CO2 and Global Temperature

Most people don't understand how an increase in CO2 can bring about a change in global temperature (or what those things mean). There are some long explanations out there, but few summaries. I will attempt one. Everything below is way oversimplified, assume I am leaving out 5 times as much qualification and quibbling. The below is subject to change.

Heat budget The atmosphere is mostly transparent to light, but NOT to heat. Some sunlight bounces off of clouds or light colored rock or ice, but about 120,000 Terawatts is absorbed near the surface and turns into heat. The Earth is a near-sphere with 510 square kilometers of area, so that is 235 million watts per square kilometer ( 955kW/acre, 235W/sq.meter, 22W/sq.foot ). The heat returns to space as "black body radiation", which is proportional to the 4th power of the Kelvin temperature. 235W/m2 corresponds to a black body temperature of 254K (-18C, -1°F). If we added 100 Terawatts to the heat budget (US levels of energy consumption for an entire world of 10 billion people) the black body temperature (and the world average surface temperature) would increase by (1/4)(100/120000)(254K) or 0.05C (0.09°F); the equivalent of moving down to a 7 meter (23 foot) lower altitude. The problem is not how much energy we consume (though we could certainly get better at it), but our sources of energy, and how much land we gobble up as we use it.

Infrared transparency -18C is NOT the temperature of the surface, but the temperature at 5000 meters altitude, about halfway up through the troposphere, where the air is more transparent to infrared heat. Some parts of the infrared spectrum are blocked by water vapor (mostly frozen out at 5000 meters), other parts of the spectrum are blocked by methane and CO2. Imagine many flows of different "colors" of heat, with different colors blocked by different gases. Water Vapor Water is always a mixture of solid or liquid and vapor. The amount of vapor (google "Vapor pressure") drops with temperature. Above about 5000 meters, the temperature is very low, the vapor pressure is very small, and "water colored" infrared is free to radiate into space. CO2 does not freeze out at normal temperatures and pressures, so all CO2 is a gas in the atmosphere. CO2 is much thinner than water vapor at lower altitudes, but a little bit of CO2 is quite effective in stopping "CO2 colored" radiation. At high altitudes, CO2 is thinned out only by the thinning of the atmosphere itself. It becomes infrared transparent at an altitude determined by its density. The thicker the CO2 is, the higher (and colder) that 'transparency' altitude is.

Heat balance Whatever happens to the various colors of infrared, the total heat emission into space must be 235W/m2 in the long run. If the CO2 transparency altitude moves higher, then the CO2 colored infrared must grow colder, because higher air is colder. To compensate, more heat must pass through the dominant water vapor window. That means the temperature at the altitude where the water vapor becomes transparent must increase, which means the air at the bottom of the column, at the surface, must increase as well. The altitude change (and heating) can be significant - if the air cools by 7.2C per kilometer, than an upwards shift of 800 meters (half a mile) is associated with 6C (11°F) of average temperature increase at the surface. The blanket gets thicker.

Effects of large temperature changes 6C is the difference between the depths of an ice age, and now. At the peak of the ice age, much of North America and Eurasia were covered with thick ice, and the land extended out hundreds of kilometers to the edge of the continental shelf, creating as much new land as Africa. Going in the other direction, when there was no polar ice, the sea moved far inland, with a shallow ocean between the Rockies and the Appalachians. We live in a very thin altitude layer, with seawater at the bottom and cold at the top.

Rising Sea Levels A warmer world will melt a lot of polar ice. Sea levels will rise, and over the course of the next century inundate places where billions of people live now. But people can move, and build new (and better) places to live. Between 1900 and 2000, the world population increased from 1.6 billion to 6.0 billion, and we found places for 4.4 billion people to live over that century. We can easily do so again, especially if the world finds new and cleaner sources of food and wealth, and all the billions of us become educated enough to access them. While we should not passively accept global scale changes to our planet, we should balance alternatives, not fear the future.

Complications The atmosphere, the ocean, and the land store gargantuan amounts of heat. There is more heating near the surface, more heating near the equator than the poles, water vapor is less dense than air (and rises as it forms), and the earth has topography and spins. All this means that the distribution of temperatures is complicated and variable. Cause and effect is delayed and obscured. If the results of creating CO2 were localized and immediate, its effect on global would be obvious. And unfortunately, many people are oblivious to the long term and indirect effects; that is why some people buy lottery tickets while others own companies.

Tradition Whether one "believes" in anthropogenic global climate change, doubling atmospheric CO2 without a clear understanding of the possible consequences is probably unwise. Meanwhile, we live in a universe with many bountiful sources of energy; sticking with traditional sources (carbon, or wind/hydro) because they haven't killed us yet is no guarantee that they won't in the future. When considering hypothetical sources (terrestrial or space solar) we cannot depend on tradition to tell us the consequences; instead, we must use the best science we know, and think about effects like deep soil and desert viability that were of little concern to our ancestors. There are 7 billion of us now, and we cannot depend entirely on habits and folklore developed for the 200BCE world of 150 million people.

Urgency In a crisis, people panic. Their brains turn off. This is convenient for politicians in a totalitarian state, and increases donations to contending politicians in a democracy. But it does not solve the crisis; expensive haste has hardly ever accomplished anything besides incinerated Japanese in Hiroshima and abandoned hardware on the moon. We have time to do this right; people will be hurt by delay, but people will also be hurt by senseless waste of resources. There is some urgency for investors; some companies will become hugely profitable and dominant in new energy industries, and the first investors will make vast windfalls. But first investors in pointless, unprofitable "sustainability theater" will lose their shirts, at best preying off taxpayers and gullible second round investors until justice catches up with them. So we should learn a lot, think about the long term future, and invest wisely, not in a panic. Those who learn now will earn more than those who wait.

TemperatureCO2 (last edited 2012-03-28 22:29:12 by KeithLofstrom)