An Ice-Free Arctic: What Could It Mean For European Weather?

WeatherVolume 76, Issue 10 p. 327-328 Brief ReportsFree Access An ice-free Arctic: what could it mean for European weather? James A. Screen, Corresponding Author James A. Screen [email protected] University of Exeter, Exeter, UKCorrespondence to: J. A. Screen [email protected]Search for more papers by this author James A. Screen, Corresponding Author James A. Screen [email protected] University of Exeter, Exeter, UKCorrespondence to: J. A. Screen [email protected]Search for more papers by this author First published: 17 September 2021 https://doi.org/10.1002/wea.4069Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL What is an ice-free Arctic? When scientists talk of an ice-free Arctic they specifically mean the loss of sea ice – floating ice formed by the freezing of seawater. An ice-free Arctic is commonly defined as being when the sea ice-extent is below 1 million square kilometres. This threshold is used, rather than zero, because remnant ice along the northern coasts of Canada and Greenland is expected to remain long after the rest of the Arctic Ocean is ice-free. When might an ice-free Arctic first occur? End-of-summer Arctic sea-ice cover has halved in the past 40 years, in line with global average warming. Climate model projections suggest an ice-free Arctic in late summer is likely to first occur before 2050, for a range of plausible scenarios of future greenhouse gas emissions (Notz and SIMIP Community, 2020). The precise timing of the first occurrence of an ice-free Arctic depends on natural fluctuations on top of the long-term human-induced decline. Looking beyond the first occurrence, the more warming that occurs, the more often ice-free spells will occur and for longer portions of the year. However, the complete loss of sea ice year-round is very unlikely this century, even under the most extreme warming scenarios. Can ice-free summers be prevented? Achieving the Paris Agreement target of limiting global warming to no more than 1.5 degC above preindustrial levels may not be sufficient to prevent an ice-free Arctic from occurring. However, ice-free summers are anticipated to be a rare occurrence in a 1.5 degC warmer world, happening around once in every 40 years (Screen, 2018). For warming of 2 degC above preindustrial levels, the expected frequency of an ice-free summer rises to once in every 3 to 5 years (Screen, 2018). Current emissions reduction pledges put us on a path to around 3 degC of warming by 2100, which would lead to ice-free conditions during most summers (Screen, 2018). How could sea-ice loss affect European weather? The most immediate effect of sea-ice loss is to enhance warming in the Arctic, as open water absorbs more sunlight than sea ice. The extra heat gained by the ocean during summer is released to the air above it in autumn and winter (Cohen et al., 2020). However, growing evidence suggests the effects of sea-ice loss will be felt well beyond the Arctic Circle – possibly including northwest Europe. As the Arctic warms much faster than places further south, the temperature difference between these regions is reduced, potentially affecting the jet stream. The jet stream is a band of strong winds at around 11km above ground, flowing from west to east. The jet stream exists largely because of the equator-to-pole temperature difference, and a lessening of that gradient may cause the jet stream to slow down and follow a more wavy path (Figure 1). Figure 1Open in figure viewerPowerPoint How Arctic sea-ice loss may affect the jet stream. A slower and more wavy jet stream can lead to extreme regional weather, for example in northwest Europe and North America. Higher up in the atmosphere, at around 30km, the stratospheric polar vortex is a ring of strong winds that encircle the pole in winter. The stratospheric polar vortex is disrupted once every two or so years, which then affects the troposphere below it, often causing the jet stream to weaken and become distorted (Figure 2). Polar vortex and jet stream disruptions can cause extremely cold winter weather in northwest Europe, such as during the ‘Beast from the East’ in early 2018 and the extremely cold winter of 2009/2010. Some scientists argue that polar vortex disruptions are made more likely by human-induced sea-ice loss. Figure 2Open in figure viewerPowerPoint How the stratospheric polar vortex influences European weather. How sure are scientists about the distant effects? Although scientists are reasonably confident that Arctic sea-ice loss could affect weather and climate in northwest Europe, the size of this effect is unknown (Cohen et al., 2020). It is unclear how much of the observed changes in the jet stream and polar vortex are caused by climate change or natural climate variability. Climate model experiments tend to suggest a modest effect of sea-ice loss in the mid-latitudes, which is small compared to natural variability. However, even a weak effect could still be important in the future, if sea-ice loss continues apace. The amount of future sea-ice loss depicted in climate models is one factor (of many) that explains their projections of mid-latitude weather but it is often not the dominant factor. Given that climate models are imperfect, the possibility remains that the real effects may be larger than models suggest. Essentially, scientists’ understanding of how on-going Arctic changes impact global weather patterns over a range of timescales from seasons to centuries is imprecise. This has led the Intergovernmental Panel on Climate Change (IPCC) to report only low confidence in the nature of the connections between Arctic sea-ice loss and mid-latitude weather (IPCC, 2019). Scientists around the world are working together (Smith et al., 2019) to determine the consequences of sea-ice loss for weather and climate beyond the Arctic. Acknowledgements This paper was developed in collaboration with the Royal Meteorological Society's Science Engagement Committee. The author is grateful for suggestions that improved the clarity of the writing and graphics from Claire Parkinson, Adam Scaife, Hannah Mallinson, Liz Bentley and Saffron O'Neill. Tara Barney is thanked for producing the graphics. References Cohen J, Zhang X, Francis J et al. 2020. Divergent consensuses on Arctic amplification influence on midlatitude severe winter weather. Nat. Clim. Change 10: 20– 29. IPCC. 2019. Summary for policymakers. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. Pörtner H-O, Roberts DC, Masson-Delmotte V et al. eds: In. https://www.ipcc.ch/srocc/chapter/summary-for-policymakers/. Notz D, SIMIP Community. 2020. Arctic sea ice in CMIP6. Geophys. Res. Lett. 47: e2019GL086749. Screen JA. 2018. Arctic sea ice at 1.5 and 2 °C. Nat. Clim. Change 8: 362– 363. Smith DM, Screen JA, Deser C et al. 2019. The Polar Amplification Model Intercomparison Project (PAMIP) contribution to CMIP6: investigating the causes and consequences of polar amplification. Geosci. Model Dev. 12: 1139– 1164. Citing Literature Volume76, Issue10Special Issue: COP26October 2021Pages 327-328 This article also appears in:Briefing Papers FiguresReferencesRelatedInformation