For example, melting of land-based polar ice caps, combined with thermal expansion of the world's oceans, could raise sea levels and flood many of the world's port cities. In addition, adjusting to rising sea levels could be difficult because the change could occur abruptly. Initially, warming may cause a gradual melting of ice, but if large chunks of land-based ice fall into the ocean, they will melt more rapidly. Apart from the impact this would have on sea levels, the resulting influx of fresh water into the oceans could affect the circulation of ocean currents, producing further changes in climates. There is geological evidence that suggests the world's climate can switch rapidly from one stable state to another.
Damage is likely to be greater when changes occur abruptly.
The amount of CO2 that must accumulate before a catastrophic event would occur, however, is unknown. The timing and severity of any potential damage, therefore,are also difficult to predict. Despite the uncertainties surrounding the causes and ramifications of global warming, governments are being urged to act. An international agreement, known as the Kyoto Protocol and as of yet to be ratified by any of its signatories, calling for the reduction of greenhouse gas emissions was signed in 1997. Carbon taxes or direct controls could be used to achieve these targets, but they are likely to be very costly.