Oceanic Changes

Oceans are connected bodies of saline water that cover about three-quarters of the Earth and comprise more than 97 percent of the world’s total water resources. Although continuous, the global ocean is divided into five oceans based on geographic location, geological barriers, and other criteria, into the Arctic, Atlantic, Indian, Pacific, and Southern oceans. Oceans have great influence on global weather patterns and climate.

This influence stems from variations in ocean temperature caused chiefly by the movement of radiant solar heat absorbed by the ocean’s surface. Recently, atmospheric changes and resultant changes in chemical and physical attributes of the oceans, including temperature, salinity, and acidity, have threatened the global ocean’s stability.

For example, sea level has been rising because of thermal expansion and melting of the world’s ice. This retreat of ice causes further absorption of radiant energy from the sun into the darker colored sea that might otherwise have been reflected by ice. This then creates a positive feedback in the warming of the atmosphere and further melting of ice. A warming atmosphere has been linked to changes in rainfall and storm frequency and length. Changes like these have the potential to cause the ocean to change dramatically over a relatively short time.

Carbon Cycling

The past two centuries have witnessed a significant rise in the amount of global carbon emissions and subsequent absorption by oceans around the Earth. The global ocean plays a vital role in the Earth’s carbon cycle, about 50 times greater than that of the atmosphere. The oceans absorb about half of carbon deposits from fossil fuel burning sources from the air. The ocean contains the largest pool of carbon on the Earth’s surface; however, these carbon concentrations are not distributed equally. The surface of the ocean exchanges carbon with the atmosphere more rapidly because of mixing by winds at the surface, and because of the temperature, chemistry, and pressure of water; all of these vary by depth, affecting the solubility of carbon dioxide and carbon-containing ions. In areas of upwelling (where colder water moves to the surface), carbon is released into the atmosphere, whereas in areas of downwelling (where water piles and sinks), carbon is absorbed from the atmosphere. Oceanic upwelling and downwelling illustrate that changes in distribution of water in one area are always accompanied by compensating water changes in another area.

This process is called mass continuity. The speed at which the ocean can exchange carbon with the Earth’s atmosphere is important in regulating the pH level (or acidity) of the ocean and its nutrient and chemical stability. Once the ocean absorbs carbon dioxide, it combines with water to form carbonic acid and a series of acidbase products. Prior to the Industrial Revolution, global carbon emissions and uptake were in relative balance, which made for a healthy chemical and nutrient stasis. Currently, however, ocean carbon absorption is at a higher rate than its atmospheric exchange capabilities. Although this mediates the amount of carbon dioxide in the atmosphere, the imbalance has increased production of carbonic acid, which has resulted in a gradual decrease in pH, or the acidification, of ocean water.

Radiant Energy and Ocean Current

Radiant energy from the sun is not uniformly distributed across the globe because the Earth is spherical. Specifically, more energy per a given surface area arrives at the tropics, where the sun is more directly overhead, than arrives at the poles where sunlight strikes the Earth at a greater angle.

This has a number of effects. Evaporation is higher in these tropical waters, thereby affecting the concentration of ions in the water. This concentration of ions, in turn, increases tropical waters’ ability to dissolve carbon dioxide. Heating and evaporation at the surface also creates chemical and physical differences in adjacent water masses by latitude and depth. These differences, along with prevailing winds caused by the Earth’s rotation, are the primary drivers behind movements of water masses in the oceans.

Currents are the steady flow of ocean water in a prevailing direction. Ocean currents have been identified as the single most influential mechanism in the global climate system via their role in transferring heat to and from geographic regions. Long-term patterns in currents are dependent upon maintenance of chemical and temperature gradients. Changes in the Earth’s atmosphere can alter these gradients.

For example, warmer air above the oceans affects evaporation, and thus the chemical content and temperature of water. A warming atmosphere can also alter the temperature and chemistry of water by diluting saline, or salty ocean with cold, freshwater from melting ice.

Water Density

The majority of the movement and circulation of water and energy throughout the ocean is driven by differences in density between adjacent water masses. Density is dependent on the salinity and temperature of the water. The thermohaline circulation, or Global Conveyor Belt, is initiated by density differences and distributes heat energy between tropical and polar regions. Rainfall is higher in the tropics, and this addition of freshwater into the ocean decreases the salinity, and therefore the density, of the ocean waters in these areas. The colder oceans receive less rainfall because of lower evaporation, and thus are saltier and denser. The higher density of colder portions of the oceans relative to the tropics creates a difference in sea height, with colder oceans depressed, and warmer oceans elevated, relative to each other.

The density of near-surface water in the tropics is further decreased by solar heating; however, its salinity is increased by evaporation. This upper, less dense, but more saline tropical water thus moves in a poleward direction over the colder water. As it moves north, it releases heat to the atmosphere and the lower ocean and is thereby cooled. As it cools, its high salt content makes it denser relative to the water it is flowing over, and therefore sinks. This overturning circulation varies from year to year and on longer timescales.

Changes in precipitation, runoff, ice melt, solar heating, and winds can strengthen or weaken the conveyor belt. This can affect short-term weather conditions, but can also alter climate in the long term if the circulation pattern settles into a new equilibrium.

Ocean Ecology

Oceans are home to the majority of plant and animal life on the planet. Climate change is expected to alter marine ecology both directly through lower pH and elevated temperatures on the organismal level, and indirectly via changes in community dynamics and food and habitat alteration on the aggregate level. A major concern is the possible effect that climate change may have on ocean productivity. Phytoplankton, small plants found near the ocean’s surface that comprise the basis for the marine food web, are the foundation for almost all ocean life and play a key role in regulating global carbon levels. It is unknown how these organisms will react to changes in ocean acidity, increased nutrient mixing, and changes in temperature. However, any dramatic short-term alterations in foundational food web interactions are expected to have cascading effects through the food chain.

Species migration and shifts or expansion in species range are also expected as global ocean temperature continues to rise. Scientists expect that some organisms will thrive, as others will suffer. This is expected to upset ecological equilibrium in certain areas and is likely to affect commercial and recreational fisheries and tourism in susceptible areas. Warm water species are expected to move toward the poles. This trend has already been witnessed over the last decade in certain fisheries closer to the equator.