. 1B) has assumed expected characteristics, with high latitude amplification and larger warming over land than over ocean, as GHGs have become the dominant climate forcing in recent decades. This pattern results mainly from the ice–snow albedo feedback and the response times of ocean and land. Hansen et al. In assessing the level of global warming that constitutes DAI, we must bear in mind that estimated climate sensitivity of 3 1°C for doubled CO2, based mainly on paleoclimate data but consistent with models, refers to a case in which sea ice, snow, water vapor, and clouds are included as feedbacks, but ice sheet area, vegetation cover, and non-H2O GHGs are treated as forcings or fixed boundary conditions. On long time scales, and as the present global warming increases, these latter quantities can change and thus they need to be included as feedbacks. Indeed, climate becomes very sensitive on the ice-age time scale, as feedbacks, specifically ice sheet area and GHGs, account for practically the entire global temperature change (17). Vegetation cover is already expanding poleward in the Northern Hemisphere causing a positive climate feedback (42). Global warming could result in release of large amounts of GHGs, e.g., from melting permafrost or destabilized methane clathrates on continental shelves (43). Some of the largest warmings in the Earth’s history and mass extinctions may be associated with such GHG releases (39, 43). Although such disastrous GHG releases may require many centuries, our ignorance of GHG climate feedbacks demands caution in estimating requirements to avoid DAI. The AS is based on the rationale that positive feedba...
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