Aaron Letterly and Jeff Key
25 April 2017, updated 3 May 2017
The cryosphere is a complex part of the climate system that experiences change on different timescales. Data collected over many years helps to distinguish long-term trends from short-term variability. The "stacked plots" below track changes for key cryospheric and cryosphere-related variables. Brief summaries of recent changes are given. The purpose is to provide a quick look at changes in the cryosphere overall. More detailed assessments of the individual cryosphere components are available through the "Other Assessments" box on the right. The information here will be updated once each year.
There have been significant changes in the Arctic climate system since 1970. At current rates, these changes may radically alter the radiative and ecological properties of the Arctic within our lifetime. Surface temperature, permafrost depth, sea ice area, snow cover, and glacial mass balance are all decisive indicators of change. Figure 1 provides time series of these variables for the Northern Hemisphere (NH) or Arctic from the beginning of their records or from 1970.
Surface Temperature - Remotely-sensed data has provided a continuous record of Arctic temperatures since 1982. The Arctic summer lasts from June-September, when sea ice reaches its annual minimum. To capture a single yearly value representing Arctic summer temperature trends, mean monthly temperatures are averaged over these four months. Above 70° N, summer temperatures have been steadily increasing since the beginning of the data record. The mean summer temperature in 2016 was 2.9°C, the highest on record.
Permafrost Thaw Depths - The “active layer” is defined as a seasonally thawed surface layer above permafrost. In-situ measurement sites in Alaska and Siberia monitor the changes in this ecologically important layer as far back as 1990. Over the last 25 years, the active layer has thawed more deeply in response to increasing surface temperatures. Alaskan thaw depths have slowly increased to 50 cm, with average site thaw depths deepening to 51.6 centimeters in 2016. Northern Siberia’s (Russia’s) array of permafrost measurement sites have measured active layers increasing at faster rate than Alaska, with a 2016 average site thaw depth of 68.3 cm.
September Sea Ice Area - Passive microwave data from satellites has observed the dramatic decrease in sea ice continuously since 1982. Higher temperatures and the increased absorption of solar radiation due to the ice-albedo feedback have been responsible for drastic decreases in September sea ice area and extent in the last 10 years. In 2016, APP-x data totaled approximately 3.83 million square kilometers of sea ice area, its second-lowest minimum in this dataset.
Spring Snow Cover - Spring snow cover, averaged from March through May in the Northern Hemisphere, has been decreasing in area and duration since measurements were first available in the late 1960s. In 2016, snow cover extent was its second-lowest in March, lowest in April, and third-lowest in May since 1966.
Glacial Mass Balance - Changes in Earth’s glaciers have been catalogued systematically since 1957 by individual scientists’ reports to the World Glacier Monitoring Service (WGMS). Over the last 45 years, glacial records show that the bodies of ice have been decreasing in size and number. Mass lost from a glacier as meltwater is measured in vertical thickness lost from the height of the glacier. In 2016, glaciers worldwide lost mass, on average, equal to 928 mm of water. This is the 37th consecutive year of negative mass balances.
Greenland Ice Sheet Mass - Over the last 15 years, the losses in mass at the edges of the Greenland Ice Sheet have outpaced the accumulation of ice at its center, resulting in immense losses in its total mass. Since their launch in 2002, NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites have been able to measure the local gravity changes near the Greenland Ice Sheet, determining an approximate amount of ice lost each year in GT (gigatons; 1x1012 kilograms). In 2016, the Greenland Ice Sheet had lost 3,539 GT since measurements began in 2002 (this is 1,904 GT below the average mass of the 2002-2016 mean, Watkins et al. 2015).
Fig. 1. Changes in the NH or Arctic from 1970-2016. From the top down: Arctic surface temperature, permafrost thaw depth in Alaska and northern Siberia, September Arctic sea ice area, March-May NH snow cover, NH glacial mass balance, and Greenland ice sheet mass anomalies. Grey lines are the 5-year running mean except for Greenland ice mass, which shows the annual averages. See below for data sources. Hover the cursor over any of the plots for a larger version. Click on the plot for a full-size version of the stacked plot in a new browser tab.
The Southern Hemisphere (SH) is dominated by oceans, and has far less land surface area than the Northern Hemisphere. The Antarctic continent, though surrounded by ice and covered in snow, is large and remote enough that data records of changes in its ice sheet are not comprehensive. Satellite-derived snow cover products have relatively large uncertainties in part because much of the SH snow cover outside of Antarctica occurs in alpine areas, which creates challenges for remote sensing. Surface temperature, sea ice area, glacial mass balance, and Antarctic Ice Sheet mass balance are decisive indicators of change to Southern Hemisphere’s climate system. Recent changes in these variables are shown below.
Fig. 2. Changes in the SH or Antarctic from 1970-2016. From the top down: surface temperature, February (summer) sea ice area, glacial mass balance, and Antarctic ice sheet mass anomalies. Grey lines are the 5-year running mean except for Antarctic ice mass, which shows the annual averages. See below for data sources. Hover the cursor over any of the plots for a larger version. Click on the plot for a full-size version of the stacked plot in a new browser tab.
Surface Temperature - The Antarctic summer lasts from December through February, when sea ice reaches its annual low. To capture a single yearly value representing Antarctic summer temperature trends, mean monthly temperatures are averaged over these four months. Below 65° S, summer temperatures prior to the early 2000s were highly variable. The mean summer temperature in 2017 was -8.9°C, which followed the recent, 5-7 year trend of warming Antarctic summers.
Sea Ice Area - Since the Antarctic continent is surrounded by ocean, sea ice in this region behaves differently than ice in the Northern Hemisphere. Each year, the sea ice in the Antarctic fluctuates significantly, where very little sea ice is present at the height of the austral summer, but winter sea ice area can be as large as the Antarctic continent itself. In 2017, APP-x data totaled approximately 1.35 million square kilometers of sea ice area, its lowest minimum in this dataset.
Glacial Mass Balance - Changes in Earth’s glaciers have been catalogued systematically since 1957 by individual scientists’ reports to the World Glacier Monitoring Service (WGMS). The Southern Hemisphere, however, has a relative lack of well-studied glaciers. Echaurren Norte, a glacier in the Chilean Andes, is one of the few southern glaciers with a significant record of mass balance changes. In Antarctica, the average mass balance of the Bahia Del Diablo, Johnsons, and Hurd glaciers were taken for the years plotted. From 2000 to 2013, the Antarctic glaciers showed a slight trend of increasing mass balance, but current data is unavailable so the fate of the trend is unknown. In 2016, the mass balance of Echaurren Norte was -1720 mm w.e. (millimeters water equivalent), its fifth consecutive year of decreasing mass.
Antarctic Ice Sheet Mass - The Antarctic Ice Sheet, covering nearly all of the Antarctic continent, is the largest single mass of ice on Earth. The ice sheet has multiple regions, each with their own unique ice dynamics. West Antarctica and the Antarctic Peninsula have generally lost mass over the last 15 years, and increasing mass balance in East Antarctica has not been enough to keep the ice sheet from losing mass as a whole (Martin-Español, 2016). Since their launch in 2002, NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites have been able to measure the local gravity changes near the Antarctic Ice Sheet, determining an approximate amount of ice lost each year in GT (gigatons; 1x1012 kilograms). In 2016, the Antarctic Ice Sheet had lost 1508 GT since measurements began in 2002 (this is 842 GT below the average mass of the 2002-2016 mean, Watkins et al. 2015).
Active layer thickness (ALT), or thaw depth, refers to the depth of the top layer of soil or rock that thaws during the Arctic summer before freezing again in the fall. Changes in temperature near the surface affect ALT, meaning that changes in ALT indicate a changing permafrost state for a given region. The depth of the ALT can range from a few meters in warmer, ice-rich environments to 20 m or greater in bedrock and the coldest permafrost regions. To best observe long-term change in the Arctic, continuous year-round ground temperature measurements within the upper 15 m can be analyzed. Active layer thicknesses are typically measured by mechanical probing at regular intervals, thaw-tube measurements, or inferring thaw depth based on ground temperature measurements.
AMAP, 2012. Arctic Climate Issues 2011: Changes in Arctic Snow, Water, Ice and Permafrost. SWIPA 2011 Overview Report.
Martín-Español, A., A. Zammit-Mangion, P. J. Clarke, T. Flament, V. Helm, M. A. King, S. B. Luthcke, E. Petrie, F. Rémy, N. Schön, et al. (2016), Spatial and temporal Antarctic Ice Sheet mass trends, glacio-isostatic adjustment, and surface processes from a joint inversion of satellite altimeter, gravity, and GPS data, J. Geophys. Res. Earth Surf., 121, 182–200, doi:10.1002/2015JF003550.
Watkins, MM, DN Wiese, D-N Yuan, C Boening, and FW Landerer (2015), Improved methods for observing Earth's time variable mass distribution with GRACE using spherical cap mascons. J. Geophys. Res. Solid Earth, 120, 2648_2671. doi: 10.1002/2014JB011547.