US and Swiss researchers have modeled two climate scenarios with very high carbon emissions—above the A1F1 scenario, currently representing the upper bound of predicted carbon emissions in the IPCC work—to investigate the resulting climate change implications. Their open access paper appears in IOP Publishing’s journal Environmental Research Letters.
...relatively little attention has been paid to the upper bound of the range of future emissions...no explicit scenario analyses, analogous to those exploring the lowest feasible emissions scenarios, have been carried out to test how high emissions might plausibly be in the future.
...There are reasons to believe that emissions higher than those in the SRES set are, if not likely, at least plausible.—Sanderson et al.
The researchers, from the National Centre for Atmospheric Research, Colorado, and the Institute for Atmospheric and Climate Science, Zurich, created two hypothetical high carbon emission scenarios and compared their effects to the existing emission scenarios. The scenarios take per capita energy demand from the A1F1 scenario, but assume slightly greater population growth and higher carbon intensity; one simulation assumes that fractional primary energy production remains at present day values while the other more extreme scenario assumes that all future energy demand is met by coal.
CurrentMix assumed that global energy behavior would remain constant but that the global population would rise to 11 billion by 2100. The increase in carbon emissions envisaged in the A1F1 scenario would be doubled by the end of the century.
AllCoal was designed as a thought experiment to exceed all likely emissions for the remainder of the century. This scenario assumed that the global population would increase to 15 billion by 2100 and that demand for fuel sources would increase, with more demand placed on coal—the fuel with the highest amount of carbon per unit of energy. This would result in four times the increase in carbon emissions envisaged in the A1F1 scenario.
According to the researchers’ computer simulations, the major differences between each scenario would begin to materialize towards the end of the 21st century.
By 2100, the AllCoal and CurrentMix scenarios would produce a warming of more than 12 Kelvin (12 °C, 21.6 °F) in the Arctic regions, with global sea levels rising by 33 cm and 27 cm respectively due to the thermal expansion of the oceans.
The A1F1 scenario showed a 21 cm increase in sea levels; however the figures did not account for melting ice-sheets, which could increase sea levels by large amounts. The AllCoal scenario projected a complete loss of summer Arctic sea ice by 2070.
Each of the scenarios showed the typical pattern of increased rainfall towards the poles and drying subtropics. For example, the AllCoal scenario showed a 30–80% precipitation reduction in Southern Europe, Central America, and Southern Australia as well as increases of 50–200% in the Arctic and Antarctic regions, Northern Canada and Siberia.
The increase in most regions’ maximum temperatures went up by a factor of two in the AllCoal scenario; however some regions showed a considerably larger increase. In particular, the maximum summer temperatures in Northern Europe increased by 6–7 K by 2100.
The team noted that the high emissions scenarios assumed that aerosol and sulfate forcing remains the same as that in A1F1. In reality, depending on the dominant energy source, there would likely be significant increases in aerosol concentrations in the two high emissions futures proposed, especially in the hypothetical AllCoal case. Higher sulfate concentrations would likely act to compensate to some of the greenhouse gas forcing, although the degree of compensation remains somewhat uncertain and model dependent, they said.
In 2010, global CO2 emissions were 96% of those found in the A1F1 scenario. The scenarios considered here are not intended to be politically, economically or technically plausible; we leave it to further study to construct self-consistent future high emission scenarios. However, the findings in this study serve to show that a future in which emissions continue exceed the full range of SRES scenarios would result in significant additional climate changes above those described in the simulations considered in the Fourth Assessment Report. Although these simulations reveal no significant nonlinearities in global climate feedbacks, the model used is not capable of simulating the biogeophysical elements of the Earth system which are the most likely candidates for large nonlinearities in response to increasing emissions. A next logical step is therefore to repeat experiments of this type using a next-generation model with a fully interactive carbon cycle.—Sanderson et al.
Benjamin M Sanderson et al. (2011) The response of the climate system to very high greenhouse gas emission scenarios. Environ. Res. Lett. 6 034005 doi: 10.1088/1748-9326/6/3/034005