Arctic Sea Ice

Climate Change Indicators: Arctic Sea Ice

Figure 3. Arctic Sea Ice Melt Season, 1979-2020
Data source: NASA, 2021
Web update: July 2022
Units: Julian days (start date and end date); days (duration)

Key points

• September 2012 had the lowest sea ice extent on record, 44% below that month’s 1981-2010 average. For March, the lowest sea ice extent on record was recorded in 2017, 7.4% less than the 1981-2010 average (see Figure 1).
• Sea ice extent in September 2021 was the 12th smallest on record. That was more than 577,000 square miles less than the historical average from 1981 to 2010 for that month (see Figure 1) — a difference more than twice the size of Texas.
• All months have shown a negative trend in sea ice extent over the past few decades. The most significant year-over-year decreases occurred during the summer and fall months.
• Evidence from the age of Arctic sea ice suggests that fewer ice patches persist for several years (see Figure 2). The proportion of five-year-old or older sea ice has declined significantly over the recorded period, from over 40% September ice on average in the 1980s to less than 10% since 2010.
  
  An increasing percentage of ice arctic sea is only one or two years old. Younger multi-year ice means the ice cover is thinning, making it more vulnerable to further melting.
• Since 1979, the length of the Arctic sea ice melt season has increased by 39 days (see Figure 3). On average, Arctic sea ice begins to melt nine days earlier and resumes freezing 30 days later than before (see Figure 3).

Background

• Sea ice is an integral part of the Arctic Ocean. During the dark winter, sea ice covers the entire Arctic Ocean. Some of this ice melts in the summer due to warmer temperatures and long hours of sunshine.
  
  Sea ice typically reaches its minimum thickness and extent in mid-September, when the area covered by ice is roughly half the size of the winter maximum. The ice then begins to expand again during the fall and winter.

• The extent of the area covered by Arctic sea ice is an essential indicator of changes in global climate, as warmer air and water temperatures reduce the amount of sea ice present.
  
  Because sea ice is light in color, it reflects more sunlight to space than liquid water, thus playing an essential role in maintaining the Earth’s energy balance and helping to keep oceans cool polar regions.
  
  Sea ice also keeps the air cool by forming a barrier between the cold air above and the warmer water below. As the amount of sea ice decreases, the cooling effect of the Arctic region is reduced.
  
  This may initiate a “feedback loop” whereby ocean warming caused by more absorption of solar energy leads to an even more significant loss of sea ice and further warming.

• The age of sea ice is also an important indicator of Arctic conditions, as ice accumulated over many years is generally thicker and more robust than younger ice.
  
  The loss of older ice suggests the Arctic ice cover is thinning. Evidence also indicates that the melt season has become longer; ice begins to melt earlier in the year and freeze later than before.

• Changes in sea ice can directly affect the health of Arctic ecosystems. Mammals such as polar bears and walruses depend on the presence of sea ice to hunt, breed and migrate.
  
  Due to reduced sea ice cover and thickness, these animals are threatened by declining birth rates and restricted access to food sources. Impacts on Arctic wildlife and the loss of ice itself are already restricting the traditional subsistence hunting lifestyle of indigenous Arctic peoples such as the Yup’ik, Iñupiat, and Inuit.

• While shrinking sea ice can have adverse ecological effects, it can also present trade and economic opportunities.
  
  For example, reduced sea ice opens shipping lanes and increases access to natural resources in the Arctic region.

About the indicator

• Figure 1 shows the extent of Arctic sea ice from 1979, when routine satellite monitoring began, to 2021. Sea ice extent is defined as the area of ​​the ocean where at least 15% of the surface is frozen.
  
  This threshold was chosen because scientists have found that it best approximates the ice’s edge. While data is collected throughout the year, this indicator focuses on months when sea ice typically reaches its minimum and maximum extent.
  
  September is usually when sea ice extent goes to its annual minimum after melting in the spring and summer. Conversely, March is generally when sea ice reaches its maximum extent after the winter freeze-up.

• Figure 2 examines the age of the ice present in the Arctic during the week of September, which has a minor extent of the ice. By combining daily satellite images, wind measurements, and data from surface buoys that move with the ice, scientists can track specific patches of ice over time.
  
  This tracking allows them to calculate the ice age in different parts of the Arctic. Although satellites began collecting data in 1979, Figure 2 only shows data from 1984. It is impossible to know the entire age distribution until the ice has been tracked for at least five years.

• Figure 3 shows the start and end dates of each year’s Arctic sea ice melt season, starting in 1979. It is based on the same satellite instruments as Figure 1.
  
  Here, the melting season’s start date is when the satellites detect constant humidity on the surface of ice and snow; the end date is when the surface air temperature remains consistently at or below freezing, and ice begins to grow in the open sea.

About the Data

Indicator Notes

Data Sources