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Perhaps the earliest attempt to quantify the effects of CO2 on climate while taking depletion into account was the work by Pushker Kharecha and Jim Hansen who produced a paper titled "Implications of "peak oil" for atmospheric CO2 and climate". This study was published in 2008 but became available on line as a working paper in April 2007. In the first version of the paper, Kharecha and Hansen start from the premise that the CO2 concentration in the atmosphere should not be allowed to exceed 450 ppm; later on they arrived to the conclusion that the dangerous limit is more likely to be around 350 ppm. So, they examine several scenarios that involve policy measures to force the reduction of emissions. They find that, if no such measures are taken, CO2 concentrations might rise to near 600 ppm by the end of the century, mainly as the result of coal combustion. Oil and gas would peak before 2030 in most of the scenarios considered and would give only a minor contribution to the total of the emissions.
Shortly after the paper by Kharecha and Hansen, David Rutledge published a post on "The Oil Drum" website with the title "The coal question and climate change" (June 2007). Later on, in December 2008, Rutledge also presented his results as an invited talk at the fall meeting of the American Geophysical Society. Rutledge set up an approach that would be used again by other authors; that is, he started with an estimate of the available resources, from that he generated a production curve that involves "peaking" and then he calculated CO2 emissions in the atmosphere. Then, by means of the software package named "MAGICC," available from NCAR, Rutledge generates climate scenarios in terms of CO2 concentrations and atmospheric temperatures. The results are that geological constraints on coal production (what he calls "producer limited" profile) would limit CO2 concentrations to about 480 ppm even without policy measures to curb emissions. Under these conditions, temperatures might rise of approximately 1.6 deg. C. Rutledge concludes that "if we wish to reduce the temperature rise, we must bury the CO2 (assuming that it will not leak out for 1,000 years), or establish preserves for fossil fuels that prevent them from being produced."
Robert Brecha examined the question in his 2008 paper "Emission scenarios in the face of fossil-fuel peaking" . His approach is very similar to that of Rutledge. Brecha calculates a series of scenarios in terms of fossil fuel production - including oil, gas and coal - on the basis of reserve estimates and logistic production curves. No policy interventions are assumed. Subsequently, he estimates CO2 concentrations and atmospheric temperatures using the MAGICC software package. His conclusions are that the world energy production could peak from 2030 to 2050, depending on assumptions, causing CO2 emissions to peak as well. The CO2 accumulated in the atmosphere would continue to grow after the energy peak, but it would be slowly absorbed by the effect of the natural "sinks" of the ecosphere. By the end of the century, CO2 concentrations would stabilize in a range from ca. 480 to 580 ppm and temperatures could rise by 1-3 deg C. Brecha's results indicate - again - that geology, alone, may not be sufficient to stop anthropogenic global warming from reaching dangerous levels.
Luis De Sousa and Euan Mearns (2008) took an approach similar to the papers by Brecha and Rutledge, but arrived at somewhat different conclusions in terms of policy recommendations. For estimating emissions, they use a model that they developed earlier and that they term "Olduvai Revisited" (2008). The model is based on resource estimates and forecasts which assume "bell shaped" behavior of the production curves. They find a global peak for fossil fuel production by 2018. Using the MAGICC software package, they find that, for this scenario, CO2 concentrations should not rise over 450-500 ppm and that temperatures should not rise over 1 deg. C. De Sousa and Mearns conclude that fossil fuel decline will keep CO2 concentrations below levels that are or were considered dangerous by climate experts and that there is no need to burden the OECD and non-OECD countries with artificial measures to mitigate emissions to achieve this end.
The latest entry on this subject published in a scientific journal is a paper by Willem Nel and Christopher Cooper which appeared in "Energy Policy" (2009) with the title "Implications of fossil fuel constraints on economic growth and global warming". The paper is very detailed and comprehensive in its estimates of fossil fuel reserves; it also includes estimates on the contribution of renewables, nuclear and unconventional fuels. The authors generate production scenarios based on logistic curves. CO2 concentrations and atmospheric temperatures are calculated by a detailed modeling approach. In what they call the Energy Reference Case, Nel and Cooper find a peak in the total world primary energy production that should take place around 2025. According to the authors' model, the peak will not slow the growth of the gross world product. It will keep growing for a couple of decades longer, peaking only around 2050. CO2 emissions are expected to peak with primary energy, that is around 2025. The calculated maximum CO2 concentrations don't exceed 500 ppm, except for the most pessimistic scenario, in which 550 ppm are reached. From these concentrations, Nel and Cooper calculate that the temperature increase prior to 2100 should not exceed 1°C. They conclude that this increase in temperature is not dangerous and that reaching these CO2 levels is preferable to facing the economic and social consequences of not fully exploiting the remaining fossil fuels.
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