Potential widespread effects of geoengineering on both climate and the carbon cycle

Global warming poses a great threat to our society, and it will continue unless a deep and rapid reduction in greenhouse gas emissions is implemented.  One alternative way to counteract some of the undesired effects of global warming is geoengineering, also referred to as solar radiation modification, or climate engineering. The basic idea of geoengineering is to cool the earth by intentionally reducing the amount of solar radiation reaching the surface. Some of the proposed geoengineering methods include deployment of sunshade materials in space, injection of sulfate aerosol or SO₂ into the stratosphere, and increasing the low-cloud albedo over the ocean.

Understanding the climate response to geoengineering relies on climate modeling. Currently, most geoengineering studies focus on the response of physical climate fields such as temperature, precipitation and sea ice, under prescribed atmospheric CO₂ concentrations, while relatively few studies have explored the   response of the carbon cycle to geoengineering. Researchers from Zhejiang University used an earth system model to investigate how geoengineering would affect the global carbon cycle, with the focus on the ocean carbon cycle. The results have recently been published in Atmospheric and Oceanic Science Letters.

The researchers simulated the effect of geoengineering by reducing solar irradiance in the model to bring down the global mean surface air temperature from a high CO₂ emission scenario ( RCP8.5) a low one (RCP4.5). The results show that geoengineering, by altering physical climate fields such as temperature and ocean circulation, would have significant effects on the global carbon cycle. Compared to RCP8.5 without geoengineering, the inclusion of geoengineering reduces the amount of atmospheric CO₂. The main reason is that geoengineering-induced cooling inhibits the amount of CO₂ released from the soil, and thus enhances net CO₂ uptake by the terrestrial biosphere.

The simulation results show that geoengineering would have small effects on the ocean carbon sink because of competing effects of reduced atmospheric CO₂ and cooling of the ocean surface. Compared to RCP8.5 without geoengineering, the inclusion of geoengineering has little effect on mitigating CO₂-induced annual mean ocean acidification. On the other hand, geoengineering somewhat attenuates the amplitude of the seasonal cycle of ocean surface acidity, which could have important implications for some marine species that are sensitive to the seasonal change in ocean acidity. The simulation results also show that geoengineering has small effects on the globally integrated ocean primary production as a result of competing effects of changes in ocean temperature and circulation.

“Geoengineering, if implemented, would have widespread effects on global climate. Our study contributes to geoengineering research by showing that geoengineering would indirectly perturb atmospheric CO₂ and the ocean carbon cycle through complex physical and biogeochemical effects,” explains the corresponding author of this study, Professor Long Cao from Zhejiang University.

“Geoengineering can be thought of as a backup plan to reduce anthropogenic warming, but a deep and rapid reduction in CO₂ and other greenhouse gas emissions is the key to mitigating global warming,” adds Prof. Cao.