GHGs, Aerosols & Cookfires

In "What's the story?" we explored the basic science of anthropogenic influence upon the South Asian monsoon, but the devil is very much in the detail, and so I'll be delving into the many whys and hows over the next few posts. So without much further ado...

Figure 1 - Aerosol-Cloud Interactions in (a) clean
 air and (b) polluted air (IPCC AR5:Chapter 7)

The concentration of Carbon Dioxide (CO_₂) has increased since ~1850 (Keeling Curve) and along with other greenhouse gases (GHGs) has driven surface warming and intensification of rainfall associated with the South Asian summer monsoon (Ueda et al., 2006). This relationship is not as simple as it seems due to aerosols, predominantly sulphate and black carbon borne from the continued industrialization of South Asia (Turner and Annamalai, 2012). Aerosols interact with the climate in two main ways; interaction with clouds (Figure 1) and interaction with sunlight (Figure 2). The total radiative forcing effect of aerosols is calculated to be -0.35 (-0.85 to +0.15) W m⁻² (IPCC AR5) and Ramanathan et al (2005) have suggested that aerosols may have masked up to 50% of surface warming from the increasing levels of GHGs. Accounting for up to 60% of black carbon emissions in Asia (UNEP, 2012) through the burning of biomass, the humble cookfire has stepped into the limelight and has recently undergone a technological make-over (BBC video). The reduction of black carbon emissions has been dubbed a saviour of glaciers (which will appear in a later post) in addition to saving lives (Ramanathan, 2013).

Figure 2 - Aerosol-Radiation Interactions. The left panels are
instantaneous & the right, overall effects (IPCC AR5:Chapter 7) 

The effects of aerosols upon the monsoon are hotly debated (Turner and Annamalai, 2012) with many opposing voices. Turner and Annamalai. (2012) suggest solar radiation could be limited by both the direct scattering effect of aerosols and by their increasing the albedo of clouds. A potential result of this is a reduction of the meridional temperature gradient, leading to a lower increase in rainfall than is expected. Ramanathan et al (2005) projected a decrease in Indian Ocean SSTs due to reduced solar radiation, resulting in decreased evaporation and thus rainfall; however Annamalai et al. (2012) note that SST over the Indo-Pacific warm pool have risen, enhancing moisture content, and making reduction in rainfall unlikely. Ueda et al. (2006) calculate the overall effect as increased monsoonal precipitation, due in the most part to the enhanced moisture transport, despite the influence of aerosols. Indeed, many coupled ocean-atmosphere models simulate this result with increases in GHG concentrations (AR5). 

Furthermore there is a suggestion that increased SSTs (Annamalai et al., 2012) along with aerosols (Bollasina et al., 2011) could cause geographic redistribution of monsoon rainfall, most notably a drying of Central India (Krishnamurphy et al., 2009 in AR5). Bollasina et al. (2011) demonstrated that along with the effect upon rainfall, aerosols have driven a weakening of monsoonal circulation. Further effects of aerosols include an increase of cloud burn-off due to increased cloud lifetime and potential aerosol driven tropospheric warming (Koch and Genio, 2010), though increases of cloud cover in some areas and an increase of extreme precipitation events (Goswami et al., 2006) suggest this effect is not a main driver. Levermann et al. (2009) noted that the South Asian monsoon has two stable states; a 'wet' state and a 'dry' state. It is the moisture-advection feedback that predominantly drives the monsoon circulation, and as such a change in radiative forcing that weakens the pressure gradient (Whats, Whys, Wheres & Hows), could prompt an abrupt transition from the current monsoon regime to one characterized by reduced precipitation. On the other hand, the effects of aerosols are somewhat constrained by those of increasing GHGs, meaning a switch in monsoon regime during the 21st century and beyond is unlikely (AR5). 

CMIP models simulate the annual precipitation and temperature cycles quite well for South Asia, but although they are improving (Sperber et al., 2012 in AR5), they are still not brilliant at simulating rainfall variability on regional and local-scales (Turner and Annamalai, 2012). One cause for this uncertainty within the models is driven by the gaps in our knowledge regarding the effects of aerosols, particularly regarding aerosol-cloud interactions, and this is a major stumbling block to our understanding the monsoon. Finding the point at which GHGs overcome the effects of aerosols will also further our understanding of the monsoon, and thus our ability to model it (Turner and Annamalai, 2012). Mankind has been running an unintentional experiment on one of the largest hydrological systems on the planet and so, if having read this you have more questions than when you started, you have likely understood the point of this post...uncertainty reigns for now.