Importance of Short-Lived Climate Pollutants

Many regions of the world suffer from accelerated climate change and severe air quality problems both. These include the Arctic, South Asia, parts of Africa, and many mountainous or densely populated coastal regions.  In the Arctic and Himalayas, warming is occurring at twice the global rate, resulting in record low sea ice extents for the last 6 years, especially in 2012. The Arctic Council estimates that global sea level is now estimated to rise by 0.9 to 1.6 meters over the next 90 years, more than three times what IPCC estimated in 2007; the IPCC raised its own estimate of this potential rise to 0.5-1.0 meters by 2100 if current warming continues. Water resources are threatened in regions depending on the run-off from glaciers. Glaciers are melting all around the world, including in the important watershed of the Himalayas, that serves over 3 million people with water, food, electricity and other resources.

Air quality is deteriorating in many developing countries because of particles from diesel and use of domestic woodstoves and cookstoves. Ozone levels and emissions of black carbon cause serious health problems and affect crop yields. Methane is an important global precursor to ozone formation, in addition to being a very potent greenhouse gas.

At the same time, the short-lived nature of these compounds means quick impacts from abating emissions, while both reducing warming and improving the air we breathe.         

Early Arctic and Himalayan Work

In the early 2000s, a number of researchers began looking at the role black carbon and tropospheric ozone were playing in Arctic and Himalayan warming and melting; and conversely, what benefits might be achieved from their reduction. Researchers gathered in New York in 2006 and Oslo in 2007 to discuss these findings; and in 2008, AMAP held the first official meeting focused exclusively on the role of these pollutants in Arctic climate change.  The Arctic Council published initial findings in 2009 and in May of that year the Council’s ministerial meeting and a related Norwegian initiative, Melting Ice, highlighted the role of black carbon, ozone, and methane globally at both the ministerial meeting and at the 15th Conference of Parties to the United Nations Framework Convention on Climate Change (COP-15). The Council commissioned AMAP and a special technical task force to continue exploring mitigation options. Additional reports were published at the Nuuk and Kiruna ministerial meetings in 2011 and May 2013, respectively.

At the same time, researchers involved in the Atmospheric Brown Cloud Programme (ABC), mainly sponsored by Swedish Sida with the United Nations Environment Programme (UNEP) as its Secretariat, began noting the potential regional climate co-benefits to the Himalayas of their work to cut pollution for health reasons in South Asia and South East Asia.  The ABC Programme, formally initiated in 2002, developed in phases and at a 2008 meeting in Kathmandu it firmly established the links between greenhouse gases, air pollution, and climate change. Countries such as India, China, and Nepal started long-term research programs on glaciology and related issues to study all aspects of Himalayan glaciers, with the impact of black carbon also a major topic of concern for the regional monsoon regime, water budgets, agricultural production, and human health.

Slowing Near-term Warming: The UNEP/WMO Assessment

In 2011, UNEP and the World Meteorological Organization (WMO) joined forces to produce the Integrated Assessment of Black Carbon and Tropospheric Ozone (the Assessment), a first effort to look at the potential to slow warming globally through reductions in particle pollution (black carbon) and ozone precursors. These pollutants, which previously had been considered as classic air pollutants only, also impact climate change in the near-term.  Methane, which is regulated under the Kyoto Protocol as a greenhouse gas, is considered increasingly relevant for air quality purposes due to its impact on ground-level (tropospheric) ozone.

The Assessment began with over 2000 measures (mostly technical ways to reduce these pollutants) and modeled their impacts on health, crop yields, and climate.  For a measure to be included in the Assessment, it needed to combine health-crop benefits with climate benefits, taking into account all emissions from that source. The first phase of the Assessment found that nearly 90 percent of the climate benefits came from just 16 of these measures. More detailed global modeling for those sixteen measures followed the initial screening process in a study involving over 150 authors and reviewers.

A number of policy actions have resulted in the years since. The Swedish government funded a UNEP Action Plan that included additional regional-level work on health and crop benefits, as well as regional applicability of the different measures. While not a full atmospheric modeling effort, it provided useful information on possible future mitigation priorities for Asia, Africa, and Latin America.

In February 2012, six nations founded the Climate and Clean Air Coalition to Reduce Short-lived Climate Pollutants,[1] headquartered at UNEP’s Paris office. This coalition includes 34 state and 368 non-state Partners. The Convention on Long-range Transboundary Air Pollution‘s Gothenburg Protocol in December 2012 agreed to a revision of the Protocol to include consideration of black carbon as a constituent of particle pollution, following the 2011 recommendations of a special black carbon working group.  At a meeting in New Delhi in January 2013, the BASIC Ministerial meeting agreed to begin a program on black carbon research and potential policy actions, coordinated by the Divecha Centre for Climate Change in Bangalore.

Neither the Assessment nor the Action Plan focused on cryosphere regions per se. The Assessment did, however, note one intriguing response based on latitude bands: a temperature decrease in the Arctic as a result of the mitigation measures was nearly twice that of the globe as a whole. Indeed, late in the evaluation stage of the Assessment, modelers were able to add one additional measure aimed primarily at a northern European source of black  carbon – replacement of wood logs by pellets in biomass stoves — and were surprised that this single measure led to about 15 percent greater cooling in modeling of the Arctic region– a full one-tenth of a degree, an extremely large result in global modeling terms (Figure 4, below).

This was the only measure modeled singly, and the Arctic was the only region characterized in the Assessment using the full suite of Assessment models: ECHAM, developed by the Max Planck Institute for Meteorology in Hamburg and run by the EU’s Joint Research Centre in Ispra, and GISS from NASA’s Goddard Institute in New York. Rapidly accelerating cryosphere changes  – including the increasingly-thin, shrinking, and unstable ice in the Arctic Ocean and Western Antarctic Ice Sheet – have highlighted the need for a cryosphere-specific study of the potential mitigation of black carbon, methane, and ozone both for the benefit of these regions and the globe.

Figure 4: Impact of SLCP Measures on Warming by Latitude from the UNEP/WMO Assessment (2011).

Why Short-lived Pollutants Have Greater Cryosphere Impact

As noted above, measures aimed at methane, ozone, and especially black carbon have a greater positive impact on slowing warming in the Earth’s cryosphere.  Much of this greater response arises from the greater impact of black carbon emissions over the highly reflective surface of ice and snow.  No source emits “pure” black carbon; instead, each source emits a different and complex mixture of pollutants that impact both health and climate. While the health impacts of these sources are well documented, uncertainty remains over the climate impacts globally, especially from biomass sources such as open burning and stoves. This is because biomass sources include more “light-colored” substances, especially organic carbon, which reflect sunlight and might therefore cool the atmosphere. Black carbon particles might also lead to greater cloud formation, and clouds also reflect sunlight.

However, black carbon researchers today have reached a general consensus that, over snow and ice, there is far less uncertainty about climate impacts from black carbon sources — even from biomass sources. This is because the cooling impacts from the lighter-colored organic carbon, sulfates, and clouds primary occur over a surface that is darker to begin with.  Such a “cooling” impact is absent over a surface that is already white and highly reflective, such as ice and snow. Thus, atmospheric scientists and modelers are more confident that measures aimed at black carbon from biomass burning (e.g., stoves for both cooking and domestic heating, field burning) will have a beneficial climate effect in high alpine and polar areas that have snow and ice cover as well as positive health impacts. These same biomass sources may also lead to higher ozone levels in the Arctic, especially, which could hasten springtime melt as ozone is a powerful short-lived climate forcer.

The ABC Programme, Arctic Council, and UNEP Assessment all pointed the way for a cryosphere-specific modeling study.  While this study – like the prior UNEP/WMO Assessment – models SLCP impacts everywhere, the focus in this report is on the cryosphere regions due to their significant regional and global impacts as well as the greater certainty attending black carbon climate impacts in these regions.

The window for cryosphere-focused action is brief.  Several studies of past warm eras have raised the risk of irreversible (within millennia timeframes) cryosphere processes that could commit us to feedbacks around sea-level rise, permafrost or albedo by mid-century within the next few decades if temperature rise in the cryosphere continues unabated.  Some cryosphere scientists – alarmed at the rapid and accelerating changes shown by their research – have begun to advocate for geo-engineering in the cryosphere and on larger scales, with extremely uncertain impacts to regional and global ecosystems. ICCI hopes instead to provide insights into more effective, sustainable, developmentally appropriate, and less uncertain mitigation options using the climate-air quality connection instead, before it becomes too late for many of these cryosphere regions to be preserved.

[1] Some hydroflourocarbons (HFCs) used in refrigeration are also very short-lived; they are included in CCAC initiatives as short-lived climate pollutants.  For near-term cryosphere benefits, they do not have the differentiated impact of black carbon over ice and snow, and their atmospheric concentrations today are relatively small. Thus their chief potential lies in avoiding future catastrophic warming by substituting other measures for cooling that do not have such negative climate impacts.