Comment on “Iceland as a heat island” by D. H. Douglass et al.

[1] Iceland has experienced considerable warming in recent decades. Figure 1, which shows an annual temperature time-series from 1831 to the present provides an historical context for this warming. While the data shown derive from only one weather station (Stykkisholmur, western Iceland), the time-series captures the salient features of temperature variability in Iceland since the early 19th century. Overall, the first half of the time series is fairly cold, but following a rapid warming in the 1920s, a mid20th century warm period is established. Cooler conditions return in the 1960s, with a minimum in 1979, followed by a strong warming trend. Only in 2003 and 2004 did the temperatures reach similar levels to those experienced in the 1930s. The figure also shows thirty year averages (solid line segments) and the standard deviation of annual temperature within the corresponding interval (dashed arrows). In the latter half of the time-series, the magnitude of the changes in the thirty-year averages is similar to the standard deviation of the annual temperature within the intervals. [2] Douglass et al. [2005] have recently hypothesised that the 1979–1996 warming trend near Iceland is due to geothermal heat flux. Furthermore, they also claim that this influence extends over an area twice the size of Iceland. [3] Estimates for the geothermal heat flux [Flovenz and Saemundsson, 1992] are in the range 0.1 – 0.15 W m 2 outside volcanic areas, with higher values locally within volcanic areas. To put these values into perspective, the DaSilva et al. [1994] atlas shows that the average net ocean to atmosphere heat flux near Iceland is 80 W m . Similar numbers are obtained from the study by Seager et al. [2002], where the atmospheric diabatic heating rate over Iceland can be seen to be in the range 30–60 W m 2 [see Seager et al., 2002, Figure 5]. [4] The hyptothesis by Douglass et al. [2005] therefore has the obvious problem that the geothermal heat flux is orders of magnitude smaller than other heat fluxes known to influence atmospheric temperatures. Furthermore, there are no indications that suggest the geothermal heat flux has increased recently. [5] It is well known that geothermal heat flux can locally reach very high values in active volcanic areas. As an example, on Iceland’s most active volcano, Grimsvotn in Vatnajokull ice cap, the heat production can be estimated from ice melt to be about 1.5–2 GW. Distributed over a 100 km area this yields a heat flux of 15–20 W m . On this particular volcano the heat goes into melting of ice, but the possibility remains that other localized heat sources can have a significant local impact on atmospheric temperatures. To examine this we calculated the 1979–1996 temperature trend for 27 weather stations from which we have annual mean temperatures. The resulting trends are shown in Figure 2. They range from 0.4 to 0.9 C/decade with an average of 0.6. The figure also shows the location of central volcanoes (ovals) and associated fissure swarms (gray areas). If the hypothesis by Douglass t al. [2005] that ‘‘non-eruptive volcanic warming’’ is a cause of temperature changes in Iceland, these are the areas where one would expect temperature trends to be the largest. The figure shows no evidence of this being the case. Indeed, the mean trend for stations within (or close to) volcanic areas is 0.01 C/decade lower than the mean trend at other stations. [6] However, the figure shows that trends in the northern part of Iceland are larger than those in the southern part. The average trend for stations north of 65 N is 0.3 C/decade higher than the average for other stations. This northward enhancement of warming trends also appears in data from weather stations from Jan Mayen and Spitzbergen. Using the NORDKLIM data set [Tuomenvirta et al., 2001] (available at http://www.smhi.se/hfa_coord/nordklim) we examined annual temperatures from Jan Mayen (75.56N, 8.40W) and Spitzbergen Airport (78.15N, 15.28E). In Jan Mayen, the trend is 0.7 C/decade and in Spitzbergen 0.9 C/decade. The larger warming trend occurs in Spitzbergen, which is not volcanically active. Figure 3 shows the temperature trends