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Climate Hustle

A residential lifetime

Posted on 1 April 2010 by Doug Mackie

Guest blog post by Doug Mackie

One argument against accelerating global warming is that carbon dioxide has a short residence time in the atmosphere. The claim goes like this:

(A) Predictions for the Global Warming Potential (GWP) by the IPCC express the warming effect CO2 has over several time scales; 20, 100 and 500 years.
(B) But CO2 has only a 5 year life time in the atmosphere.
(C) Therefore CO2 cannot cause the long term warming predicted by the IPCC.

This claim is false. (A) is true. (B) is also true. But B is irrelevant and misleading so it does not follow that C is therefore true.

The claim hinges on what life time means. To understand this, we have to first understand what a box model is: In an environmental context, systems are often described by simplified box models. A simple example (from school days) of the water cycle would have just 3 boxes: clouds, rivers, and the ocean.

A representation of the carbon cycle (ignore the numbers for now) would look like this one from NASA.

In the IPCC 4th Assessment Report glossary, "lifetime" has several related meanings. The most relevant one is:

“Turnover time (T) (also called global atmospheric lifetime) is the ratio of the mass M of a reservoir (e.g., a gaseous compound in the atmosphere) and the total rate of removal S from the reservoir: T = M / S. For each removal process, separate turnover times can be defined. In soil carbon biology, this is referred to as Mean Residence Time.”

In other words, life time is the average time an individual particle spends in a given box. It is calculated as the size of box (reservoir) divided by the overall rate of flow into (or out of) a box. The IPCC Third Assessment Report 4.1.4 gives more details.

In the carbon cycle diagram above, there are two sets of numbers. The black numbers are the size, in gigatonnes of carbon (GtC), of the box. The purple numbers are the fluxes (or rate of flow) to and from a box in gigatonnes of carbon per year (Gt/y).

A little quick counting shows that about 200 Gt C leaves and enters the atmosphere each year. As a first approximation then, given the reservoir size of 750 Gt, we can work out that the residence time of a given molecule of CO2 is 750 Gt C / 200 Gt C y-1 = about 3-4 years. (However, careful counting up of the sources (supply) and sinks (removal) shows that there is a net imbalance; carbon in the atmosphere is increasing by about 3.3 Gt per year).

It is true that an individual molecule of CO2 has a short residence time in the atmosphere. However, in most cases when a molecule of CO2 leaves the atmosphere it is simply swapping places with one in the ocean. Thus, the warming potential of CO2 has very little to do with the residence time of CO2.

What really governs the warming potential is how long the extra CO2 remains in the atmosphere. CO2 is essentially chemically inert in the atmosphere and is only removed by biological uptake and by dissolving into the ocean. Biological uptake (with the exception of fossil fuel formation) is carbon neutral: Every tree that grows will eventually die and decompose, thereby releasing CO2. (Yes, there are maybe some gains to be made from reforestation but they are probably minor compared to fossil fuel releases).

Dissolution of CO2 into the oceans is fast but the problem is that the top of the ocean is “getting full” and the bottleneck is thus the transfer of carbon from surface waters to the deep ocean. This transfer largely occurs by the slow ocean basin circulation and turn over. This turnover takes 500-1000ish years. Therefore a time scale for CO2 warming potential out as far as 500 years is entirely reasonable (See IPCC 4th Assessment Report Section 2.10).

This post was written by Doug Mackie, a research fellow in the department of chemistry at the University of Otago.

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Comments 1 to 50 out of 85:

  1. Time constants associated with CO2 in the atmosphere are all over the map. I really like the bold speculations such as those offered by David Archer.

    Archer thinks big. He suggests that mankind may have the ability to postpone the next Ice Age almost indefinitely.
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  2. GC, Archer suggests that we're altering the climate for the long term, but can hardly be said to recommend this as a planned course of action. I wonder what he'd say about your strangely sanguine interpretation of his work?
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  3. Here is a link to a similar post trying to explain the immense timeframes involved in anthropogenic climate change and ocean acidification based on David Archer's work:

    "... the IPCC (2007) concluded that natural processes in the carbon cycle will be slow to remove the current levels of CO2 from the atmosphere. Following perturbation of the natural Carbon Cycle about 50% of an increase in atmospheric CO2 will be removed within 30 years, a further 30% will be removed within a few centuries and the remaining 20% may remain in the atmosphere for many thousands of years (IPCC 2007: 514).

    Archer and Brovkin (2008) reviewed long-term carbon cycle models from the recently published literature. They noted, “carbon cycle models respond to a release of new CO2 into the atmosphere in a series of several well-defined stages lasting for many millennia.” In the first stage, fossil fuel CO2 released into the atmosphere equilibrates with the ocean, which takes centuries or a millennium due to the slow overturning circulation of the ocean.

    Archer and Brovkin (2008: 284) noted that the lifetime of individual CO2 molecules released into the atmosphere may only be a few years because of the copious exchange of carbon with the ocean and the land surface. However, the CO2 concentration in the air remains higher than it would have been, because of the larger inventory of CO2 in the atmosphere/ocean/land carbon cycle.

    That is, the equilibrium processes removing fossil fuel CO2 emissions from the atmosphere operate at a system-wide level and individual CO2 molecules do not last for millennia in the atmosphere. Thus today’s fossil fuel CO2 emissions will not be “in” the atmosphere (literally) for a long period but they will continue to “affect” the atmosphere, the climate, and the oceans for many thousands of years.

    The equilibrium processes have a major negative side for the oceans. A consequence of the oceans acting as a “sink” for CO2 emitted from burning fossil fuels is ocean acidification, discussed in several recent posts here.

    Archer and Brovkin (2008: 288) point out, “after the invasion of fossil fuel CO2 into the ocean, the acidity from the CO2 provokes the dissolution of CaCO3 from the sea floor. … In the models it takes thousands of years for this imbalance to restore the pH of the ocean to a natural value.”

    After fossil fuel CO2 in the atmosphere equilibrates with the oceans, atmospheric CO2 will still remain about 20-25% higher than pre-industrial levels. Archer and Brovkin (2008: 287) note that, “eventually, the excess CO2 will be consumed by chemical reactions with CaCO3 and igneous rocks, but this takes thousands of years.”

    In an earlier publication, Archer (2005) found that the immense longevity of the tail on the lifetime of CO2 released into the atmosphere means 7% released by burning fossil fuels today will still be affecting the atmosphere in 100,000 years, and the mean lifetime of CO2 in the atmosphere is 30,000-35,000 years. He suggested an appropriate approximation of the lifetime of CO2 released by the burning of fossil fuels for public discussion is “300 years, plus 25% that lasts forever”.

    We commonly think of our children and grandchildren to appreciate the consequences of our present actions but as our present emissions of fossil fuel will continue to affect the atmosphere for over 100,000 years, we should appreciate the decisions on climate policies today will affect the next 5,000 generations of humanity and beyond."
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  4. In other words there is a difference between the lifetime of a given CO2 molecule and the lifetime of the perturbation of (increase in) the CO2 concentration.
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  5. Yep #4 kmcolo is right:

    As long as there is extra CO2 in the atmosphere then it will have an extra warming effect.

    Reproduced here is my reply to a question over at the i-phone ap page.

    The issue is the difference between residence time and lifetime.

    Residence time is the average time a given molecule with (if they had them) a given serial number stays in the atmosphere. CO2 is constantly undergoing exchange processes. That is, a plant takes up a molecule of CO2 and is removed from the atmosphere. At the same time an animal may be breathing out a molecule of CO2 produced by “burning” some plant matter. So long as total biomass is roughly balanced this causes no net change in atmospheric CO2. (Indeed, the sawtoothing in the Keeling curve shows what happens each Northern Hemisphere spring as the plants grow their leaves back and suck up CO2 released by their leaves rotting the previous autumn).

    It turns out that the average time a given molecule of CO2 spends in the atmosphere is only a few years. BUT residence time is meaningless in this concept. My bank manager does not care how much I spend so long as I have money coming in to cover outgoings. However, if IN
    Lifetime is how long before a molecule is removed permanently and not just exchanged. Some molecules are removed by undergoing change – methane is oxidised to CO2 for example. However, CO2 is (almost) chemically inert and so is only removed by an increase in total biomass or by dissolution in the oceans. The dissolution process has a bottleneck and it will be centuries before total CO2 in the atmosphere decreases. (Even then we will be in trouble as the oceans undergo acidification).

    See, for example the
    AR4 FAQ 10.3 :
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  6. duog_bostrom (#2),
    There is no need to speculate; just cough up $23 for a copy of "The Long Thaw".
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  7. GC, is it your claim that David Archer is recommending we take a random, unplanned and accidental stab at geoengineering? Is that what you took from his book, that Archer says we have a C02 shortage and should remedy it thereby avoiding another stade?
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  8. The mixing of individual destiny with all is not only for CO2 a popular argument of denier. There would be a saturation of the greenhouse effect: photons from the surface do not reach the space. But there, where the atmosphere absorbs strongly, it also emits strongly. That's photons from the surface not reach the space, but others - who cares?

    Unfortunately, the climate-scientists support that with transparency curves.
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  9. More CO2 and CH4 -> higher temperatures.
    Higher temperatures -> more extra CO2 and CH4 by releases from the ocean and the land and its flora and fauna.
    What is the goal of this escalating effect?
    Why does it happen never in the last several billion years?
    Is the climate a ball on a knife edge?

    The last 65 million years demonstrates that the temperature was explicitly forced by the continental drifts, by the distance between the sun and the earth and by the solar activity. All that time the CO2 and CH4 concentrations in the atmosphere was following the temperature situation. To argue the converse is definitely impossible!

    Why should it happen today in another way like in former times?
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    Response: This is a good question and I strongly recommend viewing the lecture The Biggest Control Knob: Carbon Dioxide in Earth's Climate History by geologist Richard Alley for an excellent overview of the whole issue. Carbon dioxide is removed from the atmosphere by rock weathering. As the earth warms, rock weathering activity removes CO2 from the atmosphere at a faster rate. This acts like a natural thermostat stopping the Earth from getting too hot.

    Conversely, when it gets colder, rock weathering slows both due to lower temperatures and ice sheets covering continental surfaces. This leads to a rise in atmospheric CO2 levels which warms the Earth. Again, CO2 acting like a natural thermostat stopping the Earth from getting too cold.

    However, rock weathering will not save us from human CO2 emissions now. It's a very slow process - the amount of CO2 removed is several orders of magnitude smaller than the amount of CO2 we're emitting into the atmosphere.
  10. Is residence time affected by temperature? And if so, what is the nature of this feedback?
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  11. re HenryH: "Why should it happen today in another way like in former times?"

    It is quite simple - we humans are pushing extra CO2 out with the result that the natural balance is upset, and heat is rapidly being absorbed by Earth. The evidence is in the rapid growth of atmospheric CO2 level. Our time scale is tens of years, the natural one is millenia or even millions.
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  12. On a lighter note, it seems that Richard Alley is something of a musician.

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    Response: That has to be one of the nerdiest songs I've ever seen. Gotta love Richard Alley, what a character.
  13. RSVP, the relationship between global temperatures and the amount of time that CO2 levels remain high is somewhat complicated.

    In general, cold water is a much better CO2 sink than warm water. This can be seen in maps showing net change in ocean pH from acidification... the greatest change is found near the poles where the water is colder and absorbs more CO2. Indeed, if the oceans got warm enough they could eventually reach the point where they were releasing more CO2 than they were sequestering.

    However, warmer temperatures and higher CO2 concentrations also mean more plant life up to a point... and while most of that plant life just briefly stores CO2 before dying and releasing it again, some percentage finds its way to the bottom of the oceans and is locked away there by cold deep sea temperatures.

    In a further complication, the 'snowball Earth' scenario shows that cold isn't an absolute benefit for sequestering CO2 because if it gets cold enough the oceans freeze over... and ice doesn't absorb CO2. This happened around 700 million years ago. The Earth nearly froze solid, but because there was so much ice all the CO2 going into the atmosphere from volcanic sources STAYED there and eventually built up enough to warm the planet.

    Setting aside the extremes and taking the balance of forces we'll see diminishing returns on carbon sequestration as the temperature increases. The most recent studies show conflicting results on whether this 'saturation' and accelerated atmospheric accumulation has begun.

    Not that it really matters on any practical timescale. One need only look at the ice core records to see that every time CO2 levels have risen ~100 ppm (usually taking a couple thousand years, but only about 150 in the current case) it has then taken around 100,000 years to slowly descend back down to that previous level. So, unless we have some way of vastly increasing the speed of carbon sequestration, comparable to the vast increase in its release we have introduced through burning fossil fuels, we've got a clear record that natural processes will require 100,000+ years to reverse the increase we have NOW (~390 ppm from 278 before the industrial revolution) and much longer as we continue to use fossil fuels going forward.
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  14. Seems that it is very difficult to explain the difference between residence time and excess lifetime. The first is governed by the exchange rate over the seasons, which is about 20% of all CO2 in the atmosphere, leading to an average residence time of a CO2 molecule (whatever the source) slightly of over 5 years. The second is governed by the sink rate, which is only 0.5% of any extra amount of CO2 in the atmosphere (whatever the source), slightly under halve of human emissions (land use changes not included).

    This sink rate remained remarkably constant in the past 110 years: 55% of the emissions (as quantity, not as individual molecules) remained in the atmosphere, 45% were absorbed by oceans and vegetation. That the sink rate remained constant was recently confirmed, which means that the oceans still are not saturating.

    The very long tail need some criticism: only when enormous quantities of oil and coal are burned (3000-5000 GtC according to Archer, we are now at a total 320 GtC), the deep oceans will increase substantially in CO2/carbonate content which only slowly will decay. With the current total of emissions, the deep oceans only increased in DIC (dissolved inorganic carbon) with less than 1%. Thus only less than 1% of the current total of emissions would stay in the atmosphere for an extremely long period.

    Further, the deep oceans are a quite good sink. The main sink of the oceans is in the NE Atlantic, where the THC moves cold water rich(er) in CO2 directly into the deep. That may come back in over a thousand years (mixed with the rest of the deep ocean) in the tropical Pacific. The surface/deep ocean exchanges are about 100 GtC/year, but based on the isotopic dilution of the fossil d13C fingerprint, some 60 GtC is directly or indirectly exchanged between the atmosphere and the deep oceans, including most of the 4 GtC sink rate. Thus it seems that the CO2 sink in the deep ocean will continue for a long period, removing most of the extra CO2 in relative short time (half lifetime around 40 years), for 99% of the current amounts, the moment we stop all emissions.
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  15. Yes, indeed, weathering is removing CO2 from the atmosphere...
    But this process cannot explain the rapid up and down in the CO2 concentration in the atmosphere.
    The glacial periods are driven by the distance between the sun and the earth, not by the CO2!
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  16. If the word "flux" was more widely used (or understood) then it might help put across these points.

    That, and the difference between "equilibrium" and "steady state".
    (As the old joke goes:
    "Old chemists never die, they just reach equilibrium")
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  17. HenryH writes: The glacial periods are driven by the distance between the sun and the earth, not by the CO2!

    First of all, it's not "the distance between the sun and the earth," it's the eccentricity of the earth's orbit, obliquity of its axis of rotation, and precession.

    More to the point, there have been plenty of other cases where changes in temperature were driven by changes in CO2. See, for example, this comment and the rest of that thread. There are lots of examples of CO2 causing warming (or decreases in CO2 causing cooling).

    The fact that some fires are caused by lightning does not mean that other fires are not caused by arson.

    The current warming is obviously caused by greenhouse gases, not by Milankovich cycles, which don't operate on decadal to century timescales.....
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  18. HenryH writes: More CO2 and CH4 -> higher temperatures.
    Higher temperatures -> more extra CO2 and CH4 by releases from the ocean and the land and its flora and fauna.
    What is the goal of this escalating effect?
    Why does it happen never in the last several billion years?

    Not sure what you mean by "goal" but this happens all the time, and not just with CO2 and CH4 but with other feedbacks.

    Some initial forcing causes the climate to change (comet impact at Chicxulub, change in solar irradiance, anthropogenic emissions of CO2, large-scale emissions of CO2 from flood basalt episodes, ...)

    This warming (or cooling) is then amplified by various positive feedbacks involving further increases (or decreases) in CO2 and CH4 as you note, as well as water vapor, albedo, etc. There are also negative feedbacks that reduce this amplification.

    The glacial/interglacial cycle is a good example of this. The initial Milankovich forcing is enough to get the process started, but the full magnitude of the swing in temperatures would not occur without the positive feeddbacks.
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  19. FerdiEgb wrote "Thus it seems that the CO2 sink in the deep ocean will continue for a long period, removing most of the extra CO2 in relative short time (half lifetime around 40 years), for 99% of the current amounts, the moment we stop all emissions."

    There is a study also described in the RealClimate post Climate Change Commitments, projecting the rate of global temperature change if humans suddenly stopped all our CO2 emissions.
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  20. Doug,

    Thanks for the guest post.

    I read the oceanic uptake can only remove part of the extra atmospheric carbon, and the rest would have to rely on the geological response of rock weathering, stretching the process to hundreds of thousands years.

    Do you agree?
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  21. doug_bostrom (#7),
    Thus far I have not offered my opinion as I see no point in annoying the thoughtful people who hang out here.

    David Archer has some very interesting speculations with huge implications if true. Archer is trying to model the effects of large CO2 emissions (up to 5,000 Gtonne C) over 500,000 years. He suggests that high emissions could delay the onset of the next Ice Age:
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  22. quokka (#12),
    Great video! I thought it was excellent.

    John Cook is right about Richard Alley's "American Idol" appeal; he will not be able to give up his day job as Harvard's Tom Lehrer was able to do.
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  23. Normally I wouldn't try to speak for David Archer, but having read quite a bit of his work (including The Long Thaw -- I concur with your recommendation of it, BTW) my guess is he'd probably say that:

    (a) Our distant descendants in AD 52010 would probably be able to prevent the onset of the next glacial advance if necessary without any help from us, and

    (b) In the meantime there's much more danger from warming (which is actually happening) than from cooling (which isn't).

    I'm all in favor of prudent concern for the future, but I think we should probably focus on the next couple of centuries rather than some hypothetical condition tens of thousands of years in the future.
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  24. If the rate of ocean transfer of CO2 is controlled by the slow circulation and turn over, taking roughly 500-1000 or so years, then surely the heat transfer will be subject to the same slow circulation and turn over.
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  25. Tom Dayton,

    Thanks for the link... It seems somewhat overdone at RC: Around 2000 there still was some heat "in the pipeline" from the oceans. But that completely disappeared in a few years time. Thus if we should stop our CO2 emissions today, in a period of about 40 years the extra CO2 (some 100 ppmv today) would be halved, including the forcing (+ feedbacks) related to that amount of extra CO2. Thus cooling the atmosphere down as can be seen in the graph for the Bern model, or maybe faster, as the Bern model is rather conservative.
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  26. In the post at RC, Matthews and Weaver appear to have not considered that if CO2 emmision is stopped, aerosol emmisions will also stop. The aerosol emmsiions currently cool the globe (amount uncertain) so temperatures would immediately rise. It looks like their model needs some more work, although it is an interesting way to look at the problem.
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  27. HenryH (#9)

    If you are asking why the earth hasn't headed toward a truly runaway greenhouse effect, another way to answer that is that the temperature increase due to an increase in a GHG is logarithmic in nature. It continually increases, but the rate of increase (1st derivative) continually declines. On the other hand, the amount of energy lost from radiation is proportional to the 4th power of the absolute temperature; energy loss accelerates rapidly with an increase in temperature. A new equilibrium, at a higher temperature, will always be reached, where the curves intersect again.
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  28. Having said that, now I'm not sure if the increase in temperature is inherently logarithmic, or if the net effect is a log because the higher increase in energy loss makes it that way. Might have to go dig that one up.
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  29. HenryH@9

    "Why should it happen today in another way like in former times?"

    There has never before in the history of the earth been a fossil fuel-burning, industrialized civilization. THAT is why what is happening today is different than in former times.
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  30. Alexandre #20:

    In the short term (centuries) CO2 in the atmosphere will slowly come to equilibrium with CO2 in the oceans. By definition this means that the final concentration of CO2 in both boxes will be greater than it was in preindustrial times. Exactly how much greater depends on the time scale we use and the total amount of fossil CO2 released.

    The oceans will acidify as a result of this process. This is a whole post in itself but I direct your attention to the
    UN Convention on Biological Diversity :

    “Ocean acidification is irreversible on timeframes of at least tens of thousands of years and is determined in the longer term by physical mixing processes within the ocean that allow ocean sediments to buffer the changes in ocean chemistry. Warming of the oceans as a result of global climate change may also reduce the rate of mixing with deeper waters, which would further delay recovery.”
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  31. RSVP #10

    "Is residence time affected by temperature? And if so, what is the nature of this feedback?"

    I assume you mean residence time and not lifetime?

    If so then, no, residence time is not directly controlled by temperature. However, while I hate to give another equivocal answer, it depends: Residence time, as explained above, is simply size of reservoir divided by rate of throughput.

    Temperature will not directly alter the size of the reservoir, i.e. amount of CO2 in the atmosphere. (Though perhaps warming will mean less burning of fossil fuels for heating but equally it may lead to increased consumption to run air-conditioners etc).

    Temperature may have secondary effects on throughput rates: One removal process for CO2 is dissolution in the ocean. This is a physical process and the bottleneck is large scale circulation. Which is, in part, driven by winds and temperature gradients. (Though as a complicating factor warm water can hold less dissolved water than cold water).
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  32. Johnd #24

    And the heat capacity of seawater is...(hint: greater than air).

    As suggested above The long thaw makes scary bedtime reading. Emphasis on the long.
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  33. CBDunkerson
    Thank you for the reply. Very interesting.

    & Doug Mackie
    Also interesting. Aside from the question, your answer brings up an idea...

    You refer to powering air conditioners and CO2 released for this. Would it make any sense then (i.e., benefit) to prohibit burning fossil fuels for energy during summer months?

    I would assume not, however, you always hear this point about air conditioners. Maybe air conditioners themselves should be completely banned to simply make people (including the elite) more conscious of global warming.
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  34. A couple of points.

    1) Re: trees. Doug Mackie talks of trees as if they have very little effect on CO2 sequestration; "Every tree that grows will eventually die and decompose, thereby releasing CO2."

    The facts are more complex. True, trees in woods and forests do live and die but in fact the organic matter on the floor of a woodland or forest is gradually increasing (let's face it, that's one of the ways peat is formed) and it ends up as a net CO2 sink. Where they are not being depleted by agriculture or deforestation the world's soils are all constantly deepening.

    Even where soils are eroding mechanically, that soil ends up washing down rivers and it's that organic matter that ends up making a significant contribution to the sediments accumulating on the bottom of our oceans.

    2) Re the CO2 lifetime. I think the discussion tends to over-complicate a simple concept.

    When describing the working of a business I always use the analogy of a bucket with a hole in the bottom. The work the company does, and product it sells, provides money to fill the bucket; while the costs -- wages, overheads, raw materials purchase -- bleeds money out of the hole in the bottom. Equilibrium is maintained while the inputs match outputs.

    The same analogy can be applied to atmospheric CO2. If we think of a molecule of CO2 as a molecule of H2O then the time a specific molecule exists in the bucket is irrelevant; all that matters is the balance between inputs and outputs. Increasing inputs will result in the water level rising; increasing output will cause the water level to fall.

    One can't push this analogy too far, but it does get the point across to some sceptics. It also helps to explain why man's relatively small contribution to the CO2 cycle is so significant.

    I hope that's useful.
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  35. One important point is this-the carbon we're currently burning comes from a time when the planet had CO2 concentrations 10x to 20x higher than at any point during the Quaternary Era. Its important to note that temperatures were a good 4-6 degrees *warmer* then than during the entirety of the Quaternary Era-in spite of the sun being cooler during this earlier time period. This should give some idea of the maximum impact we could expect from raising CO2 emissions *if* we were somehow to burn every ounce of coal & oil ever created. Even if we don't burn that much (indeed, economics will probably demand that we stop burning coal long before this because it will become more expensive to mine than what it can ever be worth-indeed, this day has almost arrived in the bulk of Europe) we'll be able to burn enough to make life on this planet extremely uncomfortable-if not downright inhospitable-for our species. That doesn't sound very intelligent to me!
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  36. Marcus (#36),
    I am a little puzzled by your conclusion that a warmer earth would be "inhospitable" given that mammals became dominant during the Eocene when temperatures were much higher than today.

    My idea of "inhospitable" would be the period ~74,000 years ago when it was so cold that the human race was near extinction. In spite of the technologies mankind is so proud, it will be very difficult for humanity when the next Ice Age arrives.
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  37. John Russell, the problem is that sequestration by trees (and plants in general) cannot keep up with CO2 increases due to humans. On the relevant thread CO2 Is Not a Pollutant there are some relevant comments containing links to sources: #3 by me followed by #4 by muoncounter.

    See also the USDA report I linked to in my comment #1 on that thread.
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  38. Tom Dayton: I know, I know; hence my point 2 about the hole in the bucket (...dear Lisa, dear Lisa).

    I thought it worth making the point about the wider ability of naturally-occurring vegetation to sequester CO2 when left alone to do its thing, which many people choose to overlook.

    The irony is that not only are humans adding to atmospheric CO2 but -- through a lack of understanding -- we're also blocking the ecosystem's correcting mechanisms which have evolved to protect the existence of life on the planet. This is the basis for Lovelock's 'Gaia' theory.
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  39. gallopingcamel writes: I am a little puzzled by your conclusion that a warmer earth would be "inhospitable" given that mammals became dominant during the Eocene when temperatures were much higher than today.

    Our agricultural system and all our infrastructure are predicated on the idea that climate is more or less constant, such that if the north-central US is a good place to grow wheat now, it will continue to be a good place to grow wheat in the future. Or, as another example, we build a fleet of ships/barges to operate on the Mississippi River or the St Laurence Seaway, with the assumption that water levels will not drop enough to prevent fully laden vessels to move through the system.

    In other words, there are a million ways in which our infrastructure is designed around a particular climate in particular places. Changes in temperature or more importantly precipitation can wreak havoc with this.

    You keep arguing that a few degrees of warming is OK because "it's better than another glacial advance." But no one is suggesting we should try to create another round of glaciation!

    The question isn't "Which is worse, too warm or too cold?" The question is "Which is better, the climate we have built our infrastructure around or a climate that's significantly warmer most places, with very different patterns of precipitation?"

    Please stop using a non-existent threat of rampaging glaciers as an excuse to ignore the actual threats associated with CO2-driven climate change.
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  40. I like 'gallopingcamel's' suggestion that humans raising the temperature by a few degrees is acceptable on the grounds that we're pre-empting an ice-age that might possibly affect us, to some unknown degree, at some unknown point in the future!

    If only sceptics were as willing for humans to take dramatic actions to lower temperature by reducing emissions, in the face of the suggested 95% chance that increased temperatures will destroy our children's future.
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  41. @HenryH... You should really watch the Richard Alley lecture that John suggested. It explains the whole issue extremely well. Google: Richard Alley The Biggest Control Knob.

    The point that Alley makes in the lecture is that Milankovitch cycles can only account for a small portion of temperature change. It's those cycles of orbit, precession, etc that set off feedbacks in CO2 and CH4 that result in the swings in global temps that we see.
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  42. robhon, if water vapour responds so strongly with positive feedback to a relatively minor change in CO2 concentration, then it must also respond as strongly directly to any changes in temperature brought about by the Milankovitch cycles, even without any changes in CO2.
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  43. John Russell @34, it is clear from the vast deposits of coal that a tremendous amount of CO2 was stripped from the atmosphere by plants. As CO2 concentrations fell, so too would temperatures as well as plant growth until a glacial period took hold. At that point with plant growth almost stalled or plants dead or dying, the process of stripping atmospheric CO2 would be at a point where decomposition of plant amterial would begin releasing previously sequestered CO2 producing conditions to allow for a recovery from the glacial conditions.
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  44. What is so hard about seeing the difference between a cooling over 10,000 years and warming over 150 in terms our capacity to adapt? If you are so desperate to avoid an ice age, then we should immediately sequester all fossil fuel so that 3000 years down the track, there will be something to burn for atmosphere enrichment.

    Frankly it seems to more like clutching at straws - believing anything rather than accept the need to deal with the problem.
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  45. #33 RSVP

    Bugger. Note to self: Use emoticons.
    My comment re NH air conditioning was jocular. Energy use is not likely to change voluntarily.

    As Working Group II reports make clear: Warming will be good for a few people. E.g. (maybe) Canadian and Siberian grain belts.

    However, it isn’t it obvious that if, for example, the USA/China endured dust bowl conditions then they would pay what the Canadians/Russians asked to avoid their people starving. I mean, sure they have nukes and all that but they would never....

    Would they?
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  46. #36 Gallopingcamel

    Mammals predate dinosaurs and lived in their shadows for 160 million years. After the KT event birds (poncy dinosaurs) dominated in many ecosystems. Mammals were very late starters.

    Mammalas have less advantage in warm conditions but do have an advantage in colder conditions. (e.g. Think cold morning start up time).
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  47. #42 John D

    Point is that water vapour is the result of temperature while CO2 causes temperature. AR4 FAQ does this in simple terms. As soon as water vapour concentration gets too high it rains.

    #43 John D

    The issue is timing. At glacial onsets/terminations it takes at least 10,000-15,000 years for CO2 to decrease/increase by ~100 ppm (and temperature change by ~10 decC). We have increased CO2 by 100 ppm (over and above the "usual maximum" of 280 ppm) in just 200 years.

    Temperature lags (but will follow CO2) and ecosystems are struggling to keep up with the rate of change.
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  48. John Russell (#40),
    Thanks but I can't accept credit for the idea that the next Ice Age can be delayed if we put 5,000 Gtonne of C into the atmosphere. Two men deserve the credit, namely Archer and Ganopolski.

    I have been very careful to avoid offering an opinion on A&G's analysis. I was hoping to get a few insights from the clientele on this blog.

    Should we try to delay or accelerate the next Ice Age? Would the idea work?

    I do not buy Ned's argument that the next Ice Age is so remote in time that we don't need to think about it.
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    Response: An ice age is only possible if northern ice sheets grow from year to year. So if you see a gigantic ice sheet creeping down northern Canada, then yes, it's time to start worrying that we're heading into a new ice age. For now, I think you can relax about an impending ice age (or as my daughter likes to say to me, chillax). Ice sheets can collapse relatively quickly because ice dynamics cause glaciers to slide faster into the ocean. But the speed that ice sheets can grow is limited by the amount of snow falling each winter.
  49. I believe that delaying the next ice age is off-topic for this "residential lifetime" thread.

    Gallopingcamel, you would be a more constructive commenter if you posted your comments in the appropriate threads instead of whichever one you happen to be reading at the moment.
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    Response: I have deleted one of Gallopingcamel's recent comments for being off-topic. However, an upcoming ice age is somewhat on-topic in the sense that residential time of CO2 does affect whether we can fall into an ice age any time soon. Archer's studies find that a significant chunk of the CO2 we emit will remain in the atmosphere for thousands of years hence an ice age is indefinitely postponed.
  50. Doug Mackie @ 47 re "Point is that water vapour is the result of temperature while CO2 causes temperature" My point was/is, CO2 is not the only mechanism that can alter temperatures that water vapour may respond to. Would the Urban Heat Island effect also qualify as a driver of water vapour? It certainly is measurable and widespread.
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