by Makari Krause
Fossil fuels, while a large part of our energy production, are not a renewable resource and will eventually be depleted. Current climate models, such as the ones used by the IPCC, use levels of future fossil fuel production that Hook and Tang (2013) think are improbable. While fossil fuel combustion currently causes a large part of anthropogenic greenhouse gas emissions, these emissions are linked to fossil fuel production and will decrease as we begin to run out of these resources. There is a multitude of different scenarios that predict future fossil fuel emission and they range hugely in their predictions. The IPCC uses a set of six scenarios called the Special Report of Emission Scenarios (SRES), which are an input for many of their aggregated climate models and influence their conclusions. Hook and Tang question the accuracy of these SRES and aim to review the assumptions that the scenarios make about fossil fuel availability.
Hook and Tang analyzed the scenarios in the SRES that took into account economic activity per capita, energy intensity, carbon intensity, and population as the main drivers of CO2 emissions and concluded that there were a number of inherent problems. The first problem is that the scenarios only break the globe into four regions: the US and Western Europe, Latin American and Africa, Asia, and countries undergoing reform. This does not allow for detailed socioeconomic and energy data for specific countries but rather for huge regions. The scenarios also do not come with likelihoods and the IPCC claims that they are all equally likely alternative futures. This triggered criticism from the outset because it makes it extremely difficult for engineers and other planners to plan for the future. The reason claimed by the SRES team for the absence of likelihoods is that social systems are based on choices people make, and it is therefore impossible to assign any likelihoods. Unlike scientific systems, the factors that influence social systems are impossible to fully determine. Hook and Tang see the omission of the depletion of fossil fuels as the largest problem with these scenarios and are frustrated that this issue has not been discussed much in the modeling community and is not used to assess future anthropogenic green house gas emissions.
Fossil fuels take millions of years to accumulate and we are extracting them at a rapid rate. This clearly leads to the conclusion that the resource is finite and no matter how fast technology and extraction techniques advance we will eventually run out. Hook and Tang state that by 2100 most of the reserves of oil gas and coal will be depleted and that this depletion is not taken into account by any of the SRES scenarios. Not one of the 40 SRES predicts that we will reach a point in the future with significantly less fossil fuel use than the current level. Hook and Tang cite a number of other studies that criticize the SRES models for being overly optimistic about fossil fuel availability and even go as far as to say that many countries have already passed their peak production levels. Consequently, they think the SRES models are outdated and result in questionable models for the impact of anthropogenic green house gas emissions on climate change.
Existing production of conventional oil is declining and if the hope is to increase or even just maintain fossil fuel production, new sources will need to be added and there will need to be a large increase in unconventional fossil fuel extraction. Hook and Tang estimate that unconventional oil production will have to increase by 10% annually in order to keep up with the decline in conventional oil production. While the reservoirs of oil that can be accessed using unconventional methods may be large, the rate at which the oil can be extracted is a limiting factor and unconventional production probably will not be able to keep up with the decline in conventional production.
A limited number of studies have been done that take depletion into account and they all come out with atmospheric CO2 concentrations much lower than those predicted by the IPCC. In general they range from 450–650 ppm by 2100 and expect that peak fossil fuel production will occur sometime before then.
In conclusion, Hook and Tang claim that the SRES scenarios are unrealistic and use overly optimistic predictions for future fossil fuel production. Some of the scenarios do not correspond to observed values over the past few years and should be thrown out because of the mismatch. They also criticize the scenarios for lacking details and being too broad in their categories. With these unrealistic scenarios being used as inputs to the IPCC models Hook and Tang question the accuracy of the aggregated models and ask that there be additional studies conducted on the topic of depletion of fossil fuels when it comes to creating these scenarios. A new set of scenarios is currently being designed but it will be a while before they are implemented.
Höök, M., & Tang, X., 2013. Depletion of fossil fuels and anthropogenic climate change—A review. Energy Policy 52, 797–809. Full paper at: http://bit.ly/1s0c7hm