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What constitutes 'safe' global warming?

Posted on 4 October 2010 by John Cook

Currently, global temperatures have warmed around 0.8°C from pre-industrial temperatures with around 0.6°C warming in the pipeline (Hansen 2005). What limit should we set on global temperatures? According to the European Union (EU), global warming should be restricted to 2°C from pre-industrial levels to be considered safe. Two degrees warming are expected to have significant impacts on ecosystems, water resources and global food production. To restrict global warming to around 2°C means we need to stabilise greenhouse gas levels at 450 parts per million CO2 equivalents.

A new study brings more empirical data to this question. It addresses the question: what will the world be like if it warms 2°C degrees from pre-industrial levels? One way to find out is to look at the last time global temperatures were at those levels. This was between 116,000 to 130,000 years ago in an interglacial period called the Eemian. Due to changes in the Earth's orbit, the amount of sunlight on the Northern Hemisphere was higher at summertime than today, triggering a series of changes that drove the planet to warmer temperatures. How warmer?

In a new study Does the Agulhas Current amplify global temperatures during super-interglacials? (Turney & Jones 2010), the authors use 263 estimates from ocean sediments and ice to reconstruct temperatures around the globe during the last interglacial. Globally, the world was around 1.9°C warmer than pre-industrial temperatures. In polar regions, temperatures were more than 5°C warmer while tropical warming was not so pronounced (similar to  warming patterns today). Global warming of 1.9°C is roughly the amount of warming expected in the more optimistic IPCC emission scenarios (to put this in perspective, we're currently tracking above the most pessimistic scenario). So the last interglacial gives us an empirical window into what our best-case future will look like.

What were conditions like at these temperatures? During the last interglacial, sea levels were 6.6 to 9.4 metres higher than current sea levels. Large parts of the Antarctic and Greenland ice sheets melted, with the southern part of Greenland having little or no ice. When we look at accelerating ice loss from the Greenland and Antarctica and wonder about its future trajectory, the past gives the answer. Metres of sea level rise.

There's a degree of uncertainty over the time frames involved. But several peer-reviewed studies, using independent methods, indicate we'll experience roughly 80cm to 2 metres sea level within this century (Pfeffer 2008, Vermeer 2009). The driving question from this study remains: can 6 to 9 metres sea level rise be considered safe?

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Comments 1 to 50:

  1. "can 6 to 9 metres sea level rise be considered safe?"
    Not for the hundreds of millions of people worldwide who live in that 6-9 metre zone who will no longer have anywhere to live.

    And when those displaced by the sea level rise are forced to migrate, the resultant social unrest will keep many others from being safe as well.

    Sell those beachfront timeshares, if ya gots 'em.

    The Yooper
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  2. I think that the sea level rise itself will drive larger problems.

    Will the relations between China, India, and Pakistan remain stable when these countries are stressed with the redistribution of water resources that this much warming will bring, at the same time that they are dealing with tens of millions of refuges, or more, from Bangladesh?

    Are there other parts of the world where this much sea level rise will add stress to already stress-filled relations?

    Honestly, I don't think that sea level rise is the biggest threat. I think the biggest threat will come from losses in agricultural productivity resulting from changes in rainfall patterns that are driven by changes in Hadley cell circulation.

    I'd like to see an study that estimates climate zone regions then and overlay that with the present zones. That would give us a better idea what will happen to our food production.

    Still, the conclusion is the same: The sooner we reduce emissions the less it will cost us.
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  3. Chris G: I think the changes are also stemming from the altered Polar vortex. One such study for 10 degrees warmer arctic:
    of course the oceanic circulation in the prev study was different than now but as the Indian Ocean warms this effect of Agulhas current may well become more common or permanent:
    One must remember "close to 40% of the Earth's land surface is presently used for cropland and pasture" so the choices of the farmers will have an effect on how the carbon on the air is used by the food plants.

    I'm expecting (natural) C4-plants becoming more common all over the planet, where it rains enough. One needs quite detailed info on the rainfall patterns, like you said, to predict what the likely biotopes are in future, but I wouldn't be surprised of rice fields in central Germany by 2050, and maize where it rains little enough.
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  4. I found this map a couple of days ago.

    I found it a bit a bit slow and clumsy to navigate until I got used to it. However, if you use it at minimum size while you get to an area, say, the Mekong delta, then maximise the view and adjust the amount of SLR you get a fairly clear, fairly depressing picture. The Nile delta is a bit of an eye opener.
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  5. adelady, I don't know what to make of the inertia in cartography... Aral Sea is not like that in the real world :-/.
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  6. I agree it's pretty crude. And I should have pointed out the provisos. It doesn't do anything for bodies of landlocked water. I confess I didn't look at the Aral Sea. By the time I'd got through the Nile, Mekong and Ganges deltas and a bit of a look at the Philippines I'd had enough.
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  7. Re: adelady (4)

    Thanks for the SLR mapping website.

    I note with amusement that windowing in on Washington DC & cranking the SLR up to 14 meters results in waves lapping at the White House and the steps of Capitol Hill. Perhaps then the denizens therein will finally start serious discussions on the issue.



    Re: jyyh (5)

    As a former cartographer, I cringed as well at certain liberties taken in inland areas (the Aral Sea's 1960 elevation above mean sea level: 53 meters). Take it as a useful reference tool, not as a map with built-in geodetic accuracy. For world sea level rise area inundations and impacts estimation: perfectly useful.

    The Yooper
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  8. From">Lowe and Gregory in Nature Reports 2010:

    'New research suggests that the possibility of sea level rise of up to two metres by 2100 should be given serious consideration. One key study examined the ice flow rates that would be required to produce substantial sea level rise by 2100 and concluded that a rise of much more than two metres would be “physically untenable” [which is from the Pfeffer 2009 abstract verbatim]...Proxy evidence from oxygen isotope ratios in Red Sea sediment cores6 suggests that sea level rose by as much as 1.6 metres per century at a time in the past when the large ice sheets covered an area similar to their present-day extent...Although increases of up to two metres this century can't be ruled out, this does not mean that they are inevitable or even likely. For climate change to produce much more than one metre of sea level rise, ice sheets would probably have to contribute considerably more to the rise than they do now... The recent acceleration of Greenland outlet glaciers and Antarctic ice streams [this is a reference to Velicogna’s GRACE study of 2009] may be due in part to natural variability, and it might not continue. Some observations indicate that a number of the outlet glaciers and ice streams that accelerated in the 1990s have since started to slow down... In this sea of uncertainty, how do we derive a better estimate of sea level rise? ...The [current semi-empirical modelling] approach is loosely based on an understanding of physical processes, but the relationship is determined by statistical methods. The general assumption is that the relationship between sea level rise and temperature (or forcing) will hold in the future and for a much greater range of warming than occurred during the period from which it was calibrated... There has already been some debate about the statistical validity of these approaches... Adding up the estimates of the various observationally derived contributions to historic sea level rise, which all have uncertainties, we find that their sum may fall short of the measured total sea level rise... The semi-empirical methods assume that any difference is due to a missing contribution that will increase with global warming. Though that assumption may be correct, without understanding/ identifying the physical processes that may make up this shortfall in sea level, there is little in the way of supporting evidence.'

    The ellipses are in the interests of attempted brevity.

    Lowe seems to have made a major contribution to the UK defra site which concludes:

    *Our analysis gives projections of UK coastal absolute sea level rise (not including land movement) for 2095 that range* from approximately 13–76 cm.

    *Taking vertical land movement into account gives slightly larger sea level rise projections relative to the land in the more southern parts of the UK where land is subsiding, and somewhat lower increases in relative sea level for the north. We have, for example, derived projected relative sea level increases for 1990–2095 of approximately 21–68 cm for London and 7–54 cm for Edinburgh (5th to 95th percentile for the medium emissions scenario).

    *A low probability High++ sea level range has been defined for vulnerability testing. For the UK this absolute SLR estimate is 93 cm to 1.9 m by 2100.

    In the case of recent iconic flooding such as Hurricane Katrina in New Orleans, we find that much of the problem emanated from the fact that some 49 percent lies below sea level, in places to 10–12 ft (3.05–3.66 m)- a problem compounded by subsidence of reclaimed land. I leave aside the contentious question of whether Katrina like events are more likely - I think they belong on another thread.

    So John, I wonder whether we really should be talking about rises of 6 - 9 metres. I do agree however that two metres would definitely be a worry. Similarly, I really have no way of knowing whether Lowe and Gregory's take on Velicogna is overoptimistic.
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    Response: Thanks for that link, Chris. The Nature commentary doesn't, however, touch on empirical determinations of Eemian sea levels. Instead, it looks at semi-empirical attempts to predict the trajectory of sea level rise over the next century. As I say above, there's uncertainty about the time-frames involved.

    But the end destination, 6+ metres sea level rise under sustained temperatures 2 degrees warmer than pre-industrial levels, is determined empirically and independently of the methods discussed in the Nature commentary. I liken it to watching James Cameron's Titanic. We know how it ends but we're not exactly sure what's going to happen along the way and whether Leonardo di Caprio gets offed or not.
  9. chriscanaris,
    "wonder whether we really should be talking about rises of 6 - 9 metres."
    It depends on our ability to discriminate between different time frames.
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  10. Actually, I'm very pragmatic on this issue. Taken for granted that we're already outside the safe operating space on a few systems, I think that the real question is "how hard can we push the brakes?".
    Politically we need to define a threshold (a goal) and 2 °C is a good one, but I do not put much value on it. Indeed, I believe that we will push the brakes anyhow and we've started doing it, although not that hard yet. Like it or not, the fossil fuel era will (relatively) soon come to an end and the very long term impacts will be avoided.
    I can't speak for John, but this is my take on this post.
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  11. chriscanaris: "So John, I wonder whether we really should be talking about rises of 6 - 9 metres. I do agree however that two metres would definitely be a worry."

    1 metre would cause many problems at Portsmouth UK, at high tides.
    2 metres would cause flooding in some places. Portsmouth is densely populated, so many homes would be at risk at regular flooding.
    3 metres would result in the Millennium tower to be closed regularly and some parts of the island to be un-inhabitable.
    4 metres would be very difficult to defend against, probably leading to serious plans for moving the 200,000 population onto new homes and towns on agricultural land.
    Although I suspect at 3 to 3 metres plans would already be in place.
    5 metres would mean abandonment of the city. About half of it is below 5m, certainly not higher.
    6 metres ...

    There would be the question of pollution, due to land fill and materials from buildings being flooded. I guess that would be planned as well.

    Total costs would be billions probably just for one city and a loss of farmland to home the displaced. The surrounding coastal area would also be affected, with similar evacuations and loss of farmland and other green spaces.
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  12. Most scenarios only go as far as 2100AD, but these all show the temperature still rising at a considerable rate. Ultimately the increase looks like being much more than 2C, even with the most optimistic model.

    We're doomed!
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  13. #12 "We're doomed! "

    For this worse case scenario, has anyone demonstrated that there will actually be less habitable land as sea level rises? Shouldnt all kinds of land tracts be getting freed up of ice compensating loss of coastal regions?
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  14. #10
    "I think that the real question is "how hard can we push the brakes?".

    If you are pushing the wrong peddal, it wont make any difference.
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  15. *@#! those links! Make this the final version and feel free to delete the others.

    John, I take your point about the Eemian period, which seems to have been a predominantly solar based event.

    Actually, being on the real Titanic would serve as a better analogy for the point you're trying to make. Ironically, because its electrical power plant functioned until the very end, many passengers were reluctant to disembark from the seeming comfort and safety of a well-lit ship. Interestingly, the Titanic was designed to float with the first four compartments flooded. Instead, the glancing blow to the starboard side caused buckling in the hull plates along the first five compartments, more than the ship's designers had anticipated.

    Coming back on topic, I looked at earlier posts on ice loss which seems to be the real polar bear in the room. It ultimately depends on whether we're looking at a CO2 forcing to equal the solar forcing of the Eemian. In the case of CO2 forcing, we're looking at concurrent H2O forcings and increased precipitation which could include increased snow cover in some settings (recall that Antarctica is the world's driest continent and hence there's some scope for negative feedbacks to GHG forcings in such settings).

    Coming back to the Titanic analogy, we don't really know whether we're flooding five compartments or four. In the former case, we would be silly to clamber into the lifeboats. In the latter, we can take cold comfort from Keynes dictum:

    'The long run is a misleading guide to current affairs. In the long run we are all dead.'

    He also said:

    'When the facts change, I change my mind. What do you do, sir?'
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  16. " temperatures have warmed around 0.8°C from pre-industrial temperatures"

    Is "pre-industrial" before or after the invention of the thermometer?

    Continutin... the link in the first sentence, takes you to a past article that starts...

    "Unfortunately, the discussion went pear shaped with some ideological anti-intellectualism "

    The word "anti-intellecutalism" in turn links to yet another article that contains the proverbial graph wherein CO2 and temperature track perfectly one on the other given two linear scales that are selected for this purpose, even though the "intellecual" theory dictates that temperature changes as the log of GHG concentration.
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  17. Chris, Keynes dictum is about _current_ affairs.

    When we're talking about warming and disruption, I actually care quite a lot about what my descendants may face long after I'm dead. I can't save them from failed love affairs or worries about passing exams.

    But I can be part of an effort to ensure that they get the best possible chance of living in a world that's congenial to healthy life and not torn by strife over inadequate water, food or living space.

    I'm not asking for utopia. Life is always difficult and dangerous. I lived through polio epidemics and the cold war and several other wars. Our generation has the opportunity to try and avoid making life more difficult and dangerous than it need be for our grandchildren's grandchildren.
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  18. "For this worse case scenario, has anyone demonstrated that there will actually be less habitable land as sea level rises? Shouldnt all kinds of land tracts be getting freed up of ice compensating loss of coastal regions?"

    How much arable land will there be at high altitude? Is it cheaper to build towns and cities on crags or on flatlands?

    As ever, it is not the change so much as the rate at which it occurs that will yield the cost. 2C over a thousand years is a much more manageable impact than 2C over a century.
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  19. RSVP #14,
    very naif and meaningless comment. Does anyone count on the right outcome from a wrong action? You need to be very lucky ...
    A brake is a brake, if you push the wrong pedal it's not a brake. As easy as this.
    I'd love to read more thoughtful comments here at SkS.
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  20. RSVP #13: "For this worse case scenario, has anyone demonstrated that there will actually be less habitable land as sea level rises? Shouldnt all kinds of land tracts be getting freed up of ice compensating loss of coastal regions?"

    Ummm... no. Not even close. The amount of land occupied by ice outside of Greenland and Antarctica is negligible and all at high altitudes which are sparsely settled to begin with. Whether Greenland will gain land from ice loss faster than it loses land from sea level rise is an interesting question, but it certainly won't outpace the rest of the world combined. Antarctica, of course, will remain uninhabitably cold unless temperatures rise so much that much of the rest of the world becomes uninhabitably warm.

    #16: "Is "pre-industrial" before or after the invention of the thermometer?"

    The thermometer predated the industrial revolution by a couple hundred years.

    "...temperature changes as the log of GHG concentration."

    No. Radiative forcing changes with the log of GHG concentrations. Temperature changes with the radiative forcing, climate sensitivity, and time.
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  21. 16: "even though the "intellecual" theory dictates that temperature changes as the log of GHG concentration."
    Over the range of 280 0 0
  • RSVP, I suggest the exercise of signing a binding commitment allowing somebody or some semi-random process to pick a date anywhere from 50-100 years in the future, a date which will not be disclosed to you, agreeing that on that date you will allow yourself or your descendants to be forced-- regardless of whether you've had a change of heart-- to leave your worldly possessions behind and quickly move somewhere else, a place that does not yet have a name or functioning economy. The commitment should entail that you or your descendants will be accompanied by a substantial collection of strangers collected from different parts of the world and of course you'll want to coexist with your new neighbors in peace and harmony regardless of accompanying cultural baggage of the portable religious, moral and ethical style.

    Alternatively, you could modify the binding agreement to entail that you wait until some bit of naked rock is exposed on the southern coast of Greenland, then take all of your worldly possessions and your family and move there in a more orderly fashion and proceed to carve out an existence. The commitment should require that you pick a date 50-100 years in the future, and again it will be enforceable on your descendants. Also, there will unfortunately be no means of picking your neighbors, as in the first example exercise.

    Does being a refugee sound good? Are you happy with facing a myriad of unresolved details each of which is crucial to your continued well being?

    Does taking on a load of uncertainty making the IPCC synthesis appear like 2+2 sound like an attractive proposition?
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  • Re: chriscanaris (15)
    "It ultimately depends on whether we're looking at a CO2 forcing to equal the solar forcing of the Eemian. In the case of CO2 forcing, we're looking at concurrent H2O forcings and increased precipitation which could include increased snow cover in some settings (recall that Antarctica is the world's driest continent and hence there's some scope for negative feedbacks to GHG forcings in such settings)."
    In the case of glaciers, Greenland and Antarctica, recall that mass-gain occurs when snowfall increases in the accumulation zone outweigh losses in the ablation zone. When the sums are out of balance in the opposite direction, mass-loss occurs. This is what is happening right now.

    Yes, even in Antarctica.

    Even in the EAIS, in spite of increased snowfall in the accumulation zone, mass-losses in the ablation/calving zones are winning the struggle.

    The current levels of CO2 were matched last in the Pliocene (approximately 3 million years ago), when temperatures were at least 2-3 degrees C warmer than today (a level our current BAU track will allow us to attain or exceed). A look at those times reveals climate patterns unlike anything existing today.

    Given the absence of some magical unknown negative feedback mechanism to offset the increased CO2, we are faced with the prospect of the loss of the entire GIS and the WAIS (if one looks beyond 2100) and an attendant 20+ SLR (as the current linear SLR won't abruptly stop at the turn of the next century).

    In your Titanic analogy (updated): we've hit the berg and the ship is listing to one side, rendering half the lifeboats inoperable. Do we stop to fill and launch the remaining boats to allow some to survive? Or do we continue at flank speed & refill the ice in our drinks?

    The Yooper
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  • I find the idea that we just allow ice to melt off Greenland and northern Canada in order to discover magically deep and fertile soil just ludicrous. Any soil that might be there will either be scraped off by the moving megatons of ice or washed away by the floods of ice melt.

    As for thawing permafrost regions, good luck cultivating soil that's releasing methane as you go. We'll need a whole new set of health and safety regulations for farm workers if anyone's mad enough to try. Any agricultural scientists on here able to tell us the likely effects on germination success and growth rates in such 'soils'? (Leave aside the chances of a whole crop being incinerated by someone accidentally igniting the methane.)
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  • adelady (you from my home town, SA?),

    to play devil's advocate, you could argue that we'd relocate farmland to the green zones as they move to higher latitudes, and relocate some of the millions of people flooded inland by rising seas to to higher altitudes. Still doesn't answer the problem of the cost/rate of change, of course, and the stress this moving around would place on societies and economies.

    When we were nomadic we could follow the weather, but as humankind's dominion is a patchwork of state borders, trouble will come when rivers dry up in one country and are filled in another. There have already been wars over water resources.
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  • Shouldnt all kinds of land tracts be getting freed up of ice compensating loss of coastal regions?"

    There's this odd tendency, on the "skeptical" side of the argument, to see all types of land as basically equivalent: If you lose some habitable space here, you simply make it up there. Wetlands flooded? Move everyone inland and northward, to the convenient "tracts" of melting permafrost!

    I generally expect "skeptics" to overlook biodiversity and ecosystem services. I'm more surprised by how often they overlook the logistical difficulty of the adaptations they suggest. Especially when it comes from people who see mitigation measures as seriously if not fatally disruptive to The Free World, the relatively sunny outlook re: climate-forced migration is really puzzling.
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  • Riccardo #19
    "I'd love to read more thoughtful comments here at SkS"

    Warmest actually have it pretty easy since they only have to convince politicians who know nothing about science. Is that thoughtful enough?
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  • @RSVP: what's a "warmest"?

    Also, convincing politicians who know nothing about science is actually pretty hard when you have a huge Climate Denial Machine funded by the Koch bros. and their ilk drowning real research with their anti-science propaganda.
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  • This article strikes me as very odd. First, by proposing that there is such as thing as "safe global warming", secondly, by writing as if it were still possible to keep the rise down to 2 deg. C.

    I thought it was a foregone conclusion: we are already far too late to keep it down to 2C. Somewhere I remember seeing 4C as being projected as more likely now.

    4C will, of course, be ugly. But since we are already over the safe limit of CO2, with no signs of slowing down, 2C just isn't possible anymore.
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  • Perhaps we've reached a state of post-denial denial, Matt. I've a kid so I've got an incentive to try keeping my innate cynicism (realism) in check, but I find it hard to see how we're going to undo the damage caused by 20 years of concerted waffling.
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  • I keep seeing articles that refer to particular atmospheric CO2 levels corresponding to particular global temperature changes. Is there a graph that shows the relationship between the two?
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  • MattJ. I'm afraid I tend to agree with you. The big issue is acting in a way to ensure that any rise over 2C is as brief as possible. If we get it 'right' the temperature spike might look a bit like that 1998 spike. It won't be a single year of course, but minimising the number of decades is worth a shot.

    It'd be nice to think that the climb up to and down from whatever maximum temperature is finally reached could be less than the lifetime of people within one generation.

    And yes Barry, I'm from the festival city.
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  • adelady, believe it or not, I also care - otherwise I wouldn't be hanging around this site. I'm not a contrarian who just wants to argue for the sake of it. You didn't notice my second quote from Keynes which is much more important than the first:

    'When the facts change, I change my mind. What do you do, sir?'

    Doug, you ask of RSVP, 'Does being a refugee sound good?' I come from a family of refugees created not by climate but by the poisonous geopolitics of the 1930s. I've never been a refugee myself but the notion is embedded in my DNA. Currently, our refugee problems worldwide are not the result of climate change but the outcome of nasty governments and equally nasty revolutionary movements. I'd hate to see climate changed added to their burdens but so far that hasn't been happening.

    That of course doesn't mean it will never happen but as the Latin tag goes, 'Homo homini lupus' - literally 'man is wolf to man.'
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  • I did notice the second quote, I just thought the idea was poorly expressed.

    When the facts change, I don't "change my mind" - I just go where the evidence leads. I get sick, the doc says here's what we have to do, I do it. I get better, I get up and get on with life.

    If the "facts" change in relation to climate science, I'll evaluate them. 25-30 years of declining temperature, coral reefs rebuilding, Arctic ice several metres thick, I won't just evaluate, I'll relax and enjoy.
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  • might be of interest.

    The debate is no longer about whether temperature increase above pre-industrial temperatures can be limited to 2C. by 2100. Given our desire to burn increasing quantities of fossil fuels, we can be certain that temperatures will rise more than 2C by 2100 and that is unavoidable.

    The effects will of course include accelerated melting of the polar ice sheets and, as noted by others above, a sea level rise of at least 1m by 2050 and over 2m. by 2100. The effects on river delta’s sustaining much of the global population is also well known.

    It is already evident that global warming is responsible for accelerated retreat of glaciers. Normally, glaciers store and release fresh water which flows into rivers and underground aquifers that are drawn on for agriculture and human consumption. This source of fresh water is imperiled by retreating glaciers and by burgeoning population growth.

    We will not have to wait until 2100 to observe the combined effects of rising sea level and retreating glaciers on the ability of humans to feed themselves. We are starting to see those affects now. They will certainly become increasingly obvious over the next 20-30 years and catastrophic long before 2100.

    And we still cheerfully increase the level of CO2-e greenhouse gas we pump into the atmosphere, despite repeated warnings by scientists of the consequences. Why?
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  • Re: ocrow (31)
    "I keep seeing articles that refer to particular atmospheric CO2 levels corresponding to particular global temperature changes. Is there a graph that shows the relationship between the two?"
    Depending upon what you're interested in:

    Near-Term (Recent History)

    Mid-Term (The Age of Civilization; i.e., Old-As-Dirt) no CO2 levels on this one, but gives a neat perspective.

    The Last 5 De-glaciation Events superimposed on each other for comparison.

    Kinda Old (400,000 BP)

    Paleo (Really Old - 800,000 BP)

    Leave it to the public servants at the EPA to give us something useful.

    Note that while other forcings exist (some positive, some negative), the resulting temperatures comes from the summary balance of all of them, and forcings can and do change over time (albeit slowly). Until humans messed with the biggest control knob of temperatures.

    The Yooper
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  • Thanks John for the timely warning. A temperature rise of 2 degrees is definitely not safe, at least not in the long term. Sea-level rise is the elephant in the room. The IPCC projections of less than one metre are based mostly on thermal expansion of the ocean, as the rate of breakup of the ice sheets is very difficult to model.

    Hansen has stated that “Proxy measures of CO2 amount and climate simulations consistent with empirical data on climate sensitivity both indicate that atmospheric CO2 amount when an ice sheet first formed on Antarctica (34–35 million years before present) was probably only 400–600 ppm" (Hansen and Sato, 2007b). Elsewhere he writes “equilibrium sea level rise for today’s 385 ppm CO2 (2008) is at least several meters, judging from paleoclimate history"

    If that is so then the CO2 concentration we are tracking to reach in this century is sufficient to eventually melt most of the ice in Greenland and West Antarctica. (East Antarctica may be somewhat safer as the ice sheet has increased the altitude.) The only consolation in this grim scenario is that the thermal inertia of the ice sheets is apparently longer than the thermal inertia of the upper layers of the ocean. In my post on this site titled "Climate Change: The 40 Year Delay Between Cause and Effect", I estimated 40 years as the time for global warming, responding to a step increase in forcing, to reach 1 – 1/e = 63.2% of its final equilibrium value. The sea level rise for the last interglacial, which John quotes as 6.6 to 9.4 metres higher than current sea level, is the equilibrium sea level rise. I expect a 2 degree warming will produce a rise of 2 or more metres by the end of this century, with the remainder being reached some centuries later.

    What we do know from paleoclimate history is that if nothing is done to reduce greenhouse gas emissions, most of the world's great cities will eventually be inundated. That is why the task facing mankind is not just to quickly move to a zero carbon economy. We will also need to extract the bulk of the CO2 emitted from 1750 up till now. That will require either carbon sequestration on an industrial scale, or geo-engineering. Both these solutions will involve decisions we make as a species, not as competing peoples. If sufficient CO2 can be removed this century, we can limit the sea level rise even if global warming temporarily reaches 2+ degrees.

    I live on the island of Tasmania. The first inhabitants of the island arrived during the last ice age. They did not arrive by boat. They were able to walk across 300 km of sea floor because the sea level was 120 metres lower than it is today. I wish our politicians could comprehend that as they push temperatures and sea level in the other direction!
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  • Re: Agnostic (35)

    The melt-curve needed to hit 1+ meter of SLR by 2050 will also deliver 3+ by 2100.

    Not saying it will happen, or not. Just pointing out the obvious.

    BTW, that jibes with Hansen's latest (catastrophic instabilities in the PIG, already underway, triggers a 5+ meter SLR deglaciation of the WAIS by 2100). If I wasn't half-asleep (and lazy) right now I'd link it. Google Hansen 2008. Think that's it.

    The Yooper
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  • No disrespect but I do find it funny the casual way you drop in the " 0.6°C warming in the pipeline" as if it's a concrete fact.
    If we were all honest about this we'd admit that at this stage that is little more than an idea which is around simply to fill an embarassing hole in an equation. There are other explanations which could replace "warming in the pipeline" which seem to me no more or no less plausable.
    The certainty with which you can drop such statements is worrying.
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    Response: Warming in the pipeline isn't a case of trying to account for less than expected warming. On the contrary, it's the inevitable consequence of a physical system with thermal inertia. If you're heating a pot of water, it doesn't instantly become hot. Similarly, if you're adding longwave radiation to the world's oceans, they don't reach equilibrium temperature instantly but take time to approach equilibrium.

    The climate time lag is an often misunderstood phenomena. I once tried explaining it but looking back at that post, I did a terrible job (for some mad reason, I thought using mathematical equations made it easier to understand - a sign of how much I had to learn about communicating science at the time). I'm planning to have another crack at it soon - using funky diagrams rather than equations.
  • HR - so what are contrary papers to this estimate then? Seems to fit well with other estimates Hare and Meinshausen, Wigley and Matthews & Weavers.
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  • HR@39
    If you see the head of a lion or a snake coming through the window, you really, really do not want to wait until the whole body is inside the room before you shout a warning.

    Once you recognise the shape, and you see/hear/smell that it isn't just a silly kid with a mask, you act. Promptly.
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  • Daniel (38)

    Thanks. The URL is

    Hansen argues, with some conviction, that CO2-e concentration of 450 ppm is too high to limit temperature increase to 2C by 2100. I think he is right and that we probably approach a dangerous tipping point when concentrations exceed 400 ppm.

    CO2-e concentration is already 388 ppm and rising at an accelerating rate because of our reliance on and increasing use of fossil fuels to meet our energy needs.

    Don’t you think it rather disconcerting that the IMF should be lending billions for the building of coal fired power stations ( at a time when the world should be contracting rather than expanding the use of coal.
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  • Agnostic, at 2+ ppm of co2 being added per year it will take 30 years to make it to 450 ppm at the current rate. We will see if that will speed up or slow down, but that is the current rate. At this rate by 210 we will near near 570 ppm. We would of doubled.
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  • HumanityRules @39

    "No disrespect but I do find it funny the casual way you drop in the " 0.6°C warming in the pipeline" as if it's a concrete fact."

    The actual figure of 0.6 is an estimate. That there must be an outstanding warming of some level that is still 'in the pipeline' is basic thermodynamics. The oceans have warmed less than th air since they have massively greater thermal mass. It takes time for them to warm to a comparable level. So as they warm more slowly, the growing temperature differential between them and the atmosphere starts to limit the temperature growth of the atmosphere. Personally I am surprised that the figure estimated for the 'in the pipeline' rise is only 0.6, I would have thoght higher. But that is only gut feel, not calculation.

    "If we were all honest about this we'd admit that at this stage that is little more than an idea which is around simply to fill an embarassing hole in an equation. "
    As I said, Its thermodynamics, not just an idea. And what is this hole you are referring to anyway?

    "There are other explanations which could replace "warming in the pipeline" which seem to me no more or no less plausable."
    Such as?
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  • Matthew @43.

    Agnostic @42 referred to CO2e, not CO2. Currently CO2e is at around 430-440 ppm. And you are right, CO2 is growing at about 2 ppm/yr at present. During the 2000's, CO2 emissions were growing at about 3%/year. That is a doubling time of 25 years. So if sustained, by 2035 we would be growing at 4ppm per year and 8 ppm per year by 2070.

    That growth rate however is made up of different contributions from different parts of the world. And the biggest growth rate is China. During the 2000's their coal consumption grew by around 7%/year, which is a doubling period of around 10 years. And as they continue to grow, their increasing size means their growth rate will come to have a bigger impact on the global growth rate figure. How long before we see 4 or 5% global growth rates. 5% growth is a doubling of around 15 years so by 2025, that would be increases of 4ppm/year and 8 ppm/year by 2040.

    So if China (and other nations like India following them) are able to sustain their economic growth rates for several decades to come and can't bring renewables / nuclear onstream at major scales incredibly fast, we could blow through the doubling level of 560ppm before mid century.

    Bringing renewables to mainstream generation levels that fast is a herculean task. And if we allow that much fossil fuel capacity to be built, decommissioning it before the end of its working life will involve capital right-offs that will make the WFC look like chickenfeed. And if the growth stalls in those countries, then the political, social and military implications of that are terrifying.

    We are in deep sh!t, right here, right now, today.

    Humanity may pay a terrible price for the fact that deep down, we just don't 'get' the Compound Interest Law!
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  • #40 scaddenp

    Another favorite paper on this website, murphy et al 2009, seems to to balance the energy budget without resorting to "missing heat". This suggests an inconsistency between Hansen 2005 and Murphy 2009.

    A new paper in press from Knox and Douglass suggests Hansen has got it wrong, questioning the TOA measurements. I realise that this part of the paper is very brief but the comment comes about from the fact that their finding seem to be moving in an opposite direction to what Hansen would like to see.

    And in a roundabout way this new paper on indirect aerosol forcing, Ruckstuhl et al 2010, which if accurate suggest IA forcing may be around 0 rather than -0.7W m2 assumed by the IPCC. Given this large potential error then my guess is the assumptions behind Hansen (and others) calculations need to be constantly re-assessed.

    (I'm happy to here how I've mis-interpreted these papers)
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  • 44.Glenn Tamblyn

    Truely basic thermodynamics?

    No assumptions about forcings and feedbacks are involved in these sorts of calculations?
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  • HR @47

    "Truely basic thermodynamics?

    No assumptions about forcings and feedbacks are involved in these sorts of calculations?"

    At the most fundamental level, yes. Just about all the forcings and feedbacks net out to some energy flux imbalance for the planet. This energy imbalance results in a net change in Total Heat Content for the climate system of which the Oceans are far and away the dominant element, but with a long time delay. Since the oceans have such huge thermal mass they take much longer to reach a new temperature than the atmosphere. And once the difference in temperature change of the oceans vs the atmosphere grows, this difference starts to alter the energy flows between atmosphere and ocean, retarding the temperature change in the atmosphere until the oceans catch up.

    And note, in this argument I haven't said warming. The reasoning is exactly the same whether the energy imbalance is positive - warming, or negative - cooling. The atmosphere leads, the oceans follow more slowly, the disparity between the two then retards the atmosphere until the oceans have caught up before a final equilibrium has been achieved. Basic Thermodynamics.

    What is more complex, and the subject of discussion is what all the forcings and feedbacks add up to and how this balance might evolve over time. But the same basic principle would apply if the Sun suddenly changed its Heat output by 1%, or if we started using geoengineering to add aerosols to the air.

    Note that I have only made this argument in qualitative terms. What the quantitative analysis is I leave to someone way above my pay grade. But if the atmospheric temperature change to date is positive, then the 'in the pipe line' warming MUST also be positive unless some process suddenly reverses the net forcing. Basic Thermodynamics. As to the quantitative estimate of how much, my get feel says that 0.6 is too low. But I am not qualifed to make that quantitave estimate. The analysis of the experts probably trumps 'my gut'

    Consider, of the heat changes measured so far, around 90% has accumulated in the oceans and only 3% in the atmosphere. But the the upper levels of the ocean have only seen temperature change of a fraction of the change in the atmosphere so far, and the abyssal oceans virtually not at all.

    To use another analogy, imagine a tribe of meerkats trying to move an elephant. As all the meerkats chatter around, pushing and shoving, there is some net direction in which they are trying to move the elephant. Even so, it takes some time for them all to move the elephant anywhere. And the Oceans are the elephant of the climate system.
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  • I understand the author's good intention, but I am afraid that the current scientific knowledge about the last interglacial (Eemian) period is too weak a link to define the dangerous level of warming. The accuracy of estimate of global mean temperature as well as of sea level seems to have improved (though I still do not think accurate enough to be used as basis of policy making). My biggest doubt is whether we can consider that global mean temperature determines sea level (in somewhat mathematical terms, whether sea level is a function of global mean temperature alone). I do think that global mean temperature matters, but I also think that spatial and seasonal distribution of temperature, or factors other than temperature, may also be crucial. So the level of global mean temperature which is likely to cause dangerous sea level rise is under-determined by Eemian data. (This is a personal opinion of a scientist but not an expert of this specific issue.)
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  • Informative post. Now a days global warming is serious problem. Global warming is certainly a global issue that needs a global solution. There are lots of reasons for Global Warming. he Global warming occurs due to rise in the temperature around the earths atmosphere. We can make efforts to stop global warming. Ways To Stop Global Warming Use solar system appliances. Plant more trees. Do not cut trees. Turn off the lights when not in room. Cover your pots while cooking. This will save energy which is require for preparation of food.
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