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Two degrees: how we imagine climate change

Posted on 14 October 2013 by dana1981

This is a re-post of a The Conversation article by David Holmes.

Two degrees – the temperature rise we need to stay under to avoid catastrophic effects of climate change – is now the catch cry for global warming. Governments and numerous NGOs have eagerly adopted the limit; whether we can meet the target is another matter. But 2C isn’t an easy concept to grasp. So, how can we imagine what 2C means for the world?

First, why is 2C so significant? At any time of the year, for most latitudes at which people live, the difference between overnight lows and daytime high temperatures can be as much as 15C. In summer it is even greater. When the day to day temperatures of weather vary so much, 2C seems insignificant.

What we’re talking about here, though, is the global average temperature, not daily variations – and we’re fast approaching 2C warmer than before the industrial revolution and emissions from fossil fuels intensified. With feedbacks, such as increased water vapour (which is a powerful greenhouse gas), loss of reflective ice surface, and potential methane pulses (a greenhouse gas 20-times more powerful than CO2), 2C could be reached before 2060.

But it’s very difficult for people to imagine what a change in average temperature of 2C means for the planet as a whole, because of that daily variation.

Art captures what New York may have looked like when it was 2C colder than it was in 1750 Flickr/Ron Wiecki

Click to enlarge

One way is to imagine what the world was like when it was 2C colder than 1750. Fortunately, the earth has already done the experiment for us, 18,000 years ago in the ice ages.

For starters, there was an ice sheet one mile thick (the Laurentide) that extended over the northern half of North America right down to New York.

But these events happened 18,000 years ago, over a timeframe of hundreds of years, as a result of changes in the earth’s orbit and other natural forces.

What should worry us today is that human-forced climate change is happening at 10,000 times the rate of climate change caused by these natural cycles.

The changes we’re seeing now may be happening quickly compared to the past, but still slowly compared to human lifespans. That’s because the climate system is “lethargic”. And here’s the problem, the inertia of climate is very difficult to imagine or visualise.

There a variety of reasons visualising the inertia of climate change is difficult; prime among them being that climate is complicated. There’s a complex interplay between climate and oceans. For instance, 90% of warming is going into the oceans – at the rate of four Hiroshima bombs per second. But even with all that heat going in the oceans, they will still take a great many decades to warm.

The other complexity is the relationship with ice and climate. Ice may be melting at the poles, but it may be having surprising effects in the Northern hemisphere, like more cold fronts.

And as the atmosphere warms it can hold more water vapour, which can lead to increased rainfall and even snow at higher latitudes.

So we need an image that captures the complexity of the climate system as it gradually approaches 2C, and that climate won’t always appear to warm in a straightforward manner. One way is to think of the planet as a kind of open icebox which has been neglected at a party. The ice inside reacts to the warmer climate outside and melts. As it melts, the resultant water remains cold until all the ice has melted. Eventually the icebox fails to prevent the drinks from heating up.

According to climate scientist James Hansen, we are never going to see another ice age, ever. When the ice melts it will not be replaced. Ice extent will fluctuate from year to year, and some climate change deniers will selectively point to recovery years, but there is only a downward escalator. Which means, unless dangerous climate change is addressed, for some of us today, and many more tomorrow, the party will definitely be over.

This article follows Four Hiroshima bombs a second: how we imagine climate change. Read it here.

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Comments 1 to 10:

  1. It might be interesting to mention Hansen and Sato's "Paleoclimate Implications for Human-Made Climate Change" where it is written :

    We conclude that the ocean core data are correct in indicating that global temperature was only slightly higher in the Eemian and Holsteinian interglacial periods than in the Holocene, at most by about 1°C, but probably by only several tenths of a degree Celsius. (p 18)

    Augmentation of peak Holocene temperature by even 1°C would be sufficient to trigger powerful amplifying polar feedbacks, leading to a planet at least as warm as in the Eemian and Holsteinian periods, making ice sheet disintegration and large sea level rise inevitable. (p 19)

    BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain. Such a huge rapidly increasing climate forcing dwarfs anything in the paleoclimate record. Antarctic ice shelves would disappear and the lower reaches of the Antarctic ice sheets would experience summer melt comparable to that on Greenland today. (p 20)

    We have presented evidence in this paper that prior interglacial periods were less than 1°C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters. It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal. (p 20)

    Conceivably a 2°C target is based partly on a perception of what is politically realistic, rather than a statement of pure science. In any event, our science analysis suggests that such a target is not only unwise, but likely a disaster scenario. (p 27)

    I don't know about his science but do you agree that the 2°C limit is indeed a politically realistic target rather than a statement of pure science ?


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  2. I believe the current status is we've already warmed nearly 1 degree with another degree already in the pipeline.  So, essentially, we're already at 2.  Is it fair to say that, even with the urgency that people are trying to invoke, it's still being understated?

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  3. Third paragraph under the graphic says:

    But these events (Laurentide icesheet) happened 18,000 years ago, over a timeframe of hundreds of years, as a result of changes in the earth’s orbit and other natural forces.

    That does not make sense. If you're talking about Heinrich events, you may say they lasted for few hundred years. But if you are talking about Milankovic cycles, this is a typo, you probably meant "hundred thousands of years".

    If my assumption about the typo is correct, then in next paragraph you state: "human-forced climate change is happening at 10,000 times the rate". That statement is not in the right ballpark (rate too fast), because in reality AGW's happening in hundreds of years, so ~1000 times faster that Milankovic forcings.

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  4. Claude @ 1

    2oC is a political target, not one based on science. Nor is it very realistic, given that there's little sign of anything other than BAU. I’d rather assume that Hansen is right – that 2oC is a prescription for disaster – than blindly accept a politically convenient target created simply so that all governments can ‘get on board’. But keeping below 2oC is going to be politically and economically very difficult. Although some people are encouraged by the increase in renewables, the impact of that increase is really very small and will not affect  the path we're heading down. Decarbonizing the grid, in itself a major undertaking, will reduce CO2 emissions by just 30%, and would merely delay the inevitable by a few years. Entire manufacturing processes need to be changed to reduce industrial CO2 emissions. But, for example, how do you produce concrete without releasing CO2? And who's going to stop impoverished people in undeveloped nations from continuing to clear land for agriculture, a practice that not only releases CO2 but reduces the ability of the biomass to act as a global sink for CO2? The 2oC 'mantra' is almost as bad as the 'tokenism' individuals are asked to perform (changing out incandescent light bulbs for CFLs or LEDs; turn down the thermostat at night; drive less etc. etc.) as if any of that will make any real difference. Nothing short of massive and rapid investment in both non-fossil fuel sources such as Thorium based nuclear power, and in effective carbon sequestration, will undo what we have done over the past 100 years – and get us back to safe CO2 levels. i.e. 350 ppm.

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  5. Chriskov @ 3

    The 'hundreds of years' and '10,000 times the rate' confused me.

    Ten thousand times the rate of 200 years would be a little over a week.

    So were going to have 2 degrees C higher average temperaures after a week?!

    Perhaps its a typo, perhaps its pure alarmism.

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    Moderator Response:

    [JH] Lose the snark, or lose your posting privilege.

  6. JH@5,

    I think panzerboy priviledges should be revoked at this point.

    not only did he badly distort the meaning of my post in a very careless manner, but he also distort my name to cap it up. Not that I particularly care about the latter but others might do. SkS standards should not tolerate such poor quality comments.

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    Moderator Response:

    [JH] I suspect that panzerboy is nothing more than a drive-by denier drone. If I had the authority to do so, I would immediately revoke his posting privileges. Alas, i do not have such authority. 

  7. Chriskoz,

    Sorry that was a typo no offense intended.


    I am a regular, daily reader of skeptical science. I'm a programmer not a scientist so I post rarely.

    I am genually confused by the statements that events of the last ice age happened 18,000 years ago over a timeframe of 'hundreds of years'. That Ice age was 2 degrees celsius different?

    Why shouln't I conclude that the author meant 10,000 times the rate of those 'hundreds of years', I don't see how to read that any differently.

    I'm sure a denier would have much fun with this arithmetic. The admittedly clumsy accusation of alarmism was intended to highlight what I think is a gift of ammunition to the deniers.

    I think I get it, climate may cool down into ice ages fast, 'hundreds of years'.

    But they recover slowly? Over thousands of years.

    The following from wikipedia suggests the opposite. The recover in a few thousand years but slide into ice ages over 1 hundred thousand years.

    So ten thousand times the rate of ? four thousand years is still a little under 5 months.

    Perhaps someone could point out where this 10,000 times rate comes from, or the error in my arithmetic?


    Jeremy Thomson

    (I'm no troll, thats my real name I just prefer to post in blogs using the panzerboy account name)

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    Moderator Response:

    [JH] Thank you for the clarification. Please take the time to carefully review your draft comments before posting them. 

  8. Alex Sen Gupta's comment on the conversation suggests the natural changes are "more likely 30x slower" than human. I particulary like

    "Exaggeration on either side of the fence is unacceptable. Please check your facts, its bad enough getting nonsense from the skeptics..."

    If only I could have said it so well.

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  9. panzerboy @7: "Perhaps someone could point out where this 10,000 times rate comes from, or the error in my arithmetic?"  

    Someone already did, chriskoz @3: "That statement is not in the right ballpark (rate too fast), because in reality AGW's happening in hundreds of years, so ~1000 times faster that Milankovic forcings."   (you responded @5)

    In other words, it was apparently an error, typographical or of calculation or understanding.  

    panzerboy @8: "exaggeration" suggests a deliberate overstatement, but if "10,000 times" were intentional "alarmist" misreporting, why would the author in the same breath understate the length of a Milankovitch cycle period by a factor of 1,000 (or of the warming swing of the cycle by a factor of about 10) by saying "hundreds of years," if he were trying to overemphasize how much slower "natural" temperature swings have been than the current anthropogenic one?  

    I rather suspect that both "hundreds of years" and "10,000 times" were honest mistakes.  I have a hypothesis of how at least the "hundreds of years" mistake may have come about - take a look at the context: 

    "But these events happened 18,000 years ago, over a timeframe of hundreds of years . . ." 

    If "hundreds" were replaced with "hundreds of thousands" the sentence would read a little funny.  It would be strange to say that events occuring over a hundred thousand years "happened" 18,000 years ago, because the events had to begin happening much longer ago than that.  On the other hand, an event that occurred over the course of a couple hundred years could be referred to as "happening" 18,000 years ago - the event could have both begun and ended 18,000 years ago plus or minus 1,000 years.  Based on that reasoning, maybe an editor at The Conversation assumed that the author meant to say "hundreds" rather than "hundreds of thousands," and made a last minute change before publication without consulting the author.  It seems to me that the change should have been from "these events happened 18,000 years ago, over a timeframe of hundreds of years" to "these conditions culminated 18,000 years ago, having occurred over a time frame of [hundreds of thousands / a hundred thousand] years."  


    Re: "more likely 30x slower" - I am not a climate scientist, but that does sound like it could be accurate based on my understanding that the warming leg of the Milankovitch cycle is typically only a few thousand years, and the current pace to +2 degrees C is about 200 years, 200 x 30 being 6,000.  However, I wouldn't jump to accuse the author of "exaggeration," if for no other reason than the fact that one who deliberately exaggerates would not tend to do so in opposite directions.  

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  10. Natural warming occurring 30 times slower than the current AGW sounds more or less consistent with the fact that today's rate of increase of CO2 in the atmosphere is about 100 times faster than the rate of increase when the last ice age ended:  See

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