What matters most for sea levels is how much ice slides off the land and melts in the sea
It was back in 250ʙⅽ when Archimedes reportedly stepped into his bathtub and had the world’s first Eureka moment – realising that putting himself in the water made its level rise.
More than two millennia later, the comments sections of news stories still routinely reveal confusion about how this same thing happens when polar ice melts and sea levels change.
This is in marked contrast to the confidence that scientists have in their collective understanding of what is happening to the ice sheets. Indeed, the 2014 Assessment Report of the Intergovernmental Panel on Climate Change reported “very high confidence” that the Greenland Ice Sheet was melting and raising sea levels, with “high confidence” of the same for the Antarctic Ice Sheet.
Despite this, commenters below the line on news stories frequently wonder how it can be true that Antarctica is melting and contributing to sea-level rise, when satellite observations show Antarctic ice expanding.
Unravelling the confusion depends on appreciating the difference between the two different types of ice, which we can broadly term “land ice” and “sea ice” – although as we shall see, there’s a little bit more to it than that. The two different types of ice have very different roles in Earth’s climate, and behave in crucially different ways.
Ice sheets form by the gradual accumulation of snow on land over long periods of time. This “grounded” ice flows in glaciers to the ocean under the influence of gravity, and when it arrives it eventually melts. If the amount of ice flowing into the oceans is balanced by snowfall on land, the net change in global sea level due to this ice sheet is zero.
However, if the ice begins to flow more rapidly or snowfall declines, the ice sheet can be out of balance, resulting in a net rise in sea level.
But this influence on sea level is only really relevant for ice that is grounded on land. When the ice sheet starts to float on the ocean it is called an “ice shelf”. The contribution of ice shelves to sea-level rise is negligible because they are already in the sea (similar to an ice cube in a glass of water, although the ocean is salty unlike a glass of water). But they can nevertheless play an important role in sea-level rise, by governing the rate at which the grounded ice can discharge into the oceans, and therefore how fast it melts.
When viewed from space, all polar ice looks pretty much the same. But there is a second category of ice that has effectively nothing to do with the ice sheets themselves.
“Sea ice” is formed when ocean water is frozen due to cooling by the air. Because it is floating in the ocean, sea ice does not (directly) affect sea level.
Sea ice is generally no more than a few metres thick, although it can grow to more than 10 metres thick if allowed to grow over many winters. Ice shelves, on the other hand, are hundreds of metres thick, as seen when an iceberg is created and rolls over.
In the ocean around Antarctica, almost all the sea ice melts in the southern hemisphere spring. This means that every year an area of ocean twice the size of Australia freezes over and then melts – arguably the largest seasonal change on our planet.
So, while ice sheets change over decades and centuries, the time scale of sea ice variability is measured in months.
The seasonal cycle of Arctic sea ice is much smaller. This is because the Arctic retains much more of its sea ice in the summer, and its winter extent is limited by land that surrounds the Arctic Ocean.
What is happening to land ice?
The two great ice sheets are in Greenland and Antarctica. Thanks to satellite measurements, we now know that since the early 1990s both have been contributing to sea-level rise.
It is thought that most of the Antarctic changes are caused by seawater melting the ice shelves faster, causing the land ice to flow faster and hence leading to sea-level rise as the ice sheet is tipped out of balance.
In Greenland, both surface and ocean melting play important roles in driving the accelerated contribution to sea levels.
What about sea ice?
Over the last four decades of satellite measurements, there has been a rapid decrease and thinning of summer Arctic sea ice. This is due to human activity warming the atmosphere and ocean.
In the Antarctic there has been a modest increase in total sea ice cover, but with a complex pattern of localised increases and decreases that are related to changes in winds and ocean currents. What’s more, satellite measurement of changes in sea ice thickness is much more difficult in the Antarctic than in the Arctic mainly because Antarctic sea ice has a lot of poorly measured snow resting on it.
The Southern Ocean is arguably a much more complex system than the Arctic Ocean, and determining humans' influence on these trends and projecting future change is challenging.
Observations of the changes happening in the Arctic and Antarctic reveal complex stories that vary from place to place and over time.
These changes require ongoing monitoring and greater understanding of the causes of the observed changes. And public confusion can be avoided through careful use of the different terms describing ice in the global climate system. It pays to know your ice sheets from your sea ice.
Matt King, Professor, Surveying & Spatial Sciences, School of Land and Food, University of Tasmania; Ben Galton-Fenzi, Senior Scientist, and Will Hobbs, Physical Oceanographer, Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania
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