The 8th Emissions Gap Report concludes that the world will not meet the Paris Agreement target to keep warming below 2ᵒ Celsius (2ᵒC) unless there is an urgent acceleration of short term action and increased longer term national ambition to reduce all climate forcing emissions.
The report, for the first time, acknowledges the potential contribution short-lived climate pollutants (SLCPs) such as methane, hydrofluorocarbons, and black carbon (soot), can play saying: “Reductions of SLCPs limit the rate of short-term warming and when sustained and combined with carbon dioxide (CO2) also help limit long-term warming”.
Short-lived climate pollutants are powerful climate forcers with global warming potentials many times that of CO2 and, because they have relatively short lifespans in the atmosphere, reducing their emissions have the potential to decrease the rate of warming in the next few decades. Reducing CO2 and other long-lived greenhouse gases reduces warming more slowly.
Chapter six of the report focuses on SLCPs potential role at limiting warming noting that they are an important part of mitigation efforts in virtually all scenarios that meet the 2ᵒC target, and especially those that meet the 1.5˚C target.
Over the period 2018-2050, stringent SLCP reductions based on existing, demonstrated technical measures could reduce warming by between 0.3 and 0.9 ᵒC relative to current emissions projections. Roughly half of the mitigation potential is associated with methane, one-third with black carbon, and the remainder with hydrofluorocarbons. As some policies that reduce carbon dioxide emissions also reduce SLCP emissions, a substantial portion of SLCP reductions could be achieved through carbon-dioxide mitigation efforts.
There are also other reasons to act quickly to reduce SLCPs. Black carbon and methane (because it is an important ingredient in the formation of ground level ozone) contribute to air pollution that is responsible for up to 6.5 million premature deaths and up to 52 million tonnes of crop losses each year. Reducing these pollutants would have significant health and environment benefits.
Fast action to reduce SLCPs would also help reduce climate change impacts caused by cumulative heat uptake (for example, sea-level rise, and glacier and ice sheet melting) and reduce the likelihood of passing irreversible temperature thresholds and triggering large positive feedbacks. Reducing black carbon may play an important role in mitigating regional impacts of climate change.
Steps to reduce these pollutants will also contribute significantly to the achievement of several of the United Nations Sustainable Development Goals (SDGs). These include: SDG 2 on sustainable agriculture, SDG 3 on health, SDG 7 on energy, SDG 11 on inclusive and sustainable cities, and SDG 12 on sustainable production and consumption.
The chapter’s authors note that there is significant potential to reduce SLCPs using available and proven technologies but in order to unlock that potential, dedicated policy action to strengthen legal frameworks and institutional capacities is required. An example of this potential is the Kigali Amendment to phase down hydrofluorocarbons (HFCs) under the Montreal Protocol. Agreed to in October, 2016, the Amendment has the potential to decrease HFC emissions by 61% and prevent up to 0.09˚C of warming by 2050. Technology alternatives to HFC cooling and refrigeration systems are also often much more energy efficient than the systems they replace, which provides additional CO2 and air quality benefits by reducing energy consumption.
Reductions in SLCP emissions cannot be considered equivalent to reductions in long-lived greenhouse gases, as many impacts are not directly proportional to global mean temperature change at a given point in time. For this reason, climate change mitigation policies need to consider these two classes of emissions separately.
The lead authors of Chapter 6: ‘Bridging the gap - The role of short-lived climate pollutants’ are Zbigniew Kilmont, Research Scholar at the International Institute for Applied Systems Analysis (IIASA) and Drew Shindell, Professor of Climate Science at Duke University and Chair of the Climate and Clean Air Coalition Scientific Advisory Panel.