Thursday, April 20, 2006
By Steven Milloy
Al Gore's global warming documentary hits theaters on May 28. Entitled, "An Inconvenient Truth," the film purports to make the case for concern over manmade emissions of greenhouse gases.
Meanwhile at JunkScience.com, we've produced "The Real Inconvenient Truth" - debunking two key myths of climate alarmism, including that the Earth's atmosphere acts like a greenhouse and that reducing carbon dioxide (CO2) emission will avert significant temperature change.
The notion that our atmosphere acts like a greenhouse - that is, so-called atmospheric "greenhouse gases," like water vapor and CO2, "trap" incoming solar radiation to warm the atmosphere - is wrong. Not only doesn't the atmosphere work that way, greenhouses don't either.
Greenhouses work by physically blocking heat transfer (by convection) from inside to outside - the same effect that heats the inside of your car when it's parked in the sun on a hot day. Opening the doors and windows allows air currents to flow and the heat to dissipate.
But neither the atmosphere nor "greenhouse gases" block convection, so there is no literal atmospheric "greenhouse effect."
Since "greenhouse effect" terminology has long been used to refer to the natural warming of our atmosphere to a habitable level, we'll stick with that incorrect, but commonly-used, terminology for ease of discussion. So how does the "greenhouse effect" actually work?
Atmospheric flows of energy are complex, but a simplified explanation - as depicted in the graphic in the upper-left of this page - is as follows.
Incoming solar radiation is partly absorbed by the Earth's surface, partly absorbed by various atmospheric gases (particularly oxygen and ozone) and partly reflected back out to space. Solar radiation isn't significantly absorbed by greenhouse gases in the lower atmosphere and so doesn't directly cause the greenhouse effect.
For our purposes, the greenhouse effect is largely caused by energy emitted by the Earth's surface, most of which is subsequently absorbed by greenhouse gases and clouds. Very simply expressed, the greenhouse gases and clouds transform that absorbed energy into heat that warms the lower atmosphere and into energy that is radiated back to space and also back to the Earth's surface.
These radiative processes, if they acted alone, would warm the Earth's atmosphere to about 77 degrees Centigrade - much warmer than the 15 degrees Centigrade the Earth actually is. Fortunately, other atmospheric processes - including updrafts and circulation carrying heat upwards and toward the poles - facilitate energy escape into space so that our atmosphere cools to around 15 degrees Centigrade.
But our focus here is CO2's role in greenhouse warming - that's what Al Gore wants us to fret.
Putting aside the cooling convection and circulation processes mentioned above, the limiting factor with respect to greenhouse warming isn't the quantity of greenhouse gases in the atmosphere; it's the energy emitted by the Earth's surface.
Different greenhouse gases absorb different wavelengths of energy emitted by the Earth. The fact that only a limited amount of the Earth's emitted energy is available for absorption by CO2 and that CO2 has to compete with water vapor and clouds for that energy, results in a crucial (but little publicized) relationship between CO2 and atmospheric warming.
The relationship between CO2 and temperature is logarithmic in nature - that is, as CO2 increases in the atmosphere, it absorbs less and less additional energy to produce correspondingly less and less additional warming. At some point, adding more CO2 to the atmosphere doesn't significantly change atmospheric temperature.
To analogize, consider a window with many shades, each blocking half the incoming light. As successive shades are pulled, the transmitted light is halved and the effect of each shade is diminished. Eventually, there's no additional effect because previous shades have already absorbed the light to all but a vanishing degree. As more shades won't block more light, more CO2 won't cause significantly more warming.
In fact, there's been more than enough greenhouse gas in the atmosphere to cause much greater warming than actually occurs since long before humans discovered fire.
"The Real Inconvenient Truth" contains interactive CO2-Greenhouse calculators so you can get a feel for the all-important logarithmic CO2-temperature relationship.
So what's the point at which more CO2 doesn't cause more warming? Are we near it? The commonly-used range of estimates of CO2's impact on global temperature should help put any worry into perspective.
A doubling of atmospheric CO2 from pre-Industrial Revolution days (280 parts per million to 560 ppm), might increase global temperature from between 0.5 degrees Centigrade to 1.5 degrees Centigrade - that is, not much.
The current atmospheric CO2 level is about 380 ppm and the estimated temperature increase since 1880 (when regular temperature recordkeeping began) is estimated to be about 0.60 degrees Centigrade.
Since at least half of this temperature increase pre-dated 1950 - prior to any significant increase in atmospheric CO2 levels - we can estimate that the 30 percent increase in atmospheric CO2 since the Industrial Revolution is associated with a temperature increase of about 0.30 degrees Centigrade. This supports the idea that doubling atmospheric CO2 from pre-Industrial Revolution levels would cause less than a one degree Centigrade increase - and we're not close to such a doubling.
Since this small variation in global temperature is well within the historical climate record, panic hardly seems warranted.
So where does all the fuss about manmade CO2 and global warming come from? Not from actual temperature measurements and greenhouse physics - rather it comes from manmade computer models relying on myriad assumptions and guesswork. Many models incorporate hypothesized "positive feedbacks" in the climate system, which tend to amplify model predictions. But no model has been validated against the historical temperature record. So they don't "radiate" much confidence when it comes to forecasting temperatures.
In preparation for Al Gore's movie, the global warming lobby has purchased lots of newspaper and TV space for an alarmist advertising blitz during May. It's comforting to know that all that hot air won't be significantly warming the planet.