Note: This is a partial assessment of trends through 2018. Not all data are currently available. Cryosphere components that are not yet updated are indicated below.
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 at least 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 - Satellite 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 2018 was 1.74°C, warmer than the average summer temperature in 2017 but still more than 1° below 2016 maximum.
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 in 2018 measured 49.92 cm, on average. Northern Siberia’s (Russia’s) array of permafrost measurement sites measured an average active layer depth of 59.94 cm. Though both regions showed a slight decrease in active layer depth since last year, the thawed layer is over 10 cm deeper than at the beginning of the data record.
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 15 years. In 2018, ice concentration data from APP-x totalled approximately 4.27 million square kilometers of sea ice area, within 39,000 km of the 2017 ice area. Only six years had lower average September sea ice area, all within the last ten years.
Spring Snow Cover - Spring snow cover, averaged from March through May in the Northern Hemisphere, has generally decreased in area and duration since measurements were first available in the late 1960s. In 2018, spring snow cover in the Northern Hemisphere averaged 30.7 million square kilometers, which was slightly greater than the 2017 average. This was the largest average area in 16 years.
Glacial Mass Balance - NOT YET UPDATED FOR 2018 - 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 2017, glaciers worldwide lost mass, on average, equal to 886 mm of water. This is the 38th consecutive year of negative mass balances.
Greenland Ice Sheet Mass - NOT YET UPDATED FOR 2018 - 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; 1 GT = 1012 kilograms). By June of 2017, the Greenland Ice Sheet had approximately 3,500 GT since measurements began in 2002. This is 1,853 GT below the average mass of the 2002-2017 mean.
Fig. 1. Changes in the NH or Arctic from 1970-2018, except where noted. 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.
Surface Temperature - The Antarctic summer lasts (roughly) from December through March, with sea ice reaching its annual minimum extent in late February. To capture a single yearly value representing Antarctic summer temperature trends, satellite-derived mean monthly temperatures for land and ocean are averaged over these four months. Below 65°S, summer temperatures prior to the early 2000s were highly variable. The mean summer temperature in 2018 was -8.9°C, continuing the 6-8 year trend of warming Antarctic summers.
Permafrost Thaw Depths - NOT YET UPDATED FOR 2018 - Measurements at the Johann Gregor Mendel site (James Ross Island, Eastern Antarctic Peninsula, 63.8S, 57.866E) show that the active layer thickness decreased in 2017, though there has been an increase in the mean annual ground temperature at 5 and 75 cm depths (not shown). The primary difference between the two measurement sites is the underlying soil composition - the Circumpolar Active Layer Monitoring Network (CALM) site sits atop a marine terrace with sandy soil, while the Johan Gregor Mendel site is over a bed of loamy sediment.
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. According to APP-x data, average February sea ice area in 2018 was 1.34 million square kilometers, very close to the record low set in 2017. The difference in area between 2017 and 2018 was less than 0.1%, drawing attention to the trend of decreasing winter ice area in the Southern Hemisphere during the last few years.
Glacial Mass Balance - NOT YET UPDATED FOR 2018 - 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 2015, the Antarctic glaciers have shown a slight trend of increasing mass balance. In 2017, the mass balance of Echaurren Norte was -2284 mm w.e. (millimeters water equivalent), its seventh consecutive year of decreasing mass.
Antarctic Ice Sheet Mass - NOT YET UPDATED FOR 2018 - The Antarctic Ice Sheet, covering nearly the entire Antarctic continent, is the largest single mass of ice on Earth. 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; 1 GT = 1012 kilograms). As of April 2018, the Antarctic Ice Sheet had lost 1869 GT since measurements began in 2002. This is 1143 GT below the average mass of the 2002-2017 mean.
Fig. 2. Changes in the SH or Antarctic from 1970-2018, except where noted. 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.
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.
Thaw depth data from J.G. Mendel Station, Antarctica are courtesy of Filip Hrbáček, Masaryk University, Brno, Czech Republic.
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