ABSTRACT
Arrhenius’s original scientific work that identified the greenhouse effect was actually meant to address the question of why this planet has the particular temperature it does. More specifically, why is it a temperature so favourable to humans and other forms of life instead of being too cold? The difference is the consequence of heat-trapping gases such as carbon dioxide but also methane and others. What was not known at that time was any detail about the previous global
temperature. But that has been revealed by tiny bubbles of air trapped in ice formations that reflect both the temperature and the composition of the atmosphere at the time of entrapment. Ice cores open that record book to science. We now know that the average temperature of the planet has been unusually
stable for the last 10,000 years. Not only does this mean that the entire human enterprise developed during and depends on a stable climate, but also that all the planet’s ecosystems have spent the same ten millennia adapting to a stable climate. The pre-industrial concentration level of carbon dioxide was 280ppm. The
current concentration is about 390ppm and will climb to 450ppm if global emissions peak in 2016 (which seems hardly even attainable at this point). The global climate system has begun to change and the average global
temperature is already 0.75°C warmer as a consequence. A fair amount of further change is built into the system. Probably the most obvious changes we are seeing are those that involve the
frozen part of the planet (the cryosphere) and, in particular, the solid and liquid phases of water. Most glaciers are in retreat and all tropical glaciers (on the very high peaks) are retreating at a rate where they will be gone in 15 years. Very dramatic changes are taking place in the northern polar regions; the sea ice expanding and contracting annually on the Arctic Ocean has been retreating to an increasing degree and reached its lowest extent this year. It has also been declining in thickness at the same time.
