In recent decades, the Arctic has indeed undergone considerable change. The sea ice and snow cover and the mountain glaciers in the Northern Hemisphere (NH) are declining and retreating, the ice sheet in Greenland has been losing mass and the permafrost in parts of North America and Eurasia has been increasing in temperature, and even thawing in some areas. These phenomena have been reported in a vast number of publications in recent years. 3 - 8 Attribution studies have been conducted using numerical modeling but results have not been consistent and more research is needed to explain the character and the magnitude of the trends. The purpose of this article is to provide an overview of the changes in the Arctic climate as reflected in its major components and as inferred primarily from satellite data.

The Arctic is an area of intense interest because climate‐change signals are expected to be amplified in the region by about 1.5–4.5 times. 1 Ice‐albedo feedback effect 1 , 2 associated with the high albedo of snow and ice which cover a large fraction of the region has been postulated as one of the key reasons for the amplification of the warming. As the snow and ice retreat, more heat is absorbed by the Earth's surface causing a warmer surface that in turn causes more melt and enhanced retreat of the snow/ice. The Arctic is also vulnerable to rapid changes because snow and ice are very sensitive to increases in surface temperature especially near melt temperatures.

The use of satellite data has revolutionized our ability to study the Arctic. The data have led to the discovery of many unexpected phenomena and events in both polar regions. However, many measurement challenges remain. Satellite‐borne passive microwave systems have provided continuous coverage of the sea ice cover for more than four decades and have revealed details on the large‐scale distribution of the sea ice cover at a good temporal resolution, though, the spatial resolution is only about 12–25 km which is good enough for most large‐scale studies, but a full characterization of some of important processes like polynya formation, Polar Lows and divergence that takes place inside the ice pack is not possible. Additionally, the retrieval of some parameters, like temperature, albedo and cloud cover, is oftentimes compromised because of persistent cloud cover in the Arctic and the difficulty of accurately discriminating clouds from snow and ice. Optimal cloud masks have been elusive but the advent of new sensors has led to considerable improvements in this regard. Despite the many challenges, however, satellite data provide a consistent, accurate, and comprehensive record suitable for climate‐change trend studies.

Adverse weather conditions, extremely cold temperatures especially in winter, thick and almost impenetrable sea ice cover, and prevailing darkness in winter are among the many challenges of making measurements in the Arctic. Logistical problems are further confounded by the challenge of designing instruments that can provide accurate measurements consistently under extreme conditions and maintaining these instruments so they continue to provide reliable and consistent information. Transport of equipment can also be a problem because of inability of most ships to navigate through sea ice and the difficulty of snow machines to get through barriers like ridges and leads in sea ice. Over ice masses and glaciers, the rugged terrain and the presence of crevasses also pose problems. Additionally, working in mountain snow covers can be hazardous due to avalanches and difficulties in traveling through deep snow. Although the recent warming of the Arctic and the rapid retreat of perennial ice has led to the opening of the Northeast and Northwest passages in some summers and made logistics in the region somewhat less difficult, measurement challenges persist.

The Arctic consists of the Arctic Ocean, the peripheral seas, the northern parts of North America (including Greenland) and Eurasia. The central region is covered mainly by sea ice and open water that is surrounded by land that is underlain by permafrost and at least partly covered by snow, glaciers, ice caps, and the Greenland ice sheet. The southern boundary of the Arctic has been loosely defined by some as the Arctic Circle (66° 33′N)—the approximate location of the edge of the midnight sun and polar night. We use in our study an extended boundary since trends in the Arctic can be evaluated more effectively by examining the changes in the various components of the cryosphere some of which are in areas found south of the Arctic Circle. The location map in Figure 1 shows the distribution of sea ice during the end of summer in 2012, the typical location of the end of summer ice edge (yellow line) and the locations or boundaries of glaciers, ice sheet, snow, and permafrost in land areas. The sea ice cover extends into the sub‐Arctic and as far south as 44°N in the peripheral seas during winter. Permafrost, snow cover, and the Greenland ice sheet all extend south of the Arctic Circle.

OBSERVED CHANGES IN THE ARCTIC

Variability and Trends in Surface Temperature Surface temperature is one of the key parameters that determines the state of the Arctic. More specifically, it is temperature that determines the thermodynamic state of the various surfaces, especially those that are ice covered. For example, the length of subfreezing (or melt) temperatures determines the extent, thickness and duration of many cryospheric parameters. In conjunction with surface albedo, temperature controls the amount of energy transferred between the ocean and the atmosphere, including turbulent, sensible, and latent heat fluxes. Using data from meteorological stations worldwide along with other sources of in situ data, global temperatures have been shown to have increased by about 0.8°C since 1900.9-12 A general warming in the Arctic using the limited and spatially sparse in situ data has also been reported.13 Warming was also reported using a pan‐Arctic data set that was created by filling in the gaps of in situ and buoy temperature data through the use of a sophisticated spatial interpolation technique.14 Reanalysis data from ERA‐40 and national center for environmental prediction (NCEP) data have also been used for temperature change studies15 but the accuracy depends on the availability of in situ data which are sparse in the region and may not be as reliable for trend studies as those derived from satellite data.16 Monthly averages of surface temperature in the Arctic region, as derived from the advanced very high resolution radiometer (AVHRR), have been compiled and analyzed for the period August 1981 through the most recent data available using different algorithms.16, 17 The results from different sources provide similar patterns of temperature distribution but yield significantly different variability and trends not only because of differences in techniques but also in resolving differences in the calibration of the different AVHRR sensors and in addressing atmospheric corrections.16 Improved surface temperature maps of Greenland using moderate‐resolution imaging spectroradiometer (MODIS) data have been generated18 but the record length of MODIS (2000–2014) is currently too short for meaningful trend analysis. However, during this period some unexpected events were observed including the occurrence of unusually extensive surface melt events in 2002 and 2012.19, 20 The historical surface temperature satellite data show large seasonality of surface temperature with the monthly averages ranging from −29°C to 0°C inside the Arctic Circle.21 The coldest regions in the Arctic are most consistently located at the high elevations of the Greenland ice sheet but there are times when surface temperatures in parts of Siberia are even colder. To gain insight into how surface temperatures have been changing in various parts of the Arctic a color‐coded map of trends in surface temperature using the 1981–2012 monthly anomaly data is presented in Figure 2(a). The monthly anomaly data which have been updated from a previous version16 were derived using satellite climatology as the baseline. Although the trends are spatially variable, they are mainly positive. Unusually high positive values are shown over sea ice but these are mainly in areas where the summer ice edges have been retreating in recent years. There are also some negative trends, predominantly located in parts of Siberia and in the Bering Sea, that are associated primarily with unusually warm conditions in the region in the 1980s. Figure 2 Open in figure viewer PowerPoint (a) Color‐coded surface temperature trends for the entire Arctic using August 1981–November 2012 advanced very high resolution radiometer (AVHRR) data; (b) plots of yearly anomalies and trends for the entire globe (black) and for the Arctic region (>64°N) using Hansen (2010) (blue) and AVHRR (red) data; and plots of anomalies (blue), 1‐year running average (red) and linear trend (black) for (c) sea ice; (d) Greenland; (e) Eurasia, >64°N; (f) North America, >64°N; and (g) SST, >60°N. Using the updated version of the Goddard institute for space studies (GISS) data set,9 the trend in global temperature has been estimated to be about 0.081°C/decade (very likely between 0.074 and 0.088°C/decade) for the period from 1900 to 2012. The ‘very likely’ range in trend corresponds to 90% confidence level. For the period 1981–2012, the trend in surface temperature using GISS data,9 was estimated, as shown in Figure 2(b), to be 0.17°C/decade (very likely between 0.14 and 0.20) for the entire globe and 0.60°C/decade (very likely between 0.48 and 0.71°C/decade) for the Arctic region (>64°C). These results represent an acceleration of the global trend and an amplification by more than three times in the Arctic region showing consistency with modeling predictions. For comparison, the corresponding trend in temperature using AVHRR data for the same time period in the Arctic (>64°C) is 0.69°C/decade (very likely between 0.56 and 0.82°C/decade). The slightly greater trend in Arctic temperatures from AVHRR is mainly due to stronger trends in the Central Arctic Basin (see Figure 2(a)) where in situ data are very sparse.16 These results indicate a general consistency of in situ/station temperature data with satellite data. The trend results are slightly different but consistent with those reported previously but using a different algorithm for temperature and a shorter time period.18 The errors quoted are for statistical errors of the trends and do not include systematic errors which are unknown but expected to change only slightly from one year to another and would not significantly affect the errors in the trends. The range of values with 90% confidence level and referred to as the very likely range is also provided. To illustrate how the temperature varied on a monthly basis from one region to another, monthly temperature anomalies over four key regions (namely: sea ice, Greenland, Eurasia, and North America) were calculated and the results together with 12‐month running averages (in red), are presented in Figures 2(c)–(f). The plots indicate that there were years when it was unusually warm (e.g., 1995) and years when it was unusually cold (e.g., 1982). The trend in temperature over sea ice covered regions is estimated to be 0.47°C/decade (very likely between 0.37 and 0.57°C/decade at 90% confidence level), whereas the trend was significantly higher at 0.77°C/decade (very likely between 0.60 and 0.94°C/decade) over Greenland. The high trend values in Figure 2(a) are over the sea ice regions that are located in areas where sea ice had been displaced by open water in later years during the ice melt season. In the region >64°N, the trend is 0.20°C/decade (very likely between 0.07 and 0.33°C/decade in Eurasia) which is low compared to 0.54°C/decade (very likely between 0.34 and 0.69) for North America because of the negative trends in Siberia as shown in Figure 2(a). Again, the errors quoted are the statistical errors and there is a 90% confidence level that the trends are within the range indicated. Changes in the ocean temperature in the Arctic region have been reported in several studies.22, 23 The temperature of the mixed layer has apparently been affected by ice‐albedo feedback on account of recent declines in the summer ice cover and increases in dynamics both for the ice cover and the underlying ocean. The mixed layer temperature has recently been affected by changes in the extent and frequency of leads in autumn and winter as may be caused by changes in atmospheric circulation.23 The changes are monitored in part using satellite data which provide comprehensive coverage of sea‐surface temperature (SST) in the region during clear‐sky conditions21 using AVHRR and MODIS data, and almost continuously using Scanning Multichannel Microwave Radiometer (SMMR), EOS/advanced microwave scanning radiometer (AMSR‐E) and AMSR2 data.23-25 Using AVHRR data, the monthly anomalies of SST are presented in Figure 2(g) for >60 N and regression analysis yielded a trend of 0.04°C/decade (very likely between 0.02 and 0.06°C/decade). The trend is modest but using AMSR‐E data, SST has been shown to increase rapidly (5°C over a 7‐day period) to an unusually high value in the western region in 2007,23 when a dramatic decline in the sea ice cover was observed, as discussed in the next section.

Variability and Trends in the Sea Ice Cover The Arctic sea ice cover is part of the polar heat sink and is considered a key component of the climate system. It has seasonal ice and perennial ice components with each component representing approximately half (about 8 × 106 km2) of the maximum extent during the 1980s when the passive microwave satellite series just began.26 The perennial ice component is defined as the ice that survived at least one summer and is usually represented by the sea ice cover during ice minimum extent.27 During the last three decades, the perennial component has been declining significantly while the total ice cover has also been decreasing but more modestly.27, 28 The net effect is an increasing extent of the seasonal sea ice cover. Changes in the sea ice cover garnered significant attention when the perennial ice was first reported to be declining rapidly from the 1980s to the 1990s,27 leading to concerns that in the foreseeable future, the Arctic Ocean would become a blue (ice free) ocean29 in the summer. There was even greater concern when the Arctic ice cover was observed to be declining faster than what modeling studies had been predicting.8 This means that there are some unexpected factors that caused the faster decline and that the models have to take into account the physics behind these factors. The concerns were exacerbated when the perennial ice extent went down drastically in 2007 and was reduced to 40% of the average extent of the perennial ice from 1979 to 2007.30 This was followed by another decline to a new low in 2012, which has been attributed in part to the occurrence of a large though rare storm in August 2012.31-33 A key issue in the assessment of interannual variability and trends in the ice cover is the estimate of ice extent and ice area which are derived from sea ice concentrations retrieved from passive microwave data. Several algorithms have been developed to retrieve sea ice concentration.34-37 There are significant differences in the ice concentration estimates38 from the various techniques but comparative studies done in different institutions have led to the selection of two algorithms for generating the standard products used by the sea ice community. The two are the NASA Team algorithm (NT1) and the Bootstrap Algorithms with NT1 subsequently improved using a different technique called NT2.39 A comparison of ice concentrations and trends from the Bootstrap and NT2 shows good consistency in derived concentrations and trends.40 In this report, we use results from the Bootstrap algorithm because it is the one that can provide consistent data from 1978 to the present, as NT2 requires the use of 89 GHz data that was not available for most of the period from 1978 to 1992. Averages of the sea ice concentrations during the first and second halves of the historical data set are presented in Figure 3(a) (1979–1995) and 3(b) (1996–2012), respectively, to illustrate typical sea ice distributions during the two periods and how they changed from one period to another. It is apparent that ice cover during the earlier period is significantly more extensive with higher concentration than that of the latter period, especially in the Arctic Basin. To quantify the difference in the distribution a difference map is presented in Figure 3(c). The difference map shows mainly negative values with the most negative located where the sea ice cover had been retreating most rapidly. There are a few areas where the difference is slightly positive (e.g., Bering Sea) but large negative values can be observed along the edges of the location of the summer ice cover. For comparison, the trends in ice concentration are estimated for each pixel and the results are depicted in Figure 3(d). As with the surface temperature trend in Figure 2, the trend map made use of monthly anomalies using as a baseline, climatologies derived from historical data from November 1978 to December 2012. The trend analysis assumes that the changes in ice concentration vary linearly with time for each data element which is not necessarily the case. It is apparent, that the trend and difference maps have very similar spatial distributions, but not always. Significant differences in the Okhotsk Sea, Chukchi Sea, and East Siberian Sea regions are apparent and are likely associated with the nonlinearity of the data used in the trend analysis in these regions. Figure 3 Open in figure viewer PowerPoint (a) Color‐coded average sea ice concentration map for (a) the period from January 1979 to December 1995; and (b) the period from January 1996 to December 2012. (c) Differences (for each pixel) of averages in (a) with averages in (b). (d) Trend map of sea ice concentration from January 1979 to December 2012. (e) Plots of daily averages of sea ice extent for the period 1979–1995 (in gray), sea ice extent from 1996 to 2012 (in black), difference of sea ice extent between the two periods (in gold, bottom plot); and daily sea ice extent for individual years in 1980 (when summer minimum was highest, in red), 2007 (when summer minimum was second lowest, in green) and 2012 (when summer minimum was a record low, in blue); (f) Ice extents and trends of yearly averages of sea ice extent (purple line), perennial ice (in blue), and multiyear ice (in green) for the 1979–2012 period using SMMR and SSM/I data. Also similar plots from AMSR‐E data from 2003 to 2012 (gold) are presented for perennial and multiyear ice. A more quantitative representation of typical seasonal changes over different time periods is presented in Figure 3(e). Averages of ice extents for each day for the period 1979–1995 are depicted by the gray line while corresponding averages for the period 1996–2012 are represented by the black line. The gray and black lines show a considerable change in the seasonality of sea ice from the first period of satellite data to the second period, especially during the summer. The differences in average extents for the two periods are better quantified by the gold line (bottom line) which shows that the difference during the summer months (including September and October) is about twice as much as for the rest of the year. For comparison, daily values are presented for 2012 (green line) which is the year with record low perennial ice extent (Figure 1), for 2007 (blue line) which is the year with the second lowest perennial ice, and for 1980 (red line) which is when the perennial ice extent was highest during the satellite era. These three plots indicate large departures from the multiyear averages and illustrate how drastically the extent of the sea ice cover has changed since 1980. The changes in the entire Arctic ice cover are quantified in the set of plots presented in Figure 3. The yearly averages (Figure 3(a)) show significant interannual variability with a trend in ice extent of −473,000 km2/decade or −3.8%/decade (very likely between −3.1 and −4.5%/decade). These values are generally consistent with those reported for a shorter period28, 41, 42but are significantly different for the percentage trends because a different baseline was used in the estimates. Our current values make use of the first data point (along the trend line) as the baseline instead of a climatological average. The first data point is regarded to yield a more accurate trend than the climatological average used in previous studies. For comparison, the trend in ice area was estimated to be −4.9%/decade (i.e., −526,000 km2/decade) using the same data with the trend very likely between −4.6 and −5.2%/decade. The yearly extent of the perennial ice cover is also shown in Figure 3(f) (blue line) with the trend line indicating a rapid decline of −11.5%/decade (very likely between −9.3 and −13.7%/decade). Such rapid rate of decline indicates that the perennial ice, which is the mainstay of the Arctic ice cover and has been known to have existed for at least 1450 years,43 is in the process of disappearing.44 In addition, the multiyear ice (or ice that survives at least two summers) as derived from passive microwave data during the winter period,45 is also declining even more rapidly at a rate of 13.5%/decade (with 90% confidence level that the trend is between −11.0 and −16.2%/decade). The AMSR‐E data (in gold) provide an independent estimate of the perennial and multiyear ice extents and show good consistency with SSM/I data. Again, the values are slightly different from those quoted in the references because we now use a different but more accurate baseline. The percentage trends are slightly less negative but the conclusions remain the same. The net effect of a declining perennial and multiyear ice cover is a decline in average thickness which means that the volume of the ice is also declining. Direct measurements using available submarine, aircraft, and satellite data indeed indicate a considerable thinning of the sea ice cover. Thinning was reported using upward‐looking sonar measurements of ice draft from submarines.46, 47 Ice, Cloud and Land Elevation Satellite (ICESat) data in combination with submarine data also show considerable change in ice thickness. In particular, measurements of ice keels from submarine upward‐looking SONARs covering about 38% of the Arctic Ocean provided an average winter thickness of 3.64 m in 1980. By 2008, the average thickness over the same region has gone down to about 1.9 ± 0.5 m48 (likely) indicating a 48% change in thickness. The impact is a more predominant seasonal (first year) sea ice cover which has been increasing at the rate of 6.0 ± 1.3% (likely)/decade from 1979 to 2012 (Figure 3(f)). This means more ice production in the Arctic Basin and therefore changes in the salinity and temperature structure for the Arctic Ocean. A thinner ice cover will also enhance ice drift velocities while more ice production will alter vertical as well as horizontal ocean circulation. The variability of the Arctic sea ice cover over a time period extending before the satellite era is poorly known. A pan‐Arctic data set on the sea ice cover for the period 1900–1979 has been put together using available data, climatology, and spatial interpolation.49 Such data which have been updated using a more robust data set that starts in 1870 showed a fairly uniform distribution of the ice cover during the period with only a very slight negative trend. As in situ observations have been available, only in some regions of the Arctic the lack of variability is in part due to the extensive use of climatological data. Using terrestrial proxies from the circum‐Arctic region, the decline of the summer sea ice cover in the last four decades has also been reported to be unprecedented for the past 1450 years.43

Trends in the NH Snow Cover Snow cover is a sensitive indicator of climate change. It is also a powerful regulator of surface and near‐surface air temperature because of its low thermal conductivity and high albedo. There is a strong positive feedback between snow cover (especially in the spring) and the radiation balance in the NH.50 Snow‐water equivalent (SWE), the quantity of water available when a snow cover melts, is a key snowpack parameter because as much as 80% of the water supply in some mountainous areas, such as the western United States, emanates from melting snow. Recent research suggests that snow cover and SWE in the NH has been decreasing, particularly in the springtime.51 Since 1966, data from geostationary operational environmental satellites (GOES), and from the very high resolution radiometer (VHRR) and its successor, the AVHRR, on polar orbiting environmental satellites (POES), have been used extensively by the National Oceanic and Atmospheric Administration (NOAA) to produce operational snow products.52 In 1997, the Interactive Multisensor Snow and Ice Mapping System (IMS) (http://www.ssd.noaa.gov/PS/SNOW/ims.html) was developed to produce operational products daily at a spatial resolution of about 25 km, based on a variety of satellite data.53, 54 Subsequent improvements have increased the spatial resolution to 4 km. The Rutgers Global Snow Lab (GSL) reanalyzed snow cover between 1966 and 1971 using the same NOAA daily gridded composites of visible imagery for the eastern and western hemispheres of the NH used in the original mapping.55 The Rutgers GSL dataset is now considered a climate‐data record (CDR). With the advent of the MODIS, first launched on the Terra satellite in 1999, and later on the Aqua satellite in 2002, daily fractional snow‐cover and snow‐albedo maps have been available at a resolution of 500 m, thus permitting the most accurate fully automated daily global maps of snow cover to be produced routinely.56, 57 Trend analysis results of the updated snow‐cover extent (SCE) series have been used58 to show that the NH spring snow cover has undergone significant reductions over the past 90 years and that the rate of decrease has accelerated over the past 40 years. In particular, the SCE has been reduced by 0.8 × 106 km2/decade (7% in March and 11% in April and very likely between 4.5 and 9.5% in March and between 8.5 and 13.5% in April) for the period 1970–2010. Higher temperatures were determined to be the dominant factor in the observed SCE decreases, with air temperature anomalies over NH midlatitude land areas explaining 50% of the observed variability in SCE.58 The uncertainty in the determination of SCE in NH spring has been estimated with 95% confidence level to be within the interval of ±5–10% over the pre‐satellite period and ±3–5% over the satellite era.58 Spring (March, April, and May) snow‐covered areas have been declining at a rate that is even greater than predicted by the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model simulations.59 SCE results from the Rutgers GSL (http://climate.rutgers.edu/snowcover) for the period 1967–2012 are presented in Figure 4(a)–(d). The trend in earlier spring snowmelt in the NH (−2.12 ± 0.45%/decade), in particular, in Eurasia (−2.72 ± 0.63%/decade) since the beginning of the SCE record in the late 1960s is noted. The trend is more modest at −1.27 ± 0.53%/decade for the North American region. Figure 4 Open in figure viewer PowerPoint (a) Years with maximum and minimum snow cover; data from the Rutgers University Global Snow Lab. Light blue represents snow cover in February of 1978 and dark blue represents snow cover in February of 1982 and white is snow in both years. (b) Northern Hemisphere snow cover in the spring, (c) Eurasian snow cover in the spring, and (d) North American snow cover in the spring, all showing declining trends in snow‐cover extent during the satellite snow‐mapping era. From November to April, snow covers an average of about 33% of the land areas north of the Equator, and the maximum snow cover occupies about 49% of the land area in the month of January (see http://climate.rutgers.edu/snowcover and Ref 2). Warming has caused a trend toward earlier snowmelt in the Arctic, first noted in Refs 60 and 61 and later by many other researchers for the NH as a whole (e.g., Refs 51, 62-64). Using a subset of the Rutgers ∼47‐year snow‐cover CDR, a decrease in the duration of the NH snow season by 5.3 days/decade between the winters of 1972–1973 and 2007–2008 has been reported.65 It was noted that the change is primarily a result of earlier snowmelt over western Europe, central and eastern Asia, and in the mountainous western United States. A key factor is the effect of surface air temperature anomalies that are known to control spring SCE.66 Use of the standard MODIS snow‐cover products over 14 winters permits detailed regional monitoring of snow‐covered mountains.57, 67-69 Using 40 years of stream discharge which provides indirect snowmelt information and meteorological station data and 10 years of MODIS snow‐cover products in the Wind River Range, Wyoming, USA, an earlier snowmelt in the decade of the 2000s as compared to the previous three decades was observed,69 but no trend of earlier snowmelt within the decade of the 2000s was observed. The earlier snowmelt is likely due to documented increasing air temperatures during the 40‐year period. Other studies have shown that snowfall may be occurring later in the autumn in Eurasia in more recent years (versus earlier years in the ∼47 year snow‐cover CDR).71 Thus, in many regions of the NH the snow‐cover season is becoming shorter. The primary effect of the decreased duration of the snow season and the diminished snow extent during the spring months (Figure 4(a)–(c)) has been on the energy balance in late spring—reduced albedo due to earlier snowmelt and therefore greater absorption of energy at the surface.51

Mass Balance Studies of Small Glaciers and Ice Caps, and the Greenland Ice Sheet Non‐Ice Sheet Glaciers The Earth's smaller glaciers and ice caps (non‐ice sheet glaciers) cover an area of approximately 785,000 km2 or a volume of ∼250,000 km3 which, if converted to meltwater, is equivalent to a rise in sea level of ∼0.7 m.72 This includes glaciers peripheral to the Greenland and Antarctic ice sheets. The current contribution to sea level from the Earth's smaller glaciers is ∼1 mm/year, but the loss of glacier ice to the oceans is accelerating.72 Through modeling, estimates show that the largest future contributions of smaller glaciers to sea level rise will be from glaciers in Arctic Canada, Alaska, and Antarctica.73 There has also been dramatic glacier recession in the European Alps.74 The Global Land Ice Measurements from Space (GLIMS) project follows the Satellite Image Atlas of Glaciers of the World multi‐decade and multinational project led by Richard S. Williams who is now an Emeritus Scientist at the U.S. Geological Survey (USGS). There are 11 ‘Atlas’ volumes (book‐length chapters),75 providing an accurate regional inventory of the areal extent of glaciers on the planet, with different chapters focusing on different geographic areas. Additionally, Chapter 1386‐A76 focuses on changes in the cryosphere, and trends related to global warming.2 Prior to that, William O. Field compiled a NH atlas of mountain glaciers.77 The goal of GLIMS is to build and populate a Web‐accessible database of glacier data for the approximately 160,000 non‐ice sheet glaciers worldwide to produce a globally comprehensive and consistent set of glacier outlines.70, 78 Like the Satellite Image Atlas series, this project is characterized by broad international cooperation. Recently, the global inventory of glacier outlines, called the ‘Randolph Glacier Inventory’, or RGI 1.0: www.glims.org/RGI, (http://www.glims.org/RGI/randolph.html) was produced as reported.79 Estimates from different methods that yield long time series indicate steady increases in ice loss from glaciers since about 1985 with a slight decline in the most recent years. The inventories indicate that since the 1990s, several hundred glaciers globally have completely disappeared. Overall, the more complete data indicate that glaciers worldwide have lost mass at an average rate of 226 ± 135Gt/year (very likely) for the period 1971–2009 while they lost about 275 ± 135 Gt/year (very likely) for the period 1993–2009.80 For reference, a mass loss of 362 Gt of land ice (to the oceans) causes about 1 mm rise in global sea level. Greenland Ice Sheet Mass loss of the Greenland ice sheet has accelerated in the last decade due to increases in ice discharge and surface meltwater runoff.6, 81-83 Enhanced melting has been reported using passive microwave data,84 Gravity Recovery and Climate Experiment (GRACE) data81 and laser altimetry data.85, 86 Thinning along the ice sheet margins and some thickening at the higher elevations was previously observed using aircraft altimetry85 and this has since been confirmed by independent studies using GRACE data81 and ICESat laser altimetry data.87 The mass loss estimated using the various techniques has not been very consistent and ranged from −6 to 74 Gt/year over the period 1992–2001 and from 157 to 274 Gt/year over the period 2002–2011. Taking a simple average, the observations indicate a change in mass loss from 34 Gt/year for the earlier period to 215 Gt/year for the latter period.80 Increased surface melt has also been measured on the Greenland ice sheet using infrared (IR) data.19, 88-90 These studies have largely been accomplished using individual sensors such as from the AVHRR and MODIS (Figure 5). In addition, AMSR‐E and the QuikSCAT scatterometer20 can provide more information, such as surface and near‐surface melt through cloud cover. Melt originating on and near the surface is especially important in the lower parts of the ice sheet where much of the surface water runs off, or percolates through the ice to the bottom and serves as a lubricant that can speed up parts of the ice sheet.91 Such phenomenon could threaten the stability of the ice sheet but the mechanism is poorly understood and may be driven by melt water supply variability.92 Figure 5 Open in figure viewer PowerPoint 19 (a) Maps of annual maximum melt extent constructed from moderate‐resolution imaging spectroradiometer (MODIS) IST data of the Greenland ice sheet for the study period (March 2000 through August 2012). The nonice sheet covered land surrounding the ice sheet is shown in green. The boundaries of the six major drainage basins of the Greenland ice sheet are superimposed on the maps. (b) Extent of maximum melt in each melt season as derived from the MODIS IST data record; colors relate to those shown in (a). Note the large amount of melt in 2012 that lasted for >2 days. The melt extent in 2002 is also notably large. (Reprinted with permission from Ref. Copyright 2007 United States Government as represented by the Administrator of the National Aeronautics and Space Administration)