What do we know for sure?  We know for a fact that CO₂ levels are rising and that human activity is the cause.

How do we know this?  A simple calculation. Let's start with the year 1750, generally accepted as the beginning of the Industrial Revolution, when the standard measure for CO₂ levels was 278ppmv. We'll begin then, add the known level of human-generated CO₂ for that year, then subtract 2.84% of the excess CO₂ – because that's how fast nature tries to restore the balance. If we do this for 255 years, up to 2005, we get the pink line in the graph below. It fits the black line – the actual CO₂ level – like a glove.

How accurate is this fit? Considering the difficulties of gathering data from centuries past, amazingly close. We know, for example, that up until 1950, deforestation was putting about as much CO₂ into the atmosphere as were burning fossils, but it's not easy to know how many trees were chopped down in, say, 1850, and how much CO₂ this put into the air. In spite of imperfect data, the fact that a simple calculation predicts the extraordinary shape of actual CO₂ so well is clear evidence this can not be a coincidental convergence. Human CO₂ emissions must have caused the upsurge is atmospheric CO₂.

So what?  The link between human activity and rising CO₂ levels is the first step. The next one is between CO₂ and global warming. That was predicted over 100 years ago, but the evidence has become convincing only very recently. ZFacts will be adding pages about this during February, 2006.

Where did the numbers come from? CO₂ Levels. These come from two sources. From 1958 forward, they are from a weather station high atop the Mona Loa volcano in Hawaii. They are so accurate, they show levels going up every autumn, when the leaves fall, and coming down every spring. Earlier data are from ice cores in Antarctica. The two sources agree remarkably well. (All actual data points are shown in the top graph.) The next graph shows the emissions data.

Fossil Fuel data are from the U.S. DOE, including an estimate that fossil fuel emissions will increase at 1.2% per year until 2030. We assume this rate to 2050. However, the average rate for the last 5 years was about 1.9%, so this may be optimistic. Cement manufacture also releases CO₂ from the rocks it processes, so that source is tracked separately.

Deforestation Data. These extended only from 1850 to 2000 so zFacts fitted a straight line to the first 100 years (1850--1950) to extend them data back to 1750. Deforestation peaked in 1991 and from 1992--2000 followed a steep and straight descent. Again a straight line was fitted to predict the future. As can be seen above, this is a dramatic and possibly optimistic prediction.