Coastal cities could see rises of more than two metres if global warming exceeds 2°C goal, new research shows.

Researchers say urgent action needs to be taken to protect shoreline cities and world heritage sites from coastal flooding, after a new study showed that sea levels could be significantly higher than previously thought if global warming exceeds 2°C.

The study, by researchers including Dr Luke Jackson is published today in PNAS, and shows that warming of 2°C, which would occur by 2040 under a business-as-usual scenario, will lead to a global average sea-level rise of 20cm.  Crucially, however, more than 90% of coastal areas will experience greater rises, and the Atlantic coast of North America double that.

Under this ‘business-as-usual’ scenario, where mitigation efforts are unsuccessful, Earth’s global surface temperature would rise by 2°C compared to pre-industrial levels around 2040, and by 5°C around 2100.

With colleagues at the National Oceanography Centre, Delft University of Technology, the University of Copenhagen, Beijing Normal University and the University of Lapland, Dr Jackson projected that, for this scenario, the rate of global sea level rise around 2040 will be about 6mm per year, a rate twice as large as present-day observations. By 2100, this rate would be around 14mm per year, where global average sea level is expected to have risen by around 90cm. This means that a global rise of 60-70cm would occur in about 60 years from 2040 to 2100, compared with 20 cm between now and 2040. It is the increasing speed of sea-level rise over the century that will reduce the time available for adaptation in low-lying coastal regions.

However, by assuming a worst case situation of this scenario, where exceptional amounts of melt-water come from Antarctica and Greenland, projections for 2100 show that global average sea-level rise would be 180cm, 60cm greater than previous estimates. In this case, rises of over two metres are projected for small-island nations such as Micronesia and the Indian Ocean’s Chagos Islands, and coastal cities along the US Eastern seaboard.

Estimates for sea level rises for coastal cities under a 5°C warming scenario include:

Bangkok – lowest projection 49cm, highest projection 191cm

Guangzhou – lowest projection 51cm, highest projection 193cm

Hamburg – lowest projection 56cm, highest projection 195cm

Jakarta – lowest projection 49cm, highest projection 180cm

Lagos – lowest projection 52cm, highest projection 192cm

London – lowest projection 38cm, highest projection 182cm

New York – lowest projection 64cm, highest projection 224cm

Manila – lowest projection 51cm, highest projection 199cm

Whilst sea-level rise increases the risk of coastal flooding, two other factors will also increase risk. First, the rise in population numbers living in coastal megacities (400 million by 2030 including 370 million in Asia, Africa and South America) and second, the effect of local land-subsidence due to the extraction of ground water and river delta sedimentation.

Many cities are building on land left previously unoccupied because of flood risk and with the added effect of subsidence the above projections worsen considerably, with Jakarta projected to see a rise of 3m by 2100. The researchers say that without restrictions on groundwater extraction and the development of alternative water supplies, parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other cities will sink below sea level.

Dr Jackson said: “Multiple factors contribute to rising sea-levels: the warming of the ocean, altered ocean currents such as the Gulf Stream, increased melting of glaciers and ice-sheets, land-water use by ground water extraction and damming, and land-motion, which can be both natural and man-made, for example the subsidence of mega-cities.

“While there are uncertainties about how much sea-level will rise, policy makers must confront two questions. Firstly, what steps need to be taken for coastal regions to adapt to the end-of-century projections? Secondly and perhaps more pertinent is, how will coastal regions adapt to the speed at which sea-level could rise after mid-century? Time is of the essence for rapidly growing coastal cities, coastal heritage sites, and vulnerable tropical ecosystems to adapt to sea-level rise, and it is time that is slipping from our grasp.” (Post originally published at the Oxford Martin School).

• The paper, ‘Coastal sea level rise with warming above 2 °C’, is published in the Proceedings of the National Academy of Sciences (PNAS).