How much CO2 can the atmosphere take?

Or, more properly, how much CO2 in the atmosphere can we, as humans, tolerate?

The EU Commission has agreed new targets for reducing carbon emissions. The EU plan is described at the Guardian as:

• To cut its greenhouse gas emissions by 40% by 2030, compared with 1990 levels.  
• 27% of energy must come from renewable sources by 2030, which is described as a binding target as well as being described as "not yet clear whether the target will be binding at the level of member states". 
• No new legislation to regulate shale gas development.
• Reformation of the EU carbon trading scheme.

There is an article at the Guardian by Fiona Harvey and Ian Traynor that describes the EU's new targets in a bit more detail and provides some context.  For example, at one point they write: "Studies show that the EU's emissions are likely to be 25% below 1990 levels by 2020, making the targets easier to meet."

It's not a done deal yet.  It is still to be debated by member state governments, which I gather will be happening during March this year via the European Council, and the European Parliament. Given that some countries are forging ahead with emission reductions through renewable energy development and other means (eg energy efficiency), I gather that the emissions target and the renewables target are thought to be achievable.

The Cumulative Carbon Emissions Budget

The plan may be thought by some people to be ambitious, certainly compared to targets in other places, but is it enough?

Damian Carrington has an article at the Guardian about this from the point  of view of the cumulative carbon emissions budget.  He wrote:

For those focusing on climate change – the atmosphere right now is half full of carbon dioxide – the European commission's plan is clearly inadequate to meet the EU's own target of limiting global warming to 2C. For those focusing on the economy of the bloc today – EU citizen's pocket are half empty after a crushing recession – the deal was the most ambitious possible.

Damian put up the UK Guardians' version of this IPCC chart, which I've annotated. (Click for larger version.)

Adapted from Figure SPM.10: Global mean surface temperature increase as a function of cumulative total global CO2 emissions from various lines of evidence. Source: IPCC AR5 WG1 Summary for Policy Makers

To keep global temperatures at or below two degrees of warming (ie two degrees above temperatures of the beginning of the twentieth century), we need to limit the total amount of carbon emissions from all sources to around 1000 GtC or less.

Temperature rises linearly with emissions

The interesting thing is that there is a robust relationship between temperature and emissions.  The temperature response to carbon emissions has been shown to be linear up to around 2,000 PgC.    It doesn't matter over what time period or what the resulting CO2 concentration is.

That's an important point, so I'll repeat it.  Whether or not we emit the next 500 gigatonnes of carbon over a decade or over a century, the temperature will rise the same amount.  And it doesn't matter if atmospheric CO2 goes up by double or a bit less or a bit more.  The relationship we're talking about is between emissions and temperature not CO2 concentration and temperature.

That is why the focus is shifting away from concentrations of CO2 to carbon emissions.

This is explained quite clearly in a paper by H. Damon Matthews, Susan Solomon and Raymond Pierrehumbert.  To get some idea of how much temperature goes up for every 1,000 PgC emitted, the above paper shows the following:

• a very likely (5–95%) uncertainty range of 1–2.5°C of global temperature increase per 1000 PgC of cumulative carbon emitted
• the observational record showed a mean value of 1.5°C per 1000 PgC emitted, and a 5–95% range of 1 to 2.1°C/1000 PgC.

I doubt the 1.5°C observed rise factors in slower feedbacks. So I suggest adding a half a degree at least.  This is a moving target because we are still increasing the rate of growth of emissions.

The EU targets and those of all other countries need to be assessed in the light of how much they will add to cumulative emissions.  If we don't keep cumulative emissions to less than 1,000 gigatonnes (1,000 PgC) then temperatures will be likely to rise above 2°C.  Obviously there needs to be a cooperative international effort to limit cumulative emissions.  That's not to say that all countries have to have exactly the same route to reducing emissions.  It is to say that taken together, the whole world needs to keep cumulative emissions to less than 1,000 gigatonnes of carbon.

With cumulative emissions approaching 500 gigatonnes and carbon emissions currently at around 10 gigatonnes a year and rising, it doesn't leave a lot of time to switch away from burning fossil fuels, such as by switching to clean energy.  This involves a massive change in electricity generation for example.

Thing is, we need to leave some spare capacity for future emissions even after we make the major switch away from burning fossil fuels.  Remember, this is a cumulative budget.  Even when we cut annual emissions to around 10% of what they are at present, we'll still be adding at least ten gigatonnes a decade to cumulative emissions.  If we use up all the budget now, then future generations will be much more limited in their options.

The chart above shows how much time we have or not, depending on which emissions pathway we choose.  But really, in the short term (say, to 2050), unless we make radical cuts in emissions in the next few years, we're heading for more than two degrees of warming this century.  If you took the cumulative emissions by the end of 2014 to be 500 gigatonnes and assumed a 2% growth in emissions each year, then we'd have reached the 1,000 gigatonne target by 2050.  Even if we were to stabilise emissions at the current approx 10 gigatonnes a year, we'd fill the 1,000 gigatonne quota by 2070.  After that we'd have to stop all emissions immediately to stick to the budget  - which just won't happen.  So the sooner we start to reduce emissions the better.

Anthony Watts goes for a literal interpretation

Damian Carrington wrote: "the atmosphere right now is half full of carbon dioxide", linking to an article about the carbon budget as described above.  Anthony put on his literal hat (as he does) and responded with (archived here):

Maybe Damian doesn’t understand that whole “parts per million” thing, if he did, he’s certainly know that 398.58 is nowhere close to “half full”. The value of 500,000 parts per million is what would be considered “half-full”. He’s only off be a couple orders of magnitude.

And showed he doesn't follow hyperlinks, by adding:

Maybe he was thinking in terms of saturation of the CO2 effect in the atmosphere on temperature? In that case, we are closer to 90%, and additional CO2 won’t make much difference.

No, Anthony.  You are very wrong when you claim "additional CO2 won't make much difference".  You are also wrong about what Damian was thinking.  Damian was referring to the cumulative carbon emissions budget, as you'd have realised if you'd read the article to which he linked.

Anthony then puts up this chart, with no humour or satire tag, so he does intend for it to be taken seriously.  Which means that post of his was meant to trick his dumber readers.  He doesn't explain what the chart is meant to represent.  What it appears to be is a chart of a log relationship of CO2 and temperature. Since Anthony chose temperature on the Kelvin scale, I've highlighted and enlarged bits of it to show just the relevant temperatures as anomalies.  I chose the baseline temperature as that when CO2 was around 280 ppm, which is what CO2 concentration was before industrialisation.  The result is shown below - as an animation:

Anthony's chart shows that with a doubling of CO2 from 280 ppm to 560 ppm, temperatures would rise by about 2 degrees Kelvin (which is 2 degrees Celsius).  So he's in the ballpark of climate sensitivity.  However what he wrote shows that either he doesn't understand that or he's deliberately deceiving his readers - neither of which says much about Anthony Watts.  He wrote:

This graph showing CO2′s temperature response to supplement the one Doug Hoffman cites from IPCC AR4. here we see that we are indeed pretty close to saturation of the response.

"Close to saturation" is a relative concept.  There is plenty of room for temperatures to rise and cause untold damage.   (Note that Anthony's chart stops at 800 ppm.  Plus he's got the Y axis at a scale that makes reading associated temperatures difficult, and hard to determine if his chart resembles reality).

Thing is, of course, that we're heading for much more than a doubling of CO2.  If we were to continue to increase emissions by two per cent a year, then by 2040 we'd have doubled the CO2 concentration in the atmosphere assuming that half the extra emissions continued to be absorbed at the surface.  Sooner if the oceans reckoned they'd already absorbed more than they could handle.  By the early 2060s CO2 would be up to around 800 ppm.  And by 2100 we'd be over 1500 ppm.

In terms of the carbon budget, that would take cumulative emissions to almost 2800 gigatonnes, which is off the chart (of the top chart SPM.10) and getting into the territory of six degrees of warming by the end of the century.  Not a pleasant thought.

In the meantime, fasten your seatbelts.  It's going to be a rough ride over the next few years.

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Matthews, H. Damon, Susan Solomon, and Raymond Pierrehumbert. "Cumulative carbon as a policy framework for achieving climate stabilization." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370, no. 1974 (2012): 4365-4379.

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From the WUWT comments

The comments are mostly of the "dumb denier" type, which isn't a surprise given Anthony's article is also of the "dumb denier" kind. (Archived here.)

Les Johnson didn't bother reading Damian Carrington's article or following the hyperlink but says what "he thinks" he meant, which was wrong.  Damian was referring to cumulative emissions not CO2 concentration:

January 22, 2014 at 11:01 am
I believe that by 1/2 full, he meant that we are 1/2 way to a ‘safe’ level of CO2. From pre-industrial levels of 260, that would mean we are at 390 (we are), and must go no further than 520.

Just a guess. But you would think a journalist would be able to communicate the concept better. Warmist wonder they have communication problems…

albertalad goes for conspiracy lingo and says:

January 22, 2014 at 11:20 am
Sadly it isn’t just this guy off his rocker – it is the EU mass insanity driving themselves into poverty led by an even crazier class of leaders without a gain of common sense between the entire lot. Genuine science has ceased to exist in the entirety of the EU – replaced by a self imposed dark ages eagerly followed by the masses.

climateismydj says dryly of Anthony's speculation:

January 22, 2014 at 11:26 am
Maybe if the author of this article followed the link under his highlighted Guardian article text, he’d find the answer staring him in the face. But then again, WUWT has journalism standards to uphold…

nutso fasst says:

January 22, 2014 at 11:38 am
Given the atomic weights of carbon and oxygen, isn’t the total carbon in the atmosphere actually closer to 0.015%?

I don't know if DirkH is trying to be clever or if he is being extra thick when he says:

January 22, 2014 at 11:59 am
He thought it’s 398 percent, not ppm. And 398 percent is about half full.
Quite a good one from the Guardian.

thebleedingobvious is right when he says CO2 isn't the same as carbon, but not right when he suggests that "alarmists" always use the wrong term.  As he rightly points out, CO2 is not the same as carbon.  However total carbon includes the carbon in CO2 (and CH4 and other molecules containing carbon).

January 22, 2014 at 1:03 pm
Carbon dioxide is not the same thing as carbon.
Why do alarmists use the wrong term?

Illiterati dbstealey says:

January 22, 2014 at 1:06 pm
thebleedingobvious says: “Why do alarmists use the wrong term?”
The answer is simple: because they are scientific illiterates, who are just repeating the standard talking points given to them.

Bruce Cobb is keen to get to six degrees of warming and says:

January 22, 2014 at 1:43 pm
Since the ideal CO2 level is probably somewhere around 1,000 ppm, at close to 400 ppm we still have about another 100 ppm to go before we’re at half full. But, close enough. Burning more coal could help us get there.

As does "So dumb" MarkW who says:

January 22, 2014 at 2:33 pm
A few million years ago, the atmosphere had almost ten times as muchCO2 in it compared to today’s atmosphere. The idea that the atmosphere is half full is something so dumb that only someone completely in thrall to his religion could believe it.

Brian Davis says:

January 22, 2014 at 3:04 pm
Anthony, do you have the source for the chart at the end of your post showing the temperature response to CO2?

REPLY: See link in the story -A

Anthony's "link in the story" is no help at all.  It doesn't provide any source of his chart either!


Critical reading is not one of Ron House's fortés - he says:

January 22, 2014 at 5:50 pm
There it says: “The world’s leading climate scientists have set out in detail for the first time how much more carbon dioxide humans can pour into the atmosphere without triggering dangerous levels of climate change – and concluded that more than half of that global allowance has been used up.”

That just makes the stupidity even more appalling than the ignorance: Half of a maximum addition to something is not in general the same as half of the total amount.