The oceans are as important in the Earth's climatic systems as the atmosphere, because the oceans absorb atmospheric heat and carbon dioxide. Major uncertainties remain about how much heat is taken down to deeper water by currents (tending to delay overall warming), and how much less effective the oceans will be at acting as a 'sink' for carbon dioxide (tending to enhance the greenhouse effect).

Worrying indicators:

August 1990:

• British scientists warn that changes in ocean circulation could cause "enormous and rapid" changes in the world's climate. Oxford University's Dr Peter Killworth suggests there is evidence from ice core data that oceans can "flip" to completely different states within 50 years or less. During this period, the oceans lose heat at a dramatic rate. An enhanced greenhouse effect could trigger such a switch, a prospect Killworth describes as "terrifying".

October 1990:

• French scientists report that water deep in the Mediterranean is 0.12 degrees Celsius warmer than in 1959 leading "to the possibility that the deep water temperature trend may be the result of greenhouse gas-induced local warming" (J.P. Bethoux and colleagues, Paris University, October 1990).

July 1992:

• Ocean temperatures off the coast of California show significant warming in recent years. Temperatures in the upper 100 metres have risen by 0.8 degrees Celsius in the last 42 years.

September 1992:
• Sea surface temperatures across a large area of the western Pacific, south of Japan, have increased by about 0.7 degrees Celsius in recent years. In reporting the increase, the Japanese Maritime Safety Agency says: "Global warming phenomena have been proved by the change in water temperature."