Introduction

In these days of discussion of the future extinction of polar bears and the use of Arctic ice-free shipping routes, it might seem strange to look at the case for a global warming influence in the Arctic in more detail. While it is predicted by models that the Arctic will warm faster than more southerly latitudes, and multiple indicators of climate change show large shifts over the last two decades, the attribution of change due to greenhouse gas increases is still difficult because of the relative influence of the large range of climate extremes from natural (intrinsic) variability. Such decadal variability is often observed as changes in wind patterns (Maslanik et al. 2007). As little as 8 years ago Serreze et al. (2000, 2007) commented that "Taken together, these results paint a reasonably coherent picture of [Arctic] change, but their interpretation as signals of enhanced greenhouse warming is open to debate." Since then evidence of an anthropogenic influence in the Arctic is emerging as discussed in this paper, along with an improved understanding of the structure of intrinsic variability and internal feedbacks within the Arctic. Such feedbacks include cloud cover and shifts in the relative area coverage of ice and ocean.

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IWD 1381 13BZ 1913 IBS« 1B5S 1BB6 1BB7 19HS lfl&S IBM 1B91 1631 1333 1SB+

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Fig. 1. Three-month running mean of the Arctic Oscillation index. (From the CPC web site.)

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Fig. 1. Three-month running mean of the Arctic Oscillation index. (From the CPC web site.)

Fig. 2. Arctic-wide and mid-latitude surface air temperature anomalies for the 20th century. A 5-year running mean is applied.

In the mid to late 1990s the climate pattern known as the Arctic Oscillation (AO) had a persistent run of positive values from 1989 to 1995, which advects warmer air into the Eurasian Arctic compared to years when it is in its negative phase (Fig. 1). There was considerable opinion that this persistence was an indication of anthropogenic climate change (Palmer 1999; Feldstein 2002), but after 1996 this argument was rejected as the phase of the AO became more random from year to year while Arctic indicators showed continued warm conditions (Overland and Wang 2005). A second example of natural variability is the 1930s warm period in certain parts of the Arctic, which appears not unlike the period of the 1990s when Arctic-wide averages are compared (Fig. 2).

As will be discussed in section "Arctic change", the cumulative impacts of the AO and other climate patterns acting in concert with external forcings such as greenhouse gases and internal feedbacks are important to explain historical temperature changes in the Arctic and the recent rapid loss of sea ice in summer. Further, while Fig. 2 looks like a climate oscillation, the true story is that the causes of the temperature maxima are different in the 1930s and 1990s. Before taking up current evidence about the Arctic, we digress and address the general issue of application of the scientific method in understanding climate change.

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