Last updated March 22, 2017 at 4:08 pm
Science Update is a monthly written series from Dr Paul Willis tackling some of the controversial topics in the public and aims to provide the current research behind these subjects.
There is some confusion about what’s happening with respect to melting ice around the globe and its effect on sea levels. This is due in part to a change in the nature of sea ice melting over the last year or so as well as deliberate misrepresentations of the data by some parties who dispute climate change. So it’s time to update the science on global ice and sea level rise.
Types of ice
There are two broad categories of ice that play differing roles on the changing environments of Earth: sea ice and land ice.
On average, sea ice covers about 2.5 times the size of Canada at around 25 million square kilometres. It is a critical component of Earth’s biosphere because it influences climate globally as well as local wildlife and people who live in the Arctic.
Sea ice grows during the winter months and melts during the summer months, but some sea ice remains all year round in certain regions. About 15% of the world’s oceans are covered by sea ice during part of the year.
Melting sea ice does not contribute to sea level rise because it floats on the sea and the volume of water generated by melting sea ice is the same as the volume of water the ice displaces when solid. However, at many locations, sea ice holds back land ice in the form of glaciers so, when the sea ice disappears, the movement of glacial ice into the sea increases. This introduction of land ice into the oceans does contribute to sea level rise. An increase of glacial movement after the collapse of the Larson B Ice Shelf in Antarctica recorded an eight-fold increase in glacial movement.
Why is sea ice melting?
Sea ice is melting due mostly to increasing oceanic temperatures melting the ice from below but also due to rising atmospheric temperatures melting it from the top.
This melting effect forms a feedback loop as explained here by the National Snow and Ice Data Center (NSIDC) in the US:
“Sea ice has a bright surface, so much of the sunlight that strikes it is reflected back into space. As a result, areas covered by sea ice don’t absorb much solar energy, so temperatures in the polar regions remain relatively cool. If gradually warming temperatures melt sea ice over time, fewer bright surfaces are available to reflect sunlight back into space, more solar energy is absorbed at the surface, and temperatures rise further. This chain of events starts a cycle of warming and melting. This cycle is temporarily halted when the dark days of the polar winter return, but it starts again in the following spring. Even a small increase in temperature can lead to greater warming over time, making the polar regions the most sensitive areas to climate change on Earth.”
Effects of melting sea ice
Sea ice can affect the movement of ocean waters contributing to the ocean’s global “conveyor-belt” circulation. Changes in the amount of sea ice can disrupt normal ocean circulation, thereby leading to changes in global climate.
Too much or too little sea ice can be a problem for wildlife and people who hunt and travel in polar regions. In the Arctic, sea ice can be an obstacle to normal shipping routes through the Northern Sea route and Northwest Passage.
Historic rates of melting sea ice
Sea ice loss in the Arctic has been fairly constant for most of the last few decades.
Source: ClimateState.com / Andy Lee Robinson on Wikimedia
Antarctic sea ice cover expanded between 1979 and 2013 probably because of warming ocean temperatures effectively thinning out the ice sheet.
However, during this period, sea ice decreased substantially in the Bellingshausen and Amundsen seas close to the Antarctic Peninsula, a region that has warmed significantly over the last decades.
Sea ice surrounding Antarctica reached its highest extent in 2014 but the amount of Arctic sea ice shrinkage was around three times greater than the amount of Antarctic spreading leading to a net global loss of sea ice. As a whole, the planet had been losing sea ice at an average annual rate of 35,000 square kilometres since 1979 and this rate had been accelerating.
In recent years the extent of Antarctic sea ice spreading has reversed from expanding to contracting while the decline of Arctic sea ice has continued unabated. Sea ice in the Arctic and the Antarctic set record low extents every day in December 2016 continuing a pattern that began in the preceding months.
Source: Daily image updates via NSIDC
For the year 2016, sea ice extent in both polar regions was at levels well below what is typical of the past several decades.
Source: Daily image updates via NSIDC
In January 2017 global sea ice (Arctic plus Antarctic) continued to track at record low levels although at a more moderate rate than preceding months.
Source: Daily image updates via NSIDC
Land ice is ice that has accumulated on land usually from snow falls. Land ice occurs in both the Arctic (particularly Greenland) and the Antarctic but can also be found at altitude in mountain ranges around the world. Usually land ice forms into glaciers which can form icebergs when they contact the sea. In polar regions land ice can form ice sheets.
Melting land ice is a source of water for surrounding areas in several parts of the world. Unlike melting sea ice, melting land ice does contribute to sea level rise.
Satellite data shows that the land ice sheets in both Antarctica and Greenland are losing mass. The continent of Antarctica has been losing about 134 gigatonnes or 152 cubic kilometres of ice per year since 2002, while the Greenland ice sheet has been losing an estimated 287 gigatonnes or 150-250 cubic kilometres per year. This loss of land ice is caused by melting due to higher atmospheric temperatures and movement of ice in glaciers into the sea.
Source: GRACE satellite data via NASA Climate Change website
Melting ice and sea level rise
As stated above, sea levels are only effected by the melting of land ice, not sea ice. So far the combined effects of melting Antarctic and Greenland ice has been around 15mm in sea level rise.
The Third Pole
The third largest accumulation of land ice in the world stretches across the Himalaya – Hindu Kush Mountains and on to the Tibetan Plateau. It is so large it is often referred to as the Third Pole. The Third Pole is the source of the 10 major river systems which provide irrigation, power and drinking water for over 1.3 billion people in Asia – nearly 20% of the world’s population.
The rate of warming in the Third Pole region is significantly higher than the global average, and the rate is higher at higher altitudes. This trend is expected to continue. In the past half-century, 82% of the plateau’s glaciers have retreated and in the past decade, 10% of its permafrost has degraded.
Globally significant deposits of ice can be divided into sea ice and land ice. Sea ice, in turn, can be divided into Arctic and Antarctic sea ice. Arctic sea ice has been steadily decreasing in extent over the preceding decades due to warming oceans and a warming atmosphere. Antarctic sea ice had been expanding in extent while thinning in depth for several decades but this expansion has reversed over the last few years.
Land ice has also been decreasing in extent at both poles and where it occurs in mountain ranges around the world. The melting of land ice is the main driver of global sea level rise while melting sea ice does not contribute to this problem.
With the prospects of further atmospheric and ocean warming in the future coupled with feedback loops that promote increased melting of sea ice, it seems likely that the melting of ice globally will continue for the foreseeable future.