The importance of non-CO2 greenhouse gases

Abstract

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). 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