A recent news article suggested that climate change may not be as bad as feared. But the report was based on one flawed study and missed a lot of important points.
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 The claim was primarily supported by a single as-yet unpublished article by a group in Norway, which attempts to use instrumental temperature evidence available back through the late 19th century to estimate the climate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.
Knutti, R., Hegerl, G. C., Nature Geoscience
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.
The instrumental temperature record alone, it turns out, is an especially poor constraint on climate sensitivity because it is so short, and because there are multiple natural and human factors at work over the past century. For this reason, there is an extremely wide spread of estimates of climate sensitivity when only information from the instrumental record is used. That spread includes estimates that are both lower and higher than the mid-range (around 3°C) estimate (see figure above).
However, there is a wealth of other sources of information that scientists have used to try to constrain climate sensitivity (see for example this discussion at the site RealClimate). That evidence includes the paleoclimate record of the past thousand years, the specific response of the climate to volcanic eruptions, the changes in global temperature during the last ice age, the geological relationship between climate and carbon dioxide over millions of years, and more.
When the collective information from all of these independent sources of information is combined, climate scientists indeed find evidence for a climate sensitivity that is very close to the canonical 3°C estimate. That estimate still remains the scientific consensus, and current generation climate models which tend to cluster in their climate sensitivity values around this estimate remain our best tools for projecting future climate change and its potential impacts.
Given that it will take a significant effort to avoid doubling the amount of carbon dioxide in the atmosphere, from a policy perspective arguments about the precise climate sensitivity are somewhat irrelevant. Even at the lower end of the estimated sensitivity range, the projected impacts of climate change are likely to be devastating to human civilisation and our environment. What it will take to avoid such a scenario is what we - and The Economist - ought to be focusing on.
Dana Nuccitelli is an environmental scientist and climate blogger for Skeptical Science and The Guardian.
Michael E. Mann is Distinguished Professor of Meteorology at Penn State University. He is author of the recent book "The Hockey Stick and the Climate Wars".