Role of the oceans in global climate

Abstract

The oceans play a major role in determining the Earth's climate and its natural variations, and also the magnitude and timing of changes that may result from human activities such as the emission of carbon dioxide and other ‘greenhouse’ gases. They do so by absorbing much of the solar radiation and of the downward infrared radiation emitted by the greenhouse gases in the atmosphere, by distributing this heat by ocean currents and eddies induced by wind stress, and by density gradients set up by variations in temperature and salinity. Descriptions of the large-scale heat transfer within the oceans and the exchanges of heat, moisture, momentum and fresh water between the atmosphere and the oceans are based partly on observation, partly on theory and partly on model stimulations. The oceans will moderate any climatic effects resulting from human emissions of carbon dioxide because they absorb about half of this carbon dioxide and also delay any additional warming because of their great thermal inertia. This is demonstrated by the results from climate models in which the global atmosphere is coupled to the deep global oceans and the atmospheric carbon dioxide is allowed to increase at 1% p.a. compound to double after 70 years. The resulting rise in globally averaged temperature predicted by three different models ranges from 2·3 to 1·3 K compared with predictions of 4–5 K from models having only a shallow ocean that does not interact fully with the atmosphere. Reliable long-term predictions of climate change will not be possible unless and until there is an adequate supply of oceanic observations to feed and validate the models. Measurements of ocean surface temperatures, winds, waves and currents are now being made by ingenious satellite-borne instruments but observations from the ocean interior are very sparse and irregular. The latter cannot be obtained in sufficient quantity by ships but could, in principle, be provided by small unmanned, remotely controlled submarines located and interrogated by satellite. An international global ocean-observing system, estimated to cost about US$2 × 10⁹ year⁻¹, is now under discussion, but the difficult question of funding has yet to be addressed.