Oceans cover 70% of our planet (more than 360 million km2). They absorb 90% of the total solar heat received and also absorb more than a quarter of the carbon dioxide we emit. The mass of the oceans is more than a thousand times greater than the atmosphere (1.3 x 1021 kg vs, 5.5 x 1018 kg) and has a much higher inertia. The top few meters of the ocean store as much heat as the planet’s entire atmosphere.
Solar radiation penetrates only the top 100-m layers. The water below is not directly heated. The water cycle on the earth (Figure 1) is started by the sun warming the surface of the oceans and causing the water to evaporate. This increases the water vapor content of the atmosphere, which increases the rate off water circulation. This cycle provides fresh water for the planet, while evaporation cools the oceans. The energy obtained travels with the water vapor until released in the process of condensation precipitation.
An increase in global warming and the resulting increased rate of evaporation influence the water cycles in the oceans. The common global warming models accurately consider the response of the atmospheric processes to the rise of global warming, but they often do not take into account the oceanic ones. So, I will focus on these often neglected oceanic processes, describing both the vertical and the horizontal "ocean conveyor belts."
The vertical ocean conveyor belt:
- Global warming increases the water vapor content of the atmosphere. When it condenses, it becomes the fresh water supply of the planet. Evaporation requires heat, which is taken from the ocean, thereby cooling it. Increased evaporation increases the rate of water circulation on earth by 7% per each °C temperature rise.
- The cooling caused by evaporation increases the density of the surface waters, making them heavier than the waters below. This causes the so called "thermohaline circulation," because as the warm surface waters are moved downward they force the cold waters below to rise. This circulation indirectly heats those layers that solar radiation cannot reach directly.
- As evaporation removes fresh water from the ocean's surface, so the remaining water becomes saltier (and therefore heavier) than the waters below. This process also increases the vertical circulation of the ocean's water, because as the saltier and warmer surface waters move downward, they heat the water layers below.
This "vertical water circulation" that brings heat to the lower levels is reduced as global warming increases. The increase causes the temperature of the surface waters to rise, reducing their density relative to that of the waters below. Therefore, if the density of the evaporating surface waters drop, the weight difference between that and the waters below drops, and this vertical circulation slows or eventually stops. The faster density increases with depth, the slower the vertical circulation becomes. As a result, less mixing takes place among the water layers, increasing global warming at the ocean's surface because it reduces the rate at which heat is transported down into the lower layers and more of it remains on the surface.
The horizontal conveyor belt is often referred to as the "great ocean conveyor belt." It starts at the tropics and moves the warm water layer on the surface from the equator to the poles and returns the cold and salty current at the bottom of the ocean back toward the equator (Figure 3). This conveyor slowly (in years to decades) turns over the water in the entire ocean, from top to bottom.
Surface currents, such as the Gulf Stream warmed Europe and the east coast of the United States as it moves north, where near Greenland it gets cold and heavy enough to sink and return to the tropics as a gigantic cold and salty stream at the bottom of the ocean. It is my view, that the various global warming models should (but do not) fully consider the changes in the flow rates of these currents because they have a growing and large impact on the heat balance of the oceans. They not only regulate global climate, but also their mixing effect supports the life of marine ecosystems which supply a substantial portion of the world's food. Their mixing effects also control how heat, carbon, nutrients, and dissolved gasses are exchanged between the upper and lower layers of the ocean and when they slow. This effect diminishes and global warmings rises.
As the ocean's conveyor belts slow, the surface temperature and global warming rise because less heat is conveyed into the lower layers. Because of the reduced mixing, less food is supplied to the algae, which under the protection of corals perform the photosynthesis (splits the water using solar energy and produces sugar from the hydrogen and CO2 while generating much of the oxygen on earth).
In the past, the ocean absorbed over one-quarter of the CO2 emitted, but as the water temperature rises the solubility of CO2 drops and the resulting "degassing" further increases the CO2 content of the air. In addition, the basic chemistry of the oceans is changing fast because the emissions during the industrial age made the water 30% more acidic. As a consequence, the coral reefs die and that destroys the healthy habitat that previously supplied food and protection to millions of plant and animal species. Based on all this, I believe that if global warming continues as it does today, the oceans soon will become too warm for coral reefs to survive.
The conclusion of my process control analysis is that the majority of global warming projection models is excessively optimistic because they do not sufficiently consider the yet to be triggered the oceanic processes. It also because they do not realize that the inertia of the oceanic processes will continue to increase global warming even after greenhouse gas emission neutrality is achieved.