By Laura Wilcox
Aerosols are tiny particles or liquid droplets suspended in the atmosphere. They can be created by human activities, such as burning fossil fuels or clearing land, or have natural sources, such as volcanoes. Depending on their composition, aerosols can either absorb or scatter radiation. Overall, increases in aerosol concentrations in the atmosphere act to cool the Earth’s surface. This can be the result of the aerosols themselves reflecting radiation back to space (aerosol-radiation interactions), or due to aerosols modifying the properties of clouds so that they reflect more solar radiation (aerosol-cloud interactions).
The cooling effect of aerosols means they have played an important role in climate change over the last 200 years, masking some of the warming caused by increases in greenhouse gases. However, the climate impact of aerosols is much more interesting than a simple offsetting of the effects of greenhouse gases. While greenhouse gases can remain in the atmosphere for hundreds of years, most anthropogenic aerosols are lucky to last two weeks being deposited at the surface. This gives them a unique spatial distribution, with most aerosols being found close to the regions where they were emitted. This is a marked contrast to greenhouse gases, which are evenly distributed in the atmosphere, and makes aerosols very efficient at changing circulation patterns such as the monsoons and the Atlantic Meridional Overturning Circulation. Although aerosols tend to stay close to their source, their influence on atmospheric circulation means that a change in aerosol emissions in one region can result in impacts around the world. Asian aerosols, for example, can influence Sahel precipitation by changing the Walker Circulation, or influence European temperature by inducing anomalous stationary wave patterns.
Figure 1: A snapshot of aerosol in the Goddard Earth Observing System Model. Dust is shown in orange, and sea salt is shown in light blue. Carbonaceous aerosol from fires is shown in green, and sulphate from industry and volcanic eruptions is shown in white. The short atmospheric lifetime of aerosols means they typically stay close to their source so that aerosol concentrations and composition varies dramatically with location. Image from NASA/Goddard Space Flight Center.
The short atmospheric lifetime of anthropogenic aerosols means that changes in emissions are quickly translated into changes in atmospheric concentrations, and changes in impacts on air quality and climate. Increases in European aerosols through the 1970s were one of the main drivers of drought in the Sahel in the 1970s and 80s. As European emissions decreased following the introduction of the clean air acts in 1979, precipitation in the Sahel recovered, and the trend became more strongly influenced by greenhouse gas increases. Meanwhile, the rate of increase of European temperatures accelerated as the cooling influence of anthropogenic aerosol was lost.
Poor air quality has been linked to many health issues, including respiratory and neurological problems, and is a leading cause of premature mortality in countries such as India, where many of the world’s most polluted cities are currently found. In recent decades, China has dramatically reduced its aerosol emissions in an attempt to improve air quality, and other countries are expected to follow suit. However, the timing and rate of reductions of aerosol emissions are dependent on a complex combination of political motivation and technological ability. As a result, our projections of aerosol emissions over the next few decades are highly uncertain. Some scenarios see global aerosol returning to pre-industrial levels by 2050, while different priorities mean that emissions continue to increase in other scenarios. While I expect that some scenarios are more likely than others, this means that for near-future climate projections aerosol may not change very much in the early twenty-first century, or may be reduced so quickly that we see the emission increases that took place over the last 200 years reversed in just 20-30 years. While this would be a great outcome for the health of those living in regions with poor air quality, it may come with rapid climate changes, which need to be considered in adaptation and mitigation efforts.
Figure 2: Global emissions of black carbon and sulphur dioxide (a precursor of sulphate aerosol) from 1850 to 2100, as used in the sixth Coupled Model Intercomparison Project (CMIP6). The rate and sign of future emission changes are still uncertain.
Unfortunately, large differences in emission scenarios aren’t the only uncertainty associated with the role of aerosol in near-future climate change. A lack of observations of pre-industrial aerosol, uncertainties in observations of present-day aerosol, and differences in the way that aerosol and aerosol-cloud interactions are represented in climate models make aerosol forcing the largest uncertainty in the anthropogenic forcing of climate. For regional climate impacts, these are compounded by uncertainties in the dynamical response to aerosol changes. In anthropogenic aerosol, we have something that may be very important for near-future climate, especially at regional scales, that is highly uncertain. For climate change mitigation and adaptation to be effective, we need to improve our understanding of these uncertainties, or, even better, reduce them.
Regional assessments of climate risk often rely on regional climate models or statistical algorithms. However, this often results in the influence of aerosol being lost. Most regional climate models do not include aerosol processes, and statistical approaches typically assume that historical relationships will persist into the future, so that the impacts of changing aerosol types and emission locations are not accounted for. Broader approaches use projections from Earth System Models to tune simple climate models or statistical emulators, which are often only able to account for the global impact of aerosol changes, neglecting their larger impacts on regional climate.
We have designed a set of experiments that we hope will improve our understanding of the climate response to regional aerosol changes, provide a stronger link between emission policies and climate impacts, and support the development of more ‘aerosol-aware’ assessments of regional climate risk. The Regional Aerosol Model Intercomparison Project (RAMIP) includes experiments designed to quantify the effects of realistic, regional, transient aerosol perturbations on policy-relevant timescales, and to explore the sensitivity of these effects to aerosol composition. Simulations are just getting underway now. Will we find that these tiny particles are having a big impact on regional climate in the near future? Watch this space!
For more details of the RAMIP experiment design, take a look at our preprint in GMD
For more thoughts on aerosol and climate risk assessments, see our recent comment