The importance of non-CO₂ greenhouse gases

The IPCC Synthesis Report states that reducing emissions of non-CO₂ agents can be an important element of global mitigation strategies. The non-CO₂ greenhouse gas symposium (NCCG7) shows how this could be possible.

Global warming is caused by several greenhouse gases. The focus of science and policy is mostly on carbon dioxide (CO₂). But there are more greenhouse gases. Non-CO₂ greenhouse gases are important contributors to radiative forcing of the atmosphere and include methane (CH₄), fluorocarbons and SF₆ and nitrous oxide (N₂O). The effects of black carbon (BC) and the so-called indirect gases, contributing to tropospheric ozone formation and aerosols add to this list.

The latest IPCC Synthesis Report states that the human influence on the climate system is clear and growing (IPCC 2014). It also states that implementing stringent mitigations activities can ensure that the impacts of climate change remain within a manageable range, creating a brighter and more sustainable future. To keep a good chance of staying below 2 °C our emissions should drop by 40–70% globally between 2010 and 2050, falling to zero or below by 2100. The chair of IPCC (RK Pachauri) added to this that “we have that opportunity, and the choice is in our hand” (press release, 2 November 2014).

Ambitious emission reduction is possible, but the window for action is rapidly closing. Leo Meyer (PBL, The Netherlands and IPCC Technical Support Unit) indicates that 65% of our carbon budget compatible with a 2° warming goal is already used (Figure 1). This implies that if we want to limit global warming to 2°, some fossil fuels will have to stay underground (Pulles 2015). The economic consequences of such measure moderate. Business as usual scenarios typically assume an economic growth of a few percent per year. In scenarios consistent with a 2° warming this economic growth is hardly affected (Meyer 2014).

Figure 1. The global carbon budget compatible with a 2° warming goal (modified from Meyer 2014).

Reducing emissions of non-CO₂ agents can be an important element of mitigation strategies. However, the IPCC Synthesis report is not very explicit about how to achieve this. Emissions of non-CO₂ gases (methane, nitrous oxide and fluorinated gases) contributed by about 27% to the anthropogenic global emissions of Kyoto gases in 2010. The week following the publication of the IPCC Synthesis Report, the 7th non-CO₂ Greenhouse Gas Symposium (NCGG7) was organized. More than 200 scientists from all over the world presented their work. Following are some highlights from the conference.

Several illustrative 2° scenarios exist according to Niklas Höhne (New Climate Institute; 2014). These typically assume that fossil CO₂ emissions are reduced to zero and that CO₂ from forestry is becoming negative (indicating a carbon sink). Non-CO₂ greenhouse gases are typically stabilizing or slightly reducing, assuming that it will be difficult to reduce emissions of CH₄ and N₂O from food production. Nevertheless, about half of the avoided warming in a recently published 2° scenario is a result of avoided non-CO₂ greenhouse gas emissions (Rogelj et al. 2014). According to Höhne it is possible to realize emission reduction scenarios that are consistent with a 2° warming (Blok et al. 2012). Non-CO₂ GHG emission reductions initiatives can make a significant contribution if framed as opportunity, and alongside of reductions of CO₂ emissions. Measures aimed at reducing short-lived climate forcers (such as black carbon, CH₄ and fluorinated gases) are to be considered complementary rather than a substitute for early and stringent CO₂ mitigation (Rogelj et al. 2014).

Business-as-usual scenarios typically assume that greenhouse gas emissions keep increasing. However, NCGG7 presenters show that technologies exist to reduce emissions considerably. They argue that it is now a matter of policy and willingness. Bottom-up initiatives may fill the gap between business-as-usual trends in emissions and environmental targets (Blok et al. 2012). Wedging the Gap is a bottom-up approach, building on all the rapid developments in technology and implementation and on the great initiatives in many places to bridge the global emissions gap. It consists of amplifying the actions of frontrunners in 21 types of activity by applying them on a large scale, under the leadership of organizations already active in the field (Blok et al. 2012).

Emissions of non-CO₂ greenhouse gases are already decreasing in several world regions. Emissions are being reported in National Greenhouse Gas Inventories. The emissions of most of non-CO₂ greenhouse gases from reporting countries decreased during 1990–2012, according to Dr Kiyoto Tanabe (IPCC Task Force on National Greenhouse Gas Inventories; 2014). Emission reduction during 1990–2012 was relatively large in fugitive emissions of CH₄ from solid fuels, CH₄ emissions from enteric fermentation, N₂O emissions from chemical industry and N₂O emissions from agricultural soils. Only emissions of hydrofluorocarbons (HFCs) increased as a side effect of the phase out of ozone depleting substances. Almost 90% of the HFC emissions were from refrigeration and air conditioning equipment in 2012.

Many scenario studies assume that it is difficult to reduce emissions from food production. However, there are possibilities to grow more food with less greenhouse gas emissions. Even emissions of N₂O are can be considerably reduced (Oenema et al. 2013; Winiwarter et al. 2014) and at relatively low costs. For instance, in Europe, emissions can be reduced by 50% at low costs (<5 euro/ton CO₂) (Sutton et al. 2014). Rudy Rabbinge (Wageningen University and Global Research Alliance chair; 2014) argues that in particular technical sciences offer opportunities to control resources in such a way that emissions are lower.

Also in the energy sector large emission reductions are possible. Lena Högland-Isaksson (IIASA) argues that climate policy is needed now, and that methane mitigation should be an integrated part of general energy systems transformation in the near term. Her cost curves for CH₄ emitting sectors in Europe indicate that for most sources, emissions can be reduced considerably at negative or very low costs (Figure 2). In other words, emission reductions lead to monetary benefits. This holds in particular for CH₄ emissions from coal mining, and from oil and gas. It also holds for CH₄ emissions from solid waste and wastewater.

Figure 2. Costs of emission reduction plotted in a cost curve that is typical for energy-related non-CO₂ greenhouse gases (modified from Eickhout 2014; Högland-Isaksson 2014).

Despite the potential for emission reductions, since 2008 not many new climate legislation proposals have been made. This is due to failure of Copenhagen, the financial crisis and doubts about climate science according to Bas Eickhout (European Parliament, Greenleft party). He argues that to reduce emissions at net negative costs, barriers to energy savings need to be removed. Internalising external costs of harmful emissions can lead to large emissions reductions at relatively low costs. On the other hand, Europe can set example with recent policy plans to phase out the F gases (2014).

Summarizing, NCGG7 showed that large emission reductions in GHG are needed to limit warming to 2°, and that the window for action is rapidly closing. The focus of climate research and policy is largely on CO₂. However, Non-CO₂ GHG emissions can play an important role, and be reduced at relatively low costs. An important conclusion of NCGG7 was that UNFCCC is far from what is needed. Non-CO₂ GHG emission reduction is very needed and possible and deserves more attention.

Professor Carolien Kroeze
Environmental Systems Analysis Group of Wageningen University and Faculty of Management, Science & Technology at the Open University of The Netherlands
[email protected]
Dr Tinus Pulles
VVM
[email protected]