Professor Sir Brian Hoskins, Imperial College, London
If you sampled CO2 in an urban area, we would have passed 400ppm some time ago, but the Mauna Loa observatory is on a volcanic island in the middle of the Pacific Ocean. CO2 mixes with other gases high up in the atmosphere so across the globe its level is fairly similar and the measurements on this remote island are more representative of the global average.
The last time in the Earth’s history when we saw similar levels of CO2 in the atmosphere was probably about 4.5 million years ago when the world was warmer on average by three or four degrees Celsius than it is today.
There was no permanent ice sheet on Greenland, sea levels were much higher, and the world was a very different place, although not all of these differences may be directly related to CO2 levels.
See also the Scripps Institute of Oceanography report: Carbon Dioxide at Mauna Loa Observatory reaches new milestone: Tops 400 ppm. Scripps, NOAA measurements cross threshold in same 24-hour period.
Quote: Before the Industrial Revolution in the 19th century, global average CO2 was about 280 ppm. During the last 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended.
NOAA senior scientist Pieter Tans, with the Global Monitoring Division of NOAA’s Earth System Research Laboratory in Boulder, Colo says: “The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration.”
See also 350.org : ATMOSPHERIC CO2 CONCENTRATIONS JUST PASSED 400 PARTS PER MILLION.