The US pulling out of The Paris Agreement has now made an already ‘challenging’ target to keep global temperature well below 2℃ above pre-industrial levels ‘very, very unlikely’, according to Professor Hugh Coe, a distinguished researcher in the field of atmospheric science at The University of Manchester.
When I asked Professor Coe whether we are on track to meet the Paris Agreement’s terms, his response was an immediate and unequivocal “no”. Though this answer was expected, the certainty with which he delivered it reinforced the stark reality: we are heading for a significant overshoot of the agreement’s aspirational 1.5°C target. Once this threshold is crossed, we risk pushing many of the Earth’s biomes beyond critical climate tipping points – irreversible thresholds that could trigger rapid and catastrophic changes. The consequences include the loss of vital carbon sinks, accelerated global warming, rising sea levels, the continued disappearance of entire island nations, and the thawing of permafrost, potentially releasing ancient microbes frozen underground for hundreds of thousands of years.
Yet, the solution has been known for well over a century. In 1896, just a year after the invention of the radio, Swedish scientist Svante Arrhenius first predicted that human-caused greenhouse gas emissions would drive climate change. From that moment, the only logical solution became clear: stop altering the composition of the atmosphere – stop burning fossil fuels. However, despite decades of scientific warnings, governments worldwide have largely ignored this advice. The reason? A stark disparity in political power between those who profit from fossil fuel exploitation and those who suffer its catastrophic consequences.
The idea of geoengineering has roots in the late 1980s, when scientists, including Professor Coe during his PhD at UMIST (University of Manchester Institute of Science and Technology – the precursor to UoM), were studying acid rain. His career has followed the trajectory of climate science for over 30 years, with a particular focus on aerosols – tiny liquid droplets in the atmosphere produced by natural processes such as volcanic eruptions, as well as by human activities like fuel combustion.
Aerosols have played a surprising role in climate dynamics. Since the Industrial Revolution, their increased presence has led to various environmental impacts, including acid rain, ozone depletion, and a cooling effect on the climate. This occurs because aerosols influence cloud reflectivity, or “cloud albedo“. When aerosols act as condensation sites for water vapour, clouds form with more but smaller water droplets, increasing their reflectivity. This enhanced cloud albedo reduces the amount of solar radiation reaching the Earth’s surface, thereby cooling the planet.
Some of this cooling effect is already observable. Global temperature records from the 1950s show an otherwise unexplained plateau in warming, coinciding with high levels of aerosol pollution from industrial activity. However, as governments later took action to reduce acid rain by curbing aerosol emissions, this cooling effect diminished, allowing warming to resume at an accelerated rate. This raises a critical question: can we deliberately harness aerosols’ cooling power to further slow climate change?
While geoengineering strategies such as aerosol injection could theoretically provide temporary relief, they come with significant risks and uncertainties. Regardless, one fact remains clear: unless governments act decisively to reduce greenhouse gas emissions, any temporary fixes will only delay the inevitable.
One proposed method of Solar Radiation Management (SRM), as Coe explains, was first introduced by his late colleague, Professor John Latham. A former head of Manchester’s Physics Department and namesake of the Latham Laboratories in the Simon Building, Latham was also an acclaimed, prize-winning poet. His idea – Marine Cloud Brightening (MCB) – involves “seeding” marine stratocumulus clouds with sea salt aerosols, generated at or near the ocean surface. Essentially, this means spraying tiny seawater droplets into low-lying clouds, increasing their reflectivity and making them appear brighter when viewed from above.
The effectiveness of MCB in enhancing cloud albedo depends on the concentration of background aerosols. Over the ocean, where human activity is minimal, natural aerosol levels are much lower, making cloud brightening significantly more effective in these areas. This also explains why burning low-quality shipping fuel has a disproportionate impact on acid rain: pollutants are introduced into an otherwise clean atmospheric environment. According to models by Latham, MCB could theoretically offset the warming effects of a twofold increase in pre-industrial CO₂ levels. While not a solution to global warming, it exemplifies how scientific innovation could help manage the climate crisis and buy time for longer-term solutions.
One of MCB’s most promising potential benefits is its ability to preserve sea ice. Their model simulated environmental and climatic changes over a 70-year period (2020–2090), assuming atmospheric CO₂ levels increase by 1% annually until doubling pre-industrial levels by 2045, after which they stabilise (2CO₂ scenario). Without MCB, the model predicted a 76% decline in Arctic minimum sea ice coverage. However, with MCB, this reduction dropped to just 3%, effectively preserving most of the Arctic’s sea ice.
The implications of this are significant. By maintaining ice coverage, MCB could help protect Arctic wildlife and slow the acceleration of global warming by preserving highly reflective icy surfaces that prevent excessive solar absorption. Additionally, the cooling effect of MCB could keep regional temperatures low enough to prevent permafrost thawing – mitigating the risks associated with its melt, including carbon release, the potential reintroduction of ancient microbes, and the destabilisation of vast land areas.
But, geo-engineering the climate does come with significant risks. An MCB project large enough to offset the warming effects of a 2CO₂ scenario could cause a globally averaged 1.3% reduction in rainfall. However, the same model also predicted a 3.5% increase in rainfall over land, attributed to the inland flow of moist ocean air caused by cooling.
Predicting changes in precipitation patterns remains one of the biggest challenges in climate modelling. While the models provide valuable insights into risk management, they are not yet precise enough to determine the full impact of MCB. One key variable is the location where aerosols are introduced into stratocumulus clouds. By carefully selecting these locations, it may be possible to mitigate negative effects on critical ecosystems such as the Amazon rainforest. However, uncertainty remains. While many models suggest that the Amazon would experience reduced rainfall, there is no consensus on the severity of this reduction – some even predict no significant change at all.
Ultimately, scientists, including Professor Coe, agree that our current understanding of climate sensitivity to aerosols is insufficient to produce a fully reliable model of geoengineering’s consequences. As it stands, attempting to manipulate the atmosphere would be a high-stakes gamble, one that could disrupt ecosystems and impact millions of lives in an effort to mitigate the greater threat of global warming.
When asked if there had been a moment in his research career that underscored the urgency of the climate crisis, rather than pinpointing a single event Professor Coe reflected on the broader trend he has observed: while international cooperation has successfully addressed environmental challenges such as acid rain, CFC (chlorofluorocarbons) pollution, and air quality, climate action has failed to follow suit. “We don’t see an evolution in our ability to get on top of the problem, and that means the pressure is building and building”, he told me.
The scientific consensus is clear: we are not acting fast enough. Across the globe, climatic tipping points are approaching, threatening irreversible damage to ecosystems and placing millions of lives at risk. We are behind in the race against climate change. The solution is straightforward: stop burning fossil fuels and achieve net-zero emissions. But with time running out, science must explore every possible avenue to buy us the time we so desperately need.
