Ocean Heat Signs of Change

Storage and Movement  The ocean covers over 70% of Earth's surface and holds 97% of its water.  Its high heat capacity, fluid motion, and abundant life mean the ocean plays a key role in regulating climate.  Sunlight, wind, and convection – the buoyant rising of warm water followed by sinking as it cools -- mix heat through the ocean vertically.  The ocean's heat content influences sea surface temperatures and exerts a strong influence on climate.  Tracking the ocean is key to understanding climate change.

Storage and Movement (cont’d)  The oceans are mostly composed of warm salty water near the surface over cold, less salty water in the ocean depths. These two regions don't mix except in certain special areas.  The ocean currents, the movement of the ocean in the surface layer, are driven mostly by the wind.  In certain areas near the polar oceans, the colder surface water also gets saltier due to evaporation or sea ice formation.

Storage and Movement (cont’d)  In the polar regions, the surface water becomes dense enough to sink to the ocean depths.  The pumping of surface water into the deep ocean forces the deep water to move horizontally.  The deep ocean water must find an area on the world where it can rise back to the surface. This closes the current loop.  This process usually occurs in the equatorial ocean, mostly in the Pacific and Indian Oceans.  This very large, slow current is called the thermohaline circulation because it is caused by temperature and salinity (haline) variations.

Adding Heat Alters Ocean Dynamics  As heat-trapping gases accumulate in Earth's atmosphere, they trap more of the Sun's energy, the ocean absorbs more heat, and its temperature rises.  Certain gases in the atmosphere resemble glass in a greenhouse, allowing sunlight to pass into the "greenhouse," but blocking Earth's heat from escaping into space.

 The gases that contribute to the greenhouse effect include water vapor, carbon dioxide (CO2), methane, nitrous oxides, and chlorofluorocarbons (CFCs). Adding Heat Alters Ocean Dynamics (cont’d)

 Added heat changes ocean dynamics, as many currents are driven by subtle differences in temperature (and salinity). Currents influence climate and ecosystems.  In addition, water expands as it warms, so as ocean temperatures climb, sea levels also rise.  At high latitudes, warming waters have additional impacts: they melt sea ice and coastal ice shelves.

 This map shows sea surface temperature. Red and yellow depict warmer waters and blue shows colder waters. Adding Heat Alters Ocean Dynamics (cont’d)

 This map shows ocean salinity. The white regions have the highest salinity and the dark regions have the lowest. Adding Heat Alters Ocean Dynamics (cont’d)

 This is a map of ocean density. Light blue regions indicate the least dense waters; dark blue regions are the most dense. Adding Heat Alters Ocean Dynamics (cont’d)

 As water temperature warms, it also expands.  Ocean temperatures are rising, resulting in sea level rise.  At high latitudes, warming waters have additional impacts: they melt sea ice and coastal ice shelves. Adding Heat Alters Ocean Dynamics (cont’d)

Boosted Heat and Ocean Life  Rising water temperatures and shifting currents affect coastal and marine water quality, altering species distribution and biological productivity.  Seabirds like this Pigeon Guillemot, have been washing up dead on beaches in California and Oregon.  These dead birds are casualties of shifts in the California Current’s primary productivity.  Marine researchers believe this marine food shortage may be linked to climate change.

Boosted Heat and Ocean Life  Because heat enters the ocean at its surface, it tends to make a warmer surface layer that is less likely to mix with deeper, colder layers, reducing mixing in the ocean.

 Warmer seas will likely produce more coral bleaching, hypoxia, disease, harmful algal blooms, and invasive species. Some species or populations may shrink or disappear unless they can relocate. Boosted Heat and Ocean Life  Some species or populations may shrink or disappear unless they can relocate. In the image below, dead fish float alongside a seagrass mat. A toxic algal bloom is the suspected cause of the fish kill.

Behind the Numbers  Upper-ocean heat content has increased significantly since 1955.

 Over the latter half of the 20th century, scientists estimate that more than 90% of the energy gained by the biosphere as a result of human activities has accumulated in the ocean.  How much the ocean warms can vary greatly from year to year despite steadily increasing greenhouse gas concentrations over the past few decades.  Actual year-to-year changes are influenced by events like volcanic eruptions and recurrent ocean- atmosphere patterns like El Niño. Behind the Numbers