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A brief history of short-lived climate pollutants in environmental science and governance

By Niklas Löther, Research Trainee

Interest in climate forcers other than CO2 is on the rise. The IPCC’s latest assessment report dedicates a full chapter to such substances, including gases like methane and HFCs but also aerosols like black carbon. Methane in particular then took centre stage at last year’s COP26, where over 100 states joined a pledge to reduce its global emissions by 30% in the next decade. What unites these forcers and makes their mitigation an attractive complement to ongoing decarbonisation efforts is their relatively short atmospheric lifetimes: For example, while CO2 can last centuries in the atmosphere, methane only stays there for about 12 years and aerosols tend to rain back to the ground after just days – so targeting these substances can yield palpable results even in the near-term. This is why they are generally grouped together under the label of short-lived climate pollutants (SLCPs), even though they are otherwise very different from one another. Furthermore, many SLCPs also have negative effects on human health through contributing to air pollution. As climate change is accelerating and air pollution remains a leading cause of premature death in many countries, it is unsurprising that these pollutants are now gaining increasing attention.

However, this is by no means the first time SLCPs have been in focus. Indeed, attention towards them has come in several waves, peaking every few years but then subsiding again. Such intermittent attention has yielded significant scientific progress and some positive political and legal initiatives, but the resultant governance landscape is as uneven as this history would suggest: Most SLCPs are not subject to any specific rules on the global level and are rather governed through regional and/or non-binding instruments. While a more unified approach to SLCPs would make little sense, given the differences in their respective sources and basic chemistry, this degree of fragmentation is still undesirable as it leaves many gaps in geographical and substantive coverage and adds to the overall ambition gap between states’ climate goals and current emissions pathways. According to the IPCC, it will be impossible to limit warming to 1.5°C without more significant action against short-lived forcers than is taking place at the moment. Furthermore, a more ambitious and global approach to SLCPs could also prevent over 3.5 million premature deaths each year.

The current moment, most notably through the Global Methane Pledge, may be seen as the third such wave of attention towards SLCPs. This article goes through these three waves to put recent achievements into a longer-term historical context and assess how far we have actually come in governing SLCPs, as well as what still remains to be done in this field. It is based on research for a Master’s thesis written for the ClimaSlow Project and available in full on UEF’s e-repository.

1. A First Wave of SLCP Science in the 1990s

Interest in the substances now called SLCPs can be traced back to two scientific discoveries around the turn of the millennium, before which climate policy was essentially seen as synonymous with CO2 mitigation. The first was made in the Indian Ocean Experiment, a large-scale, multi-year study of air pollution over South Asia conducted in the late 1990s. Unlike previous studies, this one measured pollution not only over inhabited areas on the Indian subcontinent, but also over the sea around it, and it examined not only the impacts on human health, but also on the climate – making this by far the most comprehensive examination of air pollution at the time. Its shocking results showed that pollutants spread far wider and accumulate in higher concentrations than had previously been anticipated, to such an extent that they seasonally create “brown clouds” covering the entire region – with severe effects for human health. Additionally, these clouds’ dark colour was found to absorb sunlight and thus cool the surface below, decreasing agricultural productivity, at the same time as heating the atmosphere above and measurably contributing to global warming. The main culprit behind both these negative consequences was black carbon or, as it is commonly called, soot. A result of inefficient burning processes, soot was then thought to be a problem of the past, but Asia’s population boom at the time – and an associated rise in agricultural burning and the use of traditional cookstoves – created the sort of pollution there not seen since the industrial age in the West. Hence, the Indian Ocean Experiment thrust black carbon back onto the global agenda, where it remains today as one of the substances we now call SLCPs.

The second significant discovery was made around the same time but on the other end of the world, in Greenland. Scientists there were analysing microscopic air bubbles trapped in the island’s ancient ice sheet to examine how the atmosphere’s composure and temperature has changed over the last millennia. As anticipated, they found that historic CO2 levels closely correlated with climactic changes, but they also discovered something unexpected: The same was true for methane. While this latter gas was already known to be a climate forcer, conventional wisdom at the time was that its effects would be negligible compared to those of CO2 as its atmospheric concentrations ­are much lower – but this thinking now had to be re-evaluated. Furthermore, like black carbon, methane is also associated with detrimental health effects because it contributes to the generation of harmful tropospheric ozone.

By the year 2000, it was therefore clear that climate change is not solely a consequence of carbon emissions, but also related to these more short-lived pollutants. However, the international community did not yet act against SLCPs, as climate politics were highly divisive at the time: Negotiations for the Kyoto Protocol had just concluded, but the US were refusing to ratify it on the basis that it did not impose substantive obligations on developing countries. In this context, some developing countries portrayed the new science into short-lived forcers as a ploy by the West to distract from its own historic responsibility for emissions of long-lived CO2 and temporarily blocked plans for a global assessment of the issue. The only international forum that actively kept working on SLCPs was consequently the Arctic Council, where countries of the global South had no influence, but which was also limited in the effect it could have by its regional nature. Hence, the first wave of global interest in SLCPs quickly subsided.

2. A Second Wave of SLCP Science and Governance Initiatives post-2009

It took until 2009 for the international community to give SLCP governance another chance: Climate negotiations in Copenhagen that year had failed to produce a much-anticipated new global treaty, leaving many frustrated with traditional climate policy and looking for alternatives. Mitigating SLCPs seemed a particularly attractive option since it can slow near-term warming to buy more time for global negotiations to fix the longer-term issue of CO2. In the meanwhile, the Arctic Council’s research had provided further evidence for this and its pronouncements helpfully cautioned that action against SLCPs must be a complement to wider decarbonisation efforts and not replace them, thereby counteracting the argument that they are just a distraction from CO2. Previous controversy was thus put aside in this moment and climate pollutants were back on the political agenda.

Additionally, yet another SLCP was discovered to significantly impact the climate around this time: Hydrofluorocarbons (HFCs). These industrial chemicals were devised as chlorofluorocarbon-replacements that do not deplete the ozone layer and became much more widely used during the 2000s. However, while they did help to solve the problem of ozone depletion, they were also highly potent greenhouse gases, with a lifetime of just a few years, but an extremely high global warming potential. The speed at which they were adopted by industries across the world surprised many and raised the prospect that HFC-emissions may reverse any progress the world had made in mitigating climate change up to that point within just a few years. This served as another wake-up call and illustration of the devastating effects even short-lived forcers can have.

The following years were consequently the most productive in global SLCP governance so far: Several large-scale scientific assessments were conducted to inform policy-makers about black carbon, methane, and HFCs, ultimately culminating in the foundation of the Climate and Clean Air Coalition (CCAC), a public-private partnership dedicated to promoting these substances’ mitigation. Initiated by the US in 2011 and since joined by over 70 states, as well as NGOs, research institutes, and corporations, the Coalition launched several sectoral programmes to reduce SLCP emissions for example in energy production or residential heating. Furthermore, the Arctic Council’s work intensified through the establishment of a framework voluntarily committing its members to reporting their black carbon and methane emissions and develop strategies for reducing them, all aided by a regular expert review, and notably endorsed by some observer states like China and India as well. Legal changes also occurred at this time: The Gothenburg Protocol, which sets pollution reduction targets for much of the Northern hemisphere, was amended in 2012 to specify that black carbon should receive special attention in its implementation and, in 2016, HFCs were added to the Montreal Protocol as a substance to be phased-down in the Kigali Amendment. Both amendments have since entered into force, but only Kigali enjoys near-global ratification, with the revised Gothenburg Protocol mostly limited to North America and Western Europe in coverage.

Unfortunately, the HFC phase-down still arguably marks the high-point of global SLCP governance and has not been followed by any other substantive legal changes. After 2016, there was a lack of leadership on SLCPs, with no powerful state expending significant political capital on further promoting their mitigation internationally. While work under the CCAC and Gothenburg Protocol continue, as did that of the Arctic Council until Russia’s invasion of Ukraine, they would need to be upscaled significantly to adequately deal with the problem. Hence, the absence of international leadership ultimately ended this second wave of interest in climate pollutants, which again left much to be desired.

3. The Global Methane Pledge – A Third Wave of SLCP Governance?

The recently-launched Global Methane Pledge may signify the beginning of a new phase of more intense interest in SLCPs. As a joint initiative launched by the US and the EU, it marks the return of major economies to the SLCP agenda. At this point, over 120 other states have endorsed this voluntary pledge to reduce global methane emissions by 30% over the next decade. This initiative has been framed by its proponents as a revolutionary change that brings the 1.5°C target within reach.

However, the nature and efficacy of this commitment are already being questioned: Firstly, as previously argued on this blog, the Pledge’s relationship to the legal climate regime is vague: While it was announced at COP26 and should contribute to the Paris Agreement’s implementation, it remains unclear whether actions taken under this new initiative will feature in states’ official contributions to the treaty and thus be subjected to its transparency and review mechanisms. Secondly, it has also been noted that some major emitters have not signed on to the Pledge, with China, Russia, Iran, and Algeria (all among the top 10 methane emitting states) being notable absences. Furthermore, the entire continents of Africa and Asia are clearly under-represented in the initiative.

In light of these valid criticisms, the Global Methane Pledge looks less like a revolution in the governance of short-lived forcers, as its initiators framed it, and more like a continuation of the above-described history of piecemeal and very gradual change. Doubtlessly this new focus on methane is a positive development, but yet again it falls short of the ambition that would have been required, given the simultaneous climate and public health crises SLCPs significantly contribute to. Meanwhile, other short-lived forcers are increasingly fading into the background, even though they still cause millions of premature deaths each year and we are still on track to overshoot the 1.5°C goal. In this context, each significant source of air pollution and decimal degree of warming that can be avoided should be avoided, which in turn requires both broader and deeper changes in the governance of SLCPs than are currently happening.

4. Taking Stock and Moving Forwards

Overall, the history shows that the intermittent attention paid to SLCPs has resulted in a gradually expanding, but very uneven and fragmented governance landscape: The only SLCPs to be covered by clear and detailed rules on the global level are HFCs, while governance instruments focusing on black carbon and methane are generally more regional and/or less specific. Progress had been held back initially by controversy and more recently by a simple lack of leadership. Even while one SLCP, methane, has attracted significant attention once again in recent months, concrete actions so far continue the piecemeal and gradual approach of the past: The Global Pledge to reduce its emissions lacks both precision and geographical coverage in some key regions. Furthermore, other SLCPs are fading into the background once again, so this third wave of SLCP governance may already be slowing down.

Still, the SLCP agenda has proven to be remarkably sticky over time, always returning eventually, even after being neglected for several years. Its promise of slowing warming in the near-term while delivering health co-benefits appears to hold inherent appeal. Discussions around how to govern these substances are thus unlikely to be completely subside anytime soon. All this goes to show that, while a lot has been accomplished, even more work still remains to be done. If future initiatives are to address and avoid the shortcomings of past actions in this field, it is imperative to learn from this history.

Photo by Kirk Cameron on Unsplash

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