Large-scale horizontal water motion within an ocean. The way energy from the sun, stored in the sea, is transported around the world. The currents explain, for example, why the UK has ice-free ports in winter, while St. Petersburg, at the same latitude as the Shetland Islands, needs ice breakers. Evidence is growing that the world's ocean circulation was very different during the last ice age and has changed several times in the distant past, with dramatic effects on climate. The oceans are vital as storehouses, as they absorb more than half the sun's heat reaching the earth. This heat, which is primarily absorbed near the equator is carried around the world and released elsewhere, creating currents which last up to 1.000 years. As the Earth rotates and the wind acts upon the surface, currents carry warm tropical water to the cooler parts of the world. The strength and direction of the currents are affected by landmasses, bottlenecks through narrow straits, and even the shape of the sea-bed. When the warm water reaches polar regions its heat evaporates into the atmosphere, reducing its temperature and increasing its density. When sea-water freezes it leaves salt behind in the unfrozen water and this cold water sinks into the ocean and begins to flow back to the tropics. Eventually it is heated and begins the cycle all over again.