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Climate change and aerosols: new research

New research published today in Nature gives us a better idea of how much aerosols produced by people are influencing climate…

How much influence do aerosols such as this smog have on climate? Flickr/freddie boy

New research published today in Nature gives us a better idea of how much aerosols produced by people are influencing climate change.

Aerosols — tiny particles produced naturally and by burning fossil fuels — work against warming by greenhouse gases to cool the Earth. This led scientists to speculate that aerosols were “masking” warming, and that without aerosols in the atmosphere the Earth would be heating up much faster.

The study released today shows that aerosols produced by people are not as important as once thought. But natural aerosols are far more important for their influence on climate, and researchers warn without better understanding of natural sources of aerosols, climate forecasts will fall short.

Aerosols have two sources: natural emissions such as those from volcanoes, bushfires and the ocean; and unnatural sources such as burning fossil fuels and sulphate emissions. They can be solids such as smoke and sea salt, liquids such as water, or gases such as sulphur dioxide (SO2).

Through their interaction with clouds, aerosols reflect more of the sun’s energy back into space, meaning that their overall effect is to cool the Earth. Scientists measure this cooling through “radiative forcing”. Aerosols from both natural and human sources have an overall forcing of -1.16 Watts per square metre.

But this cooling effect of aerosols is dwarfed by the warming effect of increased greenhouse gases. That’s why the Earth is heating up. Knowing these influences is vital for understanding the Earth’s “climate sensitivity” — how much the planet will warm in the future.

The research published today uses statistical analysis to show that human aerosols have likely had no change in cooling effect since the 1980s, even as aerosol emissions have increased. And cleaning up aerosols, such as those that are the source of dramatic air pollution in China, would likely have little impact on climate.

Dr Matthew Woodhouse, research scientist at CSIRO and an author on the paper, explained:

At high concentrations of aerosols, like the present, the change in forcing for a given change in anthropogenic aerosols is actually quite small. It’s only when you get down to what is quite a clean atmosphere anyway that you see big changes in forcing. Cleaning up 10% of aerosols won’t necessarily make a big difference to the forcing.

Measuring and modelling these natural sources is the next challenge for climate scientists, Dr Woodhouse said. Calculating the effect of aerosols is very complicated. For example aerosols such as sea salt are highly influenced by winds, which are difficult to model.

Professor Andrew Glikson at Australian National University said the research was solid, but had its limitations in focusing on short-term climate effects. Aerosols have a short lifetime in the atmosphere before they fall back to Earth, which means if aerosol emissions were halted they would quickly cease their cooling effect.

Professor Glikson also said new research means clean air policies will need to account for variations in natural aerosol emissions.

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26 Comments sorted by

  1. Keith Thomas

    Retired

    No, atmospheric aerosols have three sources: the third is biological processes. These natural aerosols include terpenes - which make the Blue Mountains blue- and bacteria. They are the products of plants and so their quantity and quality has an anthropogenic component, varying as it does according to land clearing, felling of old-growth forests etc. And wasn't there animal dander found in the upper atmosphere a few years ago?

    Bacterial aerosols include a significant proportion falling within the size range that can nucleate water vapour into rain droplets. It is these bacterial aerosols that enable the precipitation of the Amazon to be about five times higher than the moisture entering the region - a fact that puzzled observers when it was first noted.

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  2. Peter Smith

    (None)

    Does this suggest sulphur geo-engineering will have little effect other than bleaching the sky?

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  3. Mike Pope

    logged in via email @hotmail.com

    Dr Glickson notes that ...Aerosols have a short lifetime in the atmosphere before they fall back to Earth.

    Very true - but what part of earth do they fall back to? If that part happens to be a snow or ice covered surface, aerosol soot particles are likely to absorb solar energy which would otherwise have been reflected by a pristine surface. In other words, aerosols contribute to reduction of albedo thereby increasing the level of energy absorbed by the Earths surface, radiated as infra-red light and re-radiated by greenhouse gases in the atmosphere.

    Aerosols definitely have a cooling effect but they also have a warming effect

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  4. Warwick Rowell

    Permaculturist

    Dr Matthew Woodhouse, research scientist at CSIRO and an author on the paper, explained:

    "At high concentrations of aerosols, like the present, the change in forcing for a given change in anthropogenic aerosols is actually quite small. It’s only when you get down to what is quite a clean atmosphere anyway that you see big changes in forcing. Cleaning up 10% of aerosols won’t necessarily make a big difference to the forcing."

    Let me check to logic: At high concentrations, a given change will…

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  5. Dianna Arthur
    Dianna Arthur is a Friend of The Conversation.

    Environmentalist

    The preceding 3 posts illustrate how complex is the science of climate.

    Everything has an effect, some effects contradict others.

    Safest course of action? Reduce the amount of pollutants; the fossil fuels, the chemical infestation of waterways and groundwater. To mitigate the effects of our behaviour is not anywhere near as difficult as finding a definition that suits everyone.

    Stop talking, start acting.

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  6. Andy Cameron

    Care giver

    Brilliant article. And timely. I recently encouraged a curious youngster, who asked me why Snooki (Jersey Shore) used so much hair spray - "why don't they tell her she's causing climate change"? A bit baffled by the question's syntax, and pre-occupied with peeling potatoes, I suggested Wikipedia's "Simple English" page on the "greenhouse effect". Two days later, the same youngster asked me, "why aren't we taxing water instead of carbon dioxide"? Crickets.

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  7. Shirley Birney

    logged in via email @tpg.com.au

    Indeed “how now brown cloud?” or more specifically atmospheric brown clouds (ABCs) that hover seasonally over Asia (and to a lesser degree, North America and Europe). Sulphate aerosols cool the atmosphere and mask the warming in black carbon particulates in brown clouds, a result of biomass burning and industry emissions including diesel fuel.

    So black carbon is, it appears, the big brown ogre. I’ve not yet read all the links provided by the authors but it appears that cleaning up dirty air…

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  8. John Carlaw

    logged in via Facebook

    -1.16 WM**-2 is hardly dwarfed by net forcing of 0.82-2.47 depending on scenario, they are similar magnitude
    per http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2-1.html
    and AR5 Wg1 ch7

    even in ar5 there are still very large unknowns
    http://www.ipcc.ch/report/ar5/wg1/#.Unu2RnAfeCU
    Chapter 7: Clouds and Aerosols
    section 3

    See also AR4WG1 ch8 ->AR5WG1ch9 both climate model evaluation
    Ar5 doesn't show a huge firming up of the uncertainties

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  9. Comment removed by moderator.

  10. Mark McGuire

    climate consensus rebel

    What happened to the thesis of Trenberth with missing heat of 4 Hiroshima bombs a minute diving to the deep oceans, doing karaoke with Elvis? That didn't work so now it is aerosols causing the un-modelled halt in global warming.
    And this passes as settled science. Much like a kidney stone.

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  11. John Crest

    logged in via email @live.com.au

    Damned street artists!

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  12. Warwick Rowell

    Permaculturist

    HI Mark

    "SETTLED SCIENCE" is a contradiction in terms. Nearly settled, unsettling, and various other partial statements seem to be the state of play with this relatively new area of scientific investigation. I am astounded by how rapidly it has become more sophisticated, and more accurately describes an incredibly complex and dynamic situation.

    Thomas Kuhn's seminal work on the Structure of scientific revolutions is just under fifty years old; how things have changed since then. More scientists, more challengers, more computing power, more integrated, systems work, all bringing the wonderous and difficult complexities of this planet to our attention.

    Now we have to act appropriately, or it seems likely the big brain experiment may prove to be a dead end. Homo sapiens needs to start doing a bit more collective, effective sapiens-ing..

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    1. Michael Shand
      Michael Shand is a Friend of The Conversation.

      Software Tester

      In reply to Warwick Rowell

      Mark is merely a troll climate contrarian, a massive hypocrite that doesn't care to actually understand anything, just wants to attack it as left wing socialist conspiracy by the united nations to make us live in hobit homes in accoradance with agenda 21 blah blah alex jones, andrew bolt, style crazy

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  13. helen stream

    teacher

    This whole piece and the research it describes, is premised on the idea that until now it was postulated that the 'missing heat' that CAGW high priest Trenberth is desperately seeking, is being masked by cooling aerosols---that if we could clean up the industrial aerosols, the world would see even more catastrophic warming than we are now warned of by the alarmists---but that now we don't have that option to cut the warming, and that we should factor that in and think, ' Shock horror, it's worse…

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    1. Shirley Birney

      logged in via email @tpg.com.au

      In reply to helen stream

      "Why do they and you---including apparently the CSIRO---- only deal with the aerosols you think might be masking warming----and not with those that world-respected climate scientists have testified to the US Congress are responsible for causing at least 50% of the warming???"

      "At least 50% of the warming?" My, that sounds like creative accounting to me. Are you alluding to black carbon? Please provide a link which validates the accuracy of this declaration to Congress.

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    2. helen stream

      teacher

      In reply to Shirley Birney

      Shirley Birney...

      I haven't got time to find all the links, but provide these comments on research that discusses the more than 45% figure, and much other research notes that the warming from black carbon is amplified as it reduces albedo, creates increasing areas of dark water and sets up a catastrophic feedback cycle that increases global warming----and black carbon also becomes the darkening part of cryoconite deposits and cryoconite holes which have an ongoing and building effect on the melt…

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    3. Shirley Birney

      logged in via email @tpg.com.au

      In reply to helen stream

      "I haven't got time to find all the links,"

      Helen I requested one link - not several. Since you appear to have quoted so efficiently and at length, I'm confident you would have access to the report submitted to Congress where you claim that "climate scientists testified to the US Congress (that aerosols) are responsible for causing at least 50% of the warming."

      Tomorrow will be fine for the link - when you're ready thanks.

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    4. helen stream

      teacher

      In reply to Shirley Birney

      Shirley Birney...

      First up, I don't work for you, and don't feel like going to any trouble at all to point you to links---especially considering your officious tone.

      Exactly what is it you're so pompously disputing----that prominent scientists including the ones I mentioned reported to the US Congress on black carbon at all?---disputing that they were non-sceptics?----disputing that they warned Congress that black carbon was a very large problem that should be mitigated in the short term…

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    5. Shirley Birney

      logged in via email @tpg.com.au

      In reply to helen stream

      "First up, I don't work for you, and don't feel like going to any trouble at all to point you to links..."

      I used to be very particular about whom I employed Helen (or should I address you as Mr Coochey?)

      Thank you for yet another verbose diatribe on the impacts of BC. However, I simply reiterated a simple request for you to supply a link in which you claim scientists testified to Congress that “other” aerosols cause at least 50% of the warming. Now you claim it is “more than (the) 45…

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    6. helen stream

      teacher

      In reply to Shirley Birney

      I'm not in favor of BAU, but of direct action and adaptation to the moderate warming that is most likely the natural result of the earth's emergence from the Little Ice Age.---but not of carbon taxes and Emissions Trading Schemes that damage our sovereignty and our economy.

      I don't deny that CO2 causes some warming, but I have never seen a refutation by Hansen of his conclusion, and that of others that...

      [ '...although the addition of carbon dioxide in the atmosphere does increase the surface…

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    7. Michael Shand
      Michael Shand is a Friend of The Conversation.

      Software Tester

      In reply to helen stream

      Helen has no time to find links, no time......unless it's time to reply to comments and then, well of course, unlimited time it seems

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    8. Michael Shand
      Michael Shand is a Friend of The Conversation.

      Software Tester

      In reply to helen stream

      "moderate warming that is most likely the natural result of the earth's emergence from the Little Ice Age"

      Are you a climate scientists Helen?

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  14. Geoffrey Sherrington

    Surveyor

    Where in the lead article do the authors note that aerosols are poorly understood to the extent that they are a main candidate for the poor performance of climate models?
    Would it not be prudent to absorb a new paper over a few weeks, rather than rushing to print with "New research published today in Nature..."
    Are the authors aware of back-and-forth between Dr Slingo of the UK Met Office and Nic Lewis, and independent climate scientist, that notes that the HadGem2 GCM has been designed with a link between aerosol effects and other variables that effectively prevents the generation of low values for ECS? As Lewis puts it -
    "As HadCM3's parameters are perturbed, the resulting changes in ECS and aerosol forcing are closely linked. When significantly lower values for ECS – as suggested by recent observational studies – are obtained, HadCM3's aerosol forcing takes on highly negative values."

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