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How clouds can make climate change worse than we thought

The amount of global warming we can expect in the future has been a tough question to pin down. A new study that I led with colleagues in France has enabled us to come up with a more accurate analysis…

Understanding clouds is crucial to understanding whether temperatures will rise quickly in coming decades. Visun Khankasem/Shutterstock.com

The amount of global warming we can expect in the future has been a tough question to pin down. A new study that I led with colleagues in France has enabled us to come up with a more accurate analysis based on a better understanding of how clouds behave. It suggests that if fossil fuel use continues unabated we can expect warming of at least 4C by 2100. This is at the high end of the warming range suggested by many existing studies.

Our study ultimately concerns “climate sensitivity” - the amount the planet warms for a given amount of greenhouse gas. Think of it this way: to plan for a pizza party, you would start with an estimate of how much pizza an average person eats, and then multiply by the number of guests you plan to have. By analogy, global warming predictions begin by working out the climate sensitivity, and then scaling it to the amount of carbon dioxide and other warming gases we expect to emit.

Uncertainty abounds

The problem is that estimates of climate sensitivity have long ranged from 1.5-4.5C for a doubling of atmospheric carbon dioxide levels. Imagine not knowing whether the average guest will eat two or five slices of pizza. Determining this number more accurately is arguably the biggest question in climate science.

Unrestricted fossil fuel use could bring the carbon dioxide concentration to roughly three times pre-industrial levels by 2100. If climate sensitivity were near the 1.5C level, this would lead to 2-3C of further global warming by the end of the century. But if climate sensitivity were closer to 4.5C, we could expect 5-6C of warming by 2100. And by 2200, the warming in the high-sensitivity case could exceed 10C.

We could probably adapt to 2C of warming, although at the least it would be disruptive to delicate ecosystems like tropical coral reefs. But 6C would be catastrophic. Even 4C would probably be enough to displace tropical human populations, destroy natural ecosystems, force changes to agriculture and our way of life, and lead to eventual loss of the Greenland ice sheet and many metres of sea-level rise. So the amount matters.

Cloud control

This is where the clouds come in. Clouds, along with other factors such as sea ice, are affected by warming in ways that then go on to influence climate sensitivity. The question is whether they increase or decrease it.

In most calculations, warming reduces the amount of clouds near the ocean surface, increasing climate sensitivity because these clouds reflect less sunlight and thus allow more warming. This effect is much stronger in some models than others.

Our study breaks new ground in two ways. First, it identifies what is causing much of the discrepancy between different models, and why most of them show clouds near the surface thinning out as the climate warms. Second, it strongly discounts the lowest estimates of future warming. I am very happy about the first result, not so happy about the second one.

The key process we have identified is mixing of air between levels near the ocean surface where these clouds form, and levels a few kilometres higher up. If this mixing is strong, it tends to thin out the cloud layer as the atmosphere warms.

On the other hand, if the mixing is very weak, the water lost from low clouds is more than matched by increased ocean evaporation. Thus the clouds are thickened and warming is reduced.

By looking at the present-day atmosphere, we can therefore work out which models will offer the best prediction of the future.

We found that the mixing is strong in the real atmosphere, implying a high climate sensitivity. Of the 43 climate models we examined, every one with realistically strong mixing had sensitivity of more than 3C.

This narrows the expected range of warming to between 3C and 4.5C for a doubling of carbon dioxide. This is a momentous and sobering result, but how reliable is it?

The big picture

Some people have interpreted the recent modest warming rates as evidence of lower climate sensitivity. It would be great if those lower values were definitive, but working them out requires us to understand all the influences on climate in recent decades. For instance, pollution particles called aerosols have changed greatly in recent decades, particularly as a result of Asia’s economic growth, with uncertain effects on climate.

In contrast, researchers looking at prehistoric warming episodes have often found evidence of higher sensitivity.

As with any single study, our work does not settle the issue of climate sensitivity. But it does open a new door in understanding the role of clouds, and it raises the likelihood that sensitivity is high. If we are lucky, future research may find something that brings our best estimates of warming back down again. However, our new study shows that such a lucky finding would have to involve some process that is currently missing from every climate model - a much taller order than before.

Some media and blog reports have claimed our work shows that climate models are wrong. This misses the point. All models have flaws, but usually these flaws tell us nothing about whether — or in what direction — their future predictions might be off. What we have found is one particular flaw that systematically causes many (but not all) models to underestimate warming.

Our result is a sobering one, but in a logical world it would not alter policy all that much. After all, if you didn’t know whether the average guest would eat two or five pizza slices, you would probably order enough pizza to cover yourself in any case. Given the appalling stakes for being wrong on carbon dioxide, it is crazy not to take the same conservative approach. But so far, the world has evidently been crazy.

Perhaps our result can serve as a reminder that not knowing everything does not justify complacency. Uncertainty may mean the problem is worse than you thought.

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44 Comments sorted by

Comments on this article are now closed.

  1. Suzy Gneist
    Suzy Gneist is a Friend of The Conversation.

    Multi-tasker at Graphic Design & Montville Coffee

    Thanks for the article and a good explanation of the meaning of 'uncertainty', yet I fear that those who equate uncertainty with being wrong will continue to barge ahead without course or speed adjustments towards the iceberg.

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    1. Mark Pollock

      Analyst

      In reply to Suzy Gneist

      The iceberg? Don't you mean the inferno :-)?

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    2. Suzy Gneist
      Suzy Gneist is a Friend of The Conversation.

      Multi-tasker at Graphic Design & Montville Coffee

      In reply to Mark Pollock

      It's a figure of speech, a metaphor, and yes, you could maybe compare some people's reactions to hunker down and hope for the best with Pompeii's demise rather than the barging on into disaster of the technologically "invincible" Titanic, if you prefer.

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    3. Mike Hansen

      Mr.

      In reply to Suzy Gneist

      Even the most optimistic take on climate sensitivity is not good news.

      "If climate sensitivity were near the 1.5C level, this would lead to 2-3C of further global warming by the end of the century."

      Hunkering down is not an option. Donning the tin-foil hat and claiming it is all a conspiracy is though, apparently.

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    4. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Suzy Gneist

      Suzy I think of the words Pompeii and iceberg are accurate to describe folly or stupidity as seen in deniers. But another thing which worries me is whether as ice becomes melted in Greenland and Iceland, will this mean there could be multiple occurrences of volcanic activity. We know that parts of northern Canada are rising and sea levels are dropping, due to the earths crust under Greenland losing the weight of ice on top of it. I expect we'll find out.

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  2. David Rennie

    IT Contractor

    Nice explanation of the methodology and meaning of the research. Much clearer that the newspaper articles I read.

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  3. David Semmens

    logged in via Twitter

    Interesting article Steve. I wonder how well the models with realistically strong mixing perform when predicting observed temperatures? Do they do better than the others?

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  4. Nick Maley

    IT Delivery Manager

    So this research appears to have an empirically testable prediction: that CO2 forcing will reduce cloud cover over the oceans. Is there any work being done, or has been done, to test this prediction?

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    1. Felix MacNeill

      Environmental Manager

      In reply to Nick Maley

      Nick, wasn't that in a sense already covered in this paragraph about midway in the article:

      "We found that the mixing is strong in the real atmosphere, implying a high climate sensitivity. Of the 43 climate models we examined, every one with realistically strong mixing had sensitivity of more than 3C."

      Not quite the same thing I realise, but isn't the point of the article that this research was based on actual observations (as above) and this was used to test the accuracy of the full set of available models, i.e. this IS empirical reasearch used to validate models, rather than something that happens to produce a testable hypothesis (your bit about cloud cover over the oceans)?

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    2. Mike Hansen

      Mr.

      In reply to Felix MacNeill

      Nick's point is back to front if I understand the paper correctly.

      "If this mixing is strong, it tends to thin out the cloud layer as the atmosphere warms."
      "We found that the mixing is strong in the **real** atmosphere" (emphasis added).

      They then looked for climate models that reflected this real world observation and discovered that "Of the 43 climate models we examined, every one with realistically strong mixing had sensitivity of more than 3C."

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    3. Steve Sherwood

      Director, Climate Change Research Centre at UNSW Australia

      In reply to Nick Maley

      That is a good point. The problem is that our observations of clouds are not stable enough (accurate enough) over time to tell one way or the other (although people have tried).

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  5. David Arthur

    resistance gnome

    Rather than all accumulating heat being retained in the atmosphere, perhaps an increasing proportion of it could be transferred to high latitudes where it 'vanishes' as sensible heat drives ice melt (latent heat) - akin to how melting ice keeps the drinks cool in a New Year's Eve ice bath?

    According to "An update on Earth's energy balance in light of the latest global observations", Stephens et al, Nature Geoscience 5, 691–696 (2012) doi:10.1038/ngeo1580, an estimate for present net accumulation…

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  6. Comment removed by moderator.

    1. In reply to Mike Hansen

      Comment removed by moderator.

    2. In reply to Mike Hansen

      Comment removed by moderator.

  7. Dale Bloom

    Analyst

    One of the most significant ways to reduce CO2 production is to reduce the Pentagon’s war machine.

    There are only 35 countries that consume more oil per day than the Pentagon.

    The Pentagon is the largest single oil consumer in the world, producing not only CO2, but also black carbon to collect in the atmosphere.

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  8. Mike Stasse

    Retired Energy Consultant

    It bothers me that none of the models take Limits to Growth, and Peak Oil in particular, seriously.

    Industrial civilisation could be all over within ten to twenty years. Or as good as... This will impact Carbon emissions tremendously, but I worry that the reduction in particulates will also suddenly unleash potential warming by CO2 already in the air when the particulates already here start precipitating in the rain.

    Uncertainty abounds alright.......

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    1. Chris O'Neill

      Retired Way Before 70

      In reply to Mike Stasse

      "I worry that the reduction in particulates will also suddenly unleash potential warming by CO2 already in the air when the particulates already here start precipitating in the rain."

      Yes, while the particulates and CO2 both increase exponentially, their forcings substantially oppose each other in a consistent way. Stop the exponential growth and this falls in a heap: the particulates soon disappear but the CO2 hangs around.

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    2. Mark Shaw

      Health Media

      In reply to Mike Stasse

      I have always wondered why HAARP is ignored in the scientific measurement and modelling?

      Excerpt from “Report 14 January 1999 on the environment, security and foreign policy”
      European Parliament website. Sorry for the long quote but it is relevant.

      http://www.europarl.europa.eu/sides/getDoc.do?pubRef=-//EP//TEXT+REPORT+A4-1999-0005+0+DOC+XML+V0//EN

      HAARP - a weapons system which disrupts the climate

      On 5 February 1998 Parliament’s Subcommittee on Security and Disarmament held a hearing…

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  9. Sherwood Botsford

    logged in via Facebook

    @Mike Stasse

    If by limits to growth, you are referring to the model and study that came out of MIT in the '70s it was a very limited model. If not, then please add a reference.

    Regarding peak oil: While conventional oil may or may not have peaked, the oil sands in Canada and those in South America both hold more oil than all the conventional fields in the world combined. Natural gas is in very large abundance right now.

    This is too bad. Running out of fossil fuel, and the subsequent rise in price might hurry us toward slowing down the emissions of greenhouse gasses.

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    1. Mike Stasse

      Retired Energy Consultant

      In reply to Sherwood Botsford

      Not only was the MIT model NOT limited......... new studies are showing that it is bang on target!
      http://damnthematrix.wordpress.com/2012/10/16/are-we-on-the-cusp-of-global-collapse/

      Re the tar sands...... they may hold more oil than what Saudi Arabia has (or had!), but the production rate is pathetically slow, will never increase sufficiently to have any impact on global oil production, and have such poor energy profit ratio that they will only go broke as the cost of energy and the resources required to extract them go up. It's a total white elephant.

      Natural gas is NOT as abundant as you might be led to believe..... even the execs who work for the gas fracking industry in the US know this...
      http://www.nytimes.com/interactive/us/natural-gas-drilling-down-documents-4-intro.html?_r=0

      Peak ALL fossil fuels (yes, even coal!) is on target to occur ~2017.
      http://damnthematrix.wordpress.com/2013/03/22/peak-fossilsuranium-in-2017/

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  10. Sherwood Botsford

    logged in via Facebook

    Re: Original article.

    Even with strong mixing, won't the long term effect be to put more water vapour into the atmosphere? This creates more latent heat transfer both between layers of the atmosphere and through air mass movement between low and high latitudes.

    My expectation is that as the temperature warms there will be more evaporation, and hence more cloud somewhere.

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    1. Chris O'Neill

      Retired Way Before 70

      In reply to Sherwood Botsford

      "as the temperature warms there will be more evaporation, and hence more cloud somewhere"

      Only if it's still cool somewhere else. The warmer it is, the more absolute humidity is needed to make clouds.

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    2. Steve Sherwood

      Director, Climate Change Research Centre at UNSW Australia

      In reply to Sherwood Botsford

      The key outcome of the stronger mixing we saw is to move water vapour from where it is more effective at forming clouds and cooling the planet, to where it is much less effective at doing so. The global hydrologic cycle (global evaporation rate) is not really affected.

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  11. Alex Cannara

    logged in via LinkedIn

    If I hear "climate sensitivity" again, I may spit.
    ;]
    The problem, folks, is the 1.5 trillion tons of fossil-fuel-produced CO2 now in air and only <30% dissolved in seas so far.

    That 1.5 trillion translates into ~500 billion tons of fossil Carbon. The natural Carbon Cycle is maintained by sea life and can handle at most 0.3 billion tons per year We are thus 1500 years behind what Nature can sequester, primarily as seafloor limestone. We emit >9 billion tons of Carbon each year. Each year…

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  12. Chris O'Neill

    Retired Way Before 70

    "Uncertainty may mean the problem is worse than you thought."

    This is a sentence denialists like Pollock don't understand.

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  13. Geoff Henley

    Research Associate in Health Statistics at Flinders University

    Steve Sherwood has been quoted as saying “Climate sceptics like to criticize climate models for getting things wrong, and we are the first to admit they are not perfect, but what we are finding is that the mistakes are being made by those models which predict less warming, not those that predict more”.

    However, real-world observations show completely the opposite - that models predicting less warming are closer to reality. The scenarios of 4C warming by 2100 are based purely on unvalidated models and are not supported by empirical data.

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    1. Peter Ormonde
      Peter Ormonde is a Friend of The Conversation.

      Farmer

      In reply to Geoff Henley

      Not really Geoff ... what is happening is that the models are getting much more accurate very quickly ... much closer to predicting the range of reality.

      The Wall Street Journal ran a graph a few years back trying to demonstrate how wrong the models were. Unfortunately when the graph was actually examined what became obvious was that the models were indeed becoming far more precise in their projections over the years and that there was indeed an overall rate of warming that fell within the…

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  14. Comment removed by moderator.