Ocean and Climate

Source: NASA Earth Observatory

For insights into what Earth might be like if there had never been an ocean, hop over to Venus where a runaway greenhouse effect has rendered the surface hot enough to melt lead. The most abundant gas in Venus’s atmosphere is carbon dioxide, a gas that is transparent to sunlight but absorbs heat radiating back towards space. In contrast, carbon dioxide makes up only 0.038 percent of Earth’s atmosphere. Carbon dioxide levels on Earth may be increasing due to the burning of fossil fuels and the felling of the tropical rainforests, but they will never approach those on Venus.

Phytoplankton, Source: NASA SeaWiFS

To understand why the two atmospheres differ so drastically, look to the ocean and the life it supports. Photosynthetic organisms, whether they are single-celled phytoplankton floating on or near the ocean surface or plants growing on land, take up carbon dioxide from the environment and use the sun’s energy to build carbohydrates. This process releases oxygen, which is the source of oxygen in the atmosphere. Other organisms eat these primary producers, and the carbohydrates are passed up through the food chain. Organisms use these carbohydrates both as fuel and as structural building blocks. Through cellular respiration, organisms break down carbohydrates for energy, a process that in most cases requires oxygen and releases carbon dioxide back into the environment. When organisms die, other organisms called decomposers break them down, again consuming oxygen and releasing carbon dioxide back into the environment.

Some parts of organisms resist decomposition. Many aquatic organisms, from plankton to corals to mollusks, build skeletons or shells made of calcium carbonate. Over millions of years, pressure and heat cement some of these calcium carbonate remains into rocks such as limestone, dolomite, and marble, locking up vast quantities of carbon. Because these remains don’t decompose, oxygen remains in the environment. Many prominent geological features are built from the remains of marine organisms. If carbon dioxide were suddenly released from these minerals, the atmosphere would more closely resemble Venus’s.

In other cases, especially in the ocean, sediment buries the remains of organisms. Deprived of oxygen needed for decomposition, these remains may transform into deposits of coal, oil, and natural gas, locking up more carbon. By burning these fossil fuels for energy, we are putting the carbon dioxide back into the environment.

People living near the ocean experience the ocean’s moderating influence on the climate because the ocean absorbs heat when the air is warm and releases heat when the air is cool. They may also experience some of the powerful storms the ocean can unleash. But the exchange of energy between the ocean and atmosphere is more than a regional phenomenon affecting local climates. In the tropics in particular where energy from the sun is at its greatest, the exchange of heat between ocean and atmosphere drives much of the global atmospheric circulation.

Anybody standing outside on a frigid winter day will find that even the bright sunshine can barely warm the air. The air is mostly transparent to the sun’s radiation. But if one were to put on a black winter jacket, the sun would heat the jacket’s surface.

Sea surface temperatures for spring 2004.
(Click image to enlarge) Credit: Norman Kuring, GSFC, NASA

Likewise, the sunshine heats up the surface of the land and ocean, although it heats the ocean more slowly than land. Most of the heating of the ocean takes place in the tropics. While ocean surface currents carry some of the heat north and south away from the tropics, the bulk of the energy is released back into the atmosphere. Some heat is released in the form of infrared radiation. Greenhouse gases, most notably water vapor from the ocean, but also carbon dioxide, trap this heat, warming the atmosphere.

The most important mechanism is latent heat release or evaporation. Over the ocean, latent heat is the engine that drives atmospheric circulation.

As the sun beats down and the ocean warms, water from the upper layer of the ocean evaporates. The conversion of liquid to vapor requires a lot of energy, so evaporation cools the top layer. (Think of how sweat evaporating from your skin cools your body.) Trade winds carry the vapor to the area where the north and south trade winds converge called the intertropical convergence zone (ITCZ). There the moist air rises and cools. The water vapor condenses on tiny particles suspended in the air called nuclei, forming clouds. This condensation releases energy, heating the surrounding air. The warmed air then rises higher, drawing up more moisture from the ocean. More vapor then condenses higher in the atmosphere and releases more heat, causing the air to rise further, and so on. The result is towering clouds that dump up to five meters of rain per year over some parts of the tropical ocean.

This same process fuels hurricanes. At the centers of these storms, moist air rising from the warm ocean heats up as the vapor condenses. Warm air is less dense, so the atmospheric pressure drops. More moist air then rushes in off the ocean due to the pressure gradient, rotating counterclockwise due to the Coriolis effect. This air rises up, and condensation releases more heat, intensifying the storm and further lowering the pressure. (Hurricanes generally don’t form close to equator because the Coriolis effect is weak.)

The tremendous amounts of energy released through condensation near the equator drives much of the atmospheric circulation that redistributes heat and moisture throughout the world. The air that has risen aloft, which by now has cooled and lost most of its moisture, expands towards the poles. At between 30 and 40 degrees latitude, this air sinks. These latitudes are known as the horse latitudes, and it’s where the world’s great deserts exist. The airflow then splits. Some air continues towards the poles, creating the westerly winds in the mid latitudes. The rest returns towards the equator. The combination of the Coriolis effect and pressure differences due to the rising air at the ITCZ where the rain occurs (low pressure) and sinking air at between 30 and 40 degrees (high pressure) steers the winds to the west, creating the trade winds. As the trade winds sweep over the ocean, they accelerate evaporation, perpetuating the cycle. This circulation of the atmosphere driven by the evaporation and condensation of water close to the equator is called the Hadley circulation.

The winds this process generates also drive surface currents and the overturning circulation. These currents carry smaller amounts of the heat the tropical ocean absorbs towards the poles, although the ocean gives up most of this heat to the atmosphere in the lower latitudes. The winds also drive mixing and upwelling in the ocean. These processes lift nutrients from the ocean bottom to the surface. Eddies that spin off of currents also transport nutrients and bring more nutrients to the surface. These nutrients are what sustain biological productivity in the ocean.

Related Links

Lesson 2: Coriolis Force (PDF)

Scientists have concluded that increasing concentrations of carbon dioxide in the atmosphere due to the burning of fossil fuels and the felling of tropical rainforests is slowly warming the climate. The ocean serves as an important buffer to climate change.

As was discussed above, photosynthetic marine organisms remove carbon dioxide from the environment to build carbohydrates. The formation of minerals such as limestone and of fossil fuels from the remains of these organisms permanently removes carbon dioxide from the environment.

Ocean water also holds tremendous quantities of carbon dioxide, 40 times more than the atmosphere. It absorbs almost half of the carbon dioxide released from the burning of fossil fuels. Carbon dioxide dissolves in the frigid waters near the Arctic and Antarctic. In the winter, this cold water sinks as part of the ocean’s overturning circulation and carries the carbon dioxide into the deep ocean. After hundreds of years, mixing from winds and tides pulls this water back to the surface. As the water warms, it releases carbon dioxide back into the atmosphere.

Ocean water also absorbs tremendous quantities of heat. As the atmosphere warms due to the buildup of greenhouse gases, it transfers some of this heat to the ocean, slowing the pace of climate change.

The tropical ocean is the source of most of the rainfall throughout the world. Some of the most consequential rainfalls are generated by the seasonal monsoons, especially over Asia. In the summer, the centers of continents heat up, drawing moist air from the cooler ocean. The heavy monsoon rains over much of Asia not only provide these countries with critical moisture, they release tremendous amounts of latent heat which helps drive atmospheric circulation. A similar process fuels the North American monsoons, which provide important summer rainfall to the southwestern United States and northwestern Mexico.

The ocean, atmosphere, and land interact in complex ways, producing a climate in which life thrives. Even seemingly small changes in one area can have a ripple effect, sparking changes in other areas. For example, changes in the distribution of warm water in the ocean, such as occurs in the tropical Pacific during an El Niño event, alter evaporation and cloud formation patterns. These changes in turn affect rainfall and wind patterns. Changes in wind patterns may affect ocean surface currents and upwelling, which may impact the availability of nutrients on which marine ecosystems depend. Understanding these connections is essential as we grapple with the implications of climate change and our actions that may contribute to it.