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Evening sun behind clouds above the sea, Shetland Islands.
© Matthias Graben/imageBROKER/Corbis
5 October 2015 16:45

Factcheck: Aerosols research misinterpreted to ‘alarming extent’’, says study author

Robert McSweeney


Robert McSweeney

05.10.2015 | 4:45pm
FactchecksFactcheck: Aerosols research misinterpreted to ‘alarming extent’’, says study author

In its online edition on Friday, the Express claimed that the discovery of a new “natural coolant” by scientists has thrown “previous estimates of rising temperatures into doubt”.

The story followed in the footsteps of similar articles on the Register and Breitbart, two websites which have a history of publishing climate sceptic articles. The Register said the new research meant “there isn’t as much urgency about the matter [climate change] as had been thought”. While a Breibart article by James Delingpole claimed it presents “further proof” that “the reason that all that predicted ‘global warming’ has failed to materialise is that it has been countered by the planet’s natural cooling effects.”

The Express and Breitbart quoted Benny Peiser, director of the Global Warming Policy Foundation, a climate sceptic lobby group, as saying: “Here is more evidence…that climate models…should never have been trusted in the first place.”

But a co-author of the study tells Carbon Brief the researchers “completely disagree” with the way their study has been reported, and that these articles are a “misuse of our research”.

The Express, online edition, Friday 2 October.

The Express, online edition, Friday 2 October.


The new study, published last week in the journal Environmental Science & Technology, concerns natural emissions of a compound called “isoprene”. Despite sounding like a material a keen cyclist would wear, isoprene is actually a gas that helps aerosols form in our atmosphere.

Aerosols are tiny liquid or solid particles in the air. They have a direct effect on temperature by scattering sunlight, and an indirect effect by stimulating clouds to form, affecting how much sunlight reaches the Earth’s surface. Overall, they generally cool the Earth’s surface, counteracting – but not offsetting – the warming impact of rising levels of greenhouse gases in the atmosphere.

Aerosols have natural sources, such as volcanic emissions and plant vapours, and manmade sources, such as car exhausts, factories and power plants.

Isoprene isn’t an aerosol in its own right, but combines with other chemicals in the atmosphere, such as oxygen, to create them. These particles are big enough for water vapour to condense onto, allowing clouds to form.

Scientists know that isoprene is produced by plants and trees on land and by plankton in the oceans. But the new study finds that it is also produced by the interaction of sunlight with chemicals in the top tenth of a millimetre of the ocean surface.

While model simulations suggest the oceans produce around 1.9m tonnes of isoprene per year, the results of the new study suggest the ocean surface could produce between 0.2-35m tonnes of additional isoprene.

Breitbart article, Thursday 1 October.

Breitbart, Thursday 1 October.

‘Crazy’ claims

So, if the oceans are producing more isoprene than scientists thought, could this “pose a serious threat to manmade global warming theory,” as Delingpole claims? The answer is “no”, Prof Piers Forster tells Carbon Brief, in no uncertain terms:

“To say that it is potentially important for climate change is so far from the mark as to be quite crazy.”

Forster, a professor of physical climate change at the University of Leeds who wasn’t involved in the study, explains that there are several reasons why.

First, the media reports confuse background levels of isoprene with a change over time, he says. In order to reduce the impacts of climate change, isoprene levels would need to rise to counterbalance increase greenhouse gases in the atmosphere. But the paper isn’t saying that isoprene levels are getting higher, just that they’ve always been high, Forster says:

“The natural aerosol cooling could be 100 times bigger than our current estimate, but it would make no difference to climate change as it would stay more-or-less constant with time.”

Second, the paper refers to the amount of isoprene, not the extent of its cooling effect, Forster points out. Isoprene helps clouds to form, but clouds can both warm and cool the surface of the Earth. So, as other evidence suggests, larger amounts of natural aerosols doesn’t necessarily result in a like-for-like decrease in temperatures.

Third, there are many different natural aerosols. So even if higher levels of isoprene caused higher levels of aerosols in our atmosphere, any reevaluation of total natural aerosol emissions “would only be revised up slightly”, says Forster.

The Register article, Tuesday 30 September.

The Register, Tuesday 30 September.

New small detail

Overall, the three media reports “misinterpret to an alarming extent” the findings of the study, co-author Dr Christian George from the University of Lyon tells Carbon Brief:

“We didn’’t make any statement about cooling effects. We showed just a new small detail that might have an impact on the forming processes of clouds.”

It is unlikely that higher-than-thought levels of isoprene are a factor in the recent slowdown in global surface temperature rise, as Delingpole’s article claims.

In fact, as isoprene only hangs around in the atmosphere for less than a month, its impact is mostly limited to regional or continental-scale climates, George says.

Similarly, these new findings are unlikely to affect projections for global temperature rise in the future, says George, though they will contribute to fine-tuning estimates on smaller geographical scales:

“Our study is a new brick that should help understanding our complex world, by providing new knowledge on air-sea exchanges, but it definitively does not question climate change, it just helps us understand its impact.”

The scientific evidence on manmade climate change is clear, concludes George, and their results do not cast any doubt on this:

“There is no question that the global climate will become warmer. The question is just how much, how fast and how the effects will change our lives.”

Ciuraru, R. et al. (2015) Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface, Environmental Science & Technology, doi: 10.1021/acs.est.5b02388

Image: Evening sun, Shetland Islands.
Sharelines from this story
  • Carbon Brief factchecks claims by The Express that the discovery of a new “natural coolant” by scientists has thrown “previous estimates of rising temperatures into doubt”

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