In early November 2004 the results of the Arctic Climate Impact Assessment (ACIA) were published, a uniquely detailed regional study compiled by 300 scientists over 3 years. The study describes the ongoing climate change in the Arctic and its consequences: rising temperatures, loss of sea ice, unprecedented melting of the Greenland ice sheet, and many impacts on ecosystems, animals and people. The ACIA is the first comprehensively researched, fully referenced, and independently reviewed evaluation of arctic climate change and its impacts for the region and for the world.
Sadly, in recent years we have become accustomed to a ritual in which the publication of each new result on anthropogenic climate change is greeted by a flurry of activity from industry-funded lobby groups, think tanks and PR professionals, who try to discredit the science and confuse the public about global warming.
An example of this is the article “Polar Bear Scare on Thin Ice” by industry lobbyist Steven Milloy , featured on Fox News. Milloy claims in his polemic that the ACIA “debunks itself” based on one graph of the 1,200-page study. This graph shows the evolution of Arctic temperatures over the past century, and the fact (well-known to climatologists) that in the 1930s similarly warm temperatures where reached in the high Arctic as at present. Milloy concludes from this that both warmings are due to a natural cycle.
Scientifically, this argument holds no water: it is simply not possible to draw conclusions about the causes of climate variations by just looking at one time series. Only considering the time series of Arctic temperature, it is impossible to tell what the cause of the 1930s warming was, what the cause of the recent warming is, and whether both have the same cause or not. Milloy’s specious argument is a characteristic example for a method frequently employed by “climate skeptics”: from a host of scientific data, they cherry-pick one result out of context and present unwarranted conclusions, knowing that a lay audience will not easily recognise their fallacy.
In fact, the conclusion of the ACIA study that the recent warming is due to anthropogenic greenhouse gases is of course not based on one particular time series, but on a host of further scientific data. For example, looking at all the temperature data rather than just one time series reveals that the pattern of warming of the 1930s was very different from the recent warming. In the 1930s, warming was localised to the high latitudes, consistent with this warming being the result of a natural oscillation (the so-called “Atlantic Multidecadal Oscillation”). Very similar natural oscillations are also found in climate models. The recent warming, in contrast, encompasses most of the planet; this is consistent with it being the result of a global forcing. A very similar pattern of warming is found in climate models as a result of rising greenhouse gases. (For full details, see the publication of Johannessen et al., Tellus 2004). Many other lines of evidence demonstrate convincingly that anthropogenic forcing was very likely the dominant factor in the warming of recent decades.
Milloy further claims that the observed global warming of 0.6-0.8 C over the 20th Century is “well within the natural variation in average global temperature, which in the case of the Arctic, for example, is a range of about 3 degrees Centigrade”. This is another misconception frequently promoted in skeptics articles: it confuses global-scale with local changes (note Milloy’s delicate phrasing of this), and hence compares apples with pears.
Local climate variations are generally much larger than global ones. The reason is simple: it is easy to generate large localised temperature changes simply by changing the atmospheric circulation patterns (as happens for example in the North Atlantic Oscillation, NAO) – this will steer the winds along a different track, causing some regions to warm and others to cool. In a global or hemispheric average, in contrast, this kind of redistribution of heat cancels out. To get global-scale variations, you need to add heat overall, not just shift it around to a different place. Global-scale variations are therefore much smaller, and they reflect changes in global climate drivers, for example in greenhouse gas concentrations or in solar activity. For this reason, an anthropogenic warming trend can only be clearly identified in hemispheric or global averages or in pattern studies. It can neither be demonstrated nor debunked by looking at individual local time series. Even as the global average temperature is rising, some regions have been cooling in recent decades (e.g., the Labrador Sea region or the Antarctic).