Sea-level rise increases the frequency of coastal flooding events and threatens coastal populations around the world.
Why is sea-level rise important?
Around 148 million people are exposed to coastal flooding events worldwide, a number that is expected to surge in the coming decades. In the UK, the replacement for the Thames Barrier and the 300 km or so of sea defences that protect London need to be overhauled by 20702. The investment required to protect London against flooding depends heavily on sea-level rise but could exceed £20 billion.
Is sea-level rising today?
Yes, sea level has risen at an accelerating rate throughout the past century. Satellite measurements since the early-1990s indicate that today it is rising at a rate of 3.7 mm per year3. The contributions to this are approximately evenly split between melting ice adding water to the ocean and the expansion of seawater as it warms globally
How do ice sheets affect sea level?
If glaciers and ice sheets shrink, ice that was held above sea level will find its way into the oceans. If ocean volume increases global sea level will rise. This has happened many times in geological history and between glacial and interglacial periods over the past few hundred thousand years.
Why are ice sheets often in the news?
For two reasons. Firstly, satellite measurements of Antarctica and Greenland suggest that the icesheets are more sensitive to climate change than once thought. Secondly, the Inter-governmental Panel on Climate Change (IPCC) highlighted the issue by suggesting that current knowledge is inadequate to estimate confidently the contribution that ice sheets might make to sea-level rise in coming centuries. While technology makes sea-level rise easier to observe, and we can predict some contributions to future sea-level rise with increasing certainty, we cannot yet fully-predict the ice sheets’ contribution. This is recognised in the most recent IPCC report by the inclusion of a “high-impact storyline”, according to which sea-level rises at approximately twice the rate of other estimates for the rest of this century.
How is Antarctica affected?
Antarctica is covered by a vast ice sheet, around the size of the USA, and it is not surprising that different areas are behaving differently. On the Antarctic Peninsula, where climate has warmed substantially, 87% of glaciers are retreating but the area is small and the contribution to sea-level rise, a few centimetres per century, is comparable to that from Alaskan glaciers. The East Antarctic ice sheet appears close to balance, although increased snowfall may cause this area to thicken slowly in future. In West Antarctica, there is an area roughly the size of Texas where the ice sheet is thinning rapidly — the Amundsen Sea Embayment (ASE). Close to the coast in ASE, thinning rates are more than 1 metre per year.
Why is the Amundsen Sea Embayment significant?
This is an area that has long caused glaciologists concern for several decades, because here the bedrock beneath the ice is a long way below sea-level and the ice is only kept in place because it is thick enough to rest on the bed. Thinning of the ice around the coast could lead to glacier acceleration and further thinning of the ice sheet. Essentially, the ice sheet may be unstable, and the recent pattern of thinning could be a precursor to wholesale loss of the ice sheet in the ASE (implying a sea-level rise of around 1.5 metres).
Are changes in the Antarctic ice sheet caused by human activities?
Recent studies have suggested that the observed atmospheric warming on the Antarctic Peninsula is the result of human activities – both greenhouse effect and ozone-loss have a role to play in this. So, it is likely that glacier retreat on the Antarctic Peninsula is attributable to human influence.
The thinning of the ASE ice sheet is not the result of atmospheric warming but is due to changing ocean circulation patterns. Although the main change in circulation that initiated rapid ice loss appears to have already happened before detailed study of the ocean in the region began in the 1990s, recent studies increasingly point to ocean changes having been caused by human-induced climate change.
What is BAS doing about it?
The ASE is a notoriously difficult place in which to undertake fieldwork, it is cold, windy and is more than 1400km from any research station. Since 2004/05, BAS has taken a lead in opening up research in this area. In 2007/08 US researchers joined BAS in collaborative projects to understand and predict changes in the ASE , leading eventually to the current jointly funded International Thwaites Glacier Collaboration (ITGC). The first phase of collaborative BAS and US work in the region identified the large Thwaites Glacier as being at greatest risk, because most of its bed lies far below sea level. The glacier holds enough ice to raise sea level by 65 cm if it were all lost to the ocean. The aim of the ITGC is to improve estimates of how much and how fast Thwaites Glacier and the wider ASE will contribute to sea-level rise.
- Ice sheet: The Antarctic ice sheet is the layer of ice up to 5000m thick covering the Antarctic continent. It is formed from snow falling in the interior of the Antarctic which compacts into ice. The ice sheet slowly moves towards the coast, eventually breaking away as icebergs which gradually melt into the sea.
- The ice sheet covering East Antarctica is relatively stable, because most of it lies on rock that is above sea level and is thought unlikely to collapse.
- The West Antarctic is less stable, because it sits on rock below sea level.
- Ice shelves float in the sea and will only increase sea-level if they break up and allow the ice they hold back to flow more quickly onto the sea.