Hansen (et al) must read: Get back to 350 ppm or risk an ice-free planet

Here is the draft of the long-awaited defense of why we need an ultimate target of 350 ppm for atmospheric carbon dioxide, by NASA’s James Hansen et al., “Target Atmospheric CO2: Where Should Humanity Aim?” [Yes, they know we’re already at 385 ppm and rising 2 ppm a year.]

The paper does suffer from one inherent analytical weakness that makes it (a tad) less dire than it appears — and some people believe the core element of this analysis is wrong (see very end of post), although I don’t.

This paper is really just a continuation of Hansen’s earlier analysis arguing that the real-world or long-term climate sensitivity of the planet to doubled CO2 [550 ppm] is 6°C — twice the short-term or fast-feedback-only climate sensitivity used by the IPCC. [You might want to read this post first since it is a bit clearer on the difference between the two sensitivities.]

The key paleoclimate finding of the article:

We infer from the Cenozoic data that CO2 was the dominant Cenozoic forcing, that CO2 was only ~450 ppm when Antarctica glaciated, and that glaciation is reversible.

That is, if we stabilize at 450 ppm (or higher) we risk returning the planet to conditions when it was largely ice free, when sea levels were higher by 70 meters — more than 200 feet!

Three years ago, Hansen (and others) argued in Science that [due to fast feedbacks], we would warm another “0.6°C without further change of atmospheric composition” [i.e. with no more CO2 emissions]. Now he’s saying “Warming ‘in the pipeline’, most due to slow feedbacks, is now about 2°C.” So the paper concludes:

An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

The inherent weakness of the paper from a policy perspective is that even if you accept their analysis (which many will not), the authors do not know how long we can overshoot 350, which is a function of not just the duration of the overshoot, but the magnitude (i.e. how high concentrations go). They note: “The time needed for slow feedbacks to ‘kick in’ is uncertain. Current models are inadequate and no paleoclimate analogue to the rapid human-made GHG increase exists.” We are truly running a first-of-a-kind experiment on the climate.

The authors write “paleoclimate and ongoing changes, and the ocean response time, suggest that it would be foolhardy to allow CO2 to stay in the dangerous zone for centuries.” Well, of course, but centuries is a long time. The authors argue:

Humanity’s task of moderating human-caused global climate change is urgent. Ocean and ice sheet inertias provide a buffer delaying full response by centuries, but there is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control.

That, of course, is a central point of this blog.

On the other hand, the authors make clear that reducing concentrations is not easy even if we do not key cross carbon cycle feedback tipping points. Moreover, recent analysis suggests that “if emissions were eliminated entirely, radiative forcing from atmospheric CO2 would decrease at a rate closely matched by declining ocean heat uptake, with the result that while future warming commitment may be negligible, atmospheric temperatures may not decrease appreciably for at least 500 years.

So I suspect the authors are right that 450 ppm is too high if maintained for even a few centuries. On the other hand, realistically, 350 ppm is simply not going to be seen again this century. The authors write:

This target [350 ppm] must be pursued on a timescale of decades, as paleoclimate and ongoing changes, and the ocean response time, suggest that it would be foolhardy to allow CO2 to stay in the dangerous zone for centuries.

The ill-defined difference between decades and centuries is key. What if we could keep the peak below 450 ppm, and start concentrations declining by 2100, which would almost certainly require near-zero if not net-negative global emissions, and then get back to near 350 ppm by, say 2150 and then even lower by 2200? Would that be good enough? As I argued in my book, I believe that with a World War II scale effort for the next few decades, we could stay below 450. My take away from this paper is that we would need to keep up that level of effort through 2100 — to get back below current levels.

The final point of the paper deserves reprinting:

Present policies, with continued construction of coal-fired power plants without CO2 capture, suggest that decision-makers do not appreciate the gravity of the situation. [Note to Hansen et al: That is the understatement of the year.] We must begin to move now toward the era beyond fossil fuels. Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects.

The most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is herculean, yet feasible when compared with the efforts that went into World War II. The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.

Okay, so we should have listened to Hansen two decades ago. The time to act is yesterday. He has been right longer than anyone I know.

One final point. Some pretty smart people think Hansen is wrong about the long-term climate sensitivity issue (start here). If I am reading that criticism correctly, then I think Hansen responds to it in his new paper.

Also, if I am reading Hansen et al correctly (and Lord knows I may not be), then I think he may be mostly right for a different reason than he thinks, which is to say, I think the carbon-cycle feedbacks (including the tundra melting and sink saturation) act as the equivalent of the amplifiers that he models (“loss of Greenland and Antarctic ice and spread of vegetation over the vast high-latitude land area in the Northern Hemisphere” — I will come back to that vegatation issue in a future post). In other words, if you get near 450 ppm and stay there for any length of time, you will shoot up to 700 to 1000 ppm, which certainly gets you an ice-free planet. Or perhaps the simplest way to put this — the IPCC is right when it says:

Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilisation level of atmospheric carbon dioxide concentration. Based on current understanding of climate carbon cycle feedback, model studies suggest that to stabilise at 450 ppm carbon dioxide, could require that cumulative emissions over the 21st century be reduced from an average of approximately 670 [630 to 710] GtC to approximately 490 [375 to 600] GtC. Similarly, to stabilise at 1000 ppm this feedback could require that cumulative emissions be reduced from a model average of approximately 1415 [1340 to 1490] GtC to approximately 1100 [980 to 1250] GtC.

We’re at 8 GtC/yr and rising 3% annually. We need to average below 5 GtC/yr — and maybe considerably less — for the whole century to avert catastrophe. We need to be near zero or below by 2100.

My Bottom Line: Let’s start working now toward stabilizing below 450 ppm, while climate scientists figure out if in fact we need to ultimately get below 350.


43 Responses to Hansen (et al) must read: Get back to 350 ppm or risk an ice-free planet

  1. Ken Levenson says:

    Great post – I agree wholeheartedly.
    If anyone is wondering what else they can be doing to help stabilize, go to:
    and download the checklist.
    copy it. edit it for your local conditions. make it your own.

  2. Bob B says:

    Ken, maybe Hansen can answer this since he believes in positive feedbacks?

  3. Bob B says:

    Of course you can take projections from the lat 7 yrs of cooling and see how we compare to Hansen and IPCC projections:

  4. Ken Levenson says:

    Bob B. – See Joe’s post from yesterday.

    I’ll take the opportunity to elaborate a bit on my previous comment. I agree we need to just start shooting for reductions whether the target is for 450ppm or perhaps a bit more or less. I would just emphasis that the cuts need to be front-loaded, and not back-loaded the way the Feds to deficit reductions. That in itself will be a revolution.

    I’m betting that the science will tell us pretty conclusively within the next couple of years that we really need to be going down to zero far sooner than 2100. So if we don’t front load, it will be another couple of wasted years. (Many wishy-washy qualifiers here, I admit, but there are many uncertainties – as Mr. Revkin tells us.)

  5. Ronald says:

    Ken Levinson,

    I’ll agree with you on what you wrote that we just need to reduce carbon dioxide release as a goal. I suppose those who are planning the numbers for what a Cap and Trade will look like when climatologists and other scientists give the world of what the numbers have to be are concerned about whether it’s 450 ppm, 500 ppm or whatever the number will be. I, by the way, don’t see how any of those numbers are going to be met, but there is always hope.

  6. Bob B says:

    Mr Revkin knows nothing. He is NOT qualified, so why listen to him?
    Why not do research and gather the latest data? The latest data is saying there in not an issue. So why waste money now?

  7. David B. Benson says:

    Bob B — There certainly is an issue. Unfortunately, Lucia did her stats wrong. Here are four examples of how to correctly determine temperature trends:

  8. PGosselin says:

    The number will not be met, but it won’t matter because CO2 is just a pee-wee driver and nothing more.

  9. Ellis says:

    That is, if we stabilize at 450 ppm (or higher) we risk returning the planet to conditions when it was largely ice free, when sea levels were higher by 70 meters — more than 200 feet!

    Does not follow from

    The key paleoclimate finding of the article:

    We infer from the Cenozoic data that CO2 was the dominant Cenozoic forcing, that CO2 was only ~450 ppm when Antarctica glaciated, and that glaciation is reversible.

    I believe you are missing a de in there. Small thing, but I am pretty sure you don’t want to misrepresent the Man’s Words.

  10. David B. Benson says:

    Bob B — Here is a link to a discussion of a recent paper in “Science’ taking the temperature trend over essentially the same period as Lucia mis-analyzed:

    PGosselin — I suggesst that you read “The Dicovery of Global Warming”, linked here:

    so that you will no longer appear to be such an ignorant fool.

  11. Joe says:

    Hansen was saying our current glaciation is reversible.

  12. Uosdwis says:

    You know, we’re going to have to FORCE power companies to sequester, physically deploy troops or police or something to watch them do it, because EVEN if laws are passed mandating the practice (and they will be appealed for YEARS, years we don’t have), they will ignore them if at all possible. I think we’re doomed as a species, we’ve probably passed the tipping point and don’t know it yet, or there is nothing meaningful that will happen to slow or reverse it.

  13. David B. Benson says:

    Ending Global Warming, Restoring the Holocene

    Hansen et al., March 2008,”Target CO2: Where Should Humanity
    Aim?” found here:

    states that we need to soon reduce atmospheric CO2 from the
    current 385 ppm to an initial 350 ppm for compeling reasons.
    Here is an outline of a plan for doing so.

    The world economy is about 67 trillion dollars (GDP).
    Imposing a VAT of 1% raises then 670 billion dollars per year.
    This sum is used to grow biomass, convert it to biocoal, and
    sequester the biocoal in carbon landfills, every year until
    the goal of 350 ppm is met.

    Using Powder River Basin style earth movement, it would cost
    about $16.50 per tonne to sequester the biocoal. In addition,
    the biomass must be harvested, moved to the hydrothermal
    carbonization facility, converted to biocoal (while generating
    some process heat for electricity generation), and then the
    biocoal moved to the landfill site. By conducting all these
    operations in parts of the world with ample excess land and
    low-cost labor (Africa) I will assume these steps can be done
    for only $33.50 per tonne, under half the amount required in
    the developed world. Thus the carbon capture and
    sequestration net costs are assumed to be $50 per tonne of

    I will assume, for simplicity, that the biocoal is 85% carbon.
    Humans are currently adding about 8.5 gigatonnes of carbon
    (GtC) to the active carbon cycle per year, mostly by burning
    fossil carbon. Just to maintain the current 385 ppm of
    atmospheric CO2 then requires producing and sequestering
    10 gigatonnes of biocoal per year. This costs $500 billion
    per year, leaving a net of $170 billion available for
    producing and sequestering additional biocoal to reduce the
    concentration of CO2 in the atmosphere.

    To reduce the concentration to 350 ppm requires removing
    about 185 GtC from the active carbon cycle. At the rate
    of ian additional 3.4 gigatonnes of biocoal per year, using
    the net funds available, we would remove 2.89 GtC from the
    active carbon cycle each year. Assuming this is done at
    a steady rate, it will require 64 years to bring about the
    desired initial atmospheric CO2 concentration of 350 ppm.

    About Biocoal

    Popular accounts:,2144,2071791,00.html

    A demonstration plant is described in the following link.

    Technical article:

    M.-M. Titrisci, et al.,
    Back in the Black: hydrothermal carbonization of plant
    material as an effiecient chemiccal process to treat the CO_2
    New Journal of Chemistry, 207, 31, 787–798 (25 references). (Linked below)

    or as a .pdf file

  14. Barry Brook says:

    Regarding the criticisms Joe cites about the long-term climate sensitivity issue.

    The main point of the new paper is to use Cenozoic data to counter the criticism that the slow forcing mechanisms of the glacial-interglacial cycles are not effective in the modern context. At 35 My there was no effective ice albedo operating because the planet was largely ice free. Climate sensitivity was at ~3C. After that point, triggered by geological drawdown of CO2 via weathering > subduction outgassing, ice albedo starts to kick in, as the Antarctic ice sheet accumulates. Figs 5 and S4 are the key to understanding this. Which is the better fit – 600, 450 or 325 ppm at the 35 Ma trigger point? Hansen et al say somewhere between the 325 and 450 model seems about right, and it is hard to disagree (look also at Fig S9 for proxy alternatives). In addition, as Joe has already said, there are plenty of other non-albedo amplifiers that are likely vulnerable at 1-2C above current temperatures but plausibly “out of reach” at Holocene-type temperatures. It is not implausible to imagine flat spots where one feedback plays out before others kick in. See Lenton et al 2008 PNAS 105: 1786 for a recent discussion of some likely candidates.

    Regarding the implied lag of 700 yrs between the solar trigger and CO2 feedbacks, the shorter (century to decadal scale response) is inferred on the basis that that there must have been some [unknown] period of time between initial forcing and sufficient CO2 feedback to create a net imbalance in airborne CO2 – probably on the order of centuries due to the relatively small size of the net global solar forcing (~0.25W/m2). There is now no need to await a feedback – we are already in substantial energy imbalance due to direct anthropogenic GHG forcing.

  15. Bob B says:

    David, you are the fool. Your hero Tamino is getting his butt kicked left and right on CA

  16. Bob B says:

    The bottom line is warming is NOT fitting the IPCC projections. And Like Joe all I can say is wait a couple more yrs and I hope all the NASA hockey team gets fired

  17. Bob B says:

    BTW in that same post Tamino’s 1975 breakpoint analysis was shown to be arbitrary

  18. Bob B says:

    David and Barry please indicate where the tipping point lies on this graph?

  19. Dano says:

    I like it that the only thing denialists have is cherry-picking. Tomorrow it will be false premises, and the day after it will be straw men, but still. They still have no testable hypotheses, model output, formulæ, equations, published papers, scribbles on a napkin, nada.



  20. Bob B says:

    Dano, Roger Pileke has asked RC multiple times for tests/benchmarks for their climate models—crickets chirping. He has asked them for a benchmark they can use to show the models could be wrong–still no answer. Hell dem darn climate models can model temps 3000yrs from now–but they can never prove it.

    Roger has used 22 GCM models and has showed them to be trash modeling the temps in the tropics

  21. Bob B says:

    It is up to the alarmest to PROVE things–not the other way around–that is how science works

  22. Bob B says:

    The AGW alarmest hold press conferences for 1 yr of sea ice melt and we hear impassioned cries for dying polar bears.

    I am sure RC and NASA will call a press conference to say the South pole sea ice is way above normal or that the ice is back in the North:

  23. jorge c. says:

    dear Mr.David B. Benson:

    you said: “By conducting all these operations in parts of the world with ample excess land and low-cost labor (Africa) I will assume…”
    do you know the opinion of of the “low-cost labor(ers)”??? i think that they are citizens of independets nations… aren’t they??? well, they are… to put it mildly “third world human beings” and so below your first world level!!
    if you think that your solution is right, why not in u.s.a.??? there are ample excess land too!!!
    sorry, i found your post a bit racist and colonialist.
    and if you and Dr.Hansen have found the cure for the global warming, why don’t you run for congress with your solutions as political platform??
    i beg all of you perdom, i’m old an spanish speaking…

  24. Dano says:

    Of course the GISS2000 GCM backcasted just fine. Do we hear about it at denialist sites? Huh. What about all the other CMIP tests? No? Golly.



  25. Bob B says:

    Dano, prove it. Please also prove the models predicted Ocean cooling since 2003

  26. David B. Benson says:

    jorge c. — Apologies, I certainly didn’t mean to seem either racist or colonialist. I happen to know a bit more about the situation in Africa than in South America. If somehow this plan was adoted, I would expect activity in both continents and also in the Commenwealth of Independent States, which probably qualifies and developed.

    The issue is the labor cost of collecting the biomass and the transportation cost of taking it to the hydrothermal carbonization facility. The labor rates in the U.S. and Western Europe are such that the price would approximately double while in Africa there are many people who would be happy to have a job which pays a steady, living wage.

    I would expect a small portion of the activity in the developed countries, in those situation where the biomass is being collected anyway. Forestry wastes are an example and the demonstration project in The Netherlands uses forestry wastes to product 75,000 tonnes of biocoal per year.

    Thank you for raising this point. I’ll rewrite that portion in an attempt to account for sensitivites.

  27. David B. Benson says:

    Bob B blithered “It is up to the alarmest to PROVE things–not the other way around–that is how science works”. Nope. You might actually go learn (1) how science works, the scientific method, and (2) some climatology. For the latter, try reading “The Discovery of Global Warming”, linked here:

  28. cloudy says:

    What is needed is a group of activists focusing on THIS PARTICULAR ASPECT of the Greenhouse Gas (GHG) issue specifically: the urgent need to aim for GLOBAL NET NEGATIVE GREENHOUSE GAS EMISSIONS. Then this group of activists must pressure other progressive groups, leaders, and institutions (such as churches and unions) to go along with embracing the need for this. Along the way, the “leaders” of this issue, like Gore, will need to shift from advocating 80% reduction of GHGs by 2050 (while we are all distracted by the debate against the global warming deniers) to what is REALLY needed as a MINIMUM.

    People interested are invited to post their concerns for the need for these demands, w/ emails, at:

    solidarity, love, AUTHENTICITY, and peace

  29. Bob B says:

    David, I know the scientific method. AGW falls into the unproven theory category

  30. David B. Benson says:

    Bob B falsely states “David, I know the scientific method. AGW falls into the unproven theory category”. (1) Obviously you don’t understand the scientific method well enough. (2) (i) GLobal warming is occuring. (ii) CO2 levels rising, causing (i). (iii) Carbon in the excess CO2 is confirmed to be from the buring of fossil fuels, by at least two different methods. (iv) Therefore AGW is confirmed.

  31. Bob B says:

    You are joking right?

  32. Bob B says:

    Maybe Climate science by the Hockey team, but NOT real science elsewhere

  33. David B. Benson says:

    Bob B — Yes, real science by actual scientists. Read “The Discovery of Global Warming”, linked here:

  34. Jeff says:

    Over a year ago I had said we needed to bring CO2 levels down to ‘358’ by this past Nov. 13th 2007, so its interesting that a lead Climatologist would now be under-cutting that number.
    Shamefully we call it’Global Warming'(or Cooling), because its an issue of the breakdown of the ‘Eco-System'(and that’s Man-Made), and if the debate was reframed this way, maybe progress would result.

    Its going to be a tough bird trying to convey that the last 200 years of understanding our lives, economy, etc.,,,, was ‘Wrong’.
    The other issue is that when you talk about addressing this issue, you’re also talking about ‘National Security’, and do that one thing alone, it would take no less than either a miracle or catastrophe to get everyone around the globe upon the same page(like in ‘War of the Worlds’).
    *I gave the Brookings Institution a scenario on how to do this(how to initiate what I call a Psychological Coup).
    The point is to enlist everyone’s immediate attention, because I believe this Planet has become ‘Neuro-Pathic’, and soon things are going to spiral out of control.

    Thing is, I believe the only answer going to be delivered is ‘War'(to both correct the collapsing Global Economy, and the ‘CO2’/Warming issue).

    Oh… that number ‘358’ is the Gematria Number of the ‘Messiah'(quite a long story on how I came around to that number as the critical number- and I won’t go into it any longer— I tried for a full year to make this understood).


  35. jorge c. says:

    mr.david b.benson:
    o.k., apologies accepted, and thank you for your reply

  36. Mauri Pelto says:

    As a glaciologist who peer reviews many papers, Hansen’s paper is very detailed, thorough and even difficult to fully appraisse, since it covers such a wide swath of material. I find it compelling and provoking. That is an important goal to provide sound science will argued, that provides much incentive for further examination. I would suggest having modelled ice sheets and reviewed ice sheet models. That the statement “Critical physics of ice sheet disintegratioin is absent in existing ice sheet models.” May not do justice to the models and research we are doing. The citation is Hansen, not a glaciologist, and not someone versed in the details of this area, which raises a question in my mind as to what this is in reference to. The no discernible lag between insolation change and ice melt, is also not referenced to specific glacial geologic data. This is not to say either of this points is off base, just that the case is not specifically made strongly. Lastly, Hansen understates the alpine streamflow issue. The glaciers many of us observe are shrinking and disappearing in some cases, which does reduce summer streamflow. However, this is related to the earlier melt of the surrounding alpine snowpack in non-glaciated regions, resulting in even greater reductions in late summer streamflow in glaciated alpine mountain regions. It also indicates the greater ratio of winter rain events that help raise winter streamflow, while reducing the aforementioned snowpack.

  37. Mule says:

    To Uosdwis,
    I think you’re right. You should go ahead and kill yourself now to forestall humanity’s destruction. Oh, but wait, you’ll release carbon when you decompose. Best sequester your body down a dry oil well.

  38. snydly says:

    Looks to me that from the IPCC ice core chart/650ky that temperature is the trigger for massive defeat and reversal of the temp dev and CO2 spikes each 110ky, for if the trigger was CO2, it would have already happened. There are other forcings beyond CO2/GHG–I include the following , although somewhat tongue in cheek, for comment:
    Hansen is a brave and good man. I would go farther (it’s just this damn mouth) and add that within three years there will be oceanic chaos as the water that used to be ice stops the thermohalene circulation of the oceanic conveyer current, then gathers by centrifugal force at the equatorial bulge thus redistributing the spinning mass of the planet, which will crack the tectonic plate boundaries (that are already twice as active as they were 2 yrs ago, see data/intern’l&8-30 days) resulting in earthquakes which will loose landslides (as on Canary Isl/Azores, I think), possibly encourage a big chunk of the Greenland ice cap to slip on its now slushy base layer, producing Atlantic tsunamis, rogue waves, methane pocket release from the ocean floor, at the same time as the mid-Atlantic ridge cracks open, and underwater volcanoes activate spewing molten lava which boils the oceans, steaming the atmosphere, triggering the monster storms depicted in Day After Tomorrow which dump a 1000 feet if ice on all the nuclear power plants above 30′ N which then do china syndrome creating radio-active steam chimneys in the ice blanketing at least the northern hemisphere with radioactivity which will freeze in the ice layers being laid down by the permanent storms, then the magnetic poles will flip in the magma of the earth’s core generating enough static electricity to cause the planet to glow like a sickly green lightbulb which is the signal for the aliens to land and start roasting weinies as they stroll through the cold, dead universe looking for something to do. (I mean, that’s bad, but you know what really gets me steamed?—The neocons will be fat, dumb and happy on their estates in Uruguay. What a pisser.)

    I wrote Al Gore 2 years ago with this information and have only heard back from a staffer thanking me for the tip on the mid-Atlantic ridge magnetic striping. I should go back and get my masters in Bizarre Behavior.
    Enjoy the deluge.
    Cheers, snydly.

  39. Kenn says:

    I cannot believe that a supposed educated man in the form of Hansen can attempt to persuade others that it would be a good idea to jail those who are not as committed to the myth of global warming as he is. This is America Mr. Hansen. If you want to force your lies on others, move to a country that openly would accept that sort of behavior. We will not put up with such nonsense in this country. Let’s put the ball in your court; I think you sir should be jailed for spreading lies and distortions about the myth of manmade global warming. Since there is absolutely no scientific evidence that man has now or ever done anything to help or harm global warming, I think, in a court of law, you would lose.

  40. Zach says:

    Has “Bob B” ever indicate what his background and credentials are?