Fossil fuels are cause the planet to heat up rapidly and the aerosols from their combustion kill millions of people every year. So we need to quickly decarbonize. But ironically, those aerosols actually have a beneficial side effect: They air freshener. It creates a strange climate contradiction. If we burned less gas, oil and coal, we would stop loading the sky with planet-warming carbon, but we would also load into the sky less planet-cooling aerosols.
But exactly how much cooling we get from aerosols, and how strong that effect will be as the world phase out fossil fuels, are big questions for climate researchers. Climate scientist Duncan Watson-Parris of the University of Oxford said: “It is thought that aerosols are very important. “And this uncertainty in the aerosol effect is a major uncertainty in climate science.”
Last week, Watson-Paris published a paper in the magazine Nature Climate change in which he offers a scenario of how aerosol concentrations will change by the end of the century. It assumes that when we burn less fossil fuels, we produce less aerosols. But he was able to adjust how much cooling those aerosols could provide in the future. In one version of the model, which assumes that aerosols have a stronger cooling effect, losing them is like shutting down the planet’s air conditioner. The resulting warming will be enough to beyond the goals of the Paris Agreement keep the global temperature from rising more than 1.5 degrees Celsius.
But if we assume that the aerosols actually have a cooling effect of less than 50%, then losing them will be less important and we will have a better chance of keeping the temperature below 1.5 degrees. He points out that determining the exact scale of this effect will be key for policymakers, who have spent the past two weeks studying. climate conference COP27 in Egypt negotiating how much carbon countries should be allowed to emit.
But determining that number is difficult, given the dizzying complexity of aerosols and Earth’s atmosphere. The burning of fossil fuels creates clouds of microscopic particles, mostly sulfates, that cool the climate in two main ways. “The particles themselves act like tiny mirrors, and they reflect some of the sunlight straight back into space,” said Watson-Parris. “So it’s a bit like an umbrella.” All these small atmospheric parachutes shield the planet’s surface from solar radiation.
The second way is more indirect: They affect cloud formation, which in turn affects the local climate. “All aerosols act as nuclei on which atmospheric water vapor condenses and forms cloud droplets,” said Watson-Parris.
Clouds do this naturally when water condenses around dust particles. But if you add more aerosols to a certain area, the droplets will be more numerous but smaller: There is only so much water vapor to go around all the particles. Smaller droplets are brighter than larger ones, whitening the cloud, causing it to reflect more solar energy into space. “If you make the droplets smaller, they are less likely to precipitate and the clouds can last longer,” says Watson-Parris. “And this—we call it the lifetime effect—is one of the most uncertain and likely one of the larger contributors to this overall cooling.”
