The Science Behind Climate Change: Oceans

Sunshine bursts through the clouds as waves crash on an Atlantic coastline.
Dougal Waters/Photographer's Choice/Getty Images

The Intergovernmental Panel on Climate Change (IPCC) published its fifth Assessment Report in 2013-2014, synthesizing the latest science behind global climate change. Here are the highlights about our oceans.

The oceans play a unique role in regulating our climate, and this is due to water’s high specific heat capacity. This means that a lot of heat is needed to raise the temperature of a certain amount of water.

Conversely, this large amount of stored heat can slowly be released. In the context of oceans, this capacity to release vast amounts of heat moderates climates. Areas that should be colder because of their latitude remain warmer (for example, London or Vancouver), and areas that should be warmer remain cooler (for example, San Diego in summer). This high specific heat capacity, in conjunction with the ocean’s sheer mass, allows it to store more than 1000 times more energy than the atmosphere can for an equivalent increase in temperature. According to the IPCC:

  • The upper ocean (from the surface down to 2100 ft) has been warming since 1971. At the surface, sea water temperatures have risen by 0.25 degrees Celsius as a global average. This warming trend was geographically uneven, with areas of greater warming rates in the North Atlantic, for example.
  • This increase in ocean temperatures represents an enormous amount of energy. In the Earth’s energy budget, 93% of the observed increase is accounted for by warming ocean waters. The rest is manifested by warming in the continents and melting of ice.
  • There have been significant changes in how salty the ocean is. The Atlantic has become saltier due to more evaporation, and the Pacific has become fresher because of increased rainfall.
  • Surf’s up! There is enough evidence to state with medium confidence that waves have gotten larger in the North Atlantic, by as much as 20 cm (7.9 in) per decade since the 1950’s.
  • Between 1901 and 2010, global mean sea level has risen by 19 cm (7.5 in). The rate of increase has accelerated in the past couple of decades. Many continental land masses have been experiencing some rebound (an upward vertical motion), but not enough to explain this sea level rise. Most of the observed rise is due to the warming, and therefore expansion, of water.
  • Extreme high sea events produce coastal flooding and are usually the result of the coinciding effects of a large storm and high tide (for example, the 2012 landing of Hurricane Sandy on the New York and New Jersey coastline). During these rare events, water levels have been recorded higher than during extreme events in the past, and this increase is mostly due to the rising mean sea levels discussed above.
  • Oceans have been absorbing carbon dioxide from the atmosphere, increasing the concentrations of carbon from man-made sources. As a result, the pH of the oceans’ surface waters has decreased, a process called acidification. This has important implication for marine life, as the increased acidity interferes with shell formation for marine animals such as coral, plankton, and shellfish.
  • Since warmer water can hold less oxygen, the concentration of oxygen has decreased in many parts of the oceans. This has been most apparent along coastlines, where nutrient runoff into the ocean contributes also to lower oxygen levels.

    Since the previous report, vast amounts of new data were published and the IPCC was able to make many statements with more confidence:  it is at least very likely that the oceans have warmed, the sea levels have risen, contrasts in salinity have increased, and that the concentrations of carbon dioxide have increased and caused acidification. Much uncertainty remains about the effects of climate change on large circulation patterns and cycles, and still relatively little is known about changes in the deepest parts of the ocean.

    Find highlights from the report’s conclusions about:


    IPCC, Fifth Assessment Report. 2013. Observations: Oceans.