Atmospheric carbon dioxide levels double

Atmospheric CO2 levels are predicted to become double pre-industrial levels by 2050.
The CO2 level is easy to predict under "business as usual" for several reasons. First, there have been very accurate, direct measurements at Mauna Loa since 1958 (above). Measurements of CO2 levels in air bubbles trapped in ice give us less accurate but still useful measurements back through many ice ages. It is relatively easy to measure and predict the CO2 level because there isn't much random variability and other factors affecting it unlike, say, the surface temperature. The figure below right gives the measured and extrapolated CO2 levels from 1900 to 2050.

The last 1.3 million years our planet has been locked in an ice age cycle, and the CO2 level varied consistently between 175 and 275 parts per million (ppm, blue lines in graph). The CO2 level was at about 275 ppm in 1800, a typical interglacial maximum. Then the industrial revolution got to work, and human activity started to increase the levels (black line in graph). The measurements at Mauna Loa (top) clearly show that the recent rise is exponential.

An exponential rise is exactly the way your money grows if you put it into a savings account with continuous compounding of your balance, leaving the interest in. It is the way the world economy grows if there is an approximately constant growth rate. So, it should be little surprise that the CO2 level has grown exponentially in recent years.

The theoretical curve (red line) that matches the observations from 1958 to the present turns out to have an annual growth rate of 2.22%. The figure shows that, if the current rate is maintained, the CO2 levels will reach exactly twice the pre-industrial level in 2050. This is twice the maximum level typically recorded during all of the previous interglacial periods. Clearly, we are leaving the ice age cycle in a big hurry.

Unfortunately, all this is unlikely to change very quickly, because the amount of CO2 added to the atmosphere is equal to the cumulative amount emitted by human activity (Science Daily June 2009). The reason is that CO2 is taken out of the atmosphere by natural processes much more slowly than we are putting it in. Therefore, even if we quit putting in CO2 today, the atmospheric concentration would not drop immediately, but decline slowly over a 100-1000 year time scale.

Note that the CO2 level from natural sources and sinks was in equilibrium in 1800, and so was the global system. The system still would be in equilibrium if we hadn't added all the unnatural CO2. This means that the natural CO2 is not the primary cause of global warming even though it still makes up most of the contribution to the atmosphere.

Next: The temperature increase by 2050 given these CO2 levels.

Details:
For the graph on the right, the measured CO2 levels (black line) at Mauna Loa are those taken by C. D. Keeling and the National Oceanic and Atmospheric Administration from 1958 on. Also see section two of the "Climate Change Debate" blog for previous CO2 levels.

The red line in the graph is the best fit to the data starting in 1958 and is specifically given by:
CO2 level = 280*( 1+exp(0.0222(year-2052)) ). This formula gives the projected CO2 level in parts per million (ppm) for any year from 1958 on, assuming the growth rate is constant at 2.22%.