Impacts of Carbon Emissions Scenarios

Michael Oppenheimer and colleague’s work has centered on analyzing the impacts of various CO2 emissions trajectories. By estimating the atmospheric CO2 concentrations likely to cause particular climate impacts, a study published early in the grant allowed climate-related damages from coral bleaching, disintegration of the West Antarctic ice sheet, and shutdown of the ocean’s thermohaline circulation to be linked to particular emissions scenarios. These ties sharpened the carbon-climate debate by illustrating the connection between energy policies and their environmental consequences.

Last year, Oppenheimer and colleagues published results of a study on the environmental and social consequences of overshooting concentration targets, then lowering emissions to meet these targets at a later date. Oppenheimer and colleague Brian O’Neill compared the consequences of three scenarios for stabilization targets of 500 – 700 ppm: a slow, almost linear increase toward stabilization over 200 years; a delayed but faster increase toward the same stabilization value; and a path that overshoots the target stabilization concentration by 100 ppm by 2100, then declines toward the stabilization value in the next 100 years.

The results suggested that even if ultimate CO2 targets are set low to avoid crossing climate thresholds, temporary overshoots could cause climate and ecosystem damage expected at significantly higher stabilization CO2 levels. The researchers also showed that the differences in the rate of warming among these scenarios could lead to significant differences in environmental damage. This work suggested that delaying emissions reductions may have unexpected consequences not predicted in conventional simulations.

This year, Simon Donner, Michael Oppenheimer and colleagues completed related work on the global impacts of climate change on the bleaching of coral reefs and implications for climate policy. The global assessment confirmed early suspicions that most coral reef ecosystems worldwide could become degraded in 30-50 years without an accelerated effort to reduce greenhouse gas emissions. The results demonstrated, for the first time, that the temperature adaptation required by corals to avoid dangerously frequent bleaching events varies widely across the tropics (see Figure 14). There is little evidence that corals can match the rate of adaptation required in many parts of the Indo-Pacific and Caribbean in the next 30-50 years. The results could help guide the design of optimal climate policy and aide coral reef conservation efforts in the highly vulnerable regions like Micronesia.

Figure 14. Temperature adaptation required to avoid dangerously frequent bleaching (using the HadCM3 climate model forced by the SRES A2 emissions scenario).


Control of sulfur and carbon emissions

David Bradford was involved with two projects involving the economic impacts of controlling gases other than CO2. In one study, Bradford and colleagues studied the outcome of including both health and climate damages related to sulfate aerosols in the DICE model. Reducing fossil fuel combustion to limit the health damages of sulfur emissions simultaneously reduces carbon emissions, but also acts to warm the atmosphere due to a decline in sulfate aerosols.

The authors carried out multiple simulations designed to quantify the economic and climate impacts of sulfur emission control. They found that in a scenario that takes both health and climate damages into account, economically optimal carbon emissions decrease by ~16% relative to a scenario considering climate damages alone. The accompanying decrease in sulfur aerosols led to a net warming of the climate by about 0.2 ºC in the short-term. In the long-term, however, the lower carbon emissions led to lower temperatures than in a business-as-usual scenario, making a health-based strategy a promising motivator for early action on CO2 emissions control.


Ozone production and NOx controls

The group has also done considerable work on damages from ozone and policies for nitrogen oxide emissions control. One study done by David Bradford and colleagues indicated that, for the same NOx emission, ozone produced can vary by more than a factor of three. In addition, the simulations showed that the variation in health damages caused by ozone depends strongly on the size of the exposed population. Combining both effects results in a factor of six difference in resulting mortalities for identical quantities of NOx emitted.

The work implies that although the cap-and-trade approach has been successful in reducing total NOx emissions, because it does not control for the location or time during the summer that emissions take place, nor for the resulting damages, it is less successful at minimizing the damages that result from emissions permitted under the cap. The group’s simulations indicate that total damage might be more effectively reduced by providing incentives to reduce NOx emissions at times and in locations where health damages are greatest.

This year, Vaishali Naik and Michael Oppenheimer examined the linkages between air pollution and climate change as a first step to evaluate the feasibility of mitigating tropospheric ozone for climate change benefits. Neither tropospheric ozone nor its short-lived precursors are directly regulated in a climate mitigation agreement, although ozone, an air pollutant, is the third most important anthropogenic greenhouse gas.

Using global three-dimensional atmospheric chemistry and climate models, the researchers showed that the global ozone distribution and its radiative forcing are most sensitive to changes in NOx emissions from tropical regions and least sensitive to changes from mid- and high-latitudes (Figure 15B). The study also concluded that simultaneous reductions of all short-lived precursors (CO, VOCs, and NOx) are necessary for net reduction in global radiative forcing from ozone (Figure 15A). Naik and Oppenheimer are following up on this work by examining the sensitivity of global ozone and aerosol distribution, and the resulting radiative forcing, specifically to the location of biomass burning.

Figure 15. Change in annual (A) absolute radiative forcing and (B) normalized radiative forcing (∆F/∆ENOx ), due to changes in ozone and methane resulting from a 10% reduction in surface anthropogenic NOx emissions from each of the nine regions and a combined 10% reduction in anthropogenic NOx, CO, and VOC emissions (three bars on the right).