2011-02-27 16:04:25Why 450 ppm is missleading


Why 450 ppm is misleading.



A target of 450 ppm CO2 in the atmosphere is widely regarded as synonymous with keeping mean global temperature by 2100 to no more than 2°C above those of 1750.  This is very misleading and dangerous.  For reasons set out below achievement of that target, probably by 2030, is likely to result in mean global temperatures dangerously in excess of the predicted 2°C. 


At present we use the concentration of CO2 in the atmosphere as the indicative measure of future temperature.  In doing so, we ignore the effect of other greenhouse gas emissions on temperature.  Why?  Because atmospheric concentration of CO2 is within our control and it is a widely endorsed IPCC finding that the cooling effect of aerosols and changed land use fully off-set the effect of other greenhouse gases. 


That contention has become a convenient convention rather than a reality.  It would be reality were the warming effects of greenhouse gas emissions and the cooling effects of aerosols relatively constant.  But that is not so.  We now know that, as a result of human activity, emission of greenhouse gases, particularly methane, is increasing and cooling off-sets are diminishing.



Although it has a relatively short residence in the atmosphere (~10 years) and low concentration, methane is a particularly dangerous greenhouse gas.  It has much greater capacity to absorb and radiate long wave energy before oxidizing to CO2.


Although produced by “natural” processes such as decay of organic material, most methane is released into the atmosphere as a result of human activity.  This includes methane produced by the massive numbers of farmed animals (cattle, chickens, pigs), coal and other mining, oil refining and, most noticeably, as a feedback from anthropogenic global warming.  The latter is responsible for thawing of permafrost and methane clathrates.


Permafrost, particularly on polar land, has resulted in organic material located on or beneath the surface being frozen.  That stopped the decay process and associated release of methane.  Global warming, particularly polar amplification, is causing permafrost to melt, initially at the surface then to an increasing depth.


This has two effects.  First, buildings and other structures located on frozen land are subject to damage and destruction as permafrost thaws.  Second, organic material also thaws and resumes decaying, releasing methane and other gases in the process.  The quantum of this material and methane produced from thawing of land based permafrost are not known but are thought to be significant.  Emissions from this source are increasing but do not pose as great a threat as thawing methane clathrate.


Methane continuously seeps from the earths crust.  When it comes into contact with very cold water it forms an ice-like substance known as clathrate.  Normally this substance is stable at depths of 360m in the Arctic, though studies in the Svalbad region of northern Norway show that stability is now maintained at depths  >400m, confirming the warming of Arctic Ocean waters.


When it melts, clathrate releases approximately 168 litres of methane for every litre of solid clathrate.  It occurs beneath sediments offshore along the coastline of land bordering the Arctic ocean and has also been found in the Antarctic.  When clathrate melts at depths <400m, methane bubbles to the surface and enters the atmosphere where over a period of 10-12 years it oxidizes to CO2.  Clathrate melting at greater depths usually oxidises to CO2 before it reaches the surface, a process which creates hypoxic conditions, inimical to water breathing animals.


Shakhova (2010) reports that permafrost under the East Siberian Arctic Shelf, is, thawing and starting to leak large amounts of methane into the atmosphere and doing so at an accelerating rate.  She estimates that 1.1 million tones of methane per annum now enters the atmosphere from this source, 3 times as much as is released from on-shore marshlands in this area.  By 2030, those emissions are expected to reach 1.5 gigatonnes/annum.


Consequently, atmospheric presence of methane in the Arctic has now reached the highest it has been for >400,000 years, 1.85ppm compared to 0.7ppm normally found during warm periods.  In parts of the East Siberian Arctic, methane in the atmosphere exceeds 2ppm. This is partly responsible for temperatures in the Arctic rising 2-3 times faster than in the tropics – the so-called Arctic amplification, expected to continue and accelerate the release of methane and ice melt.


Carbon Dioxide

Methane oxidises to CO2 which has a residence in the atmosphere of ~100 years.  In so doing it reduces the concentration of oxygen in the atmosphere and seawater.  Amounts of CO2 entering the atmosphere from this source will increase as the rate of methane emissions increases.


Vegetation sequesters CO2 from the atmosphere and long-lived plants would normally retain it for several hundred years, possibly longer if the decaying process is arrested through development of anoxic conditions such as burial under sediment.


Rising global temperature is contributing to a large number of trees and other vegetation being killed by insect and other pathogen such as Phytophthora, as well as drought and fire.  Trees which are burned release their stored CO2 immediately.  Those killed by drought and infestation decompose more slowly and take longer to emit CO2.


As temperature continues to increase, damage to vegetation and release of CO2 will also increase.  Forests previously thought of as carbon sinks are becoming a major source of carbon emissions.  Damage to the Amazon rainforest by the 2010 drought is estimated to have caused reduced ability to absorb CO2 and increased emissions resulting in net release into the atmosphere of 2.2 gigatonnes of CO2. Drought events of this magnitude are expected to occur more frequently.


In addition, as surface seawater temperature increases it becomes less able to absorb atmospheric CO2, effectively increasing the amount remaining in the atmosphere.  This is exacerbated by further warming causing the oceans to emit CO2 already stored in seawater.



Human release of chloroflourocarbons (CFC’s) prior to enforcement of the Montreal Protocol (1989) caused depletion of stratospheric ozone above the Antarctic and to a lesser extent the Arctic, causing increased exposure to health- damaging ultra-violet radiation.  Ozone depletion also caused cooling of Antarctic surface temperature.


Over the last 20 years of curbing CFC emissions, damage to the ozone layer has been partly reversed and is expected to be fully recovered by 2050.  Recovery will restore the radiative forcing of the ozone layer and have a warming effect globally and particularly over the Antarctic Region.




The radiative effects of aerosols is complex and short-lived but overall, they have a cooling effect.  Increased industrial aerosol emissions 1940-70 were probably responsible for the global cooling trend which occurred in that period.  Since 1970, industrialised nations have significantly reduced aerosol emissions through legislation, regulation and, following collapse of the Soviet Union, by closure of old, high emitting factories.


More recently, industrial aerosol emissions have been reduced by India and China where they continue to pose a health problem.  As a result, the cooling influence of aerosols has diminished and continue to do so.




As global temperature rises it causes fast (loss of albedo) and slow (clathrate melting) feedbacks.  A lot of work has been done on examining the effects of fast feedbacks – much less on slow ones, even though their effect on global temperature is becoming increasingly evident.


Slow feedbacks are increasing at an accelerating rate, as are their warming effects.   Stratospheric ozone is increasing reversing the cooling effect caused by its loss, particularly over Antarctica.  Cooling aerosols are diminishing as countries reduce their emissions because of their health effects.


Prior to these developments atmospheric CO2 concentration of 450ppm was equated as limiting average global temperature to 2°C above pre-industrial levels by 2100.  This can no longer be maintained.  Hansen and Sato (2011) using paleoclimate data rather than models of recent and expected climate change warn that “goals of limiting human made warming to 2°C and CO2 to 450 ppm are prescriptions for disaster”.


They predict that pursuit of those goals will result in an average global temperature exceeding those of the Eemian, producing decadal doubling of the rate polar ice loss, resulting in sea level rise of 5m. by the end of this century.


That prognosis in one which can not be ignored.  Atmospheric CO2 concentration of 450 ppm may be an icon to which politicians and others cling but it is wrong and dangerously so.  Of course the research undertaken by Hansen et al and the conclusions reached by them could be wrong.  No one has suggested this.


Taking into account all of the above matters, what concentration of CO2 will limit global warming to less than 2°C above pre-industrial levels by 2100?  Hansen suggests 350 ppm, warning that anything above this is dangerous.

2011-02-27 16:39:09papers?
Dana Nuccitelli
Do you have any specific papers you'll be referencing?
2011-02-27 18:45:33
Rob Painting

Release of CO2 from oceans??. Don't think so.

450ppm would put us back at the Pliocene, with eventually 25 meters of sea level rise.

Also a likelihood we may pass the threshold that the Greenland ice sheet collapses   http://www.skepticalscience.com/What-CO2-level-would-cause-Greenland-ice-sheet-collapse.html

We are very fubar'ed at 450ppm. 

2011-02-28 18:29:39

dana 1981thank you for your response.  I have been reading papers by JHansen and NShakhova on CH4 because I am simply not convinced that arctic amplification can be attributed primarilly  to loss of snow and ice cover.  That sparked my interest in the reliability of 450 ppm.  I am also reading blogs on CO2 feedbacks.  I don’t have access to AAAS/PNAS papers which is rather limiting but the RS offers a lot of free stuff.

2011-02-28 18:41:12

Rob Painting, I agree - so why is the public pressed with the need to keep mean global temp increase  to less than 2C or CO2 concentration to no more than 450 ppm?  As for the GIS, there is no longer anything we can do to prevent its collapse which seems likely to occur before WAIS - but probably not by much.  My guess is that they will contribute 2.5-3.0m to SL by 2100 and both could be gone within 1500 years.
2011-03-01 17:03:54
Chris Colose


There's a lot of topics outlined in the introductory post so it might help to narrow them down.  It's worth keeping in mind too that when talking about the impacts of x ppm of CO2 in the atmosphere, the amount of time it stays there is also relevant.  You probably don't lose an ice sheet if we hit 450 ppm and then suddenly turn everything off, but you might if it stays at 450 ppm for a few hundred years (there's different estimates on this though and modeling glacier dynamics is still in its infancy, I wouldn't put too much stock in one particular estimate). It's also not obvious that going back to the same CO2 concentrations as some time in the past will give you the same climate (e.g. the Pliocene).  There's hysteresis in the system so the climate state is path-dependent, and past times also had different things going on (like ocean dynamics in the Pliocene).  I do think there's a lot of things to worry about in the >400 ppm CO2 range though, but there's a lot of context behind all of it.

Also, some "targets" have political and economic utility even if they are not very meaningful scientifically.  A stabization target might help emission strategies.

I agree that arctic amplification is not only about the ice-albedo feedback, since even ice-free climates have higher latitude amplification in simulations.  There's some amount of work here concerning dynamics, some of which from Jianhua Lu or Ray Pierrehumbert (his 2002 paper is good) that I can think of.  I don't really see how methane fits into that though.

2011-03-29 12:51:37


First draft of article completed and critiques requested

2011-03-30 01:30:12
James Wight


“They predict that pursuit of those goals will result in an average global temperature increase of 6°C above preindustrial levels by 2100”

Again, I can’t find anywhere this claim is made in Hansen and Sato’s paper. Where does this number come from?

2011-03-31 17:27:49


James Wright,

Thank you for commenting

Have I misunderstood Hansen (2011) where he writes … ” BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain”.  

I assumed Hansen was referring here to doubling of CO2 + slow feedbacks, each contributing 3C.  You are right that he does not specify 2100 but then again, it is reasonable to assume that both factors will have occurred by 2100, if not sooner.  How else could we have decadel doubling of polar ice loss?  Perhaps I should ask him?

Then again, his Paper argues the danger posed by not limiting further global temperature to an increase of less than 1 degree C to avoid events characteristic of the Eemian max. So I will delete the offending sentence rather than risk an error.

Is there anything else which looks "suspect"?

2011-03-31 19:54:34


Title clarification: "Why 450 ppm is missleading" =>"Why 450 ppm is not a safe target"

2011-04-26 15:18:38Blog post
John Cook


You want to put this into a blog post and let me know if you're happy for it to go live?

2011-04-26 23:52:03One small voice
Daniel Bailey
Daniel Bailey

How else could we have decadel doubling of polar ice loss?

Keep in mind that ice sheet loss, once it gathers momentum, is decidedly non-linear. That is Hansen's & Sato's main point there.  Once enough "Momentum" builds; any perceived linearity between temperature anomalies and mass loss of the ice sheets disappears.

The CO2 bolus mankind has injected into the Carbon Cycle & requisite warming of the system attendant to that bolus has now guaranteed the eventual loss of both the Greenland Ice Sheet and the West Antartic Ice Sheet.  That is my understanding of the literature field today as it presently sits.  All that remains to be debated is how quickly the loss will occur and what that will translate into sea level rise (and what that SLR will look like on the regional level, as it will not be uniform globally).

What that means in terms of the warming of the Arctic & attendant destabilization of the immense reserves of clathrates/hydrates located therein is daunting (to put it mildly).

Pandora's Box: The forces we have put into motion can no longer be stopped by a mere cessation of anthropogenic CO2.

2011-04-27 00:00:47Suggestion
Daniel Bailey
Daniel Bailey

Someone should synthesize the themes of:

  • Burning of fossil fuels & subsequent impact to the Carbon Cycle, global temperatures & climate
  • Polar amplification
  • Resulting warming there
  • Arctic Sea Ice loss
  • Resulting albedo changes
  • Melting permafrost
  • Warming Antarctic Circumpolar Current
  • Melting GIS & WAIS
  • Sea level rise
  • Destabilization of methane hydrates/clathrates
  • Impacts of above on severe weather events (droughts AND deluges)
  • Impacts of above on food production
  • Impacts of above on world economy & global political instabilities
  • Impacts of all of the above on humanity

Into a cohesive whole.

2011-04-27 05:37:57
Rob Painting

Whoa, that'll need to be a collective effort and would span a large number of posts (to do it any justice). I agree with you whole heartedly though. I don't think people comprehend the scale of the changes we have wrought on the planet. 

2011-04-27 07:06:31We are entering into a new regime
Daniel Bailey
Daniel Bailey

I was thinking of a comprehensive overview, with links to existing SkS posts where possible, and to supporting literature for the rest.  Kind of like a Summary for Policy Makers, to tide people over until the 5th IPCC is done & finishes the job (yeah, right).

"Back To The Future..." (Catchy title suggestion)


"I don't think people comprehend the scale of the changes we have wrought on the planet."


That's because it's only just begun.  The changes have just started, after millennia of stability.

2011-05-09 05:44:47Hansen and 5 m sea level rise


The following statement is dangerously misleading in the current post, and provides fodder for people to shout "alarmist," and to paint Hansen as a fringe extremist:

[Hansen and Sato] predict that pursuit of those goals will result in an average global temperature exceeding those of the Eemian, producing decadal doubling of the rate polar ice loss, resulting in sea level rise of up to 5m by the end of this century.

This statement is too easily simplified in the reader's mind to say "Hansen predicts 5m sea level rise by 2100."

To my knowledge he has not done so, and since using the 5m-by-2095 argument in his 2007 paper to demonstrate the extreme dangers presented by a non-linear sea level rise, I believe he has been working as hard as possible to avoid making any specific predictions in any specific time frame.

He predicts a "muti-meter" sea level rise based on the presumption that ice melt will be non-linear.  He also makes the argument that Eemian temperatures probably only differed from 2000 temps by 1C or less, meaning that AGW temps will approximate or exceed Eemian temps.  This point is made to emphasize that we would then be (eventually) in store for Eemian sea levels (5m or more higher than today), but without reference to time frame.

But his paper also qualifies these statements, with references to Alley 2010 and Pfeffer et al 2008.

This correction to the post is all the more important since Hansen and Sato 2011 is as yet unpublished (to my knowledge), and so not yet really part of the "body of literature."

Certainly, the statement as it stands is true... but any reader's instinctive interpretation is not going to match the more nuanced truth that one gets from reading the actual paper.

I believe this aspect of the post should be clarified as quickly as possible.  I think it is harmful to the AGW "position" to allow vague, extreme positions to appear to be the main line of thinking of some or most climate scientists.