Carbon dioxide is present at a very small 383 ppm (.000383) of the volume of the earth's atmosphere, but it is a very powerful greenhouse gas and so has a large effect upon climate. It is also essential to photosynthesis in plants and other photoautotrophs.
Despite the low concentration, CO2 is a very important component of the Earth's atmosphere because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode) and enhances the greenhouse effect to a great degree.[8]
Although water vapour accounts for up to 90% of the greenhouse effect, there is no real way to control the amount of water vapour in the Earth's climate system and it is short-lived in the atmosphere. In addition, water vapour is almost never considered a forcing, but rather almost always a feedback.
On the other hand, carbon dioxide is a very powerful forcing, and it also lasts far longer in the Earth's atmosphere. With a radiative forcing of about 1.5 W/m2, it is relativly twice as powerful as the next majorly forcing greenhouse gas, methane, and relativly ten times as powerful as the third, nitrous oxide. Carbon dioxide contributes up to 12% to the greenhouse effect.
The 20 year smoothed Law Dome DE02 and DE02-2 ice cores show the levels of CO2 to have been 284.3 ppm in 1832.[9] As of January 2007, the measured atmospheric CO2 concentration at the Mauna Loa observatory was about 383 ppm.[10]
This rise of 98.7 ppm is 25.7% of the 1832 level, or an average of about .15% a year. However, that is not the entire story, as measurements show the rise has been accelerating a great deal in recent times. Since 1960, the measured atmospheric concentrations of carbon dioxide have risen 70 ppm, or about .67% a year; more than 400% of the 175 year average. While the ultimate effects are unknown, this level is thought to be a higher level of carbon dioxide in the atmosphere than ever before.
Were I tend to be uncomfortable is that the data points are rather limited and could be affected be secondary phenomena. After all, we were not thinking about this problem a hundred years ago and more particularly, there has been some work suggesting that ice cores are a lot less reliable than is supposed from a theoretical basis.
1st source: 1 trillion barrels of oil burned in the past one hundred years with another trillion to burn in the next fifty years.
2nd source: Billions of tons of coal burned in the past 150 years with easily as much to be burned in the next fifty. The available resource is measured at around 1 trillion tons
3rd source: Land clearing releases an average of fifty tons of carbon per acre of woodland. This process is now stabilizing and I believe is about to dramatically reverse as agricultural practices radically change.
Vincent Grey 's summary on ice core data from 1999 is well worth reading:
It is back to the drawing board for carbon cycle models. Atmospheric carbon dioxide concentration varies in a manner which has not been predicted successfully by existing models. There was significant variability before there could have been a human contribution. This variability appears to have followed temperature changes, rather than being responsible for them .Although there has been an increase during the period of industrial development, the increase has not been uniform. Thus, the period between 1935-45 showed no change. The period since 1972, when the increase has been linear despite an increase of over 45% in emissions, suggests that there are new carbon sinks being established in the ocean and in the terrestrial biosphere to absorb the increases. This behavior plays havoc with previous predictions of global warming, but it is difficult to know how long the present apparently stable rate of increase will continue.
So we know that the CO2 level climbed from the pre industrial levels of around 250 to the 300 level by 1960. Since then it has climbed linearly by 80. And it may well have been because the earth got warmer rather than the other way around.
