How high will seas rise?

As governments, businesses, and homeowners plan for the future, they should assume that the world’s oceans will rise by at least two metres - roughly seven feet - this century. But far too few agencies or individuals are preparing for the inevitable increase in sea level that will take place as polar ice sheets melt.

The reports from the Intergovernmental Panel on Climate Change (IPCC) are balanced and comprehensive documents summarising the impact of global warming on the planet. But they are not without imperfections, and one of the most notable was the analysis of future sea level rise contained in the latest report, issued in 2007.

Given the complexities of forecasting how much the melting of the Greenland and West Antarctic ice sheets will contribute to increases in global sea level, the IPCC chose not to include these giant ice masses in their calculations, thus ignoring what is likely to be the most important source of sea level rise in the 21st century. Arguing that too little was understood about ice sheet collapse to construct a mathematical model upon which even a rough estimate could be based, the IPCC came up with sea level predictions using thermal expansion of the oceans and melting of mountain glaciers outside the poles. Its results were predictably conservative - a maximum of a two-foot rise this century - and were even a foot lower than an earlier IPCC report that factored in some melting of Greenland’s ice sheet.

The IPCC’s 2007 sea level calculations - widely recognised by the academic community as a critical flaw in the report - have caused confusion among many in the general public and the media and have created fodder for global warming sceptics. But there should be no confusion about the serious threat posed by rising sea levels, especially as evidence has mounted in the past two years of the accelerated pace of melting of the Greenland and West Antarctic ice sheets.

The message for the world’s leaders and decision makers is that sea level rise is real and is only going to get worse. Indeed, we make the case in our recent book, The Rising Sea, that governments and coastal managers should assume the inevitability of a seven-foot rise in sea level. This number is not a prediction. But we believe that seven feet is the most prudent, conservative long-term planning guideline for coastal cities and communities, especially for the siting of major infrastructure; a number of academic studies examining recent ice sheet dynamics have suggested that an increase of seven feet or more is not only possible, but likely. Certainly, no one should be expecting less than a three-foot rise in sea level this century.

In the 20th century, sea level rise was primarily due to thermal expansion of ocean water. Contributions of melting mountain glaciers and the large ice sheets were minor components. But most climate scientists now believe that the main drivers of sea level rise in the 21st century will be the melting of the West Antarctic Ice Sheet (a potential of a 16-foot rise if the entire sheet melts) and the Greenland Ice Sheet (a potential rise of 20 feet if the entire ice cap melts). The nature of the melting is non-linear and is difficult to predict.

Seeking to correct the IPCC’s failure to come up with a comprehensive forecast for sea level increase, a number of state panels and government committees have produced sea level rise predictions that include an examination of melting ice sheets. For example, sea level rise panels in Rhode Island and Miami-Dade County have concluded that a minimum of a three- to five-foot sea level rise should be anticipated by 2100. A California report assumes a possible 4.6-foot rise by 2100, while the Dutch assume a 2.5-foot rise by 2050 in the design of their tidal gates.

Given the growing consensus about the major sea level rise on the way in the coming century or two, the continued development of many low-lying coastal areas - including much of the US east coast - is foolhardy and irresponsible.

Who is at risk?

Rising seas will be on the front lines of the battle against changing climate during the next century. Our great concern is that as the infrastructure of major cities in the industrialised world becomes threatened, there will be few resources left to address the dramatic impacts that will be facing the citizens of the developing world.

The ramifications of a major sea level rise are massive. Agriculture will be disrupted, water supplies will be salinised, storms and flood waters will reach ever further inland, and millions of environmental refugees will be created - 15 million people live at or below three feet elevation in Bangladesh, for example. Governments, especially those in the developing world, will be disrupted, creating political instability.

The most vulnerable of all coastal environments are deltas of major rivers, including the Mekong, Irrawaddy, Niger, Ganges-Brahmaputra, Nile, and Mississippi. Here, land subsidence will combine with global sea level rise to create very high rates of what is known as “local, relative sea level rise”. The rising seas will displace the vast majority of people in these delta regions. Adding insult to injury, in many parts of Asia the rice crop will be decimated by rising sea level - a three-foot sea level rise will eliminate half of the rice production in Vietnam - causing a food crisis coincident with the mass migration of people.

The Mississippi Delta is unique because it lies within a country with the financial resources to fight land loss. Nevertheless, we believe multibillion-dollar engineering and restoration efforts designed to preserve communities on the Mississippi Delta are doomed to failure, given the magnitude of relative sea level rise expected. Former Secretary of the Interior Bruce Babbitt said in 2008 that it was an “ineluctable fact” that within the lifespan of some people alive today, “the vast majority of that land will be underwater.” He also faulted federal officials for not developing migration plans for area residents and for not having the “honesty and compassion” to tell Louisiana residents the “truth”: Someday, they will have to leave the delta. The city of New Orleans can probably be protected into the next century, but only at great expense and with little guarantee that future storms like hurricane Katrina will not inundate the city again.