Carbon dioxide concentrations in the Earth's atmosphere are on the cusp of reaching 400 parts per million for the first time in 3 million years.
The daily CO2 level, measured at the Mauna Loa Observatory in Hawaii, was 399.72 parts per million last Thursday, and a few hourly readings had risen to more than 400 parts per million.
''I wish it weren't true but it looks like the world is going to blow through the 400 ppm level without losing a beat,'' said Ralph Keeling, a geologist with the Scripps Institution of Oceanography in the US, which operates the Hawaiian observatory.
''At this pace we'll hit 450 ppm within a few decades.''
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The 450 ppm level is considered to be the point at which the world has a 50 per cent chance of avoiding dangerous climate change. Any higher and the odds of avoiding searing temperature rises of 4 or 5 degrees by the end of the century become prohibitively risky.
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| The last time CO2 reached the symbolic milestone of 400 parts per million in the atmosphere - in the Pliocene era - temperatures rose by between 3 and 4 degrees and sea levels were between five and 40 metres higher than today. |
The rise in greenhouse gases corresponds with the extra amount of CO2 known to have been emitted by human activities, such as burning fossil fuels and cutting down forests. More greenhouse gases means more heat builds up at the Earth's surface.
The last time CO2 reached the symbolic milestone of 400 parts per million in the atmosphere - in the Pliocene era - temperatures rose by between 3 and 4 degrees and sea levels were between five and 40 metres higher than today. Carbon dioxide levels have been rising steadily since constant measurements began at the Hawaiian observatory in 1958, when the level was about 317 parts per million.
Levels of more than 400 parts per million have been recorded at a few polar monitoring stages in the past year but the Mauna Loa Observatory readings are considered the most definitive.
The finding comes as Australia's Climate Commission was set to issue a new report into global action to reduce emissions.
The US and, particularly, China are starting to move into leadership positions on greenhouse gas reductions, the report, titled The Critical Decade: Global Action Building on Climate Change, says.
The rise in coal use in China had slowed substantially and renewable energy had expanded on a massive scale, the report said. Wind power generation in China had increased almost 50-fold between 2005 and 2012, and new solar power capacity had risen by 75 per cent last year and was expected to triple by 2015.
Australia had doubled its renewable energy capacity between 2001 and 2012 but was at some risk of being left behind by other nations, Climate Commission chief commissioner Tim Flannery said.
"There are a lot of opportunities for Australia but the world is changing quickly and we need to be prepared,'' Professor Flannery said. ''We are the 15th largest emitter in the world, larger than 180 other countries. We are more influential than most of us think."
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