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6.1.2.c
Projected Global Effects of Climate Change and Sea Level Rise on Coastal Zones and Marine Ecosystems
The level of the sea at the shoreline is determined by many factors. .These factors function on a range of time scales. For example, tides work on a scale of hours whereas ocean basin changes due to tectonics and sedimentation change over millions of years.
Some of the greatest influences on the average levels of the sea are linked to climate and climate change processes. They operate on a time scale of decades to centuries. (See Figure 1.)
Thermal Expansion: You know from experience that substances expand when heated. This is true for the water in the ocean. Scientists believe thermal expansion is one of the major contributors to historic sea level changes. It is expected to be the major contributing factor to sea level rise over the next hundred years. Deep ocean temperatures change very slowly, therefore, thermal expansion will likely continue for many centuries, even if the atmospheric concentrations of greenhouse gases stabilize.
The amount of warming and the depth of water affected vary with location. Factors such as thermal expansion, changes in salinity, winds, and ocean circulation vary from region to region and all affect the sea level changes. There is a wide range of differences in sea level changes across regions of the planet. These differences are sizable compared with the global average sea level rise.
Water from the Land: Water can be added to or removed from the ocean over time. Rivers flow into the ocean and icebergs break off glaciers into the sea. A huge amount of water is stored on land frozen in glaciers or ice sheets. Actually, the sea level was lower during the last glacial period because so much water was stored in the large extension of the ice sheets on the continents of the Northern Hemisphere. The melting of mountain glaciers and ice caps is expected to make the second largest contribution to the rise of sea level over the next hundred years. These melting glaciers and ice caps will have more effect on sea level than the larger ice sheets in Greenland and Antarctica, because they are more sensitive to climate change. The ice sheets in Greenland and Antarctica are in colder climates with low precipitation and low melting rates.
Other Factors: Sea level is also influenced by other processes. Human activities such as extraction of ground water for irrigation, building reservoirs, and land use changes that affect surface runoff, all affect water stored on land, and therefore sea level.
Vertical land movements caused by natural processes, such as slow movements of the Earth’s tectonic plates, can affect local sea levels. Sea level responds to interactions between the atmosphere and ocean. The most noticeable effect is during El Niño events.
These factors will result in differences in sea level rise in different regions of the world. (See Figure 2.)
Figure 1. What causes the sea level to change?
Source: IPCC Climate Change 2001 Synthesis Report p. 67
How much? Global mean sea level is projected to rise by 0.09 to 0.88 m between the years 1990 and 2100. The two main factors are thermal expansion of the oceans and melting of glaciers and ice caps. (See Figure 3.) Nearly all models project a greater than average rise in the Arctic Ocean and a less than average rise in the Southern Ocean. There will be environmental, social, and economic impacts. (See Table 1.)
Figure 2. The rate of Sea Level Rise in different regions. Source: Source: Hengeveld, H.G. 2000. Projections for Canada’s Climate Future: A Discussion of Recent Simulations with the Canadian Global Climate Model. Meteorological Service of Canada, Environment Canada. 27pp.
Figure 3. Global average sea level rise 1990 to 2100 for the SRES scenarios. Source: IPCC Climate Change 2001 Synthesis Report p. 212.
- coloured bars represent the range for each of the six separate SRES scenarios.
- dark grey area is the range of the average of the scenarios,
- lighter gray is the complete range of the scenarios,
- white is the range of all models including uncertainties in land-ice changes and permafrost changes.
Effects
People on islands and coastal areas: Particular risk of severe social and economic effects from sea-level rise and storm surges. Many human settlements will face increased risk of coastal flooding and erosion, and tens of millions of people living in deltas, in low-lying coastal areas, and on small islands will face risk of displacement. (See Figure 4.)
High-latitude coasts will experience added impacts because of the higher wave energy, reduced sea-ice, and thawing of permafrost. This will result in rapid coastal retreat and have a severe impact on coastal settlements.
Low-latitude tropical and sub-tropical coastlines, especially where there is large human population pressure, are very susceptible. Human activities such as draining wetlands, reducing natural sedimentation from rivers, and withdrawing water from underground affect the water balance in these areas. Problems such as salinization (salt contamination) of drinking water will increase. Especially at risk are the large river delta regions of Asia and small islands.
Resources critical to island and coastal populations such as beaches, freshwater, fisheries, coral reefs and atolls, and wildlife habitat would also be at risk.
Marine and Coastal Ecosystems: Sensitive to climate change. Climate trends and variability are now recognized to strongly affect fish abundance and population dynamics, with significant impacts on fish-dependent human societies. If warm events that accompany El Niños happen more often, plankton and fish larvae abundance will decline and this will affect fish, marine mammals, seabirds, and ocean biodiversity.
Coastal ecosystems such as coral reefs, atolls, and reef islands, salt marshes and mangrove forests are highly biodiverse and productive. How much they are affected by sea level rise will depend on several factors.
- Speed of the sea level rises,
- Species growth rates keep up with the changes,
- Species migration
- Changes climate-ocean environment such as sea surface temperatures and storminess.
- Pressures from human activities in coastal zones
Healthy coral reefs can likely keep up with sea level rise, but many are already severely degraded. They are stressed by coral bleaching, UV-B radiation, and pollution.. Episodes of coral bleaching over the past 20 years have been associated with several causes, including increased ocean temperatures. Future sea surface warming will very likely increase stress on coral reefs and result in increased frequency of marine diseases.
Figure 4. Coastal Areas Highly Susceptible to a 44 cm Sea Level Rise by the 2080’s. (Assuming 1990’s level of flood protection) Source: R. Nicholls, Middlesex University. Meteorological Office. 1997. Climate Change and Its Impacts: A Global Perspective
Table 1. Potential Impacts of Climate Change and Sea-Level Rise on Coastal Systems Source: Climate Change 2001 WG II section 6.4.1