New research: climate may be more sensitive and situation more dire

July 5, 2016
Submitted by: Love Knowledge
Category: Science

This NOAA/NASA image shows Earth using near-infrared and shortwave infrared energy instead of the standard red, green, and blue light that the human eye has evolved to detect. By using infrared energy rather than visible light, the colors indicate differe Photograph: HANDOUT/AFP/Getty Images

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When comparing apples to apples, a new study finds energy budget climate sensitivity estimates consistent with climate models.

Scientists use a variety of approaches to estimate the Earth’s climate sensitivity – how much the planet will warm as a result of humans increasing greenhouse effect. For decades, the different methods were all in good general agreement that if we double the amount of carbon dioxide in the atmosphere, Earth’s surface temperatures will immediately warm by about 1–3°C (this is known as the ‘transient climate response’). Because it would take decades to centuries for the Earth to reach a new energy balance, climate scientists have estimated an eventual 2–4.5°C warming from doubled atmospheric carbon (this is ‘equilibrium climate sensitivity’).

However, a 2013 paper led by Alexander Otto disrupted the agreement between the various different approaches. Using a combination of recent climate measurements and a relatively simple climate model, the ‘energy budget’ approach used in Otto’s study yielded a best estimate for the immediate (transient) warming of 1.3°C and equilibrium warming of 2.0°C; within the agreed range, but less than climate model best estimates of 1.8°C and 3.2°C, respectively.

This new energy budget approach, which was replicated by several subsequent studies, seemed to indicate the Earth’s climate is a bit less sensitive to carbon pollution than previously thought. As a result, the IPCC adjusted its estimated range for equilibrium climate sensitivity from 2–4.5°C in its 2007 report to 1.5–4.5°C in its 2014 report. This suggested perhaps a slightly less dire climate situation.

New finding: disagreement due to apples-to-oranges comparison

Later in 2013, Kevin Cowtan and Robert Way published a paper finding that climate scientists had been underestimating global surface warming, largely because of a lack of measurements in the rapidly-warming Arctic. Additionally, while climate models simulate surface air temperatures (the temperature of the air a few meters above the Earth’s surface), over the oceans, climate scientists measure sea surface temperatures. It turns out that the water surface isn’t warming quite as fast as the air above it. Thus looking at modeled surface air temperatures versus measured global land-ocean surface temperatures is an apples-to-oranges comparison.

A new study in Nature Climate Change led by Mark Richardson in collaboration with Kevin Cowtan, Ed Hawkins, and Martin Stolpe accounts for these differences to make an apples-to-apples comparison. They find that the use of sea surface temperatures biases the Otto result low by about 9%, and the lack of Arctic observations by another 15%. When observations are adjusted to estimate surface air temperatures (red bars in the figure below), or when models are adjusted to estimate land-ocean surface temperatures (blue bars), the estimated transient climate response from climate models and the Otto approach are in close agreement.

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Like-with-like comparisons of transient climate response estimates between models and observations. In the upper two bars, the observed estimates are adjusted to match the method used for the models. In the lower three bars the model outputs are treated in the same way as the observations. Illustration: Richardson et al. (2016); Nature Climate Change.

Courtesy of Guardian News & Media Ltd

Last modified on Saturday, 01 October 2016 01:21

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