The Known Knowns of Climate Change

POTSDAM – The philosopher Daniel Dennett once compared science to the construction of a huge pyramid. Its base comprises the mass of well-established knowledge – no longer controversial and seldom discussed outside academia. More recent research is piled toward the top of the pyramid, where most public debate takes place. It is an apt metaphor for climate-change research, and one worth bearing in mind with the publication of the latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC).

The IPCC’s fifth report, the product of several years of work by hundreds of climate scientists around the world, reviews our established understanding of climate change and explains more recent findings. The media understandably tend to focus on the latter – like the much higher sea-level rise predictions compared to the previous IPCC report of 2007. But let us step back from the news cycle to look at the solid knowledge base of our pyramid.

Climate research dates back at least two centuries, to Joseph Fourier’s discovery of the effects of greenhouse gases on planetary climates; in 1859, John Tyndall demonstrated in his laboratory which gases cause this effect. Detailed radiation measurements on the ground and from satellites have since proved the greenhouse effect’s existence.

We also know beyond doubt that emissions from human activities have substantially increased the amount of greenhouse gases (especially carbon dioxide) in our atmosphere. When the first IPCC report was published in 1990, the concentration of CO2 in the atmosphere had reached 354 parts per million (up from the preindustrial baseline of 280 ppm). This year, the concentration of atmospheric CO2 crossed the 400 ppm line for the first time. CO2 levels are already far higher than they have been in a million years, as ancient air bubbles trapped in the Antarctic ice show.