In a report, EPA provides a stark contrast between business as usual, G8 only action and world-wide aciton. The data confirm the stark reality about the prospective role of the Developing World. What follows is (edited) from the report ( Economic Impacts of S.1733: The Clean Energy Jobs and American Power Act of 2009,October 23, 2009):
In previous analyses, EPA has looked at the impact of U.S. policy combined with the policies assumed for developed and developing countries on global greenhouse gas
concentrations. However, the assumptions used in earlier analyses for what policies other countries would adopt are not consistent with the recent G8/Major Economies Forum goal discussed above. EPA has now analyzed, using the MiniCAM and MAGICC models, how U.S. targets consistent with the President’s FY 2010 budget proposal (14% below 2005 in 2020, and 83% below 2005 in 2050)22 combined with international action consistent with the G8 agreement could affect global CO2e concentrations and temperatures.
EPA presents data on global CO2e concentrations through 2100 assuming a climate sensitivity (CS) of 3.0. with three scenarios. (The CS is the equilibrium temperature response to a doubling of CO2, and a CS of 3.0is deemed the “best estimate” by the IPCC.):
(1) Reference: no climate polices or measures adopted by any countries.
(2) G8 -International Assumptions: consistent with G8 agreement to reduce global emissions to 50% below 2005 levels by 2050. U.S. and other developed countries reduce emissions to 83% below 2005 levels by 2050, and developing countries cap emissions beginning in 2025, and return emissions to 26% below 2005 levels by 2050. All countries hold emissions targets constant after 2050.
(3) Developing Countries After 2050: US and developed countries same as G8 scenario. Developing countries adopt policy in 2050 holding emissions constant at 2050 levels.
In the reference scenario, CO2e concentrations in 2100 would rise to approximately 936 ppm.25 If the U.S. and other developing countries took action to reduce emissions to 83% below 2005 levels by 2050, and developing countries took no action until 2050, then CO2e concentrations in 2100 would rise to approximately 647 ppm. If the G8 goals are met, then CO2e concentrations would rise to approximately 485 ppm in 2100. It should be noted that CO2e concentrations are not stabilized in these scenarios. To prevent concentrations from continuing to rise after 2100, post-2100 GHG emissions would need to be further reduced. For example, stabilization of CO2e concentrations at 485 ppm would require net CO2e emissions to go to zero in the very long run after 2100.
Given the CO2e concentrations for the various scenarios, we can also calculate the observed change in global mean temperature (from pre-industrial time) in 2100 under different climate sensitivities. Assuming the G8 goals (reducing global emissions to 50% below 2005 by 2050) are met, warming in 2100 would be limited to no more than 2 degree Celsius (3.6 degrees Fahrenheit) above pre-industrial levels under a climate sensitivity of 3.0 or lower.
It should be noted that the temperature change in 2100 in this scenario is not stabilized, so the observed change in global mean temperature in 2100 is not equal to the equilibrium change in global mean temperature. There are two reasons for this. First, while the G8 international goals stabilize global GHG emissions at 50% below 2005 levels, CO2e concentrations and temperature are not stabilized. Determining an equilibrium temperature under any scenario requires additional assumptions about post¬2100 emissions. If emissions remain constant post-2100, CO2e concentrations will continue to rise. Equilibrium temperature would only be achieved after CO2e concentrations are in equilibrium. Second, the inertia in ocean temperatures causes the equilibrium global mean surface temperature change to lag behind the observed global mean surface temperature change by as much as 500 years. Even if CO2e concentrations in 2100 were stabilized, observed temperatures would continue to rise for centuries before the equilibrium were reached.
Continued GHG emissions reductions after 2100 could stabilize CO2e concentrations at the 485 ppm levels achieved in 2100 in the G8 scenario. In order to achieve an equilibrium temperature change of 2 degrees (assuming CS = 3.0), CO2e concentrations must be stabilized below 485 ppm, requiring continued abatement beyond the level needed to stabilize concentrations at 2100 levels. It would be possible to reduce CO2e concentrations after 2100 below 485 ppm by even further reducing GHG emissions in the next century. An ‘overshoot’ scenario such as this would further reduce the equilibrium temperature change, making it possible to achieve the 2 degrees C target even with a climate sensitivity of 3.0.