That's 400 ppm of atmospheric CO2, of course as a symbolic (though hardly milestone) level is reached, and exceeded. \u00a0The 400.03 ppm figure was reported by the US\u00a0National Oceanic and Atmospheric Administration [NOAA] on May 10th. \u00a0For the record, these are NOAA's figures over the last 1000 or so years.<\/p>\n
Year \u00a0 \u00a0CO2 ppm<\/strong><\/span><\/p>\n
1000 \u00a0 \u00a0 \u00a0 \u00a0275<\/span><\/p>\n
1750 \u00a0 \u00a0 \u00a0 \u00a0280<\/span><\/p>\n
1880 \u00a0 \u00a0 \u00a0 \u00a0285<\/span><\/p>\n
1960 \u00a0 \u00a0 \u00a0 \u00a0315<\/span><\/p>\n
2010 \u00a0 \u00a0 \u00a0 \u00a0394<\/span><\/p>\n
2013 \u00a0 \u00a0 \u00a0 \u00a0400<\/span><\/p><\/blockquote>\n
~275 ppm was the level for most of human history it seems. \u00a0Uncertainty figures are never quoted, but they must be small.<\/p>\n
So what about temperature changes? \u00a0And what about the link between CO2 and temperature change? \u00a0For example, how much does temperature change if CO2 levels are doubled (the climate sensitivity)? \u00a0And is this consistently found? \u00a0Good questions!<\/p>\n
This is all much more difficult and controversial than talking about CO2 levels.\u00a0 Here are some data about the last 100 years:<\/p>\n
Averaged over all land and ocean surfaces, temperatures have warmed ~0.74\u00baC over the last century. \u00a0More than half of this warming, about 0.4\u00b0C, has occurred since 1979.<\/span><\/p>\n
Source:\u00a0Intergovernmental Panel on Climate Change<\/a><\/p>\n
Average global temperature on Earth has increased by ~0.8\u00b0C since 1880. \u00a0Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20\u00b0C per decade<\/span><\/p>\n
Source:\u00a0NASA<\/a><\/p>\n
Global warming is now 0.8\u00b0C in the past century. \u00a00.6\u00b0C in the past three decades<\/span><\/p>\n
Source:\u00a0US National Academy of Sciences<\/a><\/p><\/blockquote>\n
These are pretty consistent, but you need to know the uncertainty to make sense of the numbers.\u00a0 This is quite hard to find data on, but it turns out to be around \u00b1 25% which is unsurprising given how difficult it is to measure Earth temperatures consistently over time.<\/p>\n
I found the UK media coverage of the 400 break through rather muted, apart from the\u00a0Guardian<\/a>, which had a full and informed account, which is more than you could say about the\u00a0'discussion' at the end of the piece which almost reached down to the standards I normally associate with the Wiltshire Times<\/a>. \u00a0The Economist<\/a> ended a recent article on climate change which discussed the models underpinning (and perhaps informing) our understanding of it, like this:<\/p>\n
As a rule of thumb, global temperatures rise by about 1.5\u00b0C for each trillion tonnes of carbon put into the atmosphere. The world has pumped out half a trillion tonnes of carbon since 1750, and temperatures have risen by 0.8\u00b0C. At current rates, the next half-trillion tonnes will be emitted by 2045; the one after that before 2080.<\/span><\/p>\n
Since CO\u2082 accumulates in the atmosphere, this could increase temperatures compared with pre-industrial levels by around 2\u00b0C even with a lower sensitivity and perhaps nearer to 4\u00b0C at the top end of the estimates. Despite all the work on sensitivity, no one really knows how the climate would react if temperatures rose by as much as 4\u00b0C. Hardly reassuring.<\/span><\/p><\/blockquote>\n
Indeed!<\/p>\n
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