Oceans, Water and Climate Patterns

Because water has a high specific heat capacity and the oceans are enormous and deep, they can soak up vast amounts of energy while warming only a little. About 90% of the extra heat trapped by the enhanced greenhouse effect over the past several decades has gone into the oceans; only a few percent has warmed the air. That makes the oceans a heat sink that slows atmospheric warming, but the stored heat does not disappear. Warmer water expands (raising sea level), fuels stronger storms, melts ice shelves from below and causes marine heat waves that bleach corals and stress fish. Large bodies of water warm slowly in spring and summer and cool slowly in fall and winter, so nearby air stays cooler in summer and milder in winter. This is the moderating effect of water. It explains why coastal climates are called maritime and inland ones continental. The Great Lakes give southern Ontario a milder climate than it would otherwise have. In the Niagara fruit belt, Lake Ontario keeps spring air cool so fruit trees blossom after the danger of frost has passed, then keeps fall air warm so the harvest season is long enough for peaches, cherries and grapes.

Surface currents are driven mainly by the prevailing winds and turned by Earth's rotation into great loops. Deep currents are driven by differences in density, which depends on temperature and salt content: this is thermohaline circulation (thermo for heat, haline for salt). In the North Atlantic, cold, salty water becomes dense enough to sink, flows south along the ocean floor, and is replaced by warm surface water flowing north. Linked together, these flows form a global ocean conveyor belt that takes roughly a thousand years to complete a loop.

Currents affect precipitation as well as temperature. Air over warm water picks up lots of water vapour, so coasts beside warm currents tend to be humid and rainy. Air over cold water is cooled from below and holds little moisture, so some coasts beside cold currents are among the driest places on Earth, like the Atacama Desert in Chile. Changes in currents shift weather far away: during El Niño, unusually warm water spreads across the eastern tropical Pacific, and much of Canada tends to have a milder winter. Scientists are also watching the Atlantic overturning circulation: the IPCC projects that it will very likely weaken this century as melting ice adds fresh water and the surface warms, which would change climates around the North Atlantic.

• Liquid water stores and moves heat: oceans and lakes are heat sinks that moderate nearby climates, and currents carry heat toward the poles.
• Water vapour is a greenhouse gas, and evaporation and condensation move huge amounts of hidden (latent) heat; warmer air holds more vapour, which amplifies warming.
• Clouds (tiny liquid droplets or ice crystals) both cool Earth by reflecting sunlight and warm it by absorbing infrared; low, thick clouds mostly cool, high, thin clouds mostly warm.
• Ice and snow have a high albedo, reflecting much of the sunlight that reaches them. Ice sheets on land also store water: if they melt, sea level rises. Melting floating sea ice barely changes sea level, because it already displaces its own weight of water.

A feedback loop happens when the result of a change affects the cause of that change. In a positive feedback, the result strengthens the original change (positive means amplifying, not good). In a negative feedback, the result weakens the original change and helps stabilize the system. In the Arctic several positive feedbacks combine, especially the loss of reflective sea ice and snow. This Arctic amplification is a major reason northern Canada is warming about three times as fast as the global average.