Understanding Decades-Long Trends in Polar Sea Ice and Their Implications for Climate Change
On September 19, 2023, the Arctic sea ice most likely reached its smallest extent for the year, marking the sixth-lowest recorded year according to findings from NASA and the National Snow and Ice Data Center (NSIDC). Simultaneously, the Antarctic sea ice reached its record lowest maximum extent on September 10. This decline occurred at a time when the ice cover should have been increasing more rapidly during the darkest and coldest months of the year.
Scientists monitor these seasonal and yearly variations because sea ice profoundly influences polar ecosystems and plays a crucial role in the Earth’s global climate. Utilizing satellite technology, researchers from NSIDC and NASA measure the process of sea ice melting and refreezing. They determine sea ice extent, which refers to the total ocean area where the ice cover constitutes at least 15%.
Between March and September 2023, the Arctic ice cover decreased from its highest point of 5.64 million square miles (14.62 million square kilometers) to a minimum of 1.63 million square miles (4.23 million square kilometers). This is approximately 770,000 square miles (1.99 million square kilometers) below the 1981–2010 average minimum of 2.4 million square miles (6.22 million square kilometers). The amount of sea ice lost during this period would be sufficient to cover the entire continental United States.
On September 10, 2023, the extent of sea ice in the vicinity of Antarctica reached its smallest winter maximum, measuring 6.5 million square miles (16.96 million square kilometers). This marked a substantial departure of 398,000 square miles (1.03 million square kilometers) from the prior record low set in 1986 – a variation approximately equivalent to the combined land area of Texas and California. In comparison, the average maximum extent during the period from 1981 to 2010 stood at 7.22 million square miles (18.71 million square kilometers).
Walt Meier, a sea ice scientist at NSIDC, remarked on this remarkable reduction, stating, “This is an unprecedented decline in Antarctic sea ice. The decrease in sea ice growth is observed across almost the entire continent, rather than being limited to any specific region.”
In the Arctic this year, scientists observed notably reduced ice levels in the Northwest Passage, as Meier noted, “The Northwest Passage has more open waters than in the past. Additionally, there appears to be a higher prevalence of loosely packed, lower concentration ice even in the direction of the North Pole. Areas that were once characterized by densely compact, solid ice sheets throughout the summer have been experiencing this more frequently in recent years.”
Meier noted that these alterations represent a fundamental, decades-long response to rising temperatures. Since the inception of satellite-based ice monitoring in 1979, sea ice has not only been diminishing in the Arctic but has also been growing younger. The earlier onset of spring melting and the progressively delayed commencement of autumn freezing result in extended melting seasons. Studies have demonstrated that, when averaged across the entire Arctic Ocean, the start of freezing occurs approximately a week later per decade, resulting in a freeze-up period that is roughly one month later compared to the conditions in 1979.
Nathan Kurtz, who serves as the head of NASA’s Cryospheric Sciences Laboratory at the Goddard Space Flight Center in Greenbelt, Maryland, emphasized that as the Arctic is warming at a rate approximately four times faster than the rest of the planet, the ice is becoming thinner. He stated, “The thickness of the ice at the end of the growth season is a crucial factor in determining the survival of sea ice. Ongoing research is employing satellites like NASA’s ICESat-2 (Ice, Cloud and land Elevation Satellite-2) to continuously monitor ice thickness throughout the year.”
Kurtz underscored the importance of long-term sea ice measurements in understanding real-time polar phenomena, stating, “At NASA, our goal is to acquire state-of-the-art measurements while also establishing links with historical data to gain a deeper insight into the driving forces behind the observed changes.”
Scientists are actively investigating the causes behind the limited growth of Antarctic sea ice, which may involve a combination of factors such as El Nino events, wind patterns, and rising ocean temperatures. Recent research suggests that the influence of ocean heat likely plays a significant role in impeding the growth of sea ice during the cold season and intensifying ice melt during warmer periods.
The record-low extent of Antarctic sea ice in 2023 continues a downward trend that commenced after a record high in 2014. Prior to 2014, the ice surrounding the continent was gradually expanding by approximately 1% per decade.
The melting of sea ice at both poles contributes to global warming through a phenomenon known as “ice-albedo feedback.” While reflective sea ice bounces back most of the Sun’s energy into space, open ocean water absorbs around 90% of this energy. As larger ocean areas are exposed to solar radiation, more heat is absorbed, causing a warming effect in ocean waters and further impeding the growth of sea ice.
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