COP21 Watch, pt.1: Air Pollution and Climate Change

Instead of further exploring sources of air pollution, I will use (probably) two posts to prep myself and you, my dear readers, on the upcoming COP21, especially the role of China in it. As a student of environmental sciences (or anyone who cares about this planet that we are living on), what could be a bigger and more important event other than the world coming together and negotiating a global and binding treaty to tackle climate change? Besides, this is not completely irrelevant to the main theme of this blog, which I will explain now.

Climate change and air pollution are obviously different phenomena, yet they share a common cause: anthropogenic emissions, primarily from combustion of fossil fuels, which alter the composition of the Earth’s atmosphere. In fact, some of the major air pollutants are also potent greenhouse gases, including tropospheric ozone and chlorofluorocarbons (CFCs). Nitrous oxide (N₂O), a gas of high global warming potential and long atmospheric lifetime, forms NOx under high pressure. Thus,climate change and air pollution are closely coupled. Yet this coupling is complex.

First, not all types of pollutants have the same climate effect. Pollutants that are also greenhouse gases absorb and emit infrared radiation and thus warm the lower atmosphere and the Earth’s surface. The climate impact of particulate matter is, as mentioned in a previous post, in itself complex. For instance, black carbon contributes to global warming both directly by strongly absorbing solar radiation and indirectly by causing cloud burn-off and melting of ice sheet and snow cover (both of which decrease the Earth’s albedo). Sulphates, nitrates and other reflective aerosols that mainly scatter solar radiation and increase cloud albedo and lifetime (by serving as cloud condensation nuclei) have the opposite effect (NASA Earth Observaory, 2010). The blue dashed curve in the following figure illustrates the probability density function of the net aerosol radiative forcing, while the vertical lines show its 90% confidence interval. Both the lower and upper confidence limits are below 0, i.e. negative radiative forcing. The fact that this curve is wider than other ones indicates that the uncertainty increases, as more aerosol effects are taken into account.

Source: Myhre et al., 2013

Though from a global perspective, the overall climate effect of particulate matter is very likely to be cooling, its local to regional impacts could be different (both in terms of the sign and magnitude), depending on the relative proportions of individual components.

Another complexity is illustrated in this figure – the varying atmospheric lifetimes of pollutants, ranging from a few days, weeks and years (short-lived climate pollutants) to centuries (eg. persistent organic pollutants) and millennia (eg. CO₂):

Source: Climate and Clean Air Coalition, n.d.

The radiative forcing of total air pollution is just as complex, if not more, as that of aerosols. This, along with other anthropogenic (eg. land use) and natural (eg. solar irradiance) radiative forcing components, drives the climate.

Kaufmann et al. (2011) studied the period between 1998 and 2008, during which the atmospheric concentration of greenhouse gases increased, but the global surface temperature indicated little warming. They found out, as shown in the following figure, the increasing anthropogenic forcing (blue line) due to rising greenhouse gas concentrations was slowed down by increasing sulphur emissions (purple line). The cooling effect of sulphur largely cancelled out the warming effect of greenhouse gases, the significance of natural forcings revealed: solar insolation (orange line) declined within its 11-year circle, amplified by increasing Southern Oscillation Index (green line) which indicated a shift from El Niño to La Niña episodes. Changes in stratospheric water vapour (another GHG) and the internal variability of the climate system were ruled out as significant factors. Kaufmann et al. (2011) attributed this rapid increase in sulphur emissions to coal consumption in Asia in general, particularly in China, whose coal consumption more than doubled during the period of study.

Source: Kaufmann et al., 2011

Changing climate may, in turn, affect air quality. The 4th IPCC report (2007) predicted “declining air quality in cities”, as climate change may alter “the dispersion rate of pollutants, the chemical environment for ozone and aerosol generation and the strength of emissions from the biosphere, fires and dust”. However, it also stated that the sign and magnitude of these impacts will vary regionally and are associated with high uncertainty.

In conclusion, air pollution and climate change are correlated in complex ways (both positively and negatively) and the uncertainty of their correlations is high. Some strategies address either air pollution or climate change separately with no impact on the other, but there are also strategies which aim for co-benefits (see figure). Integrated and consistent policymaking is thus essential. So in the next few posts, I will look at both China’s pollution-control policies and climate and energy policies, especially its pledge ahead of COP21.