We often get questions from readers about Earth’s sea ice in the Arctic and the Antarctic, and the differences between those areas. Arctic sea ice has declined over the past five decades, while Antarctic sea ice has increased, and then declined. Why do they behave differently?
How They’re Different
The primary difference between the Arctic and Antarctica is geographical. The Arctic is an ocean, covered by a thin layer of perennial sea ice and surrounded by land. ("Perennial" refers to the oldest and thickest sea ice.) Antarctica, on the other hand, is a continent, covered by a very thick ice cap and surrounded by a rim of sea ice and the Southern Ocean.
The Arctic Ocean is very deep and closely linked with the climate systems around it, making it more sensitive to climate changes than Antarctica.
During the centuries of human exploration in the Arctic, sea ice covered the Arctic Ocean well year-round, up until recent decades. But satellite observations show that Arctic sea ice has been declining in extent*, thickness and volume since 1979.1 Average Arctic sea ice extent is at its lowest since 1850.
During the summer melt season, the sea ice’s edge retreats toward the North Pole, only to re-grow during the Arctic winter. As a result of ongoing warming driven by human activities, the trend toward summer sea ice loss (from July to September, followed by a winter re-growth) continues.
Recent research suggests that there is a relationship between Arctic sea ice losses and the human burning of fossil fuels in all months.2 Aerosols (tiny particles suspended in the atmosphere) tied to human activities have offset some of the Arctic sea ice extent loss trend; a reduction in aerosol pollution will likely see a sea ice loss acceleration.3 Ice loss at the sea ice’s margins** results in winds driving warmer water beneath the Arctic sea ice, increasing the amount of heat the Arctic Ocean stores 4 and priming conditions for further sea ice loss.
A figure showing current Arctic sea ice extent can be found here.
Antarctic Sea Ice
Antarctic sea ice expands during the winter, only to melt back largely to the continent’s edge in summer.
Antarctic sea ice extent is currently below the long-term average of all decades prior since 1979. Previously, Antarctic sea ice extent had been above that long-term average due to long-term, large-scale wind circulation patterns that drove sea ice away from Antarctica5, making room for more sea ice to form nearer to the continent.6 Climate models, or computer simulations that incorporate all the factors that affect Earth’s climate, predicted this behavior.7 These long-term wind patterns reversed several years ago, resulting in a significant sea ice decline surrounding Antarctica. Values since then have been hovering around the average of all years prior in the satellite record. A figure showing current Antarctic sea ice extent can be found here.
Why Sea Ice Matters
Some of the questions we receive ask why we should care about the polar regions. These regions are very important in regulating global temperature. Because sea ice has a bright surface, 50-70 percent of incoming energy is reflected back into space. As sea ice melts in the summer, it exposes the dark ocean surface. Instead of reflecting 50-70 percent of the sunlight, it absorbs 90 percent of the sunlight. As the ocean warms, global temperatures rise further.
Also, what happens in the polar regions doesn’t stay in those regions. Their changes affect global temperatures and can even change ocean circulation. Earth’s sea ice is very attuned and responsive to even small changes in global surface and ocean temperatures.
- New Year Lows Once Again, NSIDC; Kwok, R. (2018), Arctic sea ice thickness, volume, and multiyear ice coverage: Losses and coupled variability (1958 – 2018). Environ. Res. Lett. 13 (2018) 105005 https://doi.org/10.1088/1748-9326/aae3ec; and
Arctic Sea Ice Volume Anomaly, Polar Ice Center
- Julienne Stroeve and Dirk Notz, Changing state of Arctic sea ice across all seasons, Environmental Research Letters, Volume 13, Number 10
- B. L. Mueller, Attribution of Arctic Sea Ice Decline from 1953 to 2012 to Influences from Natural, Greenhouse Gas, and Anthropogenic Aerosol Forcing, https://doi.org/10.1175/JCLI-D-17-0552.1
- Mary-Louise Timmermans, John Toole and Richard Krishfield, Warming of the interior Arctic Ocean linked to sea ice losses at the basin margins, Science Advances 29 Aug 2018: Vol. 4, no. 8
- All About Sea Ice, NSIDC
- Gerald A. Meehl et al, Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability, https://doi.org/10.1038/ngeo2751
- A Tale of Two Poles, Earth Observatory, 2014
*Sea ice extent is a measurement of the area of ocean where there is at least some sea ice.
**Margins are transition regions between the ice-covered and ice-free portions of the ocean.
Overnight, I got to take part in a truly historic Operation IceBridge (OIB) mission, and I couldn’t be happier and more excited to tell you all about it! This mission, called Mid-Weddell, was probably the most complex not only of the fall 2018 Antarctic campaign, but all of IceBridge.
To add to this, some unforeseen issues made this particular mission difficult. Upon landing after our previous mission, we were informed that there was a local fuel trucker strike. This meant NO FUEL for all of Punta Arenas, Chile. So, we had no fuel for our plane, which meant we couldn’t fly the next day and had no clue when this strike would be resolved.
The strike was resolved after a few days, but the Mid-Weddell mission was again delayed when we found out that there were cracks in the NASA DC-8 pilot’s window. A new one had to be sent from Palmdale, California, and installed before we could fly again.
After all of these added stressors, we began to worry that we wouldn’t even be able to pull off this mission because it was an overnight flight and had to be timed perfectly with an ICESat-2 satellite overpass. These two mandatory factors are not so easy to achieve because:
- The weather in the Weddell Sea has to be clear (as in no low or high clouds), so ICESat-2 can see the sea ice that we are flying over;
- There has to be a crossover of ICESat-2 in the middle of the night and in the middle of the Weddell Sea.
In order to make things "easier" on ourselves (please note my sarcasm here), we were also “chasing the sea ice” during this flight. Why do we need to chase the sea ice, one might ask? Because sea ice (frozen floating sea water) is constantly in motion, being forced around by winds and ocean currents. This makes it rather difficult to fly over the same sea ice as ICESat-2 because the satellite can fly over our entire science flight line in about nine seconds, whereas it takes us multiple hours by plane. Thus, in order to fly over the same sea ice, the sea ice must be chased during flight.
Chasing the sea ice is essentially my OIB baby project. Before this campaign, I diligently worked on writing code that would take in our latitudes and longitudes along our flight path, and, depending on the wind speed, wind direction and our altitude from the plane, determine where the sea ice that ICESat-2 flew over would have drifted by the time our plane got there. This way we could essentially fly over the same sea ice that the satellite flew over.
To do this, we asked the pilots to take the plane down to 500 feet (yes, 500 FEET!) above the surface and stay there for roughly a minute in order to take wind measurements. Then I plugged these values into my code program and changed our flight path so we could fly over the same sea ice. We monitored the winds during our flight, and if they changed significantly, we would do this maneuver again. Now how cool is that? I was in charge of changing our flight path as we flew! Can’t say I’d ever “flown” a plane before.
Since our flight was a low-light flight, it had to be conducted at night. So, we took off from Punta Arenas at 7pm for an 11-hour flight, heading south to the Weddell Sea. During our flight, and because of our flight path, we were able to see multiple sunsets and sunrises as the sun bobbed up and down across the horizon. Because of the low lighting, the sky changed from oranges to pinks to blues, making for quite the show from the DC-8’s windows. Even the land-ice lovers [on our flight] enjoyed it.
Right before 1:35am local time, John Sonntag began a 10-second countdown. When zero was reached, ICESat-2 crossed directly above our plane, thus “playing tag with the satellite” and making history. It was the first time this was done since the satellite’s launch a little over a month ago. We all began chatting on our headsets about how awesome it was to be part of this mission and to be able to witness this moment. This is what OIB had been working toward since its beginning in 2009. The data gap was now successfully bridged between ICESat and ICESat-2.
Later, during the flight, I began to think about how everyone on the team really stepped up and how easily we were all able to work together to make this mission happen. I mean, we literally chased sea ice and played tag with a satellite during this flight! It took the pilots’ maneuvering, the aircraft crew’s hard work, the instrument teams’ and scientists’ steady collecting of data—everyone working together all night long—for this mission to run smoothly. I am truly grateful for everyone’s hard work and dedication and was so happy to be there that night. As we on OIB say, “Team work makes the dream work.”
This piece was adapted from NASA's Earth Expeditions blog.