Black carbon and tropospheric ozone are harmful air pollutants that also contribute to climate change. Reducing black carbon and tropospheric ozone now will slow the rate of climate change within the first half of this century, the study said. Climate benefits from reduced ozone are achieved by reducing emissions of some of its precursors, especially methane which is also a powerful greenhouse gas. These short-lived climate forcers—methane, black carbon and ozone—are fundamentally different from longer-lived greenhouse gases, remaining in the atmosphere for only a relatively short time.

The identified measures complement, but do not replace, anticipated carbon dioxide reduction measures. Major carbon dioxide reduction strategies mainly target the energy and large industrial sectors and therefore would not necessarily result in significant reductions in emissions of the black carbon or ozone precursors; methane and carbon monoxide. Significant reductions of black carbon and O₃, require a specific strategy, as emissions come from a large number of small sources.

Full implementation of these measures would reduce future global warming by 0.5 °C (within a likely range of 0.2-0.7 °C). If the measures are implemented by 2030, this could halve the potential increase in global temperature which is projected for 2050. The rate of regional temperature increase would also be reduced.

Both near-term and long-term strategies are essential, the report concludes. Reductions in near-term warming can be achieved by control of black carbon and O₃ whereas CO₂ emission reductions, beginning now, are required to limit long-term climate change. Implementing both reduction strategies is needed to improve the chances of keeping the Earth’s global mean temperature increase to within UNFCCC’s 2 °C target.

The implementation of the identified measures would have substantial benefits in the Arctic, the Himalayas, and other glaciated and snow-covered regions. These measures are all in use in different regions around the world. Much wider implementation is required to achieve the full benefits identified in the assessment.

The assessment team examined policy responses, developing an outlook to 2070 illustrating the benefits of political decisions made today and the risks of delaying action for climate change, human health and crop yields over the next decades. Placing a premium on sound science and analysis, the assessment was driven by four main policy-relevant questions:

• Which black carbon and O₃ control measures are likely to provide significant combined climate and air quality benefits?

• How much can the implementation of the identified measures reduce the rate of global mean temperature increase by mid-century?

• What are the multiple climate, health and crop-yield benefits (by region) of implementing the measures?

• By what mechanisms could the measures be rapidly implemented?

In order to answer these questions, the assessment team determined that new analyses were needed. The assessment therefore relies on published literature as much as possible and on new simulations by two independent climate-chemistry-aerosol models: one developed at the NASA-Goddard Institute of Space Studies (NASA-GISS) and another developed by the Max Plank Institute in Hamburg and implemented at the European Commission’s Joint Research Centre. The specific measures and emission estimates for use in developing this assessment were selected using the International Institute for Applied Systems Analysis’Greenhouse Gas and Air Pollution Interactions and Synergies (IIASA GAINS) model.

The Summary of this assessment for decision makers was presented at the 26^th session of the Governing Council / Global Ministerial Environment Forum (GC/GMEF) of UNEP from 21-24 February 2011 in Nairobi, Kenya.

Resources

• Summary for decision makers of the integrated assessment of black carbon and tropospheric ozone