Interesting new study with some interesting conclusions, using a new method:
Knutti and Huber found that greenhouse gases contributed 0.6–1.1 °C to the warming observed since the mid-twentieth century, with the most statistically likely value being a contribution of about 0.85 °C. Around half of that contribution from greenhouse gases — 0.45 °C — was offset by the cooling effects of aerosols. These directly influence Earth's climate by scattering light; they also have indirect climate effects through their interactions with clouds.
The authors calculated a net warming value of around 0.5 °C since the 1950s, which is very close to the actual temperature rise of 0.55 °C observed over that period. Changes in solar radiation — a hypothesis for global warming proffered by many climate sceptics — contributed no more than around 0.07 °C to the recent warming, the study finds.
To test whether recent warming might just be down to a random swing in Earth’s unstable climate — another theory favoured by sceptics — Knutti and Huber conducted a series of control runs of different climate models without including the effects of the energy-budget parameters. But even if climate variability were three times greater than that estimated by state-of-the-art models, it is extremely unlikely to have produced a warming trend as pronounced as that observed in the real world, they found.
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