Division of Energy
Global Climate Change: Oceans' Effect on Climate
Oceans and clouds are the two largest and least understood components of the climate system. What effect do oceans have on climate? What effect will oceans have on climate change?
Energy may be transferred from one object to another without the space between them being heated. This is the mechanism where earth receives energy from the sun as radiation. Radiation travels in the form of waves that release energy when they are absorbed by an object. Waves of radiation can be distinguished by wavelength, the distance between two successive crests of the wave.
The solar radiation reaching earth is in a three wavelength spectra. The sun emits 44 percent of its radiation in the visible spectrum, seven percent in the ultraviolet spectrum (short wave), and 49 percent in the infrared spectrum (long wave).
The Balancing Act
The Earth and all things continually radiate energy. If an object radiates more energy than it absorbs, it gets colder; if it absorbs more energy than it emits, it gets warmer. When an object emits and absorbs energy at equal rates, its temperature remains unchanged. Absorption and emission are in balance.
The region near the equator (low latitudes) gains more solar radiation than is lost. The regions near the poles (high latitudes) lose more energy than they receive. To maintain an energy balance, the atmosphere and the oceans transfer the excess energy from the equator to the poles. The oceans are responsible for transferring about 30 percent of this excess energy. Any alteration of the energy-transfer system may change the climate.
Energy from the sun evaporates large quantities of water from the oceans. This water vapor is then transported by winds to other regions where it condenses into clouds and precipitation.
Precipitation runoff from land surfaces eventually returns to the oceans. This cycle is called the hydrologic cycle. Of the total water vapor content of the atmosphere, 85 percent evaporated from the oceans.
Water has a high capacity for storing energy. It requires a relatively large amount of heat energy to bring about a small temperature change in water. Thus water has a high specific heat. The same can be said about the reverse - water cools slowly as well. The percentage of radiation reflected from a surface is called the albedo. Water surfaces reflect only a small amount of solar energy. An object that reflects very little sunlight absorbs a great deal. Water has a low albedo but high absorption. Because of these unique properties and because oceans cover about 70 percent of the earth, water has a strong modifying effect on weather and climate.
Enhancement of the Greenhouse Effect
Some computer models predict that doubling atmospheric carbon dioxide, a greenhouse gas, would cause rising ocean temperatures, which will cause an increase in evaporation. The added water vapor, also a greenhouse gas, will enhance the greenhouse effect, which would further increase global warming. This is a positive feedback mechanism. However, there is a negative feedback theory. The increased evaporation would produce more clouds, which reflect and radiate away more energy than they retain. Thus increased clouds could have a cooling effect.
Oceans and Climate Change
There are four factors that control temperature at any given location on earth: 1) latitude, 2) elevation, 3) ratio of land and water, and 4) ocean currents.
The heat equator, the region of highest mean annual temperature, is about 10 degrees north latitude. Thus, the Northern Hemisphere is warmer than the Southern Hemisphere. This is a result of the Antarctic having a higher albedo than the Arctic. The Northern Hemisphere has a greater fraction of land at tropical latitudes, and ocean currents transport more warm water to the Northern Hemisphere. There is an exchange of heat and moisture between the ocean surface and the atmosphere. Because of the difference in heat capacity between water and air, even a small change in surface ocean temperatures could modify atmospheric circulation, which could have far reaching effects on global weather patterns.
Most interactions between the oceans and the atmosphere are not understood. It is known, however, that ocean currents are caused by the wind. Winds blow over the ocean, causing surface water to drift along with them. Moving water piles up, creating pressure gradients within the water. These pressure gradients produce currents that transfer heat from the equator to the poles. As surface water drifts away with the wind, cold nutrient rich water from below rises in a process called upwelling. This also is an example of heat transfer.
The oceans play a major role in the climate system, yet the exact effect they will have on rising levels of carbon dioxide and global warming is uncertain. The oceans are huge storehouses of carbon dioxide. Microscopic plant (phytoplankton) extract carbon dioxide, a greenhouse gas, from the atmosphere during photosynthesis. Warmer ocean temperatures could produce increased numbers of these plants, which could reduce carbon dioxide in the atmosphere. This is a negative feedback mechanism.
On the other hand, warmer ocean temperatures could increase the carbon dioxide in the air due to the fact that warmer water cannot dissolve as much carbon dioxide as colder water. This is a positive feedback mechanism.
Ocean-atmosphere interactions are being investigated. Current studies are trying to determine the correlation of surface-water temperatures in the Pacific Ocean with weather in the United States. A series of barometric pressure changes alter the directions in which the trade winds blow. The warmer surface-water temperature (El Nino) and the colder surface water temperature (La Nina) are indicators of this wind shift called the Southern Oscillation. The oceans have a huge capacity for storing heat energy. As they slowly warm, they should retard the rate at which the atmosphere warms. The response of ocean temperatures, ocean circulation, and sea ice to global warming will probably determine the global pattern and speed of climate change.
The ocean-atmosphere system is a delicate balance between incoming and outgoing energy. If this balance is upset, even slightly, global climate can undergo a series of complicated changes.