Predictions About Sea Levels

Although air and land surfaces are quick to warm with an increase in average global temperature, the same is not true of seawater. It takes many centuries for an increase in air temperature to gradually make its way down deeper and deeper into the ocean. For this reason, the rise in sea levels resulting from any particular amount of global warming is largely delayed for many years. Consequently, even if atmospheric carbon dioxide levels did not increase at all beyond today's values and no further global warming occurred, sea levels would continue to rise for centuries to come, as lower layers of the oceans became heated—and expanded—by air that has already been warmed.

Sea levels are predicted to rise by about half a meter by 2IOC—in addition to the 10-25-cm rise experienced over the last 100 years—though there is a large uncertainty in this value. About half of the predicted rise in sea level is due to the melting of glaciers, and the other half arises from the thermal expansion of seawater. The expansion occurs because the density of water decreases gradually as water warms beyond 4°C, the temperature at which it reaches its maximum density, as illustrated in Figure 7-13. Since density is mass divided by volume, and since the mass of a given sample of water cannot change, the volume it occupies must increase if its density decreases. Thus, as seawater warms (beyond 4°C), the volume occupied by a gram or kilogram of it increases; the only way that this can occur is if the top of the water—the sea level—increases.

Scientists predict that the Greenland Ice Sheet may eventually melt entirely due to global warming, raising ocean levels by about seven meters, but they are uncertain as to the fate of the Antarctic Ice Sheet. However, complete melting will require millennia to complete. Parts of both ice sheets sit above sea level on land. Consequently, the transfer, by the melting of their surface ice and the resulting draining of liquid water into the oceans, causes an increase in sea levels, as does the transfer into the oceans of icebergs broken off from the ice sheets. The collapse of ice shelves, which are the extensions into the sea of glaciers, such as the Larsen-B shelf in the Antarctic that collapsed in the early 2000s, allows glaciers that had been blocked to migrate more quickly toward open water.

Although an increase of half a meter in sea level does not seem very large, there are countries, like Bangladesh and the small island nation of Tuvalu in the South Pacific, in which much of the population currently lives on land that would be flooded by a rise in sea level of this amount. Damage from tropical storms would increase because of these higher sea levels.

In the long term, the most dramatic—though unlikely—effect of substantial global warming would be a change in the circulation patterns of water in the Atlantic Ocean. Currently, warm surface waters flow northward from the tropics into the North Atlantic, bringing heat to Europe and to a lesser extent to eastern North America. Some scientists have speculated that a rapid rise in temperature and rainfall levels could weaken or even eliminate this circulation pattern, as historical geological records indicate has happened in the past.

FIGURE 7-13 Density of liquid water versus temperature.

Climate Predictions for Specific Regions

It is much harder for scientists to make specific, reliable predictions for individual regions than for the globe as a whole. The climate changes that seem likely for the various continents, according to the 2007 1PCC report, are summarized in Table 7-3.

The higher latitudes of the Northern Hemisphere are expected to experience temperature increases substantially greater than the global average. Warming over some land areas, including the United States and Canada, should be noticeably faster than the average rate for the globe.

Significant Impacts of Climate Change That Will Likely Occur in the Continents in the Twenty-First Century


North America


Central and Northern Asia

Central America and West Indies Southern Asia

Pacific and small islands

Global oceans "Africa

South America

Australia and New Zealand

Antarctica and Southern Ocean

Significant retreat of ice; disrupted habitats of polar megafauna; accelerated loss of ice from Greenland Ice Sheet and mountain glaciers; shifting of fisheries; replacement of most tundra by boreal forest; greater exposure to UV radiation Reduced springtime snowpack; changing river flows; shifting ecosystems, with loss of niche environments; rising sea level and increased intensity and energy of Atlantic hurricanes increase coastal flooding and storm damage; more frequent and intense heat waves and wildfires; improved agriculture and forest productivity for a few decades

More intense winter precipitation, river flooding, and other hazards; increased summer heat waves and melting of mountain glaciers; greater water stress in southern regions; intensifying regional climatic differences; greater biotic stress, causing shifts in flora; tourism shift from Mediterranean region Widespread melting of permafrost, disrupting transportation and buildings; greater swampiness and ecosystem stress from warming; increased release of methane; coastal erosion due to sea ice retreat

Greater likelihood of intense rainfall and more powerful hurricanes; increased coral bleaching; some inundation from sea-level rise; biodiversity loss Sea-level rise and more intense cyclones increase flooding of deltas and coastal plains; major loss of mangroves and coral reefs; melting of mountain glaciers reduces vital river flows; increased pressure on water resources with rising population and need for irrigation; possible monsoon perturbation Inundation of low-lying coral islands as sea level rises; salinization of aquifers; widespread coral bleaching; more powerful typhoons and possible intensification of ENSC extremes

Made more acidic by increasing C02 concentration; deep overturning circulation possibly reduced by warming and freshening in North Atlantic Declining agricultural yields and diminished food security; increased occurrence of drought and stresses on water supplies; disruption of ecosystems and loss of biodiversity, including some major species; some coastal inundation Disruption of tropical forests and significant loss of biodiversity; melting glaciers reduce water supplies; increased moisture stress in agricultural regions; more frequent occurrence of intense periods of rain, leading to more flash floods Substantial loss of coral along Great Barrier Reef; significant diminishment of water resources; coastal inundation of some settled areas; increased fire risk; some early benefits to agriculture

Increasing risk of significant ice loss from West Antarctic Ice Sheet, risking much higher sea level in centuries ahead; accelerating loss of sea ice, disrupting marine life and penguins

Source; Scientific Expert Group Report on Climate Change and Sustainable Development, Confronting Climate Change, United Nations Foundation (2007):

In the U.S. Midwest and the area just north of it in Canada, as well as in southern Europe, the soil will probably become much less moist because of increased rates of evaporation in the warmer air and ground. This could affect the continued suitability of these areas for the growing of grain. High-latitude regions, however, could experience increased productivity, at least where the soil is suitable for agriculture. In areas that become drier, the positive C02 fertilization effect on plants will cancel some of the negative effects of decreased rainfall. There will be longer frost-free growing seasons at northern latitudes but increased chances that heat stress will affect crops grown there. Food production in temperate areas will probably also be affected by the attack of insects that in the past have been largely killed off during the winters but that could survive and flourish under warmer conditions.

Temperature and moisture changes will occur quickly compared to those that have taken place in the past, and consequently some ecosystems will be destabilized. Coastal ecosystems such as coral reefs are particularly at risk. The species composition of forests is likely to change, especially in regions far removed from the Equator. For example, the; hardwood forest in eastern North America may be at risk of extinction if climate zones shift more quickly than their migration can keep up with. The boreal forest of central Canada could be eliminated by fire by 2050; indeed, the frequency of fires in these woodlands is already climbing.

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Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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