Carbon dioxide ppm levels and targets
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Level of carbon dioxide in the world's atmosphere, expressed as parts per million (ppm), have been the subject of significant scientific research and modelling to better understand the effects of climate change and make predictions for future trends.
 Notable CO2 levels
|280||Pre industrial level; temperature 0C and sea level of 0m|
|280-300ppm||Equates to 1.7 to 2.7C and possible sea level rise of 4-6m|
|350||Level suggested by climate scientist James Hansen for a safe climate future|
|380||2008 level, 2.7 to 3.7C global temperature rise possible, 15m to 35m possible sea level rise possible|
|450||Level targeted for international negotiations in 2008, but considered dangerous due to risk of exceeding 2 to 3C global temperature rise|
|550||Considered very dangerous due to likely global temperature rise of 4C or higher|
|650||Level predicted for 2050 based on current carbon emissions. Considered extremely dangerous. 5.7C or higher temperature rise possible. Possible sea level of 75m.|
 Rate of CO2 level increase
Scientists at the Mauna Loa observatory in Hawaii say that CO2 levels in the atmosphere now stand at 387 parts per million (ppm), up almost 40% since the industrial revolution and the highest for at least the last 650,000 years.
The figures, published by the US National Oceanic and Atmospheric Administration (NOAA) on its website, also confirm that carbon dioxide, the chief greenhouse gas, is accumulating in the atmosphere faster than expected. The annual mean growth rate for 2007 was 2.14ppm – the fourth year in the past six to see an annual rise greater than 2ppm. From 1970 to 2000, the concentration rose by about 1.5ppm each year, but since 2000 the annual rise has leapt to an average 2.1ppm." 
 550ppm CO2 equates to dangerous climate change
A major conclusion arising from the study of the recent history of the atmosphere is that carbon cycle feedbacks coupled with ice melt/water feedbacks constitute major amplifying mechanisms of initial relatively minor solar and greenhouse triggers (forcings). This observation is central to the view of the acceptability, or otherwise, of a 550 ppm CO2 atmosphere, for the following reasons:
- Based on a climate sensitivity of 3 degrees for a doubling of atmospheric CO2 levels, at 550 ppm CO2 which is twice the pre-industrial level of 280 ppm CO2, mean global temperatures will rise to about 3 degrees Celsius
- A rise of global temperature of 3 degrees Celsius implies sea level rise of about 25 metres +/- 12 metres, as recorded from the mid-Pliocene (3 million years ago) and consistent with sea level rise/temperature relations during glacial terminations
- Temperature rises to 3 degrees Celsius imply widespread desertification of mid-latitudes the agricultural centres of the world
- Natural sequestration of greenhouse gases occurs over time frames of centuries to millennia and no atmospheric mechanism is known which will stabilize CO2 levels over shorter periods
- In terms of the longevity of civilization, allowing CO2 levels to rise further than they already are (387 ppm) would prove to be a unidirectional process
 CO2 levels for a safe climate
United States climate scientist James Hansen has called for a maximum upper limit of 350ppm, 30ppm below current levels:
- "The evidence indicates we've aimed too high -- that the safe upper limit for atmospheric CO2 is no more than 350 ppm"
Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany, has stated that only a return to pre-industrial levels of CO2 would be enough to guarantee a safe future for the planet. He said that current political targets to slow the growth in emissions and stabilise carbon levels were insufficient, and that ways may have to be found to actively remove CO2 from the air.
Schellnhuber, who has advised the German government and European Commission on climate, said:
- We have to start pondering that it might not be enough to stabilise carbon levels. We should not rule out that it might be necessary to bring them down again.
- It is a compromise between ambition and feasibility. A rise of 2C could avoid some of the big environmental disasters, but it is still only a compromise.
He said even a small increase in temperature could trigger one of several climatic tipping points, such as methane released from melting permafrost, and bring much more severe global warming.
- It is a very sweeping argument, but nobody can say for sure that 330ppm is safe. Perhaps it will not matter whether we have 270ppm or 320ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms.