The question of whether reduced aerosol loading contributes to global warming is not new to atmospheric scientists, but it has recently resurfaced with the extreme heatwaves across the North Atlantic and many areas of Europe. In this analysis, Copernicus Atmosphere Monitoring Service (CAMS) and Copernicus Climate Change Service (C3S) scientists conclude that it is too early to attribute the recent exceptional warming to a reduction in shipping emissions undertaken since 2020.

Research suggests that the reduction in emissions of pollutants in Europe, thanks to regulations, i­­s leading to reduced amounts of aerosols in the atmosphere. This is leading to increased solar radiation reaching the surface, with potential impacts on the intensity of heatwaves and surface water temperatures, which could increase the severity of droughts.

Scientists such as the Met Office’s Ben Booth have suggested for some time that anthropogenic aerosols, such as those resulting from industrial and shipping emissions, had been a key driver of climate variability in the North Atlantic. Climate models have since been improved to integrate atmospheric aerosols interactions.  

In 2020, the International Maritime Organization adopted its ‘IMO 2020’ regulation to drastically reduce shipping-related sulphur dioxide (SO2) emissions. Studies have concluded that the drop in emissions significantly reduced the formation of clouds over shipping lanes. An analysis by Carbon Brief estimated that that “the likely side-effect of the 2020 regulations to cut air pollution from shipping is to increase global temperatures by around 0.05C by 2050. This is equivalent to approximately two additional years of emissions.” However, linking SO2 reductions directly to the recent extreme marine heatwaves omits part of the complexity of using models to calculate sulphate aerosol interactions in the atmosphere or estimating the effective application of the IMO 2020 regulation, and, more generally, the complexity of climate and atmospheric chemistry.

Atmospheric aerosols are microscopic particles, solid or liquid, suspended in a gas, (our atmosphere in this instance). There are many natural sources of atmospheric aerosols, such as desert dust, sea spray and salt from the oceans, biogenic aerosols from vegetation, wildfire smoke, or volcanoes to name a few. The main anthropogenic source of aerosols is emissions from combustion of fossil fuels, which emit a wide range of atmospheric pollutants including particulate matter, nitrogen dioxide, and SO2. Sulphur dioxide emissions are the precursor of sulphate aerosol, which is a key player in Earth’s energy balance.  

Aerosols, by scattering, reflecting or absorbing sunlight, reduce the amount of solar radiation reaching the lower layers of our atmosphere. The Intergovernmental Panel for Climate Change (IPCC) considers that there is “strong evidence for a substantive negative total aerosol effective radiative forcing”, that is, the amount of energy entering and leaving the atmosphere, but also acknowledges that “considerable uncertainty remains”.

While aerosols have a direct cooling effect by filtering solar radiation, their effective contribution to global cooling, or warming when they are reduced, also referred to as negative or positive radiative forcing of aerosols, is still a matter of research, and not the easiest, due to the uncertainties of indirect effects such as sulphate aerosol impacts in cloud droplet formation.  

Particles and pollutants play a role in increased and reduced cloud formation, complicating the situation further. Generally, sulphate aerosols are considered to act as cloud condensation nuclei, favouring cloud formation, therefore reducing the amount of solar radiation that reaches the surface.

CAMS aerosol optical depth forecast 13 October 2017. Orange - dust, red - biomass burning, blue- sea salt, grey - sulphates, yellow - fires Credit: CAMS/ECMWF. 

The role of SO2 in climate has long been debated, and there are no clear conclusions. If it is assumed that all ships in the world adopted the IMO 20 and the more recently introduced IMO 23 regulations, this would certainly have an impact in reducing emissions, and therefore aerosols and cloud formation. But shipping is only one of the SO2 emissions sources, and only accounts for 3.5% of global emissions, according to some estimates.  

Europe has nevertheless drastically reduced its industrial SO2 emissions amid some years of record temperatures, which tends to support the positive radiative forcing theory of reduced aerosols. The drop since 2020 is remarkable, according to data from the European Maritime Safety Agency but the reduction coincides with the COVID pandemics and the related reduction in transport activity.

The Copernicus Atmosphere Monitoring Service (CAMS) has state-of-the-art models for the analysis and forecast of atmospheric pollutants. Evaluating the impact of aerosols in reducing solar radiation, and understanding their chemical interactions, is key to providing accurate data about the current state and evolution of our atmosphere.  

While the impact of natural aerosols has generally a more direct, easily observable impact on radiation, the role of anthropogenic aerosols is more complex due to chemical interactions.

The CAMS global model currently takes into account the impact of natural aerosol events, such as smoke from wildfires or large dust plumes. When these events are present in the simulations (from observations or calculated from meteorological parameters in the model), they automatically calculate a reduction of solar radiation at surface level. These reductions in radiation are generally short lived and constrained to the area where the episode takes place.

Shipping emissions are generally estimated from inventories based on the information provided by shipping companies, or measured via satellite data, and as such, not 100% accurate.

The CAMS shipping emissions inventory is based on the Automated Identification System (AIS), which provides information about the size and routes of major cargo vessels, taking into account meteorology to calculate consumption. In the forecasts, emissions are used as monthly means instead of using the real time AIS information. The recent upgrade of the CAMS system includes a new, more up to date, inventory. The shipping emissions, as the whole CAMS system, is being continually developed to providing more accurate operational data.

Reviewing the record North Atlantic Sea surface temperatures in June 2023, a preliminary analysis from CAMS scientists found a significant negative anomaly in Saharan dust aerosol transport over the tropical Atlantic Ocean, and an increased anomaly in biomass burning aerosol over the North Atlantic, coming from the massive Canadian wildfires. These aerosol anomalies are much bigger than the sulphate change from shipping emission reductions. This makes the estimation of the impact of reduced sulphate aerosol emissions on the sea surface temperatures very challenging.

The Copernicus Climate Change Service (C3S) also suggested that, among other factors, the reduced winds of a weakened Azores anticyclone - an extensive wind system that spirals out from a centre of high atmospheric pressure - could have reduced the ocean-atmosphere exchange and the vertical mixing of the ocean between colder and warmer waters, as well as reducing Saharan dust transport over the Atlantic, all of which has the potential to increase the ocean surface temperature.

“There will be, no doubt, long-term impacts from the reduced SO₂ emissions, but it will demand dedicated research to understand the impact of sulphur changes. The changes in dust or black carbon have a more tangible effect in the short term”, says Richard Engelen CAMS Deputy Director.

CAMS aerosol loading data together with C3S climate records can help understand some short-term effects of reduced aerosols, and the longer-term consequences for the climate.

In recent years, Europe has experienced some of the highest sunshine duration anomalies (and conversely, record lowest cloud coverage) on record, as shown in the Copernicus Climate Change Service’s (C3S) European State of the Climate. The report found that solar radiation across Europe was at its highest level observed since satellite data records began in 1983.

This situation coincides with the warmest summer ever recorded in Europe and widespread drought conditions in Europe, roughly matching the areas that saw higher solar radiation anomalies.

Europe has reduced SO2 emissions significantly, although not only for shipping. As described above, the introduction of the IMO 20 regulation coincided with the emissions reduction linked to the COVID-19 crisis. The effects of international shipping regulations have been measurable.

The efforts of the shipping industry to introduce less pollutant fuels and increase fuel efficiency, and the regulations to reduce fossil fuel emissions globally are an international success regardless of the possible reduction in aerosol loads, with considerable benefits for the environment and human health in the form of reduced risk of acid rain and improved air quality.

A decade ago, a scientific paper coordinated by the Met Office’s Ben Booth in Nature found that anthropogenic aerosol emissions had played an important role in North Atlantic climate variability through the 20th century.

But recent research suggests that the effect of has been overestimated, at a moment where some geoengineering projects study the possibility of releasing SO2 aerosol into the atmosphere as a disputed global warming mitigation strategy.

Experiments using MIROC-SPRINTARS, a coupled atmosphere-ocean general circulation model, found that reducing SO₂ emissions can favour the formation of the North Atlantic Warming Hole, an area in the North Atlantic Ocean warming less than the surrounding waters. This pattern is also seen in simulations of reduced CO₂ emissions. It is believed that it is linked to changes in oceanic circulation, including a slowdown of the Atlantic meridional overturning circulation (AMOC), and the addition of freshwater from the melting Arctic and Antarctic ice sheets, but the process is not well understood.