The world ocean has warmed. Global sea surface temperatures for the last 100 years show two distinct warming periods: one from 1920 to 1940 followed by a period of cooling, and a second period of warming from 1970 to the present. On average, temperatures of both surface and deep ocean water have increased during the last thirty years by 0.3 degrees Celsius. The ocean drives the Earth's climate system. The movement of warm water from the tropics towards the poles causes the ocean currents and weather patterns we are familiar with. An understanding of how the ocean stores and transports heat is necessary to understand, model, and predict how the Earth climate system will behave.

Newly available information reported in Science magazine indicates the world ocean system has warmed substantially in the last fifty years . How much of the observed heat increase is due to natural variability and how much due to our pumping greenhouse gases into the atmosphere is not known. The observed warming of the ocean is consistent with the idea that the warming of the entire Earth climate system, including the ocean, the atmosphere, and the cryosphere is due not to natural variability but more likely caused by fossil fuel-generated greenhouse gases entering the atmo- sphere. We spoke with Sydney Levitus about his work in documenting the increase in the oceans' temperature.

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