Various estimates of climate sensitivity. Graphic: Knutti and Hegerl, Nature Geoscience

By Dana Nuccitelli and Michael E Mann
12 April 2013

(ABC Environment) – The Economist recently published a lengthy article about Earth's climate sensitivity — how much the planet's surface will warm in response to the increased greenhouse effect if the amount of carbon dioxide in the atmosphere doubles relative to pre-industrial levels (something that will happen in a matter of decades if we continue with business-as-usual fossil fuel burning).

While we are pleased that The Economist brought attention to this important topic, we were disappointed by the shortcomings and inaccuracies in the piece with regard to the current state of scientific understanding.

The article focused heavily on claims that the slowed warming of Earth's surface in recent years implies a dramatically lowered estimate of climate sensitivity. The claim was primarily supported by a single as-yet unpublished article by a group in Norway, which attempts to use instrumental temperature evidence available back through the late 19th century to estimate the climate sensitivity. The authors of that article conclude that use of data to the year 2000 yields a climate sensitivity of 3.9°C, which is at the high end of the generally accepted 2 to 4.5°C range. Yet they find that by including just an additional decade of data (i.e., using observations available through 2010), the estimate falls by nearly half, to 1.9°C.

It should be a red flag that an estimate of climate sensitivity would change by a factor of two based only on the addition of a decade of data. In reality, the climate sensitivity now is not half what it was a decade ago. So where did the Norwegian study go wrong?

One likely culprit is that the role of natural climate variability, which is particularly important on timescales of a decade or less, was not properly accounted for in the analysis. One recent article published in the Journal of Geophysical Research found that internal natural variability (for example, natural oscillations in the climate like those associated with the El Niño phenomenon) can result in a sizable discrepancy (errors approaching 1°C) between the true climate sensitivity and the value of climate sensitivity derived from the instrumental record alone.

Yet another recent study published in the journal Geophysical Research Letters has argued that previously unaccounted-for effects of low-level volcanic eruptions may have offset more of the warming than scientists realised over the past decade.

And still another study published recently in Geophysical Research Letters suggests that any slowing of surface warming during the past decade may have been associated with a recent accelerated penetration of heat into the deeper oceans.

This conclusion is consistent with other recent studies finding unprecedented warming taking place in the deep oceans. According to the Intergovernmental Panel on Climate Change (IPCC), about 90 per cent of overall global warming goes into heating the oceans, while just two per cent heats the atmosphere. So the climate continues to warm, and all we may be seeing is a small change in how that warmth is being distributed between the ocean and atmosphere.

It is unfortunate that none of these studies and findings, each of which conflict with the dominant narrative of The Economist piece, were cited or discussed beyond a brief mention. It is further unfortunate that the piece provided so little of the larger scientific context necessary for readers to appreciate the current state of scientific knowledge about climate sensitivity. Most critically, the article didn't address why it is that the consensus estimate of climate sensitivity remains around 3°C. [more]

How The Economist got it wrong



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