Various predictions have forecast that within the next five to twenty years greenhouse gases may push global warming past the 1.5 degrees Celsius limit set by the Paris Agreement in 2015. However, greenhouse gas emissions remain unabated due to increasing population, continuing economic development and fossil fuels consumption, and destruction of natural carbon stocks in forests and soils. So what is our solution to combat climate change in this reality? Scientists have proposed solar radiation management and greenhouse gas removal as alternative measures, also known as geoengineering, to limit global warming and alleviate its impacts. If we continue to live in a high CO2 world, solar geoengineering or solar radiation management could be an optional strategy for offsetting the warming effects of greenhouse gases.
However, the idea of solar geoengineering is highly controversial. Some argue that solar geoengineering is a cheap and effective solution to global warming. Others warn it may bring unintended consequences and even catastrophes. In order to reach a consensus on this issue, we must investigate the promises and caveats of solar geoengineering before implementing it. In this effort, scientists from the Bjerknes Centre for Climate Research have tested several solar radiation management methods with the Norwegian Earth System Model. These methods include stratospheric aerosol injection (SAI), marine sky brightening (MSB), and cirrus cloud thinning (CCT). These “cooling” methods have the same objective: to cool down air temperatures by reducing net solar energy absorbed by the Earth’s surface (Fig. 1).
Figure 1. An example of solar radiation management method called “Stratospheric aerosol injection” that is aimed to reflect part of incoming solar radiation and cool down the Earth surface. Source: Hughhunt, SPICE SRM overview, CC BY-SA 3.0
These modeling experiments show that all the three solar radiation management techniques effectively slow down global warming as if the atmospheric CO2 level switches from an intensively rising pathway to a middle-of-the-road pathway. However, notable differences exist in other climate variables such as precipitation, humidity, and surface radiation due to the nature of the different interventions applied to the atmosphere. The ocean carbon cycle analyses reveal the controversial fact that these geoengineering endeavors do not reduce CO2 buildup in the air and water. Ocean acidification continues, which harms marine ecosystem biodiversity and productivity. We still do not know how land ecosystems will respond to altered regional climates under the geoengineering scenarios. Triggered by this question, the Bjerknes Centre has conducted additional modeling experiments. These experiments focus on the land systems in order to figure out the potential impacts of solar geoengineering on functions and services of ecosystems including agricultural production across the globe.
Figure 2. Solar radiation, air temperature, and precipitation changes after implementing (2020-2100) and terminating (2100-2150) the three geoengineering methods (SAI, MSB, CCT). RCP4.5 and RCP 8.5 are the middle-of-the-road and intensively rising greenhouse gas concentration pathways, respectively. Source: Muri et al. (2018)
Initial results show that the shifts in radiation and precipitation patterns across different countries and regions in the 21st century vary widely between the management methods although they achieve the same global mean temperature target (Fig. 2). Changes and differences in regional climate, including different aridification trends under the geoengineering scenarios also impact ecosystem services, such as vegetation productivity or crop yield, soil carbon stock, and runoff or water provision. None of the three methods consistently benefit or harm different ecosystem services across different regions. On the global scale, a general finding is that land ecosystems will increase gross primary production in a high CO2 world compared to the middle-of-the-road CO2 pathway. But it is unclear which species will flourish and which will perish. Limiting global warming by enforcing solar radiation management is particularly beneficial for the accumulation of soil carbon stocks. Otherwise, the rising temperature and increasing aridity will exacerbate land degradation and diminish the soil carbon pool, which holds greater amount of carbon than do global vegetation and atmosphere pools combined.
Reining in global greenhouse gas emissions is still needed even if we adopt geoengineering, because these emissions are closely tied to land use change, especially deforestation and agricultural expansion. A high-emission pathway implies intensive land use and land use changes, which are shown to severely destruct the vegetation carbon stock by our experiments. This would also impair biodiversity and the environment. Our analyses help evaluate the balance between below- and above-ground carbon pools, including soil, vegetation, and the atmosphere. Understanding the interplays between climate change and land use change is critical for achieving a balance between the temperature target and maintenance of ecosystem integrity and services. The “cooling” effects of solar geoengineering techniques should not be an excuse for not limiting anthropogenic emissions.
The tradeoffs of solar geoengineering on the land ecosystems and food production concerns the Sustainable Development Goals of the United Nations and the future of the Earth environment for every living species. Although we find some benefits of solar geoengineering such as global mean temperature reduction and protection of soil carbon pools, we have not fully understood the impacts on regional climate and different ecosystem services including crop yield. We are running additional crop simulations under geoengineered climate scenarios and will analyze the new results in the near future. The scientific community is also investigating other potential consequences and hazards of geoengineering for other systems. Conventional climate change mitigation and adaptation solutions like cutting emission and reshaping global economy might still be necessary pursuits, which have to depend on the fundamental way how every human being could limit their footprint on this planet.
If geoengineering continues to be a debatable option, then we must continue to research its potential impacts. Only then will we have a solid base of knowledge to plan for the rapidly changing future.
Acknowledgement: this article benefited from the works of Jerry Tjiputra, Helene Muri, and ChangEui Park
Citation: Muri, H., Tjiputra, J., Otterå, O. H., Adakudlu, M., Lauvset, S. K., Grini, A., … & Kristjánsson, J. E. (2018). Climate response to aerosol geoengineering: a multi-method comparison. Journal of Climate, (https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0620.1).
Photo credits: Homepage photo by Geraint Rowland (CC BY-NC 2.0)