Tsunami

Tsunami, Japanese word meaning “harbor wave,” used as the scientific term for a class of abnormal sea wave that can cause catastrophic damage when it hits a coastline. Tsunamis can be generated by an undersea earthquake, an undersea landslide, the eruption of an undersea volcano, or by the force of an asteroid crashing into the ocean. The most frequent cause of tsunamis is an undersea earthquake. When the ocean floor is uplifted or offset during an earthquake, a set of waves is created similar to the concentric waves generated by an object dropped into the water. Most tsunamis originate along the Ring of Fire, a zone of volcanoes and seismic activity, 32,500 km (24,000 mi) long, which encircles the Pacific Ocean. Since 1819, about 40 tsunamis have struck the Hawaiian Islands.

A tsunami can have wavelengths, or widths (the distance between one wave crest to the next), of 100 to 200 km (60 to 120 mi), and may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/h (about 450 to 500 mph). A tsunami is not one wave but a series of waves. In the deep ocean, the waves may be only about half a meter (a foot or two) high. People onboard a ship passing over it would not even notice the tsunami. Upon entering shallow coastal waters, however, the waves may suddenly grow rapidly in height. When the waves reach the shore, they may be 15 m (50 ft) high or more. Tsunamis can also take the form of a very fast tide or bore, depending on the shape of the sea floor.

Tsunamis have tremendous force because of the great volume of water affected and the speed at which they travel. Just a cubic yard of water, for example, weighs about one ton. Although the tsunami slows to a speed of about 48 km/h (30 mph) as it approaches a coastline, it has a destructive force equal to millions of tons. Tsunamis are capable of obliterating coastal settlements.

Tsunamis should not be confused with storm surges, which are domes of water that rise underneath hurricanes or cyclones and cause extensive coastal flooding when the storms reach land. Storm surges are particularly devastating if they occur at high tide. A cyclone and accompanying storm surge killed an estimated 500,000 people in Bangladesh in 1970. Tsunamis are sometimes called tidal waves, but they have nothing to do with the gravitational forces that cause tides.

Global Warming

Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last century.[1][A] The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century.[1] The IPCC also concludes that natural phenomena such as solar variation and volcanoes produced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward.[2][3] These basic conclusions have been endorsed by more than 45 scientific societies and academies of science,[B] including all of the national academies of science of the major industrialized countries.[4]
Climate model projections summarized in the latest IPCC report indicate that the global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century.[1] The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.[5][6]
An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[7] The continuing retreat of glaciers, permafrost and sea ice is expected, with warming being strongest in the Arctic. Other likely effects include increases in the intensity of extreme weather events, species extinctions, and changes in agricultural yields.
Political and public debate continues regarding climate change, and what actions (if any) to take in response. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions.