Global Is the New Local

In the previous post, I presented the impacts of China’s air pollution on human health and the environment as well as the associated economic loss. This week, I wanted to explore some of its regional to global impacts and defend my choice of blog topic – in case you have been wondering, because the module is called “Global Environmental Change”. Air pollution is by no means a single case. China is a microcosm (not literally of course:-P) of many countries going through industrialisation and motorisation. China’s experiences and lessons in controlling air pollution are useful for other developing countries. Furthermore, pollutants emitted in China do not stay within national boundaries forever, nor do they disappear right into the thin air, instead they enter the Earth’s atmospheric circulation and can be spread around the globe. This video made by NASA shows the aerosol emission and transport from September 2006 to April 2007:

There is an evident outflow of aerosol from Eastern China towards North Pacific, which is due to the prevailing west (southwest in the Northern Hemisphere) wind in the middle latitudes. This long-range transport of air pollutants has been backed up by various studies. Oh et al. (2015) provided strong evidence that the occurrence of multi-day (>4 days) high PM₁₀ concentration (>100 μg/m³) episodes in cold seasons from 2001 to 2013 in Seoul is correlated with the pollution emitted in Northeast China and high pressure anomalies over the region. Lin et al. (2014) calculated that on a daily basis, China’s air pollution contributed at maximum 12-24% of the sulphate, 2-5% of O₃, 4-6% of CO and up to 11% of black carbon concentration over the western United States:

Source: Lin et al. (2014).

China is indeed responsible for some of the air pollution over the US. Yet, on the other hand, American consumer demand for cheap goods is what fostered the pollution in China in the first place. Lin et al. (2014) also found that production for export was responsible for 36% of SO₂ emission, 27% of NO_X, 22% of CO and 17% of black carbon in China in 2006. In other words, if emission were measured using the consumption-based approach, it would be much higher for many trade partners of China. For example, the US emission for SO₂, NO_X, CO and black carbon would be 6-19% higher in that year. This finding underlies again the global relevance of the subject matter: Outsourcing of manufacturing does not necessarily outsource the pollution as well, for pollutants can be transported and redistributed via the atmospheric circulation.

Meanwhile, China’s air pollution is likely affecting the global climate, as aerosols modify the Earth’s energy budget in two ways: either directly by absorbing solar radiation or scattering it back into space, or indirectly by influencing the formation, characteristics and dynamics of clouds:

Source:Stocker et al. (2007).

Though these aerosol-cloud interaction mechanisms are known, their magnitude is still poorly quantified. Thus the radiative forcing of aerosols contributes the largest uncertainty to the overall uncertainty in anthropogenic forcing projections (Randall et al. 2013). Using a multi-scale global aerosol-climate model, Wang et al. (2014) simulated two aerosol scenarios – one for present day and one for preindustrial level – for Northwest Pacific. The main findings are summarised in this figure:

Source: Wang et al. (2014)

Anthropogenic emission of aerosols increases the amount of cloud condensation nuclei, which results in a 108% increase of cloud droplet number concentration (A) and a 13% decrease of the cloud effective radius. Therefore, the conversion from cloud droplets to rain drops is suppressed. Consequently, the liquid water path (B) and the ice water path (C), which measure the weight of the liquid water droplets and ice water droplets in the atmosphere above a unit surface area, increases by 9.8% and 8.9 % respectively, indicating a delay in warm precipitation of low-level maritime clouds.

On the one hand, clouds reflect incoming solar radiation, thus its shortwave radiative forcing at the top of atmosphere (E) cools the Earth’s surface. It is predicted to decrease by 6.7% (2.5 W/m²). On the other hand, clouds absorb and re-emit outgoing electromagnetic radiation, hence its longwave radiative forcing (F) has a heating effect. This is predicted to increase by 6% (1.3 W/m²). The net cloud radiative forcing, which is overall negative, is thus weakened.

The fraction of high-level cloud (D) increases by 2.6%. High-level clouds have a low albedo; this is outweighed by its ability to trap outgoing heat. On the contrary, low-level clouds strongly reflect incoming sunlight. Increasing high-level cloud fraction indicates that the warming effect of clouds is strengthening, confirming the change in net cloud radiative forcing.

The response of precipitation (G) is not uniform over the region of study. Overall, it increases by 2.5 %. The transient eddy meridional heat flux (H), a measure of the poleward heat transport, which is largely carried out by mid-latitude storms, is simulated to increase by 5%. Both increased precipitation and transient eddy meridional heat flux indicate the intensification of the Pacific storm track.

In summary, air pollution in China has impacts on both regional air quality and global climate. However, China is not the only one responsible for its pollution. Of course, this is not to defend China for contaminating other countries’ air or altering the global climate, but rather to emphasise the fact that while air pollution is of local origin, it requires global solutions.