What you need to know about climate sensitivity

This is a partial re-post of my latest in the Guardian's Climate Consensus – the 97%.  It's intended as a basic primer to reference the next time somebody tells you global warming is nothing to worry about because climate sensitivity is low.

What you need to know about climate sensitivity

It's a critical aspect of the climate system, but the basics are simple

geoengineering : SPICE , Stratospheric Particle Injection for Climate Engineering : clouds and sun
Clouds are the only plausible feedback that could significantly dampen future global warming. Photograph: Graham Turner for the Guardian

Climate sensitivity is a subject sometimes explored in mainstream media articles. For example, The Economist tried to summarize some recent research on the subject, although as climate scientist Michael Mann and I noted in an article for ABC, they made some key mistakes.

What is climate sensitivity?

We know that humans have increased the greenhouse effect due to the carbon emissions associated with burning fossil fuels. This increased carbon dioxide traps more heat in the Earth's atmosphere, causing a global energy imbalance. There is more energy incoming than escaping, and as a result the planet will warm until it reaches a new balanced energy state (equilibrium), with equal incoming and outgoing energy.

We also know that if we double the amount of carbon dioxide in the atmosphere, the increased greenhouse effect will cause the planet's average surface temperature to warm about 1.2°C (2.2°F) in response. That may not sound like very much, but the difference between an ice age and the current warm period is only about 5°C (9°F). Seemingly small temperature changes make a big difference in the Earth's climate.

In addition, there are feedbacks that can dampen or amplify the warming from the increased greenhouse effect. For example, when ice melts it makes the Earth's surface less reflective, causing it to absorb more sunlight and warm further. A warmer atmosphere will also hold more water vapor, and water vapor is another greenhouse gas.

The term "equilibrium climate sensitivity" refers to the total amount of warming that will occur at the Earth's surface once it reaches a new balanced energy state, including from the increased greenhouse effect from a doubling of atmospheric carbon dioxide, and including these feedback effects.

What are the biggest feedbacks?

Water vapor is probably the largest individual feedback. A 2009 study published in the prestigious journal Science by Andrew Dessler and Steven Sherwood found that, as climate scientists expected, the amount of water vapor in the atmosphere is increasing by enough to double the warming from the increased greenhouse effect of carbon dioxide.

As discussed above, melting ice is another significant warming feedback. Releases of stored carbon from beneath permafrost and methane from the deep ocean would also amplify global warming.

Unfortunately we don't know of many large negative feedbacks that would dampen global warming. Global plant growth has increased so far, and plants absorb carbon dioxide. But that trend probably won't last as extreme weather events like heat waves and droughts that damage plant life become more common.

Clouds are really the only plausible feedback that could significantly dampen future global warming. They're tricky because clouds cause both warming by increasing the greenhouse effect, and cooling by reflecting sunlight away from the Earth's surface. High clouds tend to have an overall warming effect, while low clouds tend to have an overall cooling effect.

So what types of clouds will become more abundant in a warming world, and what will the net effect on temperatures be?

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Posted by dana1981 on Friday, 10 May, 2013

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