THE DRIVING force of the Great Heat Conveyor Belt (GHCB) is water density, which decreases when the ocean’s salinity is reduced. This occurs when cold, northern winds become warmer, and when the fresh water flow from the poles’ melting ice increases. The quick response of an ocean current’s gigantic process is caused by the minute temperature and salinity differences that keep it going. When the inertia of the GHCB flow is dissipated, it’s as if the motor of a hot water pump was turned off, and the flow stops rather quickly. At that point, no more heat is sent from the equator to the North, and a new ice age can begin in the North Atlantic region.
Global warming does three things:1) it makes cold, north winds warmer and the water at the northern end of the heat conveyor belt warmer; and 2) it melts the ice and causes fresh water will pour into the ocean at Greenland, which makes the northern end of the belt less salty. Both of these effects reduce the density of the water. Once that density drops to that of the lower layers, the stream will no longer sink, and the conveyor stops. Finally, 3) as the ice melts, the level of the water in the Atlantic also rises.
From a process control point of view, the two manipulated variables that are available to keep the GHCB circulation going are the temperature of the Northern winds and the flow rate of fresh water from the melting ice. Today, both of these variables are increasing because the global heat balance has been upset by general warming.
So, what kind of contribution can process control make to stabilize this process? We can train a process control model (See Figure 1 below) on historical data, and determine the gains, time constants, dead times, inertias and interactions of this process. By doing so, we can replace a political and emotional debate with scientific predictions based on facts. By training artificial neural network (ANN) models on historical data, we can accurately predict the timing and size of future events, and hopefully motivate society to take timely corrective actions.
Hurricanes, Typhoons, Tropical Cyclones
Winds are caused by convection, and convection is caused by temperature differences. Archimedes’s principle says, because warmer air is less dense, it tends to rise through surrounding cooler air. Many aspects of the weather are a consequence of convection. If the surface of the ocean is warmer than the land, ocean air will rise and colder, heavier air from the land will blow in to take its place. Inversely, if the land’s surface is warmer than the ocean’s, the wind will blow from the ocean to the land.
The wind patterns we call hurricanes in the North Atlantic are known elsewhere as typhoons or tropical cyclones. They’re all initiated by hot spots and pressure depressions that evolve on the surface of warm oceans, and supply the energy (the jet fuel) of hurricanes. For hot spots to cause air to rise and start forming hurricanes, the ocean’s water has to be over 80 °F to a depth of about 200 feet.
The air density above a hot spot drops for two reasons. First, because the density of the air is reduced as its temperature rises. Second, because a rise in temperature causes an increase in vaporization at the ocean’s surface, and the increased moisture content again decreases the air’s density. Above these hot spots, the rising hot and moist air causes a chimney effect, which reduces the pressure below atmospheric. The vacuum developed ranges from -1 in. to -4 in. of Hg. As this pressure depression and convection pull in more hot and moist air from the ocean’s surface, the hurricane grows stronger and stronger.
As the air at the center of the hurricane rises, the earth’s rotation causes it to start spinning. This, in combination with the forces of gravity, initiates the circulatory motion that is typical of all cyclones and hurricanes. On the northern hemisphere, the direction of this rotation is counter-clockwise. A cyclone or tropical storm becomes a hurricane when its spinning velocity reaches 75 mph. Hurricane velocities of up to 200 mph have been recorded.
As this spinning, warm, moist air rises into the cooler, upper regions of the atmosphere, it cools. As a result, the moisture in the air condenses, and great quantities of energy are released into the atmosphere. The latent heat released by condensation provides the energy which accelerates this powerful dynamic process. This energy drives the convection cycle, which increases the hurricane’s lethal spinning velocity. The energy content and the velocity of the hurricane keep increasing as long as it is above warm waters.
At the outer perimeter of the hurricane, the condensed moisture returns to the ocean as rain. The temperature difference between the core and the perimeter of the hurricane is about 15 °F. Once the hurricane moves over land, its energy supply (the hot and moist air from the surface of the ocean) is cut off, its own “engine” stops, and it dies.
Hurricanes move in the direction of the prevailing trade winds. These winds are caused by the pressure differences between the atmospheric “belts” around the globe. These belts are results of the Coriolis effect, which is caused by the faster movement of the earth’s surface (due to the larger diameter of the globe) near the equator than toward the poles. For this reason, in the Northern hemisphere, the air masses moving towards the pole tend to wear to the East.
So, why does global warming make the hurricanes more powerful and more destructive? The answer is two-fold. First, overall ocean warming increases the energy supply of hurricanes, which is the temperature difference between their core and perimeter. Second, the melting ice causes ocean levels to rise, which exposes larger coastal land areas to the potential destruction from flooding. If global warming caused the two-mile high ice cap at the North Pole to melt, the levels of the oceans would rise by 20 feet.
[Editor’s note: this is the second installment of Béla Lipták’s three-part series on using process control principles to understand and perhaps control global warming. The first part, “A Process Only Mankind Can Control,” appeared in CONTROL’s Jan. ’06 issue. The third part, "Why Do We Have Global Warning?" will appear in the May 2006 issue.]
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