The World is a Giant Water Wheel
I got an inspiration today about the cyclical nature of climate change, specifically how ice ages come and go. A while back I learned about Methane Hydrate, a type of ice which contains 168 times as much methane per unit volume as the gas itself. It forms under pressure in the deep waters of the ocean, fed by falling organic debris from the life that teems there. There are enormous deposits of the stuff distributed throughout the world, making oceans the earth’s largest carbon sink by far. It’s thought that the energy contained in methane ice deposits is twice as large as all land based fossil fuels combined!
I’ve been familiar with the prevailing theory of ice age formation for some time, at least the North Atlantic version which maintains that the trigger event comes when the Gulf Stream shifts south. The gulf stream is the local expression of the Ocean Conveyor Belt, a moving circuit of water which helps distribute the heat of the tropics to the rest of the world. It’s the gulf stream giving up it’s warmth in the North Atlantic that produces the mild climate Europe enjoys. If the gulf stream were to shift southward, weather in places like England and Scandinavia would become downright inhospitable. Apparently this has happened repeatedly over the last three million years. We’re stuck in a cycle of ice ages and inter glacial periods, which is what we’re in now.
Most folks think CO2 is the most important greenhouse gas. But in fact water vapor plays a larger role in maintaining the earth’s temperature. That’s something we can appreciate when local humidity rises and makes the weather oppressive. We also know that when it’s cold outside the air can’t hold as much water. So when an ice age begins and the temperature drops, the atmosphere loses some of it’s water vapor, thereby reinforcing the cooling trend. Later on as the ice spreads, its Albedo reinforces this trend further still by reflecting more of the sun’s light back into space.
If all these cooling factors are reinforcing each other, how does the earth ever get warm again? Until today I didn’t know the answer to that question. But I do now. One effect an ice age has is to pull water out of the ocean and deposit it on dry land. At various points in the distant past the earth was much warmer than it is today and there was little, if any, ice. Sea levels back then were 800 to 900 feet higher than they are now. But today that water is locked up on land, primarily in Antarctica which has over 13 million square miles of ice at an average depth of 1.6 miles, all of it Above sea Level! – Astonishing!
The turning point in the cycle stems from the fact that ocean levels drop as the ice age continues. As the waters recede pressure on methane hydrate deposits is reduced. Not surprisingly, something that contains so much gas in a small space turns out to be pretty unstable! If it’s not under serious pressure methane hydrate tends to bleed off as methane gas. Eventually the pressure on methane ice deposits shrinks to the point where large amounts of methane gas starts escaping into the atmosphere. This is a crucial event because methane is twenty times more potent than CO2 as a greenhouse gas! So when large amounts of it escape to the surface, the greenhouse effect kicks back in, at which point everything starts getting warmer again.
The reverse happens as things warm up. Glaciers melt and sea levels rise. This increases the ocean’s capacity to form methane hydrate due to climbing pressure in the depths of the sea. The capacity of the world’s oceans to act as a carbon sink rises with the water. Eventually the oceans take enough carbon out of the atmosphere to send the greenhouse effect into another tailspin. At that point a new ice age begins. To make a long story short: The world is a giant water wheel!
(Originally posted by R. Guenette on 08./28/.06)