The Impacts of sea-level rise

Contributor: Robert Nicholls, Middlesex University, London

Summary
Introduction
Coastal flooding
Impacts on coastal wetlands
 

Summary

With unmitigated emissions, sea level will be about 40 cm higher than today by the 2080s, and this is estimated to increase the annual number of people flooded from 13 million to 94 million. 60% of this increase will occur in southern Asia (along coasts from Pakistan, through India, Sri Lanka and Bangladesh to Burma), and 20% will occur in South East Asia (from Thailand to Vietnam including Indonesia and the Philippines).

The flood impacts of sea-level rise are reduced by the emissions scenarios leading to stabilisation of CO2; by the 2080s, the annual number of people flooded is estimated to be 34 million under the 750 ppm scenario and 19 million under the 550 ppm scenario. Again, most of those flooded will be in southern and South East Asia.

Under all emissions scenarios, sea-level rise will compound the existing decline of coastal wetlands due to direct human destruction. Under the stabilisation scenarios, wetlands will have longer to adjust and this will significantly reduce losses and aid their long-term survival.

The continued rise in sea level even under the stabilisation scenarios would produce a range of progressive impacts on coastal lowlands and on low-lying coastal islands around the world without appropriate human adjustment and adaptation.

Introduction

Global warming causes sea level to rise by thermal expansion of the ocean waters and the melting of land-based ice, producing a range of impacts. In this study, we examine and compare the impacts of global mean sea-level rise due to unmitigated emissions, emissions leading to stabilisation at 750 ppm and 550 ppm CO2. By the 2080s, the Hadley Centre predictions (illustrated in detail in the first section of this report) show a 41 cm global rise (compared to the level for 1961–90) in the case of unmitigated emissions, while the 750 ppm and 550 ppm stabilisation scenarios show rises of 30 cm and 27 cm, respectively. By the 2230s, even though CO2 concentrations have stabilised, sea level will be higher by 94 cm and 75 cm, respectively, and still rising, due to its long adjustment time. This analysis considers, firstly, the increased risk of coastal flooding due to storm surges to the 2080s and, secondly, increased losses of coastal wetlands to the 2230s.

Coastal flooding

In assessing the risk of coastal flooding, the incidence of storms is assumed not to change, and the increase in frequency of high-water events arises only because of the rise in sea level. Population growth in the coastal zone is assumed to be double the national average, a projection of existing trends. The standard of flood protection is assumed to evolve from that of 1990 in line with the projected increase in GDP/capita, but with no measures introduced specifically to combat sea-level rise; historical precedent shows this to be a reasonable assumption. Calculations are made of the average number of people who are estimated to experience flooding per year by storm surges.

Globally, the average annual number of people flooded is calculated by the model to be 10 million in 1990; at the national level, these calculations agree well with available estimates. In the absence of any sea-level rise, this increases to 27 million in the 2050s, and then decreases to 13 million in the 2080s. Sea-level rise increases these numbers as shown in in the diagram below. Under the unmitigated emissions scenario, 94 million people will be flooded annually by the 2080s; an additional 81 million due to sea-level rise. The two stabilisation emissions scenarios result in smaller increases in numbers of people flooded, particularly after the 2050s. In the 2080s, 60 million fewer people are estimated to be flooded under stabilisation at 750 ppm, and 75 million fewer for stabilisation at 550 ppm.

The most vulnerable regions under the unmitigated emissions scenario are southern Asia (along coasts from Pakistan, through India, Sri Lanka and Bangladesh to Burma), South East Asia (Thailand to Vietnam, including Indonesia and the Philippines), with lesser but still substantial numbers in eastern Africa (South Africa to Sudan, including Madagascar), the Mediterranean from Turkey to Algeria and western Africa (Morocco to Namibia). By the 2080s, under the unmitigated emissions scenario more than 90% of the average annual number of people flooded would be found in these five regions with southern Asia contributing the great majority. The diagram below and the maps overleaf show the extent to which the numbers of people who experience flooding are significantly reduced in all these regions under the stabilisation scenarios, compared to unmitigated emissions.

In relative terms, the island states in the Caribbean, the Indian Ocean (excluding Sri Lanka and Madagascar) and the Pacific Ocean appear particularly vulnerable to unmitigated emissions. The diagram at the top of the next column shows clearly how the numbers of people who experience flooding are significantly reduced in all these regions under the scenarios leading to stabilisation of CO2.

Impacts on coastal wetlands

Direct human reclamation is reducing the global stock of coastal wetlands (i.e. saltmarshes, mangroves and intertidal areas) by about 1% per year. Even if this rate of decline were to be moderated substantially, wetlands could still be reduced by about 40% by the 2080s. Added to this reduction will be a further loss due to sea-level rise from human-induced global warming. Coastal wetlands are sensitive to sea-level rise as their location is intimately linked to present sea level. For wetland loss, the rate of sea-level rise is more important than the absolute rise as wetlands have some capacity to respond to sea-level rise by vertical accretion due to sediment and organic matter input. As shown in the graph below, under the unmitigated emissions scenario, the rate of global sea-level rise increases with time approaching 60 cm/century by the 2080s. Under the stabilisation emissions scenarios the rate of global sea-level rise increases much more slowly and levels off at just over 40 cm/century for the stabilisation at 750 ppm scenario and about 30 cm/century for the stabilisation at 550 ppm scenario, beginning to decrease in the 23rd century.
 

Average annual number of people flooded for three island regions in the 2080s under unmitigated emissions (red), stabilisation at 750 ppm (blue) and stabilisation at 550 ppm (green). Also shown is the case with no climate change (grey).

Rate of sea-level rise under unmitigated emissions (red), emissions leading to stabilisation at 750 ppm (blue) and stabilisation at 550 ppm (green).

The direct wetland response to sea-level rise is modelled by selecting a critical rate of long-term sea-level rise as a proportion of local tidal range above which wetland losses commence. Wetland losses are assumed to occur 30 years after any sea-level rise, reflecting response lags. The potential for wetland migration on to adjacent low-lying upland is evaluated, based on coastal morphology and the occurrence or absence of coastal protection (estimated from the predicted coastal population density in the 2080s). This latter factor restricts the potential for wetland migration compared to earlier periods of climate change and sea-level rise in the earth’s history. While there are significant uncertainties in the calculation a best estimate of wetland losses (shown in the graph below) illustrates the relative impacts for the different emissions scenarios. Note that these losses are in addition to the direct effect of wetland reclamation mentioned earlier.

Under unmitigated emissions, losses increase rapidly with time and, by the 2080s, 13% of the 1990 global wetland stock would be destroyed by sea-level rise. Substantial additional losses would be expected in the 22nd century. The stabilisation scenarios show a large reduction in the losses in the 2080s to 7% (CO2 stabilisation at 750 ppm) and 6% (CO2 stabilisation at 550 ppm). After the rate of sea-level rise levels off, no further wetlands are lost. This happens by the 2200s for the stabilisation at 750 ppm scenario when losses reach 12%, and in the 2140s for the stabilisation at 550 ppm scenario when losses reach 7%. These losses are less than those under the unmitigated scenario in the 2080s. After the rate of sea level-rise has stabilised, a slow recovery of the wetlands might begin. Therefore, stabilisation of CO2 concentrations could make an important contribution towards the long-term survival of coastal wetlands. The coastal wetlands of the Mediterranean, the Baltic and the Atlantic coast of North and Central America appear particularly vulnerable to sea-level rise, and will suffer substantial losses by the 2080s under all climate change scenarios.

An important aspect of rising sea level is its long timescale. As seen in the first section of this report, even if climate change is halted, the warming already incurred will progressively penetrate deeper and deeper causing sea level to continue to rise for many centuries. This will threaten coastal lowlands and low-lying islands with a range of impacts including erosion, inundation, increased flooding and salinisation. Some human adjustment and adaptation to sea-level rise would appear to be essential even under stabilisation scenarios.

The total annual number of people flooded in the 2080s, along the coastlines shown, under unmitigated emissions (top). The percentage reduction in numbers of people flooded (including in the island areas shown) under emissions leading to stabilisation of CO2 at 750 ppm (middle) and 550 ppm (bottom).