BackgroundPredicting extreme events in the marine environment is essential in order to quantify the risk to personnel and to measure the economic viability of activities such as marine transportation, oil and gas exploration and the development of coastal areas.
Almost 30% of civilization lives within 100 km of the coast and this percentage is expected to approximately double over the next 50 years. Although the coastal environment provides access to a wide range of natural resources, efficient transportation pathways and relatively mild coastal climates, it can be a hazardous place to work, live and play. A recent, high profile example is provided by Hurricane Katrina, which struck New Orleans in 2005; it resulted in the loss of almost 2,000 lives and damage to infrastructure, including offshore oil and gas platforms and pipelines, exceeding $90 billion. An example from eastern Canadian waters is the offshore drilling rig Ocean Ranger, which sank on the Grand Banks in February 1982 leading to the loss of the entire crew of 84. Gale force winds and high waves, reported to be as high as a five-storey building, were implicated in the loss of rig and crew. A final example, this time from Australian waters, is the oil spill disaster resulting from the Pacific Adventurer encountering tropical cyclone Hamish off the coast of Brisbane. This was an extremely rare event in which the storm remained tropical cyclone strength as it propagated along the east coast of Australia. All of the operational forecasts predicted landfall and a reduction in intensity, however it kept reforming supported by an anomalously warm boundary current (highlighting the important role played by coupling of atmosphere and ocean in the evolution of extreme marine events like tropical cyclones). All of the above examples illustrate the need for accurate forecasts of the atmosphere and ocean in order to mitigate damage due to extreme marine events, and also reduce loss of life.
The Marine Environment is Changing
There is mounting evidence that the marine environment is undergoing significant change (International Panel for Climate Change (IPCC) Assessment, 2007). For example, global sea level rose at a rate of 1.8 mm per year from 1961 to 2003. This rate almost doubled from 1993 to 2003 due primarily to thermal expansion of the oceans, with additional contributions from retreating glaciers and ice caps, and losses from the polar ice sheets. The IPCC Assessment also noted it is likely that the incidence of extreme high sea levels has increased worldwide since 1975. Local rates of relative sea level rise can be significantly higher than the global average rise if the land is subsiding due to natural processes (e.g., post glacial adjustment) or the impact of human activities like water withdrawal and oil and gas extraction in the coastal zone.
A possible acceleration in the present rate of rise of global sea level is causing considerable concern due to the increased risk of flooding in low lying islands such as the Maldives and overtopping of coastal defences in vulnerable countries like the Netherlands. Increasing sea levels and larger waves will also result in greater potential for the collapse of fixed offshore platforms, due to large impact forces when waves hit the topsides; they may also change bridge clearances in tidally dominated rivers and ports thereby impacting ship routing at particular stages of the tide.
Sea ice extent in the Arctic over recent decades has decreased sharply in all seasons, with the largest changes observed in summer. Nearly 40% of the Arctic sea ice area present in the 1970s was lost by 2007 and ice-free conditions were observed in 2008 in both the Northeast and Northwest passages. Sea ice has also become thinner making it increasingly vulnerable to melting.
The IPCC Assessment also provides observational evidence that intense tropical cyclone activity in the North Atlantic has been increasing since about 1970 although there remains significant debate within the research community. There are also suggestions of an increase in the frequency of intense tropical cyclone activity in other regions. The rarity of tropical cyclones and the limited length of quality observation records make definitive assessment of the impacts of climate change difficult in other regions. However, none of the records indicate that there has been a reduction in activity and the potential risks from only a small increase in tropical cyclone activity are great. The total liability is increasing due to the growth in urbanization along the coasts (i.e., an increased area for strikes), the growth in capital value (i.e., the supply and demand along the coast), and the heightened risks from storm surge due to sea level rise both from ice melt and thermal expansion.
Changes in the Marine Environment over the Next Century
Empirical and physically-based models are now used routinely to make projections of climate variability and change over the next century. Recent projections of global sea level rise are quite variable and strongly dependent on the assumed emission scenario for greenhouse gases (e.g., IPCC Assessment). Such projections are recognized to be somewhat speculative because they do not allow for uncertainties in carbon cycle feedbacks or ice sheet flow dynamics, and the downscaling of climate models to weather systems remains unproven. More recent studies suggest that global sea level will rise more quickly than IPCC projections and will result in an increase over the next century of about 1 metre leading to a reduction of the return period of the present day 100 year flood levels to a return period of several years for many mid-latitude stations. Model based projections of Arctic sea ice suggest that it may disappear completely by the end of this century (IPCC, 2007). There are also indications, based on results from a range of models, that future tropical cyclones will become more intense, although there is less confidence in projections in the numbers of tropical cyclones. Impacts of increased tropical cyclone intensity include disruption by flood and high winds, possible withdrawal of risk coverage in vulnerable areas by private insurers, potential for population migrations, and damage to coastal and offshore infrastructure. The tracks of extra tropical storms are projected to move poleward over the next century, and this will lead to changes in wind, precipitation and temperature patterns.