ITCZ Cause and Effect

I recently had a lecture about Black Carbon (BC), a component of fine particulate matter which is produced from the incomplete combustion of organic matter. As well as climate change, BC is responsible for detrimental impacts on human health, transporting pollution, and damaging stone buildings. Figure 1 provides a summary of the global climate system effects of BC.

Figure 1: A summary of the climatic impacts of black carbon. (Source: Bond et al., 2013).

Of particular interest to me was the effect of BC in shifting the Intertropical Convergence Zone (ITCZ) to the north. Studies have shown that this happens as a result of BC strengthening the Hadley cell in the northern hemisphere but weakening the Hadley cell in the southern hemisphere (Wang, 2007; Jones et al., 2007). 

Just a few days ago, a study led by Dr Anthony Jones of the UK Met Office was published in Nature, examining how stratospheric aerosol injection (SAI), a type of SRM, could increase the frequency of hurricanes by shifting the ITCZ (Jones et al., 2017). Figure 2 illustrates the modelled impact of SAI on hurricane / tropical cyclone frequency, which has a large dependency on the hemisphere which SAI is undertaken. 

Figure 2: Modelled hurricane / tropical cyclone frequency in response to stratospheric aerosol injection (SAI) for years of geoengineering indicated by black lines between 2020-2070. Including no geoengineering (purple), annual SAI in southern hemisphere (red), annual SAI in northern hemisphere (blue), and annual global SAI (turquoise). (Source: Jones et al., 2017).

Essentially, preferential SAI in a single hemisphere alters sea-surface temperature gradients and shifts the ITCZ towards the opposite hemisphere. So, solar geoengineering in the south causes an ITCZ shift to the north, providing optimal conditions for hurricane formation near the United States from African easterly waves in an area in the North Atlantic known as the hurricane main development region (MDR) but would have the benefit of enhancing precipitation over the Sahel. On the other hand, solar geoengineering in the north would cause the ITCZ to shift to the south, which would increase wind shear over the MDR and reduce the number of hurricanes in the north, but would reduce precipitation over the Sahel and could cause droughts. 

Speaking with Carbon Brief, Dr Anthony Jones notes his concerns that positive regional impacts could motivate nations with greater influence to deploy solar geoengineering in a single hemisphere at the expense of nations in the other hemisphere. Both the BC and SAI studies highight the global impacts of regional actions and stress that global cooperation is crucial in tackling climate change, especially in a geoengineered world.