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Saturday, May 1, 2010

J. A. Screen & I. Simmonds, Nature 464 (2010), The central role of diminishing sea ice in recent Arctic temperature amplification

Nature, Vol. 464, pp. 1334-1337 (29 April 2010); doi: 10.1038/nature09051

The central role of diminishing sea ice in recent Arctic temperature amplification

James A. Screen* and Ian Simmonds

School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia


The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades1,2,3—a feature known as ‘Arctic amplification.’ Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming1,4; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover5,6,7 and changes in atmospheric and oceanic circulation8,9,10, cloud cover and water vapour11,12 are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss13,14. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice–temperature feedbacks have emerged in the Arctic15, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic2.

The Arctic region has long been expected to warm strongly as a result of anthropogenic climate change [1,2], owing to positive feedbacks in the Arctic climate system. It is widely accepted that changes in the surface albedo associated with melting snow and ice enhance warming in the Arctic [3,15,16], but other processes may contribute. In some global climate models, changes in cloud cover and atmospheric water vapour content are more important for Arctic amplification than the surface albedo feedback [17–19]. However, the same climate models significantly underestimate the recent Arctic sea ice decline [5] and surface warming [20], in part due to unrealistic negative feedbacks [20].

One reanalysis data set suggests that Arctic warming may have been enhanced by an increase in the atmospheric poleward transport of heat and moisture [8]. However, another reanalysis data set reveals a decrease in poleward heat transport since the early 1980s [21], which was a period of rapid sea ice declines [5–7]. Changes in Arctic storm behaviour [9] may have also enhanced the warming. The vertical profile of recent warming can provide insight into its underlying causes. For instance, retreating snow and sea ice cover is expected to induce maximum warming at the surface [15,22], whereas changes in atmospheric poleward heat transport may cause warming with large vertical extent [8]. The ERA-40 reanalysis has been used to show [8] that Arctic warming trends aloft were of equal or greater magnitude than those at the surface, leading to the conclusion that atmospheric circulation changes were a more important cause of recent Arctic amplification than retreating snow and sea ice cover. However, notable discrepancies exist between the vertical profiles of warming in different reanalysis data sets [15]. The findings of ref. 8 have been contested [15,23–25], and concerns have been expressed over the validity of trends in ERA-40 that may reflect inhomogeneities or artefacts in the reanalysis rather than true climate signals [23,24].

Here we present results from a new reanalysis data set, ERAInterim [26]. Some of the key improvements over the ERA-40 data set include higher resolution, improved model physics, a better hydrological cycle, four-dimensional variational data assimilation and variational bias correction of satellite radiance data [26.]. The last feature is of particular relevance for this study because the scarcity of direct temperature measurements over the Arctic Ocean dictates that the majority of observations come from satellite radiances. The variational bias correction of satellite radiance data accounts for biases that change in time, for instance owing to changes in the observing network or drift of satellite orbits. ERA-Interim depicts more realistic Arctic tropospheric temperatures and probably suffers less from spurious trends than any previous reanalysis data set [26] (Supplementary Information). Furthermore, we build on the results of ref. 8 by including the post-2001 period, during which sea ice retreat has accelerated [5–7].
Arctic amplification is a clear feature of the warming over the 1989–2008 period based on the ERA-Interim reanalysis (Fig. 1). We diverge considerably from ref. 8 in finding that the maximum Arctic warming is at the surface and that warming lessens with height in all seasons except summer. This vertical structure suggests that
changes at the surface, such as decreases in sea ice and snow cover, are the primary causes of recent Arctic amplification. The trends at the near-surface (herein the atmospheric levels at 950–1,000 hPa) are 1.6, 0.9, 0.5 and 1.6 °C per decade, averaged over the Arctic (herein latitudes 70–90° N) during winter, spring, summer and autumn, respectively. The near-surface warming is modest in summer because energy is used to melt remaining sea ice and warm the upper ocean [3,15]. The surface amplification, defined here as the ratio of the near-surface warming to that of the whole tropospheric column (below 300 hPa), averaged over the Arctic, is greatest in autumn, with a value of 2.3. The surface amplification is aided by strong low-level stability that limits vertical mixing. The corresponding values of surface amplification for winter and spring are 2.1 and 1.8, respectively.

We note that amplified Arctic warming, above ~700 hPa, is confined to winter and is still consistently weaker than the nearsurface warming (Fig. 1a). However, the presence of amplified warming aloft hints that processes in addition to the increased transfer of heat from the ocean to the atmosphere resulting from sea ice loss have
had a contributing role in winter.

The surface amplified warming is closely linked to diminishing sea ice cover over the 1989–2008 period (linear trends of 22.6, 21.4, 25.8 and 27.9% per decade relative to the 1989–2008 means for winter, spring, summer and autumn, respectively). The components of the seasonal temperature trends that are linearly congruent with changes in sea ice (Fig. 2) show remarkable resemblance to the vertical profiles of the total temperature trends (Fig. 1). North of 70° N, a large portion of each total trend is linked to reduced Arctic sea ice cover (Fig. 2). The majority of the winter warming is associated with changes in sea ice cover (Fig. 2a) even though the sea ice declines are relatively small and the albedo feedback is weak during this season.
Strong winter warming is consistent with the atmospheric response to reduced sea ice cover [22,27] and reflects the seasonal cycle of ocean–atmosphere heat fluxes [22]: during summer, the atmosphere loses heat
to the ocean whereas during winter the flux of heat is reversed. Thus, reduced summer sea ice cover allows for greater warming of the upper ocean but atmospheric warming is modest (Fig. 2c). The interaction is undoubtedly two-way because warmer upper-ocean temperatures will further enhance sea ice loss. The excess heat stored in the upper ocean is subsequently released to the atmosphere during winter [20,22]. Reduced winter sea ice cover, in part a response to a warmer upper ocean and delayed refreezing [6,7], facilitates a greater transfer of heat to the atmosphere. The observed thinning of Arctic sea ice [28,29], albeit not explicitly represented in ERA-Interim, is also likely to have enhanced the surface heat fluxes.

Another potential contributor to the surface amplified warming could be changes in cloud cover. Clouds decrease the incoming short-wave (solar) radiation. However, this shading effect is partly offset, or exceeded, by a compensating increase in incoming long-wave radiation. In the Arctic, this greenhouse effect dominates during autumn, winter and spring (Fig. 3), in agreement with in situ observations [30]. In summer, the shading effect dominates in the lower-latitude regions of the Arctic basin whereas north of 80° N the two competing
effects approximately cancel out (Fig. 3c). Spring is the only season that exhibits significant trends in Arctic average cloudiness in ERA-Interim, and these are negative (the ERA-Interim cloud-cover trends are consistent with satellite estimates; see Supplementary Information).

Rather than contribute to the warming, decreased cloud cover would be expected to promote surface cooling because clouds have a warming influence in spring (Fig. 3b). It is likely that the temperature response to reduced cloud cover is exceeded by warming due to other processes. The radiative effect of cloud-cover changes is small in comparison with compensating changes in the temperature and humidity profiles associated
with varying ice conditions [11]. We find that the large majority of spring warming occurs in the Siberian sector of the Arctic basin (not shown), where ice clouds are the predominant cloud type [12]. In ice cloud-dominated regions, the radiative effects of changes in cloud cover are less important than changes in water vapour content [12]. In short, we find no evidence of changes in cloud cover contributing to recent near-surface Arctic warming.

A final consideration arises from model simulations which suggest that changes in atmospheric water vapour content may amplify Arctic warming [17–19]. Increases in water vapour are expected with increasing air temperatures and reduced sea ice cover [19,27]. In turn, water vapour is a powerful greenhouse gas1 and can lead to further warming and sea ice loss. In ERA-Interim, specific humidity trends are found only during the summer and early autumn, and are confined to the lower part of the atmosphere (Fig. 4a). The largest humidity increases are found in the Arctic basin. An associated increase in incoming long-wave radiation has probably enhanced warming in summer and early autumn. It is of further interest to determine whether these increases in humidity are locally driven or are a result of increased moisture transport into the Arctic. It is worth noting that the humidity trends coincide with the months of lowest sea ice coverage and largest sea ice declines. The pronounced warming in winter and spring is not accompanied by increases in humidity. A large portion of each total humidity trend is linked to changes in sea ice (Fig. 4b) and, furthermore, to significant increases in the surface latent-heat flux (that is, evaporation) in the Arctic basin (Fig. 4a). The humidity increases at latitudes 50–65° N show weaker links to sea ice and are probably influenced by other processes. However, within the Arctic these lines of evidence support the notion that part of the humidity increase is driven by enhanced surface moisture fluxes associated with sea ice reductions.
The evidence from the past two decades, based on ERA-Interim, reveals that recent reductions in sea ice cover and thickness have been great enough to enhance Arctic warming strongly during most of the year. Our results suggest that the majority of the recent Arctic temperature amplification is due to diminishing sea ice cover. The amplification is strongest in the lowermost part of the atmosphere, where modified surface heat fluxes have their greatest influence. The emergence of strong ice–temperature positive feedbacks increases the likelihood of future rapid Arctic warming and sea ice decline.

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