Introduction: Background: Oceans and Water

Background: Oceans and Water

What We Know: Underlying Processes

The oceans play a key role in regulating the Earth’s climate—and yet they remain mysterious, because so many of the basic processes underlying ocean dynamics are still poorly understood. Their fundamental role in climate is based largely on their storage and transport of heat around the globe. The oceans store vast amounts of heat, much more than the heat stored by the atmosphere, because water is 1,000 times more dense and has a heat-holding capacity four times that of air. Ocean currents are primary highways for the transport of heat around the globe.

Differences in water density drive ocean currents. The density of seawater is controlled by the water’s temperature and salinity, a combination summed up in the term thermohaline (thermo refers to temperature, haline to salinity).

The Gulf Stream, for example, is a wind-driven surface ocean current originating in the Gulf of Mexico and terminating in northwest Europe. When water from this warm current evaporates, it warms the air—which is why northwestern Europe enjoys a milder climate than Canada at the same latitude.

The driving force behind the Gulf Stream and other ocean currents is simple physics: In the waters west of Europe, the evaporation of water makes the sea saltier and colder, which makes the water more dense.

The denser water sinks and warmer surface water streams in to replace it, providing the current’s sustaining pull. Since this simple physical process is based on the interaction of warmer and colder masses of water, global climate change could seriously disrupt that interaction.

The warm ocean current known as the Gulf Stream, shown in this satellite image of sea surface temperatures, transports heat from the Gulf of Mexico to northwestern Europe. In this image, yellow indicates a temperature of 23ºC (73ºF), green is about 14ºC (57ºF), and blue is about 5ºC (41ºF).

Evidences and Uncertainties

Changes in water temperature and/or salinity could disrupt ocean currents, slowing or even shutting them down. A shutdown of the Gulf Stream would drastically change the climate of the countries ringing the North Atlantic Ocean, leading to significant cooling in these areas. Some fossil evidence suggests that past shutdowns in the Gulf Stream were associated with previous ice ages. For this reason, some researchers argue that a small increase in global temperatures could actually initiate a sudden cooling trend in parts of the world. Another possible effect of continued warming on the oceans is a significant rise in sea levels. Some of this would result from the melting of polar ice, and some would be caused by thermal expansion. (Water, like all things, expands as its temperature rises.)

Evidence suggests that sea levels have risen by 17 cm (about the width of your spread hand) over the past 100 years. Continued rises in sea levels could have devastating effects on islands whose entire living area lies only a few meters or less above sea level. The Indian Ocean’s Maldive Islands, for example, have a mean height of 1 meter above sea level; even small increases in sea levels could completely devastate tiny nations like this. But forecasting the amount of sea level rise must allow for a wide range of uncertainty: The 2014 report by the Intergovernmental Panel on Climate Change (IPCC) estimates a possible range of between 0.44 and 0.74 meters  (1.4 and 2.4 feet) of increase over the 21st century, depending largely on future carbon emissions.