THEGREENGROK    Planetary Watch

We Don’t Know Clouds After All

by Bill Chameides | July 27th, 2009
posted by Erica Rowell (Editor)

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Clouds have been the hook climate skeptics have been hanging their “you-can’t-predict-future-climate” hat on for years. That hook appears to be melting as a result of new research.

It’s about feedbacks. Positive feedbacks are like a vicious circle. (Some call these virtual circles, but I just don’t think that appellation fits.) In a vicious circle an initial change is amplified. For example, a little climate warming from greenhouse gases is made larger by a positive feedback. A negative feedback does the opposite. A little warming is decreased or even canceled out.

Positive Feedbacks Amplify an Initial Change

In the past decade or so, climate scientists have come across lots of positive feedbacks — obviously not the kind we want to find. Examples include:

    1. An increase in carbon dioxide (CO2) increases temperature;
    2. An increase in temperature increases water vapor (which, like CO2, is a greenhouse gas);
    3. An increase in water vapor increases temperature;
    4. Go back to #2.
    1. An increase in carbon dioxide increases temperature;
    2. An increase in temperature melts sea ice;
    3. Melting sea ice exposes the dark, more absorbing ocean water;
    4. The ocean water absorbs more sunlight;
    5. More absorption means an increase in temperature;
    6. Go back to #2.

One worrisome aspect of predicting climate with higher concentrations of carbon dioxide is our imperfect knowledge of these positive feedbacks. What if we have underestimated their strength? Worse yet, what if we have failed to account for one? Things could turn out a lot worse than we predicted if either of these possibilities comes to pass.

Negative Feedbacks Decrease an Initial Change — Sometimes Even Canceling It Out

On the other side are negative feedbacks. One such feedback was proposed by Dick Lindzen, the renowned climate skeptic from the Massachusetts Institute of Technology. The feedback involves clouds and Lindzen calls it the “iris effect.” Just as your eye’s iris closes when light is shined on it, low-level cloud cover, so Lindzen’s theory went, will increase as CO2 warms the atmosphere. The iris effect’s increased cloud cover lets less sunlight in to warm the Earth, leading to a negative feedback. It would work like this:

  1. An increase in carbon dioxide increases temperature;
  2. Increased temperature increases the dynamics that lead to more low-level clouds;
  3. An increase in clouds reflects more sunlight back to space;
  4. More reflection lowers temperature, largely cancelling the warming from carbon dioxide.

While there has been little or no evidence that the iris effect works the way Lindzen has predicted, climate skeptics have continued to point to clouds and their potential to drive a negative feedback as a reason to dismiss concerns about global warming. (See point and counterpoint, also here, here and here.)

New Paper Puts the Kibosh on the Irish Effect

A new paper in Science by Amy Clement of the Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, and colleagues looks like it might put the kibosh on the iris effect. Not only don’t clouds drive a negative feedback; the data analyzed by Clement et al. suggest that they may actually drive a positive feedback.

The authors used two datasets to establish the decadal variability of clouds over the Pacific Ocean: one based on visual observations of cloud cover and the other based on satellite measurements. They then correlated the cloud data with variations in sea surface temperatures and other meteorological variables for the same region.

The results are rather startling. A very clear, negative correlation is found between sea surface temperature and low-level clouds over the northeast Pacific Ocean.

All in all, Clement and her colleagues found that for each one degree Celsius increase in sea surface temperatures, loss of clouds led to a net additional warming of six watts per square meter over the ocean region. By comparison, the total warming due to the increase in CO2 between now and pre-industrial revolution is about 1.7 watts per square meter (for the entire globe not just the northeast Pacific Ocean). (See an previous post or Chapter 2 here [pdf].)

The Scary Takeaway: Decreased Cloud Cover Could Exacerbate Warming

Now the scary part of this finding. Of the 18 climate models Clement and her team analyzed (these models are used to simulate how climate will evolve in the coming decades), only two accurately predicted the negative relationship between clouds and sea surface temperatures over the northeast Pacific Ocean. Of the two, only one also gets other key atmospheric changes right. This model — the HadGEM1 — predicts that an increase in global temperatures from greenhouse gas warming will decrease cloud cover over the Pacific Ocean, leading to a positive amplification of carbon dioxide warming.

Now some caveats should be noted:

  • a correlation does not necessarily mean there is a cause-and-effect relationship;
  • the mechanism for the negative correlations is not well understood; and
  • low-level cloudiness in other parts of the Pacific Ocean showed a weak positive correlation with sea surface temperature.

Still, it looks like clouds will continue to add uncertainty to the climate debate — but not as a negative feedback, quite possibly as another of those very worrisome positive feedbacks.

filed under: carbon dioxide emissions, climate change, faculty, global warming, Planetary Watch, science
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