The polar regions vary from -1.9-5.0 degrees (centigrade) in the northern hemisphere and -1.9-2.0 degrees in the southern hemisphere. Cold temperate zones vary from 5-10 degrees in the northern hemisphere and 2-10 degrees in the southern hemisphere. Temperatures range from 10-20 degrees in the warm temperate oceans and 20 degrees plus in tropical waters.

Sea temperature is determined by solar radiation. Day length, cloud cover, latitude and time of year affects how much sun the water gets. Water temperature is highest at the equator, where it may reach 40 degrees in shallow coral reef lagoons. It is lowest in the polar regions where it can be as cold as -1.9 degrees. 98 per cent of the sun’s radiation doesn’t go further than the first metre of water.

Warm-blooded (homeothermic) animals can regulate their body temperatures and can therefore withstand moderate fluctuations in water temperature. They are able to migrate across the oceans, encountering different water temperatures. Cold-blooded (poikilothermic) animals are unable to regulate their temperatures and are more severely affected by external changes. Water takes a long time to heat up and cool down, giving animals a chance to adapt to the change. The average daily fluctuation of ocean surface waters is less than 0.3 degrees.