» Ocean Circulation
The Role Of Oceans In Climate Change
Wed, 24 Jul 2019 | Ocean Circulation
The oceans and atmosphere are closely linked to each other and form the most dynamic part of the climate system. In the atmosphere, external forcings such as variations in the Sun's energy and concentration of greenhouse gases directly affect the circulation patterns and temperature of the ocean-atmosphere system. There is also an internal relationship. Because both the ocean and atmosphere are constantly in motion, they generate their own internal fluctuations. Short-term fluctuations in wind and temperature patterns directly influence the ocean's surface waters and are what cause local storm fronts. Fluctuations in the ocean have the ability to magnify, modify, or minimize atmospheric fluctuations. A small change in just one property of the ocean's characteristics (transportation, temperature, upwelling, currents) can result in major climate changes over large regions of Earth's surface.
According to Raymond Schmitt, a senior scientist in the Department of Physical Oceanography at the Woods Hole Oceanographic Institution, until the scientific world began to research and really understand the complexities of oceanography, the ocean was not given much weight in the study and prediction of climate. Initial models simply treated the oceans as a "shallow swamp," a source of moisture lacking any serious significance.
After years of extensive research, the oceans were viewed in their proper perspective and finally considered an equal partner to the atmosphere, with the realization that they both work in tandem to create the present climate by transporting heat from the equator to the polar regions. Oceans also have a much higher heat capacity than the atmosphere—roughly 1,100 times more.
The World Ocean Circulation Experiment (WOCE), a project supported by the NSF, has also provided overwhelming evidence that the oceans play a critical role in the climate. Based on the results of this experiment, it has been determined that the deep regions of the oceans have warmed significantly since the 1950s. In fact, roughly half of the increase in greenhouse warming predicted in models that has not been observed in the atmosphere has been absorbed by the world's oceans and held in this immense reservoir. Prior to this, it had not been considered that the oceans could store so much of the greenhouse gases on such short timescales.
Oceans have such a high heat-holding capacity that, according to Schmitt, it would take 240 years for the continual deposition of greenhouse gases to raise the ocean 1.7°F (1°C). It is because of this that
Schmitt stresses that the oceans be recognized for the critical role they play in global warming. Their involvement goes well beyond just delaying the process by sequestering CO2. Research done at the Woods Hole Oceanographic Institution has also made clear how the slow movement of cold and warm regions of ocean water can play a key role in weather on a cyclic basis for months at a time. A prime example of this is El Niño.
Because oceans are dynamic, they interact with the atmosphere in two distinct ways—physically and chemically. The physical component occurs through the exchange of heat, water, and momentum. Because the oceans cover more than 70 percent of Earth's surface, their effect is significant; they act as the storage bins of huge amounts of energy—heat energy. Unlike land, which heats up and cools rapidly, water changes temperature much more slowly. It takes much longer for a body of water to heat, but once it is heated, it will stay heated for a much longer time than land will, even if the source of heat is removed. This is called a large temperature inertia. Because of this inertia, oceans play an overwhelming part in the balance of Earth's heat energy. Earth's oceans are its global heat engine.
In this dynamic ocean-atmosphere system, Earth's oceans heat up over time and store heat, which eventually escapes into the atmosphere, warming it. As the air above the ocean warms in relation to adjacent colder air over land, it creates a temperature gradient, which in turn causes wind. When wind blows across the surface of the ocean, it moves the surface and begins moving the ocean in a horizontal current pattern. But heat is not the only force at work on the water. Temperature and salinity are also working on the vertical column, or depth, of the water. This causes vertical current patterns because warmer, less salty water flows upward and colder, saltier (denser) water sinks.
In deep ocean waters, the density of the water, which is directly controlled by temperature and salinity, controls circulation patterns. The vertical circulation cells that form as a result allow heat to be stored in the great depths of the ocean where they can be later released back into the atmosphere.
With all these vertical and horizontal processes working together, a complex circulation is formed that causes the warm surface waters to travel toward the polar regions. These currents on the surface release heat to the atmosphere during their journey, effectively heating the nearby landmasses. In order to offset these warm surface currents, cold deep currents form and travel from the poles back toward the equator to repeat the cycle.
According to scientists at NASA, the oceans and atmosphere work together to distribute heat and regulate climate. Because the oceans have such immense thermal capacity, it allows them to slow the rate of climate change. The upper 10 feet (3 m) of ocean water hold as much heat as the entire atmosphere. This is what makes some coastal areas warmer, even though they may be located fairly far from the equator. A prime example is western Europe. While it is quite far north, it has a much more mild climate because of the warmth from the passage of the Gulf Stream. Without this warm northward current delivering equatorial heat on its way toward the North Pole, Europe would be much colder. As a comparison, the Scandinavian countries are at the same latitude as Alaska but have a milder climate.
The world's oceans also store and transport CO2. The oceans have absorbed about half of the total CO2 added to the atmosphere during the last 100 years by human activities such as the burning of fossil fuels and deforestation. This "sequestering" of carbon is a slow process, however, and will not keep up with current rates of CO2 input into the atmosphere. Phytoplankton in the ocean also stores CO2 from the upper layers of the ocean in their carbonate shells. Eventually, this CO2 settles to the ocean floor and gets buried in the sediment there.
According to NOAA, heat and CO2 are also exchanged vertically in the ocean from the surface to the deep bottom layers through a process called "upwelling." This vertical circulation occurs in coastal areas and is the mechanism that makes many coastal areas very productive in fishing resources. The longer-term trends in upwelling may be related to global warming. The upwelling process moves the cooler nutrient-rich water up toward the surface, where it replaces the original surface water. This is the mechanism that causes the west coasts of all continents to have relatively cool surface waters. It also provides fertilization to these waters, increasing their biological productivity.
Climate modelers at NASA have done a considerable amount of work relating the ocean's physical properties to global warming. They have determined that during the 1900s, the ocean played a significant role in regulating the atmospheric temperature due to global warming. In fact, if the oceans were not so efficient at absorbing CO2 and heat, the temperature rise experienced in the past 100 years on land would have been doubled due to increased greenhouse gases. NASA scientists have concluded that the ocean's tremendous storage capacities have offset-so far—some of the negative effects of anthropogenic global warming. What they do not know, NASA scientists stress, is whether the ocean's role as "climate moderator" will persist over the long term. There is evidence that Earth's climate has reached strategic tipping points in the past and has not been able to respond fast enough, triggering a glacial period or a drought. One thing NASA scientists caution is that as the
Working as a massive conveyor belt of heat, the oceans' thermohaline circulation has a significant effect on weather worldwide.
rate of freshwater increases near the North Pole, it will raise the freezing point. More surface ice would keep heat from being released to the atmosphere. If this occurred, it would make it more likely that the North Atlantic conveyor belt would be slowed or halted, triggering a climate change disaster. (See chapter 6 for a more detailed discussion on abrupt climate change.)
The second way the ocean and atmosphere interact is chemically. When water evaporates from the ocean's surface, clouds are formed. Water vapor has a twofold effect: 1) water vapor is a greenhouse gas, so it plays a role in heating the atmosphere, but 2) it also forms clouds, which block incoming solar radiation, thereby cooling Earth. Over an extended period of time, it is not known if the net effect from water vapor on global temperatures will be cooling or heating.
Even more important is the ocean's role with CO2. According to NASA, most of the world's carbon is located in the ocean. Because of this, the exchanges that happen between the upper and lower levels of the ocean, as well as the ocean surface and the atmosphere, are very important. Natural chemistry processes play a large part in what happens to some of this carbon, but biological processes also play a factor, and they are important to climate change. The process of photosynthesis turns CO2 into organic material. When it is in the ocean, it sinks to the ocean floor in a process that scientists call the biological pump. The way it works right now is as a carbon sink: CO2 entering the ocean is stored on the bottom. Scientists have proposed that if the ocean's circulation patterns were disrupted, this carbon could be released back into the atmosphere, making the oceans a CO2 source instead of a sink.
One of the main issues climate modelers are working on today is how the physical and biological processes of the ocean will respond to chemical and physical changes in the atmosphere. According to experts at NASA, this raises the following questions:
1. Will an increase in storms cause the upper ocean waters to mix more?
2. How will the phytoplankton react? Will they be more or less productive?
6 Infobase Publishing
Top: The ocean acts as a global heat engine. Energy from the ocean forms heat and water vapor. As the atmosphere warms, pressure gradients form surface winds that drive ocean currents. Bottom: The oceans and atmosphere interact chemically, enabling the ocean to absorb more than 90 percent of the world's carbon. This transfer process is referred to as a biological pump.
3. Will sediments high in iron content blown into the ocean by wind act as "fertilizer" to the phytoplankton and cause a rapid growth?
4. Will climate change increase the dust added to the ocean?
5. Will these changes increase activity of the biological pump, which will increase the removal of CO2 from the atmosphere by the ocean?
One theory, indicated by research, is that if phytoplankton increased in the ocean, it would counteract the increase of CO2 in the atmosphere, slowing it down and offsetting global warming. One area that researchers agree needs a lot more research in order to answer critical questions concerns the marine food chain and its vulnerability to short- and long-term climate change. Scientists at NASA are currently working to find answers to these questions.
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