How hot will it get?

Lately, there’s been much discussion about this thing called climate sensitivity. But what the devil is it? Climate sensitivity refers to the amount by which the temperature is expected to increase with each doubling of atmospheric carbon dioxide (CO2) from pre-industrial levels. It’s important because if it turns out that the climate is very sensitive, then the temperature increase we can expect will be higher rather than lower. Pre-industrial CO2 levels were 280 ppm (parts per million). They have almost reached 400 ppm today.

Current climate sensitivity estimates range from 1°C – 5°C. As we are getting close to the 1°C mark already, I think this estimate is a little on the low side.

What I find missing from many of the discussions in the blogosphere and mainstream media is evidence from the fossil record. A little while ago I wrote about A world of giant snakes, which described a period in Earth’s history when there was no ice at either pole and where crocodiles lived in the Arctic along with frost-intolerant subtropical plants. This is very strong evidence for a climate sensitivity at the high end of estimates. A climate sensitivity of only 1°C or 2°C will not provide an equable habitat for crocodiles in the Arctic.

If Earth’s climate had remained fairly stable over millions of years, then this would suggest it was less sensitive to changes in atmospheric CO2. But the climate has shifted dramatically in the past, from ice age to hot house and back again.

When models are used to generate conditions during the hot house period of the Eocene (~50 million years ago), the results are not hot enough to reproduce the conditions that the fossil record suggests for that time1. This problem is known as the equable climate problem. While the models can match conditions at the tropics reasonably well, they do not accurately represent the climate at the higher latitudes and poles, which, according to the fossil record, were significantly warmer than the models suggest.

If we’re placing bets on how much the temperature will rise with each doubling of atmospheric CO2, I’m putting my money on 3-4°C.

Let’s say humans decide to continue with business as usual and make no attempt to wean ourselves off fossils fuels.  Then in one hundred years there will be as much CO2 in our atmosphere as there was 50 million years ago2, when crocodiles lived in Greenland and giant snakes nested in the tropics. It is worth noting that humans did not exist at this time.


1. Progress in greenhouse climate modeling Mathhew Huber
2. The early Eocene equable climate problem revisited M. Huber, R. Caballero

11 Replies to “How hot will it get?”

  1. Interesting article, Rachel. I really wouldn’t care what humans did if it weren’t for the fact that we’re ruining it for other creatures. Polar bears in some areas are starting to die from exhaustion looking for food because there’s less ice around to use as ‘platforms’ from which they can hunt. Just so awful!

    1. I completely agree, Bronwyn. The generations before us have given the subsequent generation a better world. I don’t want my generation to do the opposite.

  2. Rachel,
    In latest news,The Australian has a report on page 3 today,(Monday ,May 20,2013),”Heat going out of temperature rises”,by John Ross.
    “Global temperature increases as a result of increased carbon dioxide levels in the Earth’s atmosphere are likely to be lower than previously thought,an international research team has found.
    The Oxford University led study found that a predicted doubling of CO2 concentrations,expected to occur later this century,is likely to raise global temperatures in the short term by between 1.3C and 2C.
    Previous estimates,based on climate data from the 1990’s predicted steeper rises of up to 3.1C.The new study published today in the journal “Nature Geoscience,” used data gathered more recently when the average rate of global warming was slowing down.
    The latest estimate is “arguably the most reliable”, the paper says ,partly because it is less affected by the 1991 eruption of Mount Pinatubo in the Philipines,but caution is still required in interpreting the available data.
    The IPCC previously estimated a temperature rise of between 1C and 3C,(?!- I thought it was between 2C and 4.5C), with increases outside that range described as “very unlikely”.The new study team which included an oceanographer from CSIRO’s marine and atmospheric research division in Hobart, estimates this rise could be as little as 0.9C.
    The researchers also found that some of the modelling being used for the fifth IPCC assessment report which is due next year,could be inconsistent with their observations.
    Ultimately,however, they found their new predictions suggested little difference to the global temperature increase in the long run.Their best estimate of the “equilibrium climate sensitivity”- the long term temperature rise once the effects of higher CO2 concentrations had bedded down-was 2C with an upper limit of 3.9C.This compares with other previous estimates, the study said.
    Steven Phipps , a research fellow with the university of NSW said the study provided the most accurate estimates yet of climate sensitivity” and ,in broad terms confirmed what has long been known.
    “Our planet faces a very uncomfortable future if our emissions of greenhouse gases continue unabated, ” he said
    UNSW professor Steven Sherwood said the lower warming estimates were based on findings that the oceans were sequestering heat more rapidly than expected.
    “This recent storage may be part of a natural cycle that will eventually reverse ,”he said.
    “The conclusions need to be taken with a large grain of salt until we see what happens to the oceans over the coming years.”
    So let me sum up .A new study finds that climate sensitivity could be as low as 0.9C ,and lies anyway between 1.3C and 2C,but if we wait for CO2 to “bed down” ( when?), then the equilibrium climate sensitivity will lie between 2C and 3.9C( 2.95 C).On all views the climate sensitivity figure lies below the IPCC mean.And anyway we should treat arguably the most reliable study yet with a grain of salt and wait to see what natural variability does to the oceans ,because otherwise the IPCC’s projections might be invalidated.

    1. Hi Doug, thanks for copying the article from The Australian because I can’t read it. I went and read the paper they are referring to – – and the article in The Australian is a little misleading.

      The first sentence of The Australian article suggests that International researchers have found that global temperature increases due to CO2 will be lower than previously thought.

      The first paragraph of the abstract says something a little different:

      “Using up-to-date data
      on radiative forcing, global mean surface
      temperature and total heat uptake in the
      Earth system, we find that the global energy
      budget implies a range of values for the
      equilibrium climate sensitivity that is in
      agreement with earlier estimates, within
      the limits of uncertainty”

      The Australian then says that global temperatures in the short term will be in the range 1.3C – 2. This is misleading. The paper provides figures for equilibrium climate sensitivity (ECS) as well as transient climate sensitivity (TCS). The ECS is the one everyone talks about. It’s the global mean temperature once equilibrium has been reached and it’s this value that the IPCC fourth assessment put at 2-4.5C. The TCS is the temperature before the inertia of the climate system has caught up. Hence its name: transient. It’s the measure of temperature over a twenty-year period. I’m actually not even sure where The Australian got their range for the transient climate sensitivity from because the range given in the paper for this is 0.9 – 2.0C with the best estimate at 1.3C.

      The range given for the equilibrium climate sensitivity is 1.2-3.9C with a best estimate of 2.0C.

      The authors then say:
      “The range derived from the 2000s overlaps
      with estimates from earlier decades and
      with the range of ECS values from current
      climate models”

      Then The Australian says that the researchers conclude that some of the modeling used in the fifth IPCC report is inconsistent with their findings. That’s not exactly true, and I don’t want to misrepresent what they say so I’ll just copy and paste below:

      “Our results match those of other observation-based
      studies15 and suggest that the TCRs of some
      of the models in the CMIP5 ensemble10
      with the strongest climate response to
      increases in atmospheric CO2
      levels may be inconsistent with recent observations —
      even though their ECS values are consistent
      and they agree well with the observed
      climatology. Most of the climate models
      of the CMIP5 ensemble are, however,
      consistent with the observations used here
      in terms of both ECS and TCR.”

      I’ve just read what other media outlets have to say about it and both the NZHerald and the ABC in Australia provide a more accurate picture in my view:

      You do realise that we’re heading for way more than a doubling of CO2? Emissions are accelerating rather than declining. CO2 could be as high as 1100ppm by the end of the century if we continue with business as usual. That’s almost four times as much as preindustrial. (

  3. Rachel,
    Prepare yourself for a shock .I agree with you that the ABC article and the New Scientist articles are better summaries of the paper than the Aus.This does not mean I agree with all the statements in the New Scientist e.g. That temperatures are rising faster than at any time for 11,000 years etc.
    But ,hey,let’s look on the bright side .We have a second chance for the human race, according to the headline.
    Yes, I do understand that we may pass 1100 p.p.m. under business as usual, by century end.But you see I don’t fear CO 2 emissions because I don’t believe they are a major driver of our climate !

  4. Rachel,
    I have 2 criticisms of the Conversation paper.Otto et al 2013 assesses Transient Climate Response (TCR) at 1.3 degrees Celsius,along with Equilibrium Climate Sensitivity ( ECS) at 2 degrees Celsius .The UN IPCC has long held that ECS is 2 to 4.5 degrees C.with a best estimate of 3degrees Celsius.You cannot then say that ” the overall picture remains UN changed”.Incidentally the BBC and Guardian coverages are alarmist hype.
    Secondly the Conversation is running the tired line that natural variability may have accelerated temperatures in the 1990’s and decelerated them in the last decade.There comes a time when you have to elect between CAGW driving the climate or “natural variability ” ruling the climate.The whole theory of CAGW was built on the premise that you could detect Greenhouse gases as the predominant cause of warming ,distinct from the “noise ” occasioned by nature.Now we are told that natural variability has been dominant for two separate ten year periods with countervailing effects.
    The point is the Santer principle from a leading paper by Santer et al 2008,”Our results show that temperature records of at least 17 years in length for identifying human effects on global mean tropospheric temperature.”
    As many sceptical scientists are pointing out the hiatus is now 17 to 24 years depending on which world temperature record you consult.
    Your cause is not helped by the BBC coverage of Otto et al stating ,”Since 1998 there has been an unexplained “standstill”in the heating of the Earth’s atmosphere “.The Santer principle Is either fast approaching or is passed.
    The real significance of Otto et al can be seen in Judith Curry’s post .There are major UN IPCC authors here including lead Authors.The leaked draft of next year’s AR5 is available in full on the web.This paper is not compatible with the ECS shown there .That 2 to 4.5 C figure is looking increasingly untenable.Otto et al has made the publication deadline (15 March last) and will be cited in AR5.

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