A significant amount of the climate change caused by greenhouse gas emissions in the past century has been hidden from us, scientists say — by another type of pollution that actually cools the climate and temporarily cancels out some of the warming. Two new studies, both released today in the journal Nature Geoscience, address the powerful influence of aerosols — fine particles or drops of liquid often released by industrial activity — on the climate, and suggest that as nations around the world work to reduce this type of air pollution, we will begin to see more rapid warming than expected. And that could hurt our ability to meet the climate goals set in last year’s Paris Agreement.
Scientists have long been aware that certain types of aerosol emissions — most notably, sulfate — can block solar radiation from getting through to the surface of the Earth, either by scattering sunlight directly or by helping to increase the extent and reflectivity of cloud cover over the planet. This results in a kind of cooling effect, especially in the areas where the pollution is heaviest, which can temporarily mask the ongoing warming effect caused by greenhouse gases in the atmosphere.
The important thing to remember about aerosols, though, is that they tend to have a rather short lifespan in the atmosphere — eventually, the rain brings them back out of the sky. If humans are continuously pouring pollution into the air, then the cooling effect will probably appear pretty constant. But as certain parts of the world start cutting down on their aerosol emissions, scientists have actually noticed a phenomenon known as “regional brightening,” in which the dimming effect on solar radiation begins to lift away.
This effect is already having a significant impact on certain parts of the world, as one of the new Nature Geoscience studies points out. That paper examines the impact of European reductions in aerosol emissions on warming in the Arctic, which is proceeding at a faster rate than other parts of the world. Out of all the regions of the world, Europe has reduced its aerosol loading the most in the past several decades, which means its policies have likely had the biggest effect on recent aerosol-related climate changes.
Using simulations from a climate model, which took changes in aerosol loading into account, the authors conclude that as much as 0.5 degrees Celsius of the warming that took place in the Arctic between 1980 and 2005 can be explained by aerosol reductions in Europe during that time. In other words, as the aerosol “mask” is being pulled away, researchers are seeing an enhanced regional warming as a result.
It’s true that aerosols produce the biggest effects in the regions where they’re emitted. Unlike greenhouse gases, which stay in the atmosphere for long periods of time and gradually diffuse out over the Earth, aerosol particles cluster in the sky where they were originally emitted — they don’t really have time to spread out much before raining back down.
The reason that changes in aerosols over Europe have such an effect in the Arctic has to do largely with the oceanic and atmospheric currents that run between Europe and the Arctic. In an accompanying commentary in Nature Geoscience, Thorsten Mauritsen of the Max Planck Institute for Meteorology — who was not involved with the study — notes that “Europe is situated right on the main pathway that air and ocean currents take from more southerly latitudes into the Arctic.”
So, he adds, “In summer, the warming of the atmosphere and upper ocean through reduced aerosol cooling over and around Europe yields a strengthened transport of heat into the Arctic.” And because the Arctic is generally so pristine otherwise, any aerosols that managed to drift up there from Europe would have likely had a more noticeable cooling effect than in other parts of the world — translating to a similarly pronounced increase in warming when they finally rained back down to Earth.
The researchers on that study are already starting an examination of whether changes in North American aerosol emissions — though less pronounced than the changes that have taken place in Europe — might have had similar effects, said the paper’s senior author Annica Ekman, a professor of meteorology at Stockholm University. But for now, the new paper points to the significant influence aerosol emissions can have on the Earth’s climate, at least regionally — and how their disappearance from the atmosphere might be revealing a more profoundly warming world than suspected.
As if that’s not enough, the second study published in Nature Geoscience on Monday points to the significance of the aerosol effect on a global scale.
Because of the major impact aerosols can have on the Earth’s temperature, many scientists agree that it’s important to consider them when making predictions about the earth’s climate future. And one important way of understanding what future warming will look like is to investigate a phenomenon known as Earth’s “transient climate sensitivity” — that is, how much the planet’s temperature will change when the amount of carbon dioxide in the atmosphere reaches double the level it was in preindustrial times.
This carbon dioxide doubling is expected to occur some time in this century, depending on how much humans are able to reduce carbon emissions in the meantime, said Trude Storelvmo, an associate professor of atmospheric science at Yale University and lead author of one of the new papers. So it’s important to figure out what kind of temperature changes we might expect to see as a result.
These calculations are generally based on the temperature responses caused by greenhouse gases in the atmosphere already. But because aerosol emissions are believed to have masked a significant amount of the warming that’s happened on Earth so far, some scientists believe that estimates of Earth’s climate sensitivity are too low — that the climate may, in fact, be more sensitive to carbon emissions than anyone realized, which would likely mean greater warming in the future.
Storelvmo and an interdisciplinary group of colleagues decided to investigate. But rather than using climate models, which they say don’t always accurately simulate the effect of aerosols on clouds in the atmosphere, they instead based their calculations entirely on records of temperature and solar radiation taken from more than a thousand measurement sites around the world between 1964 and 2010. They then performed a statistical analysis designed to help distinguish between the temperature changes that were caused by greenhouse gas emissions and those caused by aerosols.
Their analysis suggests that about a third of the continental warming that occurred between 1964 and 2010 was masked by the cooling effect caused by aerosols. In other words, greenhouse gas emissions during that period had a bigger effect on the climate than they actually appeared to at the time. Taking this into account, the researchers then calculated Earth’s transient climate sensitivity and found that at the time of carbon dioxide doubling — whenever that occurs — we should see a temperature increase of about 2 degrees Celsius above preindustrial levels.
Previous estimates of the transient climate sensitivity have produced a wide range of results, anywhere from below 1 degree to above 3 degrees Celsius, the authors point out, although they note that most other observational studies have produced central estimates below 2 degrees. Storelvmo suggests that some of these studies may have underestimated the influence of aerosols, or that their methods were too sensitive to short-term fluctuations in the climate, such as the so-called warming hiatus over the past decade.
On that note, this paper’s results are significant because of the goals that were set during December’s UN climate conference in Paris. Until that point, many climate activists had pushed to keep warming below a 2-degree limit, citing the potentially catastrophic climate effects that could occur otherwise. But more recently, scientists have suggested that allowing even 2 degrees of warming might be too much. So during the Paris conference, world leaders agreed to make an effort to keep warming below 1.5 degrees Celsius.
“Obviously, [our study] has the implication that we can’t allow for carbon dioxide doubling to happen if we care deeply about these warming limits,” Storelvmo said. “And so it has really implications in the sense that the higher the transient climate sensitivity is, the more fossil fuels will have to stay in the ground.”
Both studies contain sobering sets of results that speak to both the urgency of reducing global dependence on fossil fuels and the importance of taking the aerosol effect into account when making estimates about global warming.
When it comes to the Arctic, one of the world’s most vulnerable and rapidly changing regions, Ekman noted that “it seems like our aerosol particles have somehow masked the amplified Arctic warming, and as greenhouse gas concentrations continue to increase, it’s going to take over — basically, it’s going to warm more and more in the Arctic.”
And as more and more parts of the world start to cut down on their aerosols as well — a process that is vital for the improvement of air quality and the protection of human health around the globe — we may start to see the mask begin to lift away in other areas as well.