Global temperatures within one degree of million-year maximum 26 September 06

The Earth's temperature has reached and is now passing through the warmest levels of the early Holocene, according to research by NASA's James Hansen and colleagues. This makes the planet warmer than at any time for 12,000 years, and leaves us just a degree shy of the highest temperatures seen on Earth for the last million years. This conclusion illustrates the startling rapidity of the warming trend over the last 30 years.

According to Hansen: “That means that further global warming of 1 degree Celsius defines a critical level. If warming is kept less than that, effects of global warming may be relatively manageable. But if further global warming reaches 2 or 3 degrees Celsius, we will likely see changes that make Earth a different planet than the one we know. The last time it was that warm was in the middle Pliocene, about three million years ago, when sea level was estimated to have been about 25 metres higher than today.”

Hansen et al conclude that global warming of another 1C relative to 2000 will constitute ‘dangerous’ climate change (a reference to the UNFCCC’s Article 2) “as judged from the likely effects on sea level and extermination of species”. Note: this is a downwards revision from the 2C-above pre-industrial target advocated by environmental NGOs and the EU. Another degree above 2000 is about 1.8C above pre-industrial temperatures – and the window we have to avoid a commitment to this temperature rise is probably about five years.

At the current rate of warming (0.2C per decade) we will pass this ‘danger’ level in 2050 – but given that the rate of warming is very likely to go on accelerating, it will probably be a good deal earlier. The real-world impact of this warming can be judged by the fact that isotherms (lines of equal temperature) are currently migrating towards the poles on average at 40 kilometres per decade in the Northern Hemisphere. That’s 4 kilometres a year, or about 11 metres per day – much faster than plant or animal ecosystems are able to migrate and adapt. Hansen and colleagues predict that a continued rapid warming will doom a majority of life on this planet to extinction.

There’s a press release on the NASA site, with illustrations, or you can see the whole paper (as published in PNAS) in PDF here.