[haku]

Originally Posted by forre

If statistics existed 600 years ago I'd love to see how it all changed.

Well, we do know how it was 600 years ago, we have data going back 650,000 years actually, from the analysis of air trapped in the Antartic ice sheet.

Here's a graph showing carbon dioxide levels for the past 400,000 years.

This figure shows the variations in concentration of carbon dioxide (CO2) in the atmosphere during the last 400 thousand years. Throughout most of the record, the largest changes can be related to glacial/interglacial cycles within the current ice age. Although the glacial cycles are most directly caused by changes in the Earth's orbit (i.e. Milankovitch cycles), these changes also influence the carbon cycle, which in turn feeds back into the glacial system.

Since the Industrial Revolution, circa 1800, the burning of fossil fuels has caused a dramatic increase of CO2 in the atmosphere, reaching levels unprecedented in the last 400 thousand years. This increase has been implicated as a primary cause of global warming.

This graph shows that there used to be a carbon cycle in sync with the glacial/interglacial cycles, carbon was high during warm periods, carbon was low during glacial periods, it was like clockwork.
We can see that the highest levels were regularly around 280ppm and the lowest levels around 180ppm. Before the industrial revolution, carbon levels were normally high for a warm period just following a glacial one (which ended 10,000 years ago), around 280ppm, but for the last 200 years, carbon levels have skyrocketed to 380ppm and continue to rise by 20ppm every 10 years.
We'll be close to 500ppm in 2050, almost twice higher than what used to be the highest levels for the last 650,000 years, which means our atmosphere will be as far from a normal warm period as a normal warm period was from a glacial age, that's a massive difference.
Such a steep unprecedented increase can only lead to a major climate change.