Oceans have a major influence on the earthâ€™s climate. They cover 70% of the earthâ€™s surface and store more energy from the sun than the atmosphere.
Oceans affect the climate in the following ways.
- The currents in the oceans flow near the surface and also deep below, thus transferring heat all over the earth.
- Some currents are warm and some are cold. In the past, these currents have been known to change directions, slow down, reverse, or even stop.
Example: The temperature in Europeâ€™s northern coastal region is higher than normal due to the Gulf Stream, a warm current that flows here.
The North Atlantic is warmer than the North Pacific. The increased evaporation in the former, therefore, serves to increase salinity, relative to the North Pacific. This salinity difference in the ocean is thought to drive the global thermo-haline ocean circulation.
Thermo-haline Circulationâ€”Purple arrows indicate cold,
deep ocean currents. Red arrows show shallow, warm
water circulation pattern.
It is so named because it involves both heat (hence thermo) and salt (hence haline). It is also known as the oceanâ€™s â€˜conveyor beltâ€™, which links major surface and deepwater currents in the Atlantic, Pacific, Indian, and Southern Oceans.
Two attributes, temperature and salinity, determine the density of seawater, and the difference in density between the water masses in the world's oceans causes the water to flow. Thus, the global ocean circulation is driven by global density gradients, which are created by freshwater flux and by surface heat. The image given alongside provides a description of the circulation.
Note: Surface ocean currents are primarily driven by winds. Deep ocean currents, on the other hand, are mainly a result of density differences.
Considerable evaporation of moisture takes place from the warm surface ocean currents as they travel towards the high latitudes. The salt which remains behind in the water after evaporation makes the water heavier or denser. As in the atmosphere, the surface and deep-water currents of the world's oceans are inter-linked forming the global ocean circulation. Scientists have proposed that changes in this global ocean circulation influence climatic changes over hundreds and thousands of years.
The following diagram shows the pathways of ocean circulation.
Pathways of ocean circulation
You have learnt about the natural drivers for climate change, which include solar activity, radiative forcing, continental drift, variations in the earthâ€™s orbit, and oceans. Let us now proceed to learn about the changes caused by human activities.