Thursday, August 25, 2011

The Coming Ice Age?

Climate changes on different time scales for different reasons. Ice ages or glacial periods are followed or led by warmer periods or interglacial periods. Pondering what causes these changes has been a pursuit of man since the first tree was found in a block of ice that is a glacier. I have proposed that the Earth has two temperature set points, one cold (ice ages) and one not so cold (warm interglacial periods), with water controlling the thermostat.

This is nothing new. I am sure there are many papers with similar ideas using a variety of triggers that starts the ball rolling. One question is how big the triggers have to be?

Water, in its three stages controls the show. As a liquid it absorbs electromagnetic radiation only reflecting about 7% and as a solid it reflects about 95% and absorbs only about 5 percent. As a gas water vapor is great for moving heat around absorbing and giving off heat by conduction, latent heat or phase change and radiation. So to me, water runs the show.

At this point in time the earth and its atmosphere reflects about 30% of the solar energy the Earth system receives from the sun. During an ice age, more is reflected because there is more snow and ice. Even in an ice age, the water near the equator which receives the most solar energy does not freeze. If it did, the Earth would reflect too much of the solar energy and reach a snowball Earth tipping point. To recover from that, something catastrophic would be required to jump start the Earth which would mean that the fossils of critters that survived the ice age would be piled up in an easy to find layer and we would be taught about the remarkable recovery from the Shit Hit the Fan (SHF) era. There is evidence of some the Shit Nearly Hit the Fan eras, but some forms of life survived to start things anew.
Modern man is a product of the last SNHF era or recovery from the most recent ice age.

The reflectivity of the Earth, called albedo, which I can mix up absorptivity, is right now 30%, with most of that caused by clouds mainly in the tropics and sub tropics. Snow and ice at the poles and on top of mountain ranges contributes a small amount to the reflectivity because most of the snow is at the poles where solar radiation is much lower. The reflectivity cannot reduce much more. If all the snow and ice on the Earth melted, that may drop the reflectivity to 20% but all that water would produce more clouds so 25% is closer to the absolute minimum.

During an ice age the reflectivity increases, but it also has a maximum limit. This may be pretty hard to estimate. It is unlikely that the glaciers would expand to the tropics. The tropics are between 23.5 N and 23.5 S, the imaginary lines of the summer and winter equinoxes. Just for simplicity, I want to use 30 N and 30 S as my estimate tropical boundaries. Why? Because the sine of a 30 degree angle is one half. Since the Earth is pretty much a sphere, from equator to 30 N receives 50% of the solar energy of the northern hemisphere and the same thing from the equator to 30 degrees S. So 50% of all the energy the Earth receives is collected between 30 N and 30 S. If we assume that the cloud cover in this region remains about the same, we can assume that even during an ice age the Earth still absorbs 50% of the energy it absorbs in an interglacial period. Since the sun pumps out about the same amount of energy all the time, the worst case glacial albedo would 70/2 plus 30 equals 65 percent. So the range of albedo is in the ballpark of 25% minimum and 65% maximum, that means that the Earth can absorb between 75% and 35% of the solar energy available.

Since NASA has been kind enough to publish the solar energy available at the top of the atmosphere (toa) and we live on a pretty close to a sphere planet, the range of energy absorbed is 256 W/m^2 to 120 W/m^2. Power varies by the fourth root of the temperature, so while there is a big power difference, the temperature difference is not so big. So just using the fourth root, the apparent temperature at the toa would range from 2% more to about 16% less. This is based on the absolute temperature, which is about 255 K at the toa right now, so we have a rough range of 260 K to 214 K. While the average temperature of the surface depends on a bunch of stuff, a rough estimate for my purpose would be the same percentages of the 288K average surface temperature or 293K to 242 K as rough maximum average temperature ranges at the surface if albedo varies from 25% to 65 percent.

These are of course very rough numbers and not intended to be accurate enough to be useful for anything more than an illustration of the relative limits of climate change based on albedo change.

242 K is pretty cold. 274 K is zero degrees C and 242 is -32 degrees C or -26 degrees F. The temperature during the glacial periods is only estimated to be 6 degrees less or 282K. So why would my worst case be so much lower than 282K? Because of the location of the oceans.

The image above is an artist's conception of an Ice Age Earth from wikipedia. The northern hemisphere took the biggest hit because it has more land mass which doesn't benefit from the thermal mass of the ocean. So the 30N to 30S could be extend to 45N to 55S as to determine a more accurate maximum ice extent, or back calculating from the average temperature of the glacial period find that an albedo increase to 37% could produce that change. That would give us a more realistic maximum change albedo range of 25% to 40%.

While these are rough estimates, they show how lucky we are to live on a water world that has reasonable limits with our stable sun. Not that a new glacial period or 5 degrees of warming would be fun, but runaway conditions up or down are avoided thanks to the amount of water and where it is located.

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