· 6 min read
Although the focus of the climate change debate is on mitigating carbon dioxide (CO₂) levels, it is important to remember that contemporary climate change is a function of multiple radiative forcings (RFs). In simple terms, RF is a concept used in climate science to quantify changes in the energy balance in the Earth's atmosphere. Several drivers contribute to the Earth’s current radiative imbalance, where the Earth’s solar energy absorption outweighs the energy radiated back into space. For instance, well-mixed greenhouse gases (GHGs) such as methane (CH₄), nitrous oxide (N₂O), and halocarbons have a RF which cannot be ignored. Even though the focus on CO₂ emissions makes sense, as these have been the main driver of global warming since the pre-industrial period (IPCC, 2021), this article will uncover the extent to which non-CO₂ drivers affect the contemporary radiative imbalance.
Methane emissions
The concentration of atmospheric CH₄ has risen more than 150% since pre-industrial times, compared to 50% for CO₂ according to the Mauna Loa Observatory in Hawaii. The main culprits behind this rise in CH₄ include the increased number of ruminants, emissions from fossil fuel extraction and use, the expansion of rice paddy agriculture and emissions from landfills, waste and biomass burning. Admittedly, humans have changed the climate by expanding, directly or indirectly, the anoxic areas where microbes known as methanogens can decompose biomass and produce methane (Dean et al., 2018). Because of its high global warming potential (more than 27 times higher than CO₂ over a 100-year period), feedback mechanisms resulting in non-linear increases of atmospheric methane are worrying. Indeed, an excessive rise in temperature may lead to increased permafrost thawing and warming of wetland peat, which can lead to the destabilisation of methane hydrates and release CH₄ bubbles (O’Connor et al., 2010). These could uncontrollably increase the levels of atmospheric CH₄, resulting in a greater positive RF, warming the planet even more and enhancing methanogenic activity once again. Major uncertainties prevail regarding the timescales and magnitudes of these methane feedback loops. However, it is imperative to recognise the far-reaching consequences of these processes. As illustrated in Figure 1 below, methane emissions have already caused a quite significant positive RF of 0.54 W/m² relative to 1750.
Figure 1. RF of different emitted compounds relative to 1750, in W/m² (IPCC, 2021)
CFC emissions
Human-made halocarbon compounds such as chlorofluorocarbons (CFC's) have also hastened contemporary climate change as they have strong infra-red absorbing capabilities and often have long atmospheric lifetimes (Forster and Joshi, 2005). Although they have caused some cooling by depleting the ozone, stratospheric halocarbon emissions are estimated to have caused a positive RF of around 0.41 W/m² (IPCC, 2021). Additionally, emissions of N₂O due mainly to nitrogen fertilisation of agricultural soils and fuel combustion are estimated to have a RF of 0.21 W/m² (IPCC, 2021). Adding up the RFs of CH₄, N₂O and halocarbons gives a positive RF of 1.16 W/m². Ozone is also estimated to add to this positive effect by increasing the RF by approximately 0.47 W/m². Comparing these with the RF of CO₂ (2.16 W/m²), non-CO₂ GHGs have a significant impact on the Earth’s radiative imbalance which should not be overlooked.
Aerosols
On the other hand, compounds having a significant cooling effect are aerosols such as sulphate, mineral dust and organic carbon, which have a net cooling effect due to their low albedo. Aerosols act as cloud condensation nuclei, raising atmospheric reflectivity and cloud formation which decreases the global RF. However, as can be seen by the confidence interval on Figure 1, the effect of aerosols on climate change and their exact RF remains immensely uncertain due to their spatial heterogeneity and the complex ways they interact with radiation and clouds (Myhre et al., 2013). The variation across time in the cooling effect of stratospheric sulfur aerosols can be seen in Figure 2, providing some interesting insights. These aerosols, which can be emitted by volcanoes, have historically led to quite abrupt negative global RFs due to their reflectivity. Indeed, major cooling episodes can be explained by the eruptions of Mount Pinatubo (1991) and Krakatoa (1883), which were accompanied by a rapid global cooling that persisted for decades after the event (Hansen et al., 2010; Gleckler et al., 2006).
Figure 2. Global climate forcings from 1880 to 2005 (Hansen et al., 2005)
Black carbon
Another important compound which can be seen on the Figure is black carbon, a byproduct of incomplete combustion, which according to some estimates is the second or third largest contributor to global warming due to its low albedo (Wallack and Ramanathan, 2009). It has been suggested to increase the melting of the cryosphere as much as CO₂ emissions, particularly in the Tibetan Plateau (Zhang et al., 2015). It is also a significant health hazard, making it a more credible and straightforward target for climate regulation.
To conclude, contemporary climate change is caused mainly by, but not restricted to atmospheric CO₂ concentrations. Many other gases and aerosols (black carbon, N₂O, CH₄, CFC’s), affect the current radiative imbalance (and subsequent global warming) of our planet. Effective climate mitigation policies must therefore address a range of greenhouse gases and aerosols, considering their diverse sources and impacts.
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