Meeting the Climate Challenge of 2 Degrees Centigrade: A Study Using an Integrated Economics and Earth System Model (Invited Presentation)

The global average temperature has increased by almost 0.9 degC between 1880 and 2014. The evidence is now very compelling that most of this increase is due to human activity, especially activity that produces carbon dioxide and many other greenhouse gases (GHGs). There are now also compelling reasons, summarized by the IPCC 5th Assessment (2013), to regard a warming of about 2 degC between 1880 and 2100 as a threshold, above which the damages globally to human and natural systems due to climate change begin to become economically and ethically less and less tenable and damages above 3 degC warming are certainly untenable. But there is also an equally compelling case for sustaining economic growth in developed and developing nations. Also, the uncertainties in climate projections mean that we must think in terms of the probability, not certainty, of reaching these thresholds. We present results using the MIT Integrated Global System Model (http://globalchange.mit.edu ) exploring combinations of low and zero-emission technologies and their costs over time that achieve the 2 and 3 degC targets with at least 50% probability. The IGSM is well suited for this study. The IGSM has two coupled sub-models: the MIT Economic Projection and Policy Analysis (EPPA) model and the MIT Earth System Model, that simulate human activity and Earth System response to this activity respectively. The EPPA simulates the evolution of the major economic, demographic, trade, and technological processes involved in the production of GHG and aerosol emissions relevant to climate and air pollution at national and global levels. It has detailed considerations of all relevant energy, industrial, agricultural and transportation sectors and uses comprehensive regional data for production, consumption and trade. The EPPA emissions of all major GHGs covered by the Kyoto and Montreal Protocols, and primary and secondary air pollutant emissions that lead to radiatively active aerosols and ozone, are inputs into the MESM, an earth system model of intermediate complexity, which has coupled sub-models of atmospheric and oceanic dynamics, atmospheric chemistry, terrestrial and oceanic biogeochemistry, terrestrial bio-geophysics, and physics of the ocean and land cryosphere. For our study, detailed chemistry and radiative forcing calculations for each major GHG and aerosol are included in MESM. To address important climate and air pollution interactions, emissions in urban areas are first input into an urban-scale air chemistry module within MESM, while those outside urban areas are input into the global MESM model. We find that as the price imposed on emissions rises, the fraction of energy from low and zero emission technologies rises relative to fossil. Also as the carbon price increases, the energy use decreases due to higher energy efficiency. The current uncertainty in the response or sensitivity of the climate system to carbon dioxide and other greenhouse gases strongly affects the needed emission prices and the resultant scales of the changes in primary energy sources and efficiency in energy use over time. For a high climate response, the average global energy use is about 30% less, the fraction of energy production from renewables is about 40% more, and the required emissions price is about 3 times higher, compared to a low climate response. The competition among these low emission technologies is largely determined by their assumed relative costs, while the total cost is determined largely by the optimism about future technologies and policy instruments chosen to achieve the 2 or 3 degC target.