Along with the atmosphere, terrestrial biosphere and the cryosphere, the oceans play a key role in the global climate system and its variability. The oceans store vast amounts of energy absorbed from the sun and this energy moves around the globe through its currents. Changes in circulation patterns affect the long-term redistribution of heat in the oceans which could have major effects on the world's climate.
Shorter-term fluctuations (years to decades) such as the El Niño-Southern Oscillation, the Pacific decadal oscillation, the North Atlantic oscillation, and the Arctic oscillation, affect the climate on short time scales.
Various international and national projects and programmes focus on observations of ocean characteristics such as sea surface temperature and absolute sea level, projecting their changes. The Global Ocean Observing System (GOOS) is a permanent global system for such observations, modelling and analysing marine and ocean variables to support operational ocean services worldwide. Projects such as the UK Permanent Service for Mean Sea Level (PSMSL) assess sea level changes on the basis of the global network of tide gauges. However, uneven geographic distribution of gauges, and movement of the land on which the gauges are located and changes in ocean circulation lead to uncertainties in the estimates.
Other national and international modelling groups have developed projections for sea temperature and sea level rise based on scenarios for thermal expansion, being the most important component. These include NASA in the USA, PIK in Germany and KNMI in the Netherlands. They also project changes in glaciers and ice caps.
Various organisations monitor other sea water characteristics, such as sea surface temperature, which is important for weather patterns and marine ecosystems (for example, see the Copernicus Marine Environment Monitoring service). Ocean acidification, or the ongoing decrease in ocean pH, can be a result of anthropogenic CO2 emissions additional to that of a direct change to the climate (for more information see the UK Ocean Acidification Research Programme or the European Project on OCean Acidification EPOCA).