Sea Level Rise
How fast will the seas rise due to global warming? We may never know until after the fact for sure. Future sea level rise due to the melting of the Greenland and Antarctic ice sheets could be substantially larger than estimated in Climate Change 2007 according to new research from the University of Bristol. The study, published today in Nature Climate Change, is the first of its kind on ice sheet melting to use structured expert elicitation (EE) together with an approach which mathematically pools experts' opinions. EE is already used in a number of other scientific fields such as forecasting volcanic eruptions.
Sea levels around the world are rising and have been since the last ice age. Between 1870 and 2004, global average sea levels rose 17 cm (6.7 in). From 1950 to 2009, measurements show an average annual rise in sea level of 1.7 ± 0.3 mm with satellite data showing a rise of 3.3 ± 0.4 mm from 1993 to 2009, a faster rate of increase than previously estimated.
Two main factors contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.
Recent studies of Roman wells in Caesarea and of Roman piscinae in Italy indicate that sea level stayed fairly constant from a few hundred years AD to a few hundred years ago.
Based on geological data, global average sea level may have risen at an average rate of about 0.5 mm/yr over the last 6,000 years and at an average rate of 0.1—0.2 mm/yr over the last 3,000 years.
Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by more than 120 meters (averaging 6 mm/yr) as a result of melting of major ice sheets. A rapid rise took place between 15,000 and 6,000 years ago at an average rate of 10 mm/yr which accounted for 90 meters of the rise; thus in the period since 20,000 years BP the average rate was 3 mm/yr.
The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth's glacier ice which would raise global sea level by some 63 meters if it were to melt completely. The ice sheets are the largest potential source of future sea level rise — and they also possess the largest uncertainty over their future behavior. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored.
One such approach is via carefully soliciting and pooling expert judgements — a practice already used in fields as diverse as eruption forecasting and the spread of vector borne diseases. In this study Professor Jonathan Bamber and Professor Willy Aspinall used such an approach to assess the uncertainties in the future response of the ice sheets.
They found that the median estimate for the sea level contribution from the ice sheets by 2100 was 29 cm with a 5 per cent probability that it could exceed 84 cm. When combined with other sources of sea level rise, this implies a conceivable risk of a rise of greater than 1 meter by 2100, which would have deeply profound consequences for humankind. The UN IPCC's report provided figures ranging only from 18 cm to 59 cm for six possible scenarios.
The researchers also found that the scientists, as a group, were highly uncertain about the cause of the recent increase in ice sheet mass loss observed by satellites and equally unsure whether this was part of a long term trend or due to short-term fluctuations in the climate system.
Professor Bamber said: "This is the first study of its kind on ice sheet melting to use a formalized mathematical pooling of experts’ opinions. It demonstrates the value and potential of this approach for a wide range of similar problems in climate change research, where past data and current numerical modelling have significant limitations when it comes to forecasting future trends and patterns."
For further information see New Levels.
Figure image via Wikipedia.