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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.Sea levels rise when ice resting on land, grounded ice, melts (often after forming icebergs). Floating sea ice that melts has a very important role in other areas of our climate system. Land ice
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.Sea ice
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.A big breakup.
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.Antarctic sea ice grows and shrinks dramatically over the course of the year. These changes do not directly affect sea level. Land ice changes are slower but do affect sea levels, at least until the land ice becomes afloat.
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 receives funding from the Australian Research Council and the Department of Environment.
Ben Galton-Fenzi works for the Australian Antarctic Division. He receives funding from the Department of the Environment.
Will Hobbs is employed by the Antarctic Climate and Ecosystems Cooperative Research Centre, and receives funding from the Australian Research Council.
US Fish and Wildlife Service to target diseased prairie dogs, food for the ferrets, via specially designed drones that shoot the candies in three directions at once
The US government is set to unleash drones that fire vaccine-covered M&Ms in a bid to save the endangered black-footed ferret, a species that is facing a plague epidemic across America’s great plains.
The US Fish and Wildlife (FWS) has developed a plan to bombard ferret habitat in Montana with the vaccine, which will be administered via specially designed drones that will be able to shoot M&Ms in three directions simultaneously.Continue reading...
Climate Central: The warming of the planet over the past few decades has shifted a key band of clouds poleward and increased the heights of clouds tops
The reaction of clouds to a warming atmosphere has been one of the major sources of uncertainty in estimating exactly how much the world will heat up from the accumulation of greenhouse gases, as some changes would enhance warming, while others would counteract it.
The study, detailed Monday in the journal Nature, overcomes problems with the satellite record and shows that observations support projections from climate models. But the work is only a first step in understanding the relationship between climate change and clouds, with many uncertainties still to untangle, scientists not involved with the research said.Continue reading...
Oil from geopolitically unstable regions such as Russia, Libya and Iraq accounts for 80% of Europe’s imports, report shows
Europe is dependent on foreign and often geopolitically unstable regions such as Russia, Libya and Iraq for 80% of its imported oil, according to a report.
Rosneft and Lukoil are the two companies benefiting most from the EU’s current oil imports regime, supplying a third of the continent’s imported crude in 2015, according to the new study. Statoil and Saudi Aramco provided another 20%, with Chevron and Exxon accounting for 12%.Continue reading...
Australians have been both helpers and victims of the fossil fuelled web of climate science denial being detailed in the U.S Senate
By the middle of this week, about 20 Democratic Senators in the US will have stood up before their congress to talk about the fossil fuelled machinery of climate science denial.
The Senators are naming the fossil fuel funders, describing the machinery and calling out the characters that make up a “web of denial”.Continue reading...
Child immediately treated for superficial wounds by first aid officers as bird removed from show
A wedge-tailed eagle that was part of a birds of prey show at Alice Springs Desert Park flew at a young boy and latched on to his head with its talons instead of flying over to a perch as it had been trained to.
The moment was captured by a visitor to the park, Christine O’Connell, who uploaded an image on to Instagram of the eagle seemingly attempting to drag away the boy, who was wearing a green hoodie.Continue reading...
A 2,000 page report by Committee on Climate Change predicts global warming will hit UK with deadly heatwaves, more flooding and water shortages
The UK is poorly prepared for the inevitable impacts of global warming in coming decades, including deadly annual heatwaves, water shortages and difficulties in producing food, according the government’s official advisers.
Action must be taken now, according to the report from the Committee on Climate Change (CCC) published on Tuesday, with more widespread flooding and new diseases among the risks in most urgent need of addressing.Continue reading...
West Sussex There’s another sound – a continuous metallic whirr, like a fishing reel spinning
The slow-moving river surface reflects the blue sky and the patches of white and gunmetal-coloured clouds passing overhead. The trees and long, damp grass rustle in the breeze.
Familiar summer-morning bird song rises from the trees, reeds and bushes – song thrush, wren, reed bunting and the rhythmic, steady, repeated “chit-chit-chit” of a reed warbler. A nearby sedge warbler competes, blaring out its more hurried, chaotic whirrs, chatters and whistles from the top of a small tree.Continue reading...