Looking at clouds from both sides.

Looking out of an aeroplane window it is easy to see how significantly clouds reduce the flow of solar energy onto the Earth's surface.

Looking out of an aeroplane window it is easy to see how significantly clouds reduce the flow of solar energy onto the Earth’s surface.

It’s the New year, but I can’t get the issue of Climate Change out of my mind.

The UK has been a bit wet over the break and there have been some storms, but nothing compared with the extremes experienced in other places. So although this weather hasn’t struck me as particularly exceptional, it makes me wonder how we would cope if we were faced with really serious climate change.

And over the break several news sources (The Guardian, Ars Technica) reported on a paper in Nature which claims to have resolved some of the uncertainty in predictions of the extent of future climate change. The uncertainty relates to role of clouds in determining the surface temperature of the Earth.

The sensitivity of the average surface temperature of the Earth to a doubling of carbon dioxide is currently estimated to be in the range 1.5 °C to 4.5 °C. That is a simple statement and big range of uncertainty – from the serious to the catastrophic.

The ‘doubling’ refers to a change from the historical CO2 concentration of 280 ppm to a hypothetical future state of 560 ppm. Currently the concentration is just approaching 400 ppm and rising at about 2 ppm per year, so if we carry on as we are then this doubling is about 80 years away.

Currently the Earth appears to have warmed by about 0.8 °C due to an additional 120 ppm of CO2. So if everything were in a steady state (which it most definitely isn’t) one might estimate the doubling sensitivity to be about 1.9 °C – at the low end of the sensitivity range.

However, the warming effect of CO2 emissions is expected to be greater than this. You can see why by considering the additional 2 ppm of CO2 emitted last year. This has not yet had time to warm the Earth very much and even if we emitted no more CO2 at all – last year’s CO2 will still keep warming the Earth for decades to come.

The source of uncertainty that the paper refers to is the role of clouds. As CO2 warms the Earth we expect to find extra water vapour in the atmosphere and thus in general we expect to find extra clouds. Depending on where and when these clouds occur they can either add to the warming (for example, if they occur at night) or provide cooling (for example, if they occur at low level in the day).

I have read the paper a  couple of times now and I think I can just about understand what they are saying, though many of the details escaped me. As far as I can tell  they examined 48 climate models to see how they predicted evaporated water should be re-distributed through the atmosphere and they noticed two correlations:

  • The models in which the water vapour went higher in the atmosphere predicted greater sensitivity to CO2 doubling.
  • The models in which the water vapour went higher in the atmosphere better matched the experimental data for the distribution of moisture in atmosphere.

In other words, the models that best describe the real distribution of atmospheric moisture (and so presumably are most reliable) also predict sensitivity to CO2 doubling away from the lower end of the range: they think 3 ° C is the lowest realistic estimate.

This is just one paper, but the analysis is clever and exploits a much larger body of work used in building the models. But if it is correct it is very bad news, because doubling of CO2 appears to be all but inevitable.

P.S. This is the ‘Editor’s Summary’ from Nature.

This paper offers an explanation for the long-standing uncertainty in predictions of global warming derived from climate models. Uncertainties in predicted climate sensitivity — the magnitude of global warming due to an external influence — range from 1.5° C to 5° C for a doubling of atmospheric CO2. It has been assumed that uncertainties in cloud simulations are at the root of the model disparities, and here Steven Sherwood et al. examine the output of 43 climate models and demonstrate that about half of the total uncertainty in climate sensitivity can be traced to the varying treatment of mixing between the lower and middle troposphere — and mostly in the tropics. When constrained by observations, the authors’ modelling suggests that climate sensitivity is likely to exceed 3° C rather than the currently estimated lower limit of 1.5° C, thereby constraining model projections towards more severe future warming.


3 Responses to “Looking at clouds from both sides.”

  1. Edmond Hui Says:

    And a happy new year to you too!
    As my old boss used to say in the morning, “What’s the news this morning?….Hmm. I don’t like that news. Give me different news!”

  2. H Stiles (@HStiles1) Says:

    Three degree C – that is a lot… I see little appetite for doing anything about it sadly, at least among those who can….

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