Consequences of a global temperature increase have provided fodder for journalists always looking for a new angle. We have all read about recent hurricanes, floods, droughts, and heat waves, as well as the dangers posed to coral, birds, and other species of wildlife. The connection to global warming is circumstantial and conjec­tural at best, but the connection with local warming can be established with more certainty. Some phenomena can be modeled quite successfully; the most certain of these is sea-level rise.

Sea level has been rising at the rate of 3 mm (1/8 in.) per year, which would amount to an inch in eight years, or about foot a century even if the rate does not increase. Low-lying places like the Netherlands, Indonesia, and Bangladesh would be the first to feel the loss of hundreds of square miles of land area. There is some evidence that the rise seen in Fig. 1.13 has accelerated since the onset of industri­alization. Most of this can be attributed to global warming.

The three main causes are thermal expansion of water as it is heated, the melting of glaciers that have slid into the sea, and the melting of ice sheets on land. The contributions from each of these sources are shown in Fig. 1.14. The bottom part of each column is the sea-rise rate averaged over the past 42 years, while the recent average is given by the total height of the column. The rate-of-rise scale is in mil­limeters per year (mm/year). The four columns add up to the 3 mm (1/8 in.) figure quoted above. In each case, it is clear that the rate of rise has accelerated. The breakdown into the four effects required computer modeling, since the water from melting glaciers, for instance, cannot be measured directly. However, the sum of the calculated effects can be shown to agree quite closely with the sea-level rise actu­ally measured. This gives us confidence in the accuracy of modeling procedures.

Icebergs that are already floating will not change sea level as they melt because the part that is underwater (85-90% of the iceberg, depending on the temperature and salinity of the seawater) occupies exactly the volume that the iceberg will fill when it melts.5 Glaciers, ice caps, and ice sheets that are on land, however, are a different story. As land ice melts, it not only adds water to the oceans, but it also wets the ground under glaciers, making them slide into the ocean faster. Glaciers are melting at the rate of two cubic miles per week,6 and the shrinking of glaciers

Fig. 1.14 Contributions to sea-level rise by glaciers, thermal expansion, and ice sheets in Antarctica and Greenland. The lower part of each column is the 42-year average rate; the most recent 10-year average is the height of the entire column. Data from Intergovernmental Panel on Climate Change [6] over the past decade can be seen in many photographs. This is direct evidence of rising temperatures, but the unseen feedback effect is more treacherous. Ice has a high albedo, reflecting sunlight efficiently. As it melts, ground is uncovered, and this absorbs more sunlight, causing higher temperatures. As permafrost in Greenland is defrosted, exposed vegetation can rot, giving off CO2 and methane. Although the total forcing from albedo change is negative, as seen in Fig. 1.3, it is the local heat­ing where ice cover is disappearing that causes the runaway effect.

Permanent ice covers only 10% of land surface and 7% of oceans, which is why the catastrophic changes in glaciers that we can see is not the main cause of sea — level rise. As seen in Fig. 1.14, the main effect is simply the expansion of water when it is warmed. Not all consequences of ice melt are negative. Ice over the North Pole is definitely getting thinner, as directly measured by submarines there.7 The long sought-after Northwest Passage is becoming a reality. Trees growing on newly exposed ground can absorb CO2. The negative aspects, however, are dominant. If all the snow and ice on Greenland and Antarctica were to melt, the sea level would rise by 7 and 57 m, respectively [6]. This has happened in geologic eras, and the earth has undergone hot and cold periods before, even in human history; but what is new here is that it is happening extremely fast, before mankind can slowly adapt to the changes as it did previously.