Two degrees is out of reach

This is the worrying news coming from the latest-generation of climate modelling efforts, just published in Geophysical Research Letters (abstract) by a Canadian team of scientists. The team use an Earth system model, fired up with a new generation of carbon emissions scenarios, to conclude that even limiting the 2100 temperature rise to 2.3C above pre-industrial would require “an immediate and rapid ramp down of emissions, followed by negative emissions (sequestration) in the later half of this century”.

As for the goal of holding “the increase in global average temperature below 2C above pre-industrial levels”, well, to get onto that trajectory we would have to take global emissions “down to zero immediately”, because the allowable cumulative carbon budget associated with that temperature target has already been emitted. Whoops! This is unfortunate because keeping below 2 degrees is now the world’s ‘official’ goal, which was agreed at Cancun amidst great celebrations (pdf here – see para 4). Now, in the cold light of day, it looks like we’re already too late.

This new piece of rather pessimistic science makes a particular mockery of one of the big wins from Cancun for small island and other vulnerable countries – the promise of a negotiating process between 2013-15 to review the adequacy of the global goal and to consider strengthening it towards limiting the global temperature rise to 1.5C. It looks like this latter target is already in fantasy land, and will be even more so in four years time.

Can we come back to this in four years?

One of the ideas of the ‘review’ is to take into account input from the IPCC’s fifth assessment report, due out early in 2015. Although this latest study is based on results from just one model (CanESM2), it is driven by the new ‘representative concentration pathways’ which will be a key aspect of the next IPCC report. These replace the old ‘SRES’ storyline emissions scenarios which were first developed back in 2001 and fed into the Fourth and Third Assessment reports. This result is especially important because it is based on the outputs of a full Earth system model, which takes into account land-ocean and and other carbon-cycle feedbacks not fully represented in earlier climate models – and hints that the 2015 IPCC report will be more pessimistic about achieving temperature goals than earlier reports have been.

It would be nice if this dose of real-world cold water could increase the sense of urgency in the climate negotiations currently underway in Bangkok. Judging from today’s opening plenary, this seems unlikely – parties were wrangling over the mere content of the agenda for the negotiations over the year ahead, and the ‘global goal’ issue is barely even up for consideration amidst the more technical issues of what a future treaty might look like and how it might work.

Even so, there is little point in giving in to despair and torpor, even with the glacial pace at which the negotiations are proceeding. Sooner or later, the world’s big emitters are going to have to come to an agreement which limits the temperature rise we can expect over the century ahead… and 2.3 degrees is still a lot better than the 4.7 degrees predicted by the model if emissions go on rising past mid-century because we all give up and do nothing. We will have to come back to this, year after year after year, until a deal is done.

Update, 6 April, 10.30am (Bangkok time):

I’ve just had a conversation with Malte Meinshausen, who contributed to the  ‘Representative Concentration Pathways’ (greenhouse forcing scenarios) that the new Canadian climate model results are based on. He emphasised that it is important to bear in mind that analyses of many different models will be feeding into the IPCC’s Fifth Assessment Report, and that some or many of these may give a less pessimistic picture than CanESM2 does. It does appear that CanESM2 has a rather high representation of 20th-century temperature rise (of 0.9C rather than the observed 0.76, as reported in the paper), so may be on the warm side regarding climate sensitivity. Like everyone here in Bangkok, he is very busy, but if I can persuade him to write a few summary paragraphs for this blog, I will post them straight up here.


  1. Icarus62

    Does anyone actually think that we could stop at 2.3°C? With all the carbon cycle and albedo feedbacks – methane clathrates, permafrost, forest die-off, ocean CO2 outgassing, ice sheet albedo and so on – apparently just waiting in the wings, and no obvious negative feedbacks to counteract them, it seems to me very unlikely that we won’t have triggered a positive feedback that exceeds our anthropogenic forcing by the time we get to 2.3°C. In that event it’s not going to stop until we get 5, 6, 7… who knows how many degrees of warming.

    Even sequestering our own several hundred billion tons of atmospheric CO2 seems a monumental task – to imagine that we can keep up with it if nature is contributing as much or more than we are, must be completely unrealistic.

    It’s hard to even imagine what would stop the process once it’s under way. We would certainly lose all the ice on the planet, all the permafrost, much of the forest… where would it end?

  2. David

    Hard not to agree with Icarus62’s comment. It really is hard to imagine what would stop the positive feedback loops once they’re underway. Perhaps some serious terra-forming work such as the one’s mentioned in the Forum For The Future post I’ve pasted below. But in the event that humanity is being overwhelmed by severely rising temperatures, massive starving populations and associated refugee crises, the oil and fertilizers running out, water scarcity and presumably a fracturing world economy and perhaps war does anyone really think we could get our act together to cooperate and pay for such concepts? Has anyone done any studies into just how much the temperatures will rise if the feedback loops all conspire together into a worst case scenario? Obviously such a scenario would most likely devastate human civilisation as we currently know it.

  3. Manoel

    I wonder: does this new model take peak-oil into account or just assume that we’ll have plenty of oil for decades?

    1. Mark Pawelek

      Manoel – peak oil is a myth.

      Global energy resources in ZetaJoules
      – chart compiled by 1984 Nobel physics laureate Carlo Rubbia

  4. Lou Grinzo


    First and foremost, thanks for everything you’ve done and continue to do in covering climate change.

    Second, I want to add a bit of context to the infamous 2C guideline. I’ve been researching the origins of this number and exactly how it came to be considered “the” magic number for some time, and it’s quite clear that it was much more a barely-educated guess chosen out of expediency than anything else. (“Barely-educated” is a reference to the state of climate science a few decades ago, which has advanced tremendously since thanks to the work of brilliant and dedicated scientists.)

    I pointed out on my own site on December 31 that there was a UN-sponsored study that included 158 people from 58 countries, a sort of primitive, proto-IPCC effort, that said:

    “Clearly man has had nothing to do with these vast climatic changes [moving in and out of ice ages] in the past. And from the scale of the energy systems involved, it would seem rational to suppose that he is not likely to affect them in the future. But here we encounter another fact about our planetary life: the fragility of the balances through which the natural world that we know survives. In the field of climate, the sun’s radiations, the earth’s emissions, the universal influence of the oceans, and the impact of the ice are unquestionably vast and beyond any direct influence on the part of man. But the balance between incoming and outgoing radiation, the interplay of forces which preserves the average global level of temperature appear to be so even, so precise, that only the slightest shift in the energy balance could disrupt the whole system. It takes only the smallest movement at its fulcrum to swing a seesaw out of the horizontal. It may require only a very small percentage of change in the planet’s balance of energy to modify average temperatures by 2°C. Downward, this is another ice age; upward, a return to an ice-free age. In either case, the effects are global and catastrophic.”

    This is from the book form of the report, titled Only One Earth, which was published in 1972(!).

    My point is that for all the difficulty we’ll have in meeting this official goal of 2C by 2100, it’s a really bad goal. Not only is 2C likely much too high, but turning the year 2100 into a magic finish line is even worse, as 40% of the warming from the CO2 we emit by that date will happen after 2100. (See David Archer’s The Long Thaw, page 162.)

  5. Martin Kellogg

    Ramanathan & Feng 2008 found that existing GHGs as of 2005 had already committed the world to a most likely warming of 2.4 degrees C, once aerosols from burning drop out and climate system equilibrium is reached.

    V. Ramanathan & Y. Feng. Sept. 2008. On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead. PNAS 105:14245-14250. Free access at .
    Abstract: The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial era has most likely committed the world to a warming of 2.4°C (1.4°C to 4.3°C) above the preindustrial surface temperatures. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4°C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols. The range of 1.4°C to 4.3°C in the committed warming overlaps and surpasses the currently perceived threshold range of 1°C to 3°C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan–Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that ≈25% (0.6°C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6°C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO2 mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4°C.
    Sample quote from text:
    “As suggested by the IPCC (12) the Greenland Ice Sheet can disappear completely if surface warming is maintained in excess of 1.9–4.6°C for millennia and raise sea level by 7 m or more.”

  6. Martin Kellogg

    Somehow the web address for PNAS didn’t make it into my comment just submitted. It’s simply Access to Ramanathan & Feng 2008 is free there.

  7. Rob Dresser

    Even more disheartening than the greenhouse gas emissions problem is the approach we take to it. We tend to treat global warming in isolation of the associated environmental calamities overtaking us such as desertification, deforestation, air/land/water contamination, the freshwater crisis, floods and droughts, the depletion or exhaustion of natural resources, species extinctions and migrations, sea level rise and the increasing severity and frequency of storm events, food security, overpopulation and population migration, the rise of failed states and various security threats including terrorism and global nuclear weapons proliferation.

    Jared Diamond makes the case that, if we are to solve any of these challenges, we must solve them all. We cannot hope to succeed if we remain tied to eighteenth century capitalism, nineteenth century industrialism and twentieth century economics and geo-politics. They were the product of a growth-based world but today we’re pushing the finite capacity of our planet. What lies past a growth-based world but an allocation-based civilization or our self-destruction? Adopting an allocation-based civilization seems, to me, to be mankind’s only real hope for within it lie the principles needed to resolve not just global warming but all of those other, potentially existential, challenges. Unfortunately we have not shown much capacity to embrace that sort of fundamental change.

  8. James

    I just can’t see how climate change will ever be slowed unless someone 30, 40, 50 years from now invents a miracle solution that is quickly able to change the temperature within or the contents of our atmosphere.

    Because human nature dictates that greed, money, power and instant gratification take priority, with the consequences only to be ignored and
    thought of later.

    I don’t believe that people (as in the general populus of all first world countries) will sit up and take action until it is too late.

    How gloomy 🙂 I hope I’m wrong!


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