The Science of Doom

The IPCC 5th Assessment Report (AR5) from 2013 shows the range of results that climate models produce for global warming. These are under a set of conditions which for simplicity is doubling CO2 in the atmosphere from pre-industrial levels. The 2xCO2 result. Also known as ECS or equilibrium climate sensitivity.

The range is about 2-4ºC. That is, different models produce different results.

Other lines of research have tried to assess the past from observations. Over the last 200 years we have some knowledge of changes in CO2 and other “greenhouse” gases, along with changes in aerosols (these usually cool the climate). We also have some knowledge of how the surface temperature has changed and how the oceans have warmed. From this data we can calculate ECS.

This comes out at around 1.5-2ºC.

Some people think there is a conflict, others think that it’s just the low end of the model results. But either way, the result of observations sounds much better than the result of models.

The reason for preferring observations over models seems obvious – even though there is some uncertainty, the results are based on what actually happened rather than models with real physics but also fudge factors.

The reason for preferring models over observations is less obvious but no less convincing – the climate is non-linear and the current state of the climate affects future warming. The climate in 1800 and 1900 was different from today.

“Pattern effects”, as they have come to be known, probably matter a lot.

And that leads me to a question or point or idea that has bothered me ever since I first started studying climate.

Surely the patterns of warming and cooling, the patterns of rainfall, of storms matter hugely for calculating the future climate with more CO2. Yet climate models vary greatly from each other even on large regional scales.

Articles in this Series

Opinions and Perspectives – 1 – The Consensus

Opinions and Perspectives – 2 – There is More than One Proposition in Climate Science

Opinions and Perspectives – 3 – How much CO2 will there be? And Activists in Disguise

Opinions and Perspectives – 3.5 – Follow up to “How much CO2 will there be?”

Opinions and Perspectives – 4 – Climate Models and Contrarian Myths

Opinions and Perspectives – 5 – Climate Models and Consensus Myths

Opinions and Perspectives – 6 – Climate Models, Consensus Myths and Fudge Factors

Opinions and Perspectives – 7 – Global Temperature Change from Doubling CO2

Opinions and Perspectives – 8 – Pattern Effects Primer

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For people with maths, physics and chemistry (and biology) backgrounds non-linear processes are familiar. For people without this background they are often quite obscure.

I’ll give a simple example. It’s not based on reality but it seems like the easiest way to explain non-linear effects.

Here we go..

Half the world is snow-covered land and half the world is ocean. Snow reflects about half of sunlight and ocean reflects no sunlight (this is not accurate, the actual figure is something like 10%, but we’ll stick with 0% for simplicity).

We also have clouds in this world. Clouds reflect 100% of sunlight.

Half of the sky has cloud cover. In our mythical world the land has cloudy skies and the ocean has clear skies.

So the cloud over the land reflects 100% of solar radiation while the ocean, with clear skies, absorbs all of its radiation.

Result – the mythical world absorbs 50% of solar radiation and so reaches some steady state temperature.

Now some climate change takes place. The winds are stronger and all the clouds move over the ocean. So the ocean has cloudy skies and the land has clear skies. Now the land reflects 50% of its sunlight (because of the snow) and the ocean region – because it’s covered by clouds – reflects 100% of sunlight.

Result – under the changed climate, the mythical world absorbs only 25% of solar radiation and cools dramatically

The important point is that clouds still cover 50% of the skies, and the ocean and land haven’t changed. But simply moving the clouds halves the sunlight absorbed.

A more realistic example is given by in Clouds & Water Vapor – Part Five – Back of the envelope calcs from Pierrehumbert which looks at regions of low humidity and high humidity.

Articles in this Series

Opinions and Perspectives – 1 – The Consensus

Opinions and Perspectives – 2 – There is More than One Proposition in Climate Science

Opinions and Perspectives – 3 – How much CO2 will there be? And Activists in Disguise

Opinions and Perspectives – 3.5 – Follow up to “How much CO2 will there be?”

Opinions and Perspectives – 4 – Climate Models and Contrarian Myths

Opinions and Perspectives – 5 – Climate Models and Consensus Myths

Opinions and Perspectives – 6 – Climate Models, Consensus Myths and Fudge Factors

Opinions and Perspectives – 7 – Global Temperature Change from Doubling CO2

Read Full Post »