The planet is warming as extra greenhouse gases in the atmosphere trap more heat. But at the same time, other tiny particles and gases known as aerosols are having a cooling effect. The big puzzle for scientists is trying to understand how big that effect is.
The tiny particles called aerosols cause clouds to reflect more of the sun’s energy back out to space. Human activities add aerosols to the atmosphere, and that spares the Earth’s surface some warming. But precisely how much is unknown.
A new study out today offers one reason why it’s hard to pin down the size of aerosols’ cooling effect. According to Professor Ken Carslaw from the University of Leeds, the problem is not knowing how much natural aerosols were affecting the climate before humans came along and started adding more to the atmosphere. Without a baseline, its hard to measure how much things have changed since.
What are aerosols?
Aerosols are microscopic particles that float in the atmosphere. They come in a range of sizes – the smallest are a few nanometers wide, smaller than the smallest viruses. Larger aerosols can be as big as the diameter of a human hair.
They come from a range of sources, both natural and manmade. Natural sources include volcanic emissions, plant vapours and chemicals released by tiny sea creatures. Since humans started industrialising, we’ve also been emitting them from car exhausts, factories and power plants.
Once in the atmosphere, aerosols cool the climate in two ways. They directly scatter sunlight and reflect it back out to space. They can also react with clouds in complex ways, causing the clouds to reflect more light back out to space. This prevents the sun’s warmth from reaching the earth’s surface.
Working out the exact size of this second, indirect cooling effect is a challenge for scientists. They know how much aerosols in clouds are cooling the earth within a range, but they want to get a more precise assessment.
Not knowing how much cooling these cloud-aerosol interactions produce is one of the biggest uncertainties limiting our understanding of how climate has changed in the past, and our ability to predict how it might change in the future. That’s because at the moment it’s hard to know precisely how much warming the aerosols are masking.
Why the uncertainty about their cooling effect?
The new research, published in the journal Nature, calculates its own ballpark range for how much cloud-aerosol interactions have cooled the climate over the industrial period. Then it works out where most of the uncertainty is coming from.
The paper estimates that 45 per cent of the variation in scientists’ estimates of cloud-aerosol cooling is linked to uncertainty around natural aerosol emissions. It’s difficult to know the exact amount of natural aerosols being emitted or where those emissions are coming from, the authors tell us.
Without a measure of the amount of natural aerosols that were present in the atmosphere a few hundred years ago, it’s hard to know how much things have changed since humans started adding manmade aerosols into the mix. That makes it difficult to calculate the exact size of the cloud-aerosol cooling effect. Professor Ken Carslaw says these natural aerosol uncertainties have “essentially been neglected in previous studies”.
Narrowing the ballpark range
By identifying where most of the uncertainty is coming from, research like this highlights where future efforts could be focused if scientists want to narrow that ballpark range.
Professor Piers Forster, a co-author on the paper, explains:
“The best way to improve our understanding of the effect aerosols have on the climate is to better understand and measure natural aerosol emissions and their processes, especially how much sulphur dioxide is produced by volcanoes.”
Getting accurate measurements of present day emissions is possible. But working out how much natural aerosol sources emitted in the past is much more of a challenge, and that’s the information scientists really need to build the aerosol baseline. Carslaw explained:
“We may have to live with this source of uncertainty, unless we can find regions of today’s atmosphere that reliably resemble the pre-industrial.”
Scientists also need to know more about the climate impacts of past aerosol emissions because in today’s polluted atmosphere, clouds are less sensitive to the presence of aerosols.
It’s not all bad news though for this area of research though. The latest climate science report from the Intergovernmental Panel on Climate Change (IPCC) refined its estimate of how much cooling aerosols cause overall, saying that the amount of warming they offset is less than previously thought.
And there are other areas future research can focus on to get a clearer idea of aerosols’ cooling effect. Dr Olivier Boucher, a lead author of the IPCC’s recent chapter on aerosols and clouds tells us that in his opinion, understanding the microphysical processes that enable aerosols to alter the reflective properties of clouds is “probably as important” as focusing on the uncertainties around natural aerosols.
Carslaw et al. (2013) Large contribution of natural aerosols to uncertainty in indirect forcing. Nature. DOI: 10.1038/nature12674