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Author(s): James Hansen, Makiko Sato, Gary Russell, Pushker Kharecha
Introduction by Kirk Smith:
Hansen’s latest is difficult to follow in detail because of the complex nature of the arguments, but is available as indicated. Much of the argument is based on the perhaps less controversial studies of historical climates, not on climate modeling of the future. Here I reproduce the conclusion only. Note that he does mention reaching intolerable conditions for working outside and other impacts that cannot be adapted to in the normal use of that word — potentially triggering what I have been calling the “Afterlife Conjecture” — and citing our good colleague Tord Kjellstrom, who has been talking about this issue for some years. Also, that on our world’s current path we would have burned sufficient fossil fuels in a century or less to have this happen
“Burning all fossil fuels would produce a different, practically uninhabitable, planet. Let us first consider a 12 W/m2 greenhouse forcing, which we simulated with 8×CO2. If non-CO2 greenhouse gases such as N2O and CH4 increase with global warming at the same rate as in the paleoclimate record and atmospheric chemistry simulations (Beerling et al., 2011), these other gases provide ~25 percent of the greenhouse forcing. The remaining 9 W/m2 forcing requires ~4.8×CO2, corresponding to fossil fuel emissions as much as ~10,000 GtC for a conservative assumption of a CO2 airborne fraction averaging one-third over the 1000 years following peak emission (Archer, 2005; Archer et al., 2009).
Our calculated global warming in this case is 16°C, with warming at the poles about 30°C. Calculated warming over land areas averages ~20°C. Such temperatures would eliminate grain production in almost all agricultural regions in the world (Hatfield et al., 2011). Increased stratospheric water vapor would diminish the stratospheric ozone layer (Anderson et al., 2012).
More ominously, global warming of that magnitude would make much of the planet uninhabitable by humans (Sherwood and Huber, 2010; McMichael and Dear, 2010). The human body generates about 100 W of metabolic heat that must be carried away to maintain a core body temperature near 37°C, which implies that sustained wet bulb temperatures above 35°C can result in lethal hyperthermia (Sherwood and Huber, 2010; Dewhirst et al., 2003). Today summer temperature varies widely over Earth’s surface, but wet bulb temperature is more narrowly confined by the effect of humidity, with the most common value ~26-27°C and highest ~31°C.
Warming of 10-12°C would put much of today’s world population in regions with wet bulb temperature above 35°C (Sherwood and Huber, 2010). Given the 20°C warming we find with 4.8×CO2, it is clear that such climate forcing would produce intolerable climatic conditions even if the true climate sensitivity is significantly less than the Russell sensitivity, or, if the Russell sensitivity is accurate, the CO2 amount needed to produce intolerable conditions for humans is less than 4.8×CO2. Note also that increased heat stress due to warming of the past few decades is already enough to affect health and workplace productivity at low latitudes, where the impact falls most heavily on low and middle-income countries (Kjellstrom et al., 2009).
Earth was 10-12°C warmer than today in the early Eocene and at the peak of the PETM (Fig. 4). How did mammals survive that warmth? Some mammals have higher internal temperatures than humans and there is evidence of evolution of surface-area-to-mass ratio to aid heat dissipation, e.g., transient dwarfing of mammals (Alroy et al., 2000) and even soil fauna (Smith et al., 2004) during the PETM warming. However, human-made warming will occur in a few centuries, as opposed to several millennia in the PETM, thus providing little opportunity for evolutionary dwarfism to alleviate impacts of global warming. We conclude that the large climate change from burning all fossil fuels would threaten the biological health and survival of humanity, making policies that rely substantially on adaptation inadequate.
Let us now verify that our assumed fossil fuel climate forcing of 9 W/m2 is feasible. If we assume that fossil fuel emissions increase 3% per year, typical of the past decade and of the entire period since 1950, cumulative fossil fuel emissions will reach 10,000 GtC in 118 years. However, with such large rapidly growing emissions the assumed 33% CO2 airborne fraction is surely too small. The airborne fraction, observed to have been 55% since 1950 (IPCC, 2007a), should increase because of well-known non-linearity in ocean chemistry and saturation of carbon sinks, implying that the airborne fraction probably will be closer to two-thirds rather than one-third, at least for a century or more. Thus the fossil fuel source required to yield 9 W/m2 forcing may be closer to 5,000 GtC, rather than 10,000 GtC.
Are there sufficient fossil fuel reserves to yield 5,000-10,000 GtC? Recent updates of potential reserves (GEA, 2012), including unconventional fossil fuels (such as tar sands, tar shale, and hydrofracking-derived shale gas) in addition to conventional oil, gas and coal, suggest that 5×CO2 (1400 ppm) is indeed feasible. For instance, using the emission factor for coal from IPCC (2007b), coal resources given by GEA (2012) amount to 7,300-11,000 GtC. Similarly, using emission factors from IPCC (2007b), total recoverable fossil energy reserves and resources estimated by GEA (2012) are ~15,000 GtC. This does not include large “additional occurrences” listed in Ch.7 of GEA (2012). Thus, for a multi-centennial CO2 airborne fraction between one-third to two-thirds, as discussed above, there are more than enough available fossil fuels to cause a forcing of 9 W/m2 sustained for centuries.
Most remaining fossil fuel carbon is in coal and unconventional oil and gas. Thus, it seems, humanity stands at a fork in the road. As conventional oil and gas are depleted, will we move to carbon-free energy and efficiency — or to unconventional fossil fuels and coal? If fossil fuels were made to pay their costs to society, costs of pollution and climate change, carbon-free alternatives might supplant fossil fuels over a period of decades. However, if governments force the public to bear the external costs and even subsidize fossil fuels, carbon emissions are likely to continue to grow, with deleterious consequences for young people and future generations.
It seems implausible that humanity will not alter its energy course as consequences of burning all fossil fuels become clearer Yet strong evidence about the dangers of human-made climate change have so far had little effect. Whether governments continue to be so foolhardy as to allow or encourage development of all fossil fuels may determine the fate of humanity.”