Ice Sheets

Ice Sheets

In glaciology, an ice cap, also known as a continental glacier, is a mass of glacial ice that covers the surrounding terrain and is more significant than 50,000 km2.

The only current ice caps are in Antarctica and Greenland; during the last ice age at the last glacial maximum, the Laurentide Ice Sheet covered much of North America, the Weichselian Ice Sheet covered northern Europe, and the Patagonian Ice Sheet covered southern North America South.

Climate Change Indicators: Ice Sheets

Key points

• Since 1992, Greenland and Antarctica have lost ice overall (see Figure 1), each losing an average of more than 100 billion metric tons annually.
• The total amount of ice lost by Greenland and Antarctica between 1992 and 2018 was enough to raise sea levels worldwide by an average of about seven-tenths of an inch. By comparison, the average global sea level during this period.
• Ice sheets naturally fluctuate with seasonal fluctuations in temperature, precipitation, and other factors. The NASA JPL data points in Figure 1 show these seasonal patterns, especially for Greenland.
  
  However, with nearly three decades of data, Figure 1 shows that the overall shrinking of ice sheets far exceeds seasonal and interannual variations.
• Observations of ice sheet mass loss are consistent with small glaciers and warming trends in global temperatures during this period.

Background

• Ice sheets store snowfall – just like small glaciers, but on a larger scale. Over time, this snow compacts into layers of ice, and the weight of this ice causes it to flow downhill and toward the ocean.
  
  When the ice sheet is in equilibrium, new snow accumulation is precisely balanced by melting at the surface, water runoff, and calving into the sea.

• A warmer climate affects glaciers in several ways. The most apparent impact observed so far is ice loss due to warmer air and ocean waters. Warmer air causes faster ice melting and flow to the sea, especially in low-elevation areas near the edges of the ice sheets.
  
  Warmer ocean waters soften the edges and base of the ice sheet and speed up the calving process. In addition, warmer ocean waters weaken the floating ice shelves that form at critical locations along the coast, particularly in Antarctica.
  
  These ice shelves help hold back the flow of ice on land, so when ice shelves shrink or disappear, ice from the ice sheet can flow faster into the ocean. Some of the losses are offset as the warmer air allows the frigid interior regions in Greenland and Antarctica to receive more snow.
  
  Overall, however, both ice sheets are experiencing net ice loss. Eventually, as the ice sheets shrink, the water they add to the ocean raises sea levels worldwide.
  
  Melting of the Greenland and Antarctic ice sheets accounted for approximately one-third of the observed global sea-level rise between 2006 and 2015. Thus, ice sheets are essential indicators because of their sensitivity to climate change and their role in global sea level rise.

About the indicator

• Because Greenland and Antarctica are so large and remote, satellite instruments offer the most effective way to measure changes in their ice sheets.

• Measure the height of the ice sheet’s surface relative to sea level. This technique is called an altimeter and usually uses radar or laser instruments.

• Measuring changes in ice mass in Greenland and Antarctica based on slight variations in the Earth’s gravity field caused by changes in the density of the Earth’s crust and the amount of ice or water that can accumulate on that crust. This technique uses a pair of satellites called the GRACE Mission, launched in 2002.
  
  
• Using an “input-output” method that combines different sources of information about the accumulation of ice and its release into the ocean.

• Figure 1 also shows estimates from a specific analysis of GRACE data by NASA’s Jet Propulsion Laboratory.
  
  These data are presented for reference and comparison because NASA JPL is a widely cited, influential source and because the results show seasonal patterns.

About the Data

Notes on the indicator

• The relationship between climate change and ice sheet mass balance is complex, particularly in Antarctica, where large areas will continue to be too cold for appreciable surface melting.
  
  Some sites may experience more snow to offset other warming effects, and ice speed loss can be strongly influenced by specific “tipping point” events such as ice shelf collapse.
  
  Scientists who study Antarctica’s ice sheets often divide the continent into three central regions—East Antarctica, West Antarctica, and the Antarctic Peninsula—because each of these regions has distinct geological characteristics and weather patterns that affect their vulnerability to climate change.

• Data for these two recommended data sets were not available before 1992. However, studies using other methods provide a longer historical perspective.
  
  Unlike smaller land ice bodies such as glaciers, the large ice sheets of Greenland and Antarctica do not have sufficiently widespread, repeated measurements collected in situ to allow reliable estimates of changes in total mass balances.
  
  This indicator presents results from multiple measurement methodologies for comparison and to extend the temporal coverage of the time series for the mass balance of the Greenland and Antarctic ice sheets.
  
  Each of NASA’s time series has a gap from mid-2017 to mid-2018, reflecting the gap between the original GRACE mission and the GRACE Follow-On satellites they launched in 2018.

Data Sources