The Liverpool View: Climate change threat is real

Phil Woodworth is a visiting Professor in the University of Liverpool’s School of Environmental Sciences

“Last week saw the publication of the 5^th Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change, usually called just the ‘IPCC Report’. The findings extend and strengthen those of the 4^th Report six years ago.

The atmosphere and ocean have warmed significantly during the 20^th century due to the radiative forcing of greenhouse gases that are unquestionably of human origin. Arctic sea ice is in retreat, ice sheets in Greenland and Antarctica and mountain glaciers on each continent are thinning, and sea level has risen by about 20 cm. Studies suggest that sea level could continue to rise by up to 82 cm for the period 2081-2100 compared to the period 1986-2005, depending on a likely range for the world’s future carbon emissions.

Central importance of oceans

The oceans are of central importance in this research, having absorbed over 90 % of the increase in the energy (heat content) of the atmosphere-ocean system due to the warming. So, questions of how the oceans actually work have never been more important in considering how life on this planet will have to change and adapt in the future. The National Oceanography Centre (NOC), based on the Liverpool campus, and the Ocean Sciences group of the University’s School of Environmental Sciences play important roles in this research, particularly with regard to sea level change.

At NOC, we operate a monitoring system of the level of the seas around UK coasts (www.ntslf.org), including a station at Gladstone Dock at Liverpool which has one of the longest sea level records in the world. This system is designed primarily for the monitoring of wintertime floods, and has existed since the devastating storm surge in eastern England and the Netherlands in 1953. However, the same data collected over many years provide information for research into climate change.

NOC also has stations for monitoring sea level in the South Atlantic, Antarctica and Gibraltar, all of which are components of the Global Sea Level Observing System. Data from all these stations go into the Permanent Service for Mean Sea Level databank (PSMSL, www.psmsl.org), hosted at NOC, which is the primary source of information on long-term global ‘mean sea level’ change used by the IPCC.

With the use of sea level data collated at Liverpool, the IPCC 5^th Report concluded that global mean sea level has risen at an average rate of 1.9 mm/year since the start of the 20^th century, but with considerable regional variations linked to the way that the ocean circulation has adjusted. At the coast, the observed sea level change will be a combination of this regional sea level change and change in the level of the land that the observer is sitting on. Land level change can be positive or negative (emergence and submergence) due to regional and local geological processes. For example, in northern Sweden, where the land is still popping up following the removal of the great ice sheets after the last Ice Age, the effects of present-day sea level rise are minimal compared to those of Glacial Adjustment. However, in low-lying areas such as Bangladesh, which is in a submerging river delta, sea level rise is already having major impacts.

Compilations of global sea level (m) estimated from historical and paleo data, and melded with future predictions for low (blue line) and high (red line) emission scenarios. The historical data includes tide gauge data collated at Liverpool (PSMSL) and reconstructions from salt marshes (purple symbols). Reproduced from the IPCC report

For most people who live near the coast, there will be less interest in the slow changes in the mean levels of the ocean taking place on timescales of decades or centuries than in the sometimes dramatic extreme events (storm surges) caused by winter storms. As far as we know, the increase of the extreme sea levels that have occurred during storms during the last few decades have been similar to those in mean sea levels, rather than due to changes in the frequency of the storms themselves. For many parts of the UK coast, the increase in mean sea level during the 20^th century (about 20 cm) will have doubled the risk of flooding.

So how much more frequently are you likely to be flooded in future? The answer to this question depends on exactly where you live, and the statistics of tides and storm surges in your area. But, as a general rule, one can say that changes in flood frequency have a logarithmic relationship to mean sea level change.

Using the above example, if mean sea level changes by 80 cm in the future then that would increase the frequency of flooding by a factor of 2⁴ = 16. In other parts of the world, such as the flat coastal delta of Bangladesh, or coral islands which have elevations of only 1-2 m, then the impacts could be considerably larger. Studies of the likely impacts are the responsibility of the IPCC Working Group II, the final report of which is planned in 2014.

NOC had major inputs to the IPCC 5^th Report, providing lead and contributing authors and a review editor to two of its thirteen chapters. The inputs were possible because of the expertise that exists in NOC in Liverpool in Sea Level Science. One of the largest sea level groups in the world is based here, collaborating closely with scientists in the School of Environmental Sciences which has its own expertise in topics such as how the oceans are warming and the mechanisms for exchange of carbon between the atmosphere, land and ocean. In addition, NOC plays an important role in advising on Government policy (Defra, DECC, DfID) on issues such as coastal protection, and has played a major part in studies such as Thames Estuary 2100 in considering how London can continue to be effectively protected from flooding.”

Do you want to know more?

Register at www.psmsl.org for a workshop celebrating the 80^th anniversary of the PSMSL, to be held on 28-29 October in the Victoria Building in Liverpool.

This workshop will contain several extensive review talks of the IPCC Report by some of the world’s leading climate scientists.

The following people contributed to this article: Dr Svetlana Jevrejeva (National Oceanography Centre, IPCC lead author for sea level change), Professor Chris Hughes (joint University of Liverpool and National Oceanography Centre) and Professor Ric Williams (School of Environmental Sciences, University of Liverpool)