State of the Climate 2018

Cryosphere

Changes have been observed in the ice sheets and ice shelves around Antarctica, with glaciers retreating in West Antarctica and melting of the underside of ice shelves to the west of the Antarctic Peninsula. Floating ice shelves help to stabilise the Antarctic ice sheet, by restricting the flow of glacial ice from the continent to the ocean. Warm ocean water penetrating below the ice shelves of the West Antarctic ice sheet is destabilising a number of glaciers, increasing the Antarctic contribution to sea level rise. Latest estimates for the Antarctic ice sheet show a loss of 2720 ± 1390 billion tonnes from 1992 to 2017, corresponding to an increase in global mean sea level of 7.6 ± 3.9 mm (approximately 10 per cent of the total). Melting of the Greenland ice sheet has increased dramatically, from an average of 34 billion tonnes (Gt) per year during 1992–2001 to 215 Gt per year over 2002–2011.

Changes in sea ice have little direct impact on sea level because sea ice is frozen sea water that floats and when it melts it returns the original volume of water to the sea. However, the presence or absence of sea ice can influence the climate and can also be an indicator of wider climate changes.

Changes in Antarctic sea ice are complex, with statistically significant increases in extent and seasonal duration in some regions—particularly in the Ross Sea, between 160° E and 150° W—and decreases in others— particularly to the west of the Antarctic Peninsula in the Bellingshausen Sea. The duration of the sea-ice season has changed by up to four months over the 38 years of record in these regions. Sea-ice extent around Antarctica has significant inter-annual variability.

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Trends in the length of the sea-ice season each year (in days per year) around Antarctica, 1979–2017. The Antarctic land-mass is shaded grey. Each year sea ice around Antarctica expands (or advances) from a minimum extent in February to a maximum extent in September. Duration is a measure of the number of days that a particular location contains sea ice.

Key points

  • The ice sheets and ice shelves of Antarctica and Greenland are losing ice due to a warmer climate; sea-ice extent has reduced in the Arctic.

Between 1979 and 2014 the net sea‑ice extent showed a positive trend, with substantial regional variations, however since then sea-ice coverage has been predominantly below average. Over the long term, sea‑ice coverage responds to large‑scale atmospheric and oceanic changes. The overall increase in sea‑ice extent since 1979 has mostly been attributed to changes in westerly wind strength. The largest recorded wintertime extent of approximately 20.2 million km 2 was recorded on 20 September 2014, closely followed by the lowest recorded summertime extent of approximately 2.1 million km 2 on 1 March 2017. These trends and the large variability are due to many factors including changes in winds over the Southern Ocean and near the Antarctic continent, and changes in ocean surface temperature and salinity. More observations are needed to determine if the recent decrease in sea-ice extent is the start of a long-term trend or a short-term change.

The sea-ice extent in the Arctic Ocean has decreased 3.5–4.1 per cent per decade since satellite records began in 1979. The four lowest wintertime maximum Arctic sea-ice extents in the satellite record have all occurred in the past four years. The record low minimum Arctic sea-ice extent occurred in September 2012.

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Antarctic and Arctic sea-ice extent (shown as the net anomaly from the 1981–2010 average) for the period January 1979 to May 2018 (106 km 2). Thin lines are monthly averages and indicate the variability at shorter time-scales, and thick lines are eleven-month running averages.

Why are Australia and the Earth warming?

Energy comes from the Sun. To maintain stable temperatures at the Earth’s surface this incoming energy must be balanced in the longer-term by an equal amount of heat radiated back to space. Greenhouse gases in the atmosphere, such as carbon dioxide and methane, make it harder for the Earth to radiate this heat, so increase the temperature of the Earth’s surface, ocean and atmosphere. This is called the greenhouse effect. Without any greenhouse gases, the Earth’s surface would be much colder, with an average temperature of about –18 °C. For centuries prior to industrialisation the incoming sunlight and outgoing heat were balanced, and global average temperatures were relatively steady, at a little under 15 °C. Now, mostly because of the burning of fossil fuels and changes in land use, the concentrations of greenhouse gases in the atmosphere are rising and causing surface temperatures to increase, leading to an ‘enhanced’ greenhouse effect. There is now an energy imbalance at the Earth’s surface of around 0.7–0.8 Wm–2 (averaged globally). The atmosphere and oceans will continue to warm until enough extra heat can escape to space to allow the Earth to return to balance. Because increased levels of carbon dioxide persist in the atmosphere for hundreds of years, further warming and sea level rise is inevitable