Introduction to Geoengineering - SRM

What is Geoengineering and Solar Radiation Management?

As mentioned in the first post, Geoengineering often refers to 'large-scale efforts to diminish climate change from greenhouse gases that have already been released to the atmosphere' (Caldeira et al., 2013). It can be classified into two major categories, solar radiation management (SRM) and carbon dioxide removal (CDR) - this post will focus on SRM. 

Solar radiation management (SRM) targets to offset the effects of anthropogenic climate change, including high atmospheric CO2 and other greenhouse gases (GHGs) by reducing the amount of solar radiation absorbed by Earth. Since atmospheric CO2 and other GHGs deploy radiative forcing on the Earth's climate system through making harder for the heat coming in to escape from Earth (Caldeira et al., 2013). SRM approaches can be divided into four major kinds: 

     Space-based approaches introduces to reduce the incoming solar radiation reaching Earth, including the installment of mirrors and rings in orbit around the Earth by constellations of spacecraft (Pearson et al., 2006) or placing a thing glass shield using lunar materials near the first Lagrange point (L1) of the Earth-Sun system (Early, 1989). 

     Stratospheric aerosol-based approaches aim to inject sulfate aerosols into the lower stratosphere to cause scattering solar radiation back to space, leading to the cooling of Earth (Caldeira et al., 2013). This idea of injecting aerosols have suggested from looking at the past volcanic eruption such as Mount Pinatubo in 1991, which caused cooling of the earth's surface in following years by almost 0.5 degrees Celsius (Cruten 2006). Even though sulfate aerosol injection could be done in troposphere to cool the earth, it is considered to be more efficient to inject in the stratospheric due to their longer residence time of about 1 - 2 years - while only a week in troposphere - hence the required sulfur for cooling would be much less in the stratosphere injection, compared to the troposphere (Dickinson, 1996). 

     Marine cloud brightening is another way of modifying the reflectivity of the earth, particularly increasing the low-level marine stratocumulus cloud reflectivity (Caldeira et al., 2013). Studies on cloud albedo such as Twomey (1977) suggest that by increasing the number of cloud condensation nuclei (CCN), greater number of cloud droplets would be produced while the droplet size actually drops, then the total cloud droplet surface area will increase and thus the cloud reflectivity. Areas with extensive marine stratocumulus clouds such as the west coast of Africa have been distinguished with potential to adopt such SRM approach (Latham et al., 2008). 

     Surface albedo enhancement will be one of the most well known SRM strategies. It aims to increase the earth's surface albedo through altering the surface reflectivity of the ocean, deserts, urban and rural regions (Caldeira et al., 2013). Methods can range from whitening the roof of the buildings to planting more reflective plants. 

Different SRM strategies: a) using satellites to reflect the incoming radiation back to space, b) injecting sulfate aerosols into the stratosphere, c) marine stratocumulus cloud brightening, d) altering the ocean surface albedo, e) growing plants or crops with high albedo and f) whitening building roofs - adopted from (Caldeira et al., 2013). 

Many scientists advocating the implementation of geoengineering as an effective measure for responding to climate change claims that it can be used with other strategies (to reduce greenhouse gas emissions and increase adaptive resilience to climate change) to accelerate the pace of 'returning' the earth back to pre-industrial level. This may in part because rather 'quick' response of climate system to geoengineering is projected (Matthews & Caldeira, 2007) in computer simulations. Especially, SRM strategies such as whitening the roofs of the building or planting crops with high albedo can be implemented in a fairly easy way, anticipating for a quick response. Hence, some implies that there may be only a little cost in delaying the geoengineering deployment 'in the field' until the earth reaches a very 'dangerous' and imminent situation of climate change (Matthews & Caldeira, 2007).

     However, many oppositions for geoengineering implementation also exist, keeping the debate ongoing. Next post will explore about such debates and the other major geoengineering scheme of CDR.