Cutting Black Carbon and Methane Emissions Would Reduce Projected Global Warming by Half

A new United Nations Environment Program report [PDF] focuses on measures that would dramatically the emissions of black carbon and methane gas by 2030. The report finds:

Full implementation of the identified measures would reduce future global warming by 0.5 ?C (within a range of 0.2–0.7 C, Figure 1). If the measures were to be implemented by 2030, they could halve the potential increase in global temperature projected for 2050 compared to the Assessment's reference scenario based on current policies and energy and fuel projections. The rate of regional temperature increase would also be reduced.

Black carbon is composed of particulates emitted by unfiltered diesel engines, biomass-fueled cookstoves, and open-air burning of agricultural wastes. Black carbon heats the atmosphere and when it settles on snow and ice in places like the Himalayas and the Arctic promotes faster melting. Methane is a potent greenhouse gas itself (about 25 times the warming potential of carbon dioxide over 100 years) but also reacts with nitrogen oxides and carbon monoxide to produce ground-level ozone which is itself a potent greenhouse gas. The key here is that ozone and black carbon get washed out of the atmosphere fairly quickly, so if emissions are cut they disappear soon.

The report identifies various measures for cutting future emissions and notes the considerable health benefits that would result from such reductions, e.g., fewer deaths from respiratory illnesses. So far so good. I was eager to compare the costs with the benefits listed. However, I found nothing on costs until reaching near the end of the report:

This Assessment did not assess the cost-effectiveness of different identified measures or policy options under different national circumstances. Doing so would help to inform national air quality and climate policy makers, and support implementation on a wider scale. Further study and analyses of the local application of BC and tropospheric O3 reduction technologies, costs and regulatory approaches could contribute to advancing adoption of effective action at multiple levels.

Well, yes.

The report did offer the intriguing observation that some regions of the globe are already projected to reduce some emissions:

The regional BC emission trends, therefore, vary significantly, with emissions expected to decrease in North America and Europe, Latin America and the Caribbean, and in Northeast Asia, Southeast Asia and the Pacific, and to increase in Africa and South, West and Central Asia.

Although contested, there is considerable evidence that pollution initially worsens but begins to decline as incomes pass various per capita income thresholds. This is the result of a combination of economic growth, technological progress, and increased regulation of the atmospheric commons.

Consider for example the trends for particulates (black carbon) and ozone in the United States. The Environmental Protection agency reports that ambient ozone is down by 30 percent since 1980 and particulates have been cut by 38 percent since 1990. Between 1980 and 2010, U.S. per capita real GDP grew from $25,000 to $42,000 per year.