1.1 The Urgent Need to Deal With Climate Change

The climate is warming, and the rate of change is highest in the Arctic, where summer ice is vanishing at an accelerating rate. According to the 2013 Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), the warming of the atmosphere and ocean system is unequivocal. From 1880 to 2012 the globally averaged combined land and ocean surface temperatures increased 0.85 ± 0.20°C, with the last three decades warmer than the previous one and warmer than any others since 1850. The increase in temperatures has been even higher in the Arctic region, with dramatic effects. On 30 December 2015, temperatures on a buoy 300 km from the North Pole shot up 29°C due to an influx of warm air from Storm Frank, and some data suggested the temperatures topped the freezing point at the North Pole in the dead of winter, an exceedingly rare event (http://www.telegraph.co.uk/news/weather/12075282/North‐Pole‐temperatures‐spike‐above‐freezing‐as‐Storm‐Frank‐sends‐warm‐air‐north.html, 10 Jul. 2016).

It can be stated with “high confidence” (see AR5 for a definition of this and similar terms) that the Greenland and Antarctic ice sheets have been losing mass and glaciers have been shrinking worldwide. Arctic sea‐ice extent (area covered by at least 15% ice) decreased from 1979 (when satellite measurements began) to 2012, at a rate “very likely” in the range of 3.5%–4.1% per decade. The pace of decrease is apparently accelerating: for the baseline 1978–1999, the rate was 3% per decade [Johannessen et al., 1999]. The loss of Arctic sea ice in the summer of 2007 was much greater than expected and attracted the attention of the scientific community and the public [Kerr, 2007; Revkin, 2007]; but the extent of sea ice in 2012 was even lower, and through July 2016, the sea ice extent was lower than ever recorded in the spring, more than 2 standard deviations below the 1981–2010 average. Figure 1 illustrates the alarming and accelerating loss of ice in the Arctic Ocean.

Figure 1 Open in figure viewer PowerPoint Arctic sea‐ice extent (area covered at least 15% by sea ice) in September 2007 (white area). The red curve denotes the 1981–2010 average. Sea‐ice extent in 2016 was the second lowest ever recorded. Courtesy of the National Snow and Ice Data Center ( http://nsidc.org/arcticseaicenews/charctic‐interactive‐sea‐ice‐graph/ ).

The loss of Arctic sea ice is due to anthropogenic effects and is therefore likely to continue to accelerate. According to the IPCC AR5 Report, the increase in global temperatures is very similar to that predicted from the cumulative anthropogenic CO 2 emissions of 2040 ± 310 Gt between 1750 and 2011. It is “extremely likely” that most of the global temperature increases are due to anthropogenic forcings. Anthropogenic influences have “very likely” contributed to Arctic sea‐ice loss since 1979. Global average temperatures have been observed to rise linearly with cumulative CO 2 emissions and are predicted to continue to do so, resulting in temperature increases of perhaps 3°C or more by the end of the century. The Arctic region will continue to warm more rapidly than the global mean. Year‐round reductions in Arctic sea ice are projected in virtually all scenarios, and a nearly ice‐free (<106 km2 sea‐ice extent for five consecutive years) Arctic Ocean is considered “likely” by 2050 in a business‐as‐usual scenario.

While the trends in sea‐ice extent are consistent with late‐summer ice vanishing from the Arctic Ocean by mid‐century, the trends in sea‐ice volume and sea‐ice thickness seem more consistent with attainment of such a state even sooner. Figure 2 shows the minimum volume of Arctic sea ice through 2016, as similarly presented by Schweiger et al. [2011], using data from the Pan‐Arctic Ice Ocean Modeling and Assimilation System [PIOMAS; Zhang and Rothrock, 2003]. Before 2007, sea‐ice volume was never known to drop below 9000 km3, but sea‐ice volume has dropped below 7000 km3 every year since then. A linear extrapolation suggests the late‐summer Arctic ice volume is decreasing by 3200 km3 per decade and will certainly vanish by the 2030s. Some studies indicate the ice may effectively vanish even sooner than these estimates, possibly before 2020 [Maslowsky et al., 2012; Wadhams, 2016].

Figure 2 Open in figure viewer PowerPoint 3 per year and will vanish by the early 2030s or sooner. Data courtesy of PIOMAS ( Monthly average Arctic sea‐ice volume for April and September. Linear fits suggest sea‐ice volume at the end of Arctic summer is decreasing by 320 kmper year and will vanish by the early 2030s or sooner. Data courtesy of PIOMAS ( http://psc.apl.uw.edu/research/projects/arctic‐sea‐ice‐volume‐anomaly/ ).

Sea‐ice volume is decreasing at a faster rate than sea‐ice extent because the ice is getting thinner over time. Newly created “first‐year” ice is typically not more than 1 m thick. Ice that survives the summer can grow and become multiyear ice, with a typical thickness of 2–4 m. During the 1980s, multiyear ice comprised 50%‐60% of all ice in the Arctic Ocean, but by 2010 only 15% of Arctic sea ice was over 2 years old [Comiso, 2012; Polyakov et al., 2012]. First‐year ice is much more susceptible to summer melting and to fracturing and dispersal by winds and waves. Warmer summers hinder the survival of ice through to the next winter and inhibit formation of multiyear ice. Figure 3 shows the mean annual thickness of sea ice in the Arctic for the period 2000–2012, as determined by combining multiple datasets using various techniques such as upward‐looking sonar, electromagnetic sensors, LIDAR, or radar altimeters, etc. [Lindsay and Schweiger, 2015]. Significantly, half the Arctic Ocean is characterized by ice that is less than 1.5 m in thickness on average; an increase of even 1 m would be significant. Over this period, the mean annual ice thickness decreased at a rate of 0.58 m per decade. A simple linear extrapolation of the data suggests an ice‐free Arctic in September as soon as the late 2020s, or earlier than that if the trends accelerate.

Figure 3 Open in figure viewer PowerPoint Lindsay and Schweiger [ 2015 Lindsay and Schweiger [ 2015 The Cryosphere 9, 269–283. Used with permission from R. Lindsay. (a) Mean annual thickness of ice in the Arctic over the period 2000–2012, as determined by a combination of datasets using different sensing techniques, from]. (b) The mean thickness over the Arctic of the ice in May, September, and averaged throughout the year. The annual average is decreasing at 0.58 m per decade, and a simple extrapolation suggests an ice‐free Arctic in summer by the 2030s. Figure taken from] “Arctic sea ice thickness determined using subsurface, aircraft, and satellite observations,”9, 269–283. Used with permission from R. Lindsay.

The Arctic is warming faster than the rest of the planet because of several polar amplification feedbacks [Holland and Bitz, 2003; Alexeev and Jackson, 2013; Pithan and Mauritsen, 2014], chief among them is the strong ice‐albedo feedback. Fresh snow reflects up to 90% of incident sunlight, sea ice reflects sunlight with albedo up to 0.7, and even sea ice with melt pools has albedo 0.2–0.6 (see Section 2.3); in contrast, open water absorbs most sunlight, having an albedo close to 0.06 (https://nsidc.org/cryosphere/seaice/processes/albedo.html, 10 Jul. 2016). As ice melts, it becomes open water capable of absorbing more sunlight, in a positive feedback. On long (104–105 year) timescales, this ice‐albedo feedback is what amplifies the external forcing due to changes in Earth's obliquity and eccentricity (Milankovitch cycles) into the large temperature variations that drive ice ages [McGehee and Lehman, 2012]. But the feedback can act on shorter timescales, and there is direct evidence that the ice‐albedo feedback enhanced the loss of ice during the 2007 Arctic summer [Lindsay et al., 2009; Perovich et al., 2008].

A simple calculation underscores the important role played by Arctic ice in Earth's climate. The Arctic acts as a major temperature regulator for the Earth [Winton, 2006]. During the summer months, Arctic sea ice historically has covered an area close to 6 million km2 (Figure 1). On average it receives sunlight for half the year, with the Sun ≈ 10° above the horizon. Total cloud cover over the ocean during the summer months averages 82% [Eastman and Warren, 2010]. Thus the surface in 1 year absorbs a total energy (6 × 1012 m2) × (1366 W m−2) × (sin 10°) × (0.18) × (0.5 year) × (1 − a), where a is the albedo. Assuming a = 0.06 for ocean water and a = 0.75 for sea ice, the difference in energy absorbed is 2.8 × 1021 J in a year. Averaged over 1 year and the total area of the Earth, 4πR E 2 = 5.12 × 1014 m2, this is equivalent to 0.17 W m−2 of radiative forcing. The current climate change is driven by about 1.2 W m−2 of anthropogenic radiative forcing, so losing summer sea ice would have a profound effect on Earth's climate, accelerating temperature increases. Conversely, preventing the loss of Arctic ice that will inevitably arise from climate change (or even increasing the amount of Arctic ice) would have a measurable impact on slowing temperature increases worldwide.

This argument is bolstered by consideration of other feedbacks unique to the Arctic. Billions of metric tons of carbon are stored in the permafrost of the tundra and would be released as CH 4 or CO 2 if the permafrost were to melt [Dutta et al., 2006; Walter et al., 2006; Schuur et al., 2008]. Already the rate of carbon release from thawing permafrost appears to be increasing [Schuur et al., 2008]. According to the IPCC AR5, it is virtually certain that near‐surface (uppermost 3.5 m) permafrost extent at high northern latitudes will be reduced as global mean surface temperatures increase. The area of permafrost is projected to decrease by 81% in a business‐as‐usual model (RCP8.5). The threat from methane emitted from the melting of offshore permafrost in the shelf seas of the Arctic may be even greater [Shakhova et al., 2014; Wadhams, 2016]. Release of billions of tons of the greenhouse gases CH 4 or CO 2 over a few decades would substantially increase Earth's mean surface temperatures further.

Other consequences of Arctic sea ice loss are severe. Decreasing sea ice may be altering weather patterns in the northern hemisphere by weakening the jet stream [Cassano et al., 2014; Overland et al., 2015]. Decreased sea ice in the Arctic also seems to have caused enhanced coastal erosion [Overeem et al., 2011]. Loss of sea ice threatens the entire sea‐ice ecosystem [Post et al., 2013], including, among other things, making it more difficult for polar bears to find food [Derocher et al., 2004].

For the preservation of the Arctic and its unique ecosystems, and especially to preserve its role in Earth's climate through the ice‐albedo feedback, and to prevent the worst positive feedbacks such as release of gigatons of greenhouse gases stored in the permafrost, it is vital to do as much as possible to prevent loss of sea‐ice in the Arctic. Moreover, the need is urgent, as the normal cooling effects of summer sea ice are already lessened and may disappear in less than two decades.