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Climate Change Commitment II

Filed under: — gavin @ 2 June 2010 - (Español)

A couple of months ago, we discussed a short paper by Matthews and Weaver on the ‘climate change commitment’ – how much change are we going to see purely because of previous emissions. In my write up, I contrasted the results in M&W (assuming zero CO2 emissions from now on) with a constant concentration scenario (roughly equivalent to an immediate cut of 70% in CO2 emissions), however, as a few people pointed out in the comments, this exclusive focus on CO2 is a little artificial.


I have elsewhere been a big advocate of paying attention to the multi-faceted nature of the anthropogenic emissions (including aerosols and radiatively and chemically active short-lived species), both because that gives a more useful assessment of what it is that we are doing that drives climate change, and also because it is vital information for judging the effectiveness of any proposed policy for a suite of public issues (climate, air pollution, public health etc.). Thus, I shouldn’t have neglected to include these other factors in discussions of the climate change commitment.

Luckily, some estimates do exist in the literature of what happens if we ceased all human emissions of climatically important factors. One such estimate is from Hare and Meinshausen (2006), whose results are illustrated here:

The curve (1) is the result for zero emissions of all of the anthropogenic inputs (in this case, CO2, CH4, N2O, CFCs, SO2, CO, VOCs and NOx). The conclusion is that, in the absence of any human emissions, the expectation would be for quite a sharp warming with elevated temperatures lasting almost until 2050. The reason is that the reflective aerosols (sulphates) decrease in abundance very quickly and so their cooling effect is removed faster than the warming impact of the well-mixed GHGs disappears.

This calculation is done with a somewhat simplified model, and so it might be a little different with a more state-of-the-art ESM (for instance, including more aerosol species like black carbon and a more complete interaction between the chemistry and aerosol species), but the basic result is likely to be robust.

Obviously, this is not a realistic scenario for anything that could really happen, but it does illustrate a couple of points that are relevant for policy. Firstly, the full emissions profile of any particular activity or sector needs to be considered – exclusively focusing on CO2 might give a misleading picture of the climate impact. Secondly, timescales are important. The shorter the time horizon, the larger the impact of short-lived species (aerosols, ozone, etc.). However, the short-lived species provide both warming and cooling effects and the balance between them will vary depending on the activity. Good initial targets for policy measures to reduce emissions might therefore be those where both the short and long-lived components increase warming.


727 Responses to “Climate Change Commitment II”

  1. 1

    I think the timing issue (i.e. commitment) is an important one to make clear to a lot of people whom I’ll term “reasonable skeptics.” That is, there are any number of people that feel that whether you call the last decade warming or cooling, the overall increase in temperatures for the last 30 years hasn’t been “too bad,” especially if you look at Spencer’s graphs which conveniently squash the Y axis to “hide the incline”, and put the zero point at the beginning of the satellite record (i.e. after some warming had already occurred). But I digress…

    My point is that the idea of “warming in the pipeline” or a “temperature commitment” doesn’t occur to a lot of people. They’re too concrete, hence the ability to distract them with a recent snowy or cold winter, or normal winter Arctic ice recovery, or a recent decade of merely moderate warming (until this year!), or most importantly an overall rate of change that is not consistent or directly proportional in time to CO2 emissions. Their argument amounts to “warming doesn’t look so bad right now, so there is no problem.”

    So this is one other way to point that out… the simple idea that even if civilization came to a complete halt right now, warming would still continue for a while before things started to settle down.

    I’d be very interested in seeing a study presenting a variety of realistic, feasible scenarios that take into account such factors as oil resource limitations (i.e. what if we really do hit peak oil in 3-4 years?) and the subsequent economic impacts — positive feedbacks on fossil fuel reductions, if you will (i.e. if fossil fuel supply drops precipitously, then disturbances in economies would reduce usage even beyond supply changes). I’m not sure that the example of a “feasible scenario” is all that feasible, given that it appears to presume a constant increase in emissions, when fossil fuel availability is going to be increasingly unpredictable in coming decades.

  2. 2

    1) This study seems to make use of the answer to a question that arose in the discussion on the last thread… that is, how long will CO2 levels remain elevated in the atmosphere after emissions completely cease? The graph presented here implies that they’ll begin to drop fairly quickly with a reverse logarithmic impact on temperatures, bringing us back to where we are now in about 100 years.

    2) How does this square with Matthews and Weaver in Nature Geoscience (RC post here, from March)? That study shows temperatures leveling off almost immediately, but then staying level for hundreds of years. Did Matthews and Weaver miss the aerosols factor, or is there a discrepancy in how CO2 residence (in terms of concentrations) is estimated in the two studies?

    3) My last point/question would be that this presumes no step changes as a result of tipping points. For example, there is some fear that Arctic ice melt may be dangerously close to a tipping point. What is the picture if, for example, summer Arctic ice completely disappears sometime soon (i.e. before an abrupt cessation in emissions)? Would/could the resulting positive feedback due to decreased albedo further increase temperatures to a point where recovery is dramatically slowed (i.e. could the entire “return to normal” curve be pushed ahead by a hundred years or more)? Or might Arctic ice recover even more quickly (as demonstrated by current winter ice growth), making it a non-factor? Are there other positive step-change feedbacks to be considered (methane release?) that are just as dangerous, and also likely?

  3. 3

    I was vague in my own question #2 above… what I should have asked was “is the only factor that Matthews and Weaver missed the aerosols issue, or are there other differences, such as the estimate of the residence/fallout of CO2 in/from the atmosphere, or the reversal of positive feedbacks such as ice melt/albedo changes?”

  4. 4
    Gilles says:

    all this assumes that the main factor to be optimized is the amount of warming, irrespectively of the amount of energy we burn. Meaning that the implicit indicator that measures the “welfare” , whatever it is (GDP, life expectancy, HDI, or any fancy index you’d like) is strongly (negatively) correlated with the average temperature, and weakly correlated with the fossil fuel consumption.

    The only problem is that all the data I’m aware of show exactly the opposite, but I may have missed some of them.

  5. 5
    Completely Fed Up says:

    “Did Matthews and Weaver miss the aerosols factor, or is there a discrepancy in how CO2 residence (in terms of concentrations) is estimated in the two studies?”

    I think the authors here think that difference is the result of considering CO2 only, which as a long residence time (note that we don’t get back to pre-industrial levels for a thousand or two years) as opposed to all GHGs some of which have much smaller residency times but are a smaller total effect than CO2′s cumulative change.

  6. 6

    4 (Gilles).

    No.

    You are putting your own political spin onto a purely scientific post. The underlying question is “how well can we control the temperature increase resulting from GHG, through emissions controls?”. As is usual with science, degrees of freedom are reduced by focusing on a specific, detailed question and reducing the complexity of the problem to include as few variables as possible. In this case, the only factors are impacts of human emissions on global temperature (explicitly aerosols and GHG effects).

    You are purposely twisting the point of the post, and the science, to promote your own agenda. This is a typical denier tactic… instantly redirect the focus of the discussion to suit your own whim.

    Stick to the science. Question: What controls do we have over our own fates? Answer: Less than expected in the short term, due to the difference in residence time of aerosols versus GHGs. Simple question, simple answer, useful information.

    Not that I care to engage you, but you also highlight the ignorance and short sightedness that understanding “temperature commitment” entails among deniers:

    The only problem is that all the data I’m aware of show exactly the opposite, but I may have missed some of them.

    And there in lies the rub. If we have committed to X (a dangerous X) degrees of increase, but have only seen a fraction of that warming to date, and if we will add to that commitment still without seeing more than a fraction of that warming, then deniers like you will say “all the data I’m aware of show exactly the opposite” because the warming is not yet showing negative impacts with easy, undeniable attribution to climate change.

    But by the time the impacts and attribution are undeniable, it’s too late. That’s the whole point!

  7. 7
    Jack Maloney says:

    The #1 and #2 commitments appear to assume that most or all global warming is anthropogenic, and that stopping anthropogenic forcing will effectively stabilize earth’s climate. ???!

  8. 8
    Ray Ladbury says:

    Gilles,
    There are plenty of studies that show a negative correlation between higher temperature and per capita GDP. Had you made even a cursory google search, you would have found them. This has been a very active area of research for decades.

  9. 9

    Re #4–

    Yes, well acceleration, velocity, and altitude don’t correlate with a person in free-fall, either–until they hit the ground.

    Different regimes, different correlations. Hence the desirability of prediction based upon physical theory, not pure stats.

  10. 10

    Re #4–

    Yes, well acceleration, velocity, and altitude don’t correlate with well-being for a person in free-fall, either–until they hit the ground.

    Different regimes, different correlations. Hence the desirability of prediction based upon physical theory, not pure stats.

  11. 11
    SecularAnimist says:

    Bob (Sphaerica) wrote: “This is a typical denier tactic… instantly redirect the focus of the discussion to suit your own whim.”

    Actually, it isn’t so much specifically a denier tactic, as it is a troll tactic — a gambit used by someone who enjoys impressing himself with his ability to deliberately waste other people’s time with BS.

  12. 12

    The figure caption for the first line should read:
    “Note that *temperature* intially rise due to ceased cooling by aerosols” (whereas now it says “emissions” instead of temperature)

    [Response: Indeed. But that is the published caption, so I can't change it.... :( -gavin]

    Bob (2),

    Indeed, Matthews and Weaver omitted the aerosol factor. As I wrote in a comment to the older RC post on their work, in M and W the CO2 emissions are set to zero, but aerosol emissions are (presumably) allowed to continue.

  13. 13
    Hank Roberts says:

    > Bart … figure caption
    Perhaps halting emissions of aerosols causes faster feedback emissions due to that faster warming? Caption puzzled me too.

    Gavin — I’d love to see the identical analysis done with ocean pH as the criterion rather than temperature. Is anyone looking at that?

    I’d love to see a list appended to the main post:
    > Good initial targets … where both the short
    > and long-lived components increase warming:
    Methane? CFCs? Bromine? HFCs? Diesel soot? Wood soot? Relative effect?

    Trolling noted; I’m not biting in this topic. Leaving it to admins.

  14. 14
    Earl Killian says:

    Ramanathan and Feng came up with 2.4°C for the zero emissions case, whereas the Hare and Meinshausen graph shows about 1.1°C. Can you explain the discrepancy?

  15. 15
    Mango says:

    true or false?

    http://climaterealists.com/attachments/database/GreenhouseGasTheoryDiscredited.pdf

    [Response: Do you really have to ask? -gavin]

  16. 16
    Colin Crawford says:

    While this is yet another in a long list of great articles on RealClimate, I’m still dismayed that it ONLY represents an “absolute BEST CASE scenario!” Sort of “following up” on Mr. Maloney’s comment (#7) it seems to me that the graph and text ONLY consider potential warming based on anthropogenic GHGs and don’t reflect “real world” conditions.

    First, “we” should all be sufficiently aware of “human nature” to understand that there will NOT be any significant, if any at all, deliberate reductions of human-produced GHGs in the foreseeable future… unless, of course, the entire global economy grinds to a complete halt, which seems more and more likely with each passing day. Regardless, and more to the point, from what I see of the SSTs from NOAA and the current news of Arctic “ice-extent” at NSIDC, it almost goes without saying that the cover/volume numbers for northern ice will be dramatically lower than at anytime in recorded history. As a result, the northern seas will be absorbing even more heat than at any time in the history of homo [not so] sapiens.

    Moreover, the research into permafrost thaw/retreat and associated increases in methane release I’ve been reading over the past couple years indicate that that process is accelerating at an “unanticipated” degree. Given the serious under-estimations (in the IPCC reports) of Arctic ice retreat witnessed over the past half-decade or so, I think it’s more “realistic” to assume that there will be a greater and sooner “contribution” to global (especially Arctic) warming than is currently expected. In other words, I find it perfectly reasonable to consider that even IF all anthropogenic GHGs did cease “today,” there is sufficient warming-momentum to expect a release of at least 10% of the CO2, CH4 and NOx from that region which, by my calculations, send global CO2-equivalent concentrations above (maybe WELL above) 500 ppm. As a result, all those curves on your graph will become distinctly more vertical! This seems practically inevitable as we approach the next Solar maximum over the next 1-2 years.

  17. 17
    Daniel Goodwin says:

    The principle factor I’d expect to motivate revision of the curves given here is methane releases from permafrost and clathrates. Have any discoveries in this vein since 2006 begun to percolate into the models, or is that question a matter of “wait and see” at this point?

  18. 18
    Edward Greisch says:

    The “feasible scenario” looks very dangerous to me. I think we will trigger in a big way the positive natural feedbacks that have already begun: Arctic ocean sunlight absorption, tundra peat bog [marsh?] melting and some of the methane hydrate melting. Won’t they will push the temperature high enough to trigger a really big ocean bottom methane hydrate release? Won’t that in turn give us a PETM-like spike that will push GW to the “Humans go extinct for sure” level of 6 degrees C?

    I think we must do the unfeasible scenario that looks more like line “0″ or line 1 on the graph. Note that denialists will change arguments again and see the continuation of warming on line 1 as proof that CO2 isn’t what causes GW.

    I have heard that tundra peat bogs can be “tamed” by turning them into grassland. Is that possible? Environmental regulations against touching Alaskan peat bogs would have to be changed because “wetlands” would have to be drained.

  19. 19
    Doug Bostrom says:

    Mango says: 2 June 2010 at 11:50 AM

    true or false?

    http://climaterealists.com/attachments/database/GreenhouseGasTheoryDiscredited.pdf

    Neither. “Peak Stupid”.

    (Hat tip on “Peak Stupid” to “J” at OpenMind

  20. 20
    Jim Eager says:

    Gavin, shouldn’t “The curve (1) is the result for zero emissions of all of the anthropogenic inputs (in this case, CO2, CH2….”

    Shouldn’t that be CH4?

    [Response: Yep. -gavin]

  21. 21

    Earl Killian (14),

    Ramanthan and Feng’s number of 2.4 degrees warming relates to the committed warming of the current level of GHG only (2.9 W/m2, ozone excluded, if I recall correctly), so without the masking effect of the cooling aerosols. The rationale behind that is that A) aerosols are shortlived and B) we’re going to clean them up sooner or later (e.g. for health reasons).

  22. 22
    CM says:

    Jack #7, yes, current global warming is mostly anthropogenic. The paper discussed assumes a (positive) solar and (negative) volcanic forcing held constant at the mean levels of recent history. The resulting natural forcing for 2005, they say, is 0.54 W/m2 over preindustrial (1770-1800) levels, which I assume is pretty generous, since it was based on work reviewed for the TAR, and the solar irradiance change was revised down considerably in AR4. The anthropogenic forcing over the same period is nearly three times that, 1.48 W/m2.

    Colin #16, Daniel #17, Ed #18, the authors note that positive terrestrial carbon-cycle feedback or releases of methane hydrates would lead to larger long-term warming for each scenario.

    PS. Colin, a Worst-Case Scenario just in…

  23. 23
    GFW says:

    At least three comments so far mention the arctic sea ice albedo effect. Has that been quantified as a function of latitude and season? What I mean is, we know that when the sun is high overhead, ice reflects 90%, and water absorbs almost that much. But at low sun angles, water reflects a lot too. Meanwhile, a dark, slightly warmer than ice ocean can radiate more energy to space than ice.

    So, is there a solid analysis of how the actual trends in arctic ice cover are affecting the year-round energy budget? I suspect the feedback is still positive, but that the strength of the feedback diminishes as the ice retreats further. But that’s just physical intuition. Someone must have done a serious calculation of this.

  24. 24
    Gilles says:

    Bob #6 : “Stick to the science. Question: What controls do we have over our own fates? Answer: Less than expected in the short term, due to the difference in residence time of aerosols versus GHGs. Simple question, simple answer, useful information.”

    I assume that in your question , “fate” is equivalent to “average temperature of the globe” ? – which is exactly what I meant… I personnally think that “fates” includes much more than the average temperature we live in – and of course there are many different ways to influence it, most of which aren’t linked with temperature.

    RL#8 : “Gilles,
    There are plenty of studies that show a negative correlation between higher temperature and per capita GDP. Had you made even a cursory google search, you would have found them. This has been a very active area of research for decades.”

    For a given energy consumption? are you sure ? where ?

  25. 25

    Mango 15,

    The paper you point to is beyond stupid. Take any layer of atmosphere, or the whole thing. It radiates from the bottom as well as the top. Thus Gavin’s multiplication by two. To say this is bogus, a fraud, a hoax, is to betray gross ignorance of very basic geometry, not to mention radiation physics.

  26. 26
    Septic Matthew says:

    16, Colin Crawford, First, “we” should all be sufficiently aware of “human nature” to understand that there will NOT be any significant, if any at all, deliberate reductions of human-produced GHGs in the foreseeable future… unless, of course, the entire global economy grinds to a complete halt, which seems more and more likely with each passing day.

    I wonder sometimes that such intemperate comments are not derided by the people here who profess concern about “trolls”.

    To start with, there is no one “human nature”, but lots of human natures: differences between cultures, differences between people within cultures, differences between epochs within cultures (exemplified by the expanding and contracting dynasties in Chinese history.)

    Second, China, the US, and the EU are already investing $billions per year in solar, wind and biofuels to supplement fossil and nuclear power, and eventually to replace fossil power as it runs out. CO2 output has been nearly constant or slightly reduced in the US and EU, and coal consumption is already declining in the US, as the renewable energy sector is growing faster than the total energy demand. Itis also true in China that the renewable energy sector is growing faster than energy consumption overall; starting from a smaller base, it will be making a substantial contribution to overall energy statistics in China by 2020 (some of the “foreseeable future”.)

    Third, there is no threat that global economies will “grind to a halt”. The worst documented recession in US history produced a 25% decline in annual GDP, not a “halt”, and the current monetary problems will produce perhaps as much a decline in the US and EU, but probably not in Japan, China, S. Korea, Indonesia, India or Brazil, though those countries all have problems of their own.

  27. 27
    Sam Yates says:

    This question may be a little bit too involved to properly answer, but I figured I’d give it a shot anyway; why, on the graph provided, is the model’s response to forcings so variable (relative to the actual temperature record)? That is, it looks like at some times the model responds too quickly, going up or down before actual temperatures did (1870-ish, for example, where there’s a peak in the model that doesn’t happen in historical data until what looks like at least half a decade later), at other times the model lags Earth’s actual response (the slow climb and peak at a little after 1950, when the historical temperatures peaked at 1940-ish), and others where the model seems to match the data pretty perfectly. Is this just a result of different forcings being accounted for to lesser or greater degrees, or…?

    …And, um, off-topic, but I just wanted to thank y’all for putting this much effort into communicating climate science to the world at large; it can’t be pleasant at times, particularly when you’ve got all manner of people accusing you of fraud, attempted global domination (seriously, now?), and other nastiness, but…thank you. If we and the world as we know it are going to survive, we need to understand that world and the laws by which it operates, and your work communicating those laws and how our understanding of them are arrived at is invaluable.

  28. 28
    Aaron Lewis says:

    Mother Nature plays by different rules than gridded models project. The thing is that whenever there is a dispute over the rules, Mother Nature always wins.

    Five years ago, the gridded models said that Arctic Sea Ice would be around for another 80 years.

    Five years ago, I said the Arctic Sea Ice would be seasonal by 2014 and I was called an “Alarmist.” I still am.

  29. 29
    Ray Ladbury says:

    Mango@15, Oh my God. The stupidity density in that piece is so dense it bends light! That has to be satire. Please tell me it’s satire!

    [Response:How about that subtitle stunner: "Shock new evidence ... shames beleaguered space administration in new global warming fraud scandal". Not just a scandal but a fraud scandal: serious stuff there. Poe's law claims another victim--jim]

  30. 30
    David B. Benson says:

    Sam Yates (26) — The climate model’s internal variability doesn’t coincide with that in the data, I believe. It is possible to more closely match the record:
    http://www.realclimate.org/index.php/archives/2010/03/unforced-variations-3/comment-page-12/#comment-168530

  31. 31
    Earl Killian says:

    Bart Verheggen (21),

    Yes, I think I understand Ramanathan and Feng. What I don’t understand is how Hare and Meinshausen’s “zero emissions” line comes up with a result so much lower. I haven’t read Hare and Meinshausen so I don’t know how their scenario differs from Ramanathan and Feng to come up with a temperature anomaly that is less than half.

  32. 32
    wili says:

    I’d like to second the points at posts #16 and 17. I know that there was a thread on them, but it does seem likely that seabed methane in the Arctic could provide a significant discontinuity to these models.

    What the curve in the graph seems to show is that aerosols are currently “masking” at least one degree of warming. Is that the case, or is some of the continued increase because of feedbacks that have already started?

    I know there have been threads on this too, but I’d like to have it clearly stated here–what is the range of uncertainty about the estimation that aerosols are masking one degree of warming? How likely is it that it is much less (presumably not very) or much more.

  33. 33
    David Stern says:

    #21 & #31 The zero emissions scenario must involve the elimination of aerosols related to fossil fuel emissions – and the key to the figure says that that is the case. Ramanathan and Feng’s scenario is for constant concentrations not emissions. But in order to explain the difference between that and the zero emissions scenario here, either GHG concentrations need to fall very rapidly once emissions stop (and maybe CH4 etc. can explain that?) or there is a difference in the climate sensitivity etc.

  34. 34
    Hank Roberts says:

    > negative correlation between higher temperature and per capita GDP.
    Yeah, look at Iceland. Oh, wait ….

    Or look at the list of countries colonized by Europeans; perhaps they got richer sooner because needing to burn coal to stay warm in winter led to the steam technology of the age of empires?

  35. 35
    Frank Giger says:

    LOL @ higher temps and GDP. Governance and the rule of law are a better correlation.

    Note that Hong Kong and Isreal outstrip their neighbors in the rankings. Plus California, Texas, and Florida would get high rankings if considered countries instead of states.

    Now, back to the science! Thanks for getting back on the information train, RC!

  36. 36
    Hank Roberts says:

    > what is the range of uncertainty about the estimation that aerosols
    > are masking one degree of warming?
    I think you’re asking for what’s called a “probability distribution”

    http://www.google.com/search?q=site%3Aipcc.ch+range+uncertainty+aerosols+masking+one+degree+warming
    leads to:
    http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-9-2.html
    “… Radiative forcings from forcing agents have been combined into their main groupings. This is particularly useful for aerosol as its total direct RF is considerably better constrained than the RF from individual aerosol types ….”
    [describing the pictures on the linked page]
    “… Adding together the anthropogenic RF values shown in panel (A) of Figure 2.20 and combining their individual uncertainties gives the probability density functions (PDFs) of RF that are shown in panel (B). Three PDFs are shown: the combined RF from greenhouse gas changes (LLGHGs and ozone); the combined direct aerosol and cloud albedo RFs and the combination of all anthropogenic RFs….”

  37. 37
  38. 38
    Mango says:

    @ #19, 25 & 29

    Just for the record, I don’t believe it’s true

    I wanted to know if Gavin was going to demand a retraction or sue the guy who printed it

  39. 39
    Completely Fed Up says:

    Hank, Gilles has displayed a complete inability to understand any responses so please do not respond to him, it only gives him what he wants: attention. He is not going to get the point and this boring and highly repetitive diversion will continue. Enough already

  40. 40
    CM says:

    GFW,

    > the arctic sea ice albedo effect. Has that been quantified as
    > a function of latitude and season?

    Layman reply, the fruits of unsystematic reading and discussion on an earlier thread (corrections and improvements welcome):

    The albedo of ocean water is not only a function of the angle of incidence of sunlight (latitude and season), but also a function of cloudiness (diffuse light); roughness of the sea; and clorophyll concentrations. I’ve seen reference works where ocean albedo approaches 100% at 90 degrees latitude, but this assumes a very calm surface on a very clear day so you can just apply the Fresnel equations. Besides, the lower the sun and the higher the percentage of incoming sunlight that is reflected, the lower the total incoming sunlight that matters to the radiation balance. Sources I’ve found put the real-world annual mean Arctic ocean albedo at 10-20%.

    Ice albedo is affected by such things as melt ponds and snow cover. Work is ongoing to improve the parameterizations of all this stuff.

    Someone else would have to speak to how this feeds into the modeling work.

    Possibly helpful links:

    Ocean Surface Albedo Lookup Table: Jin and Charlock
    http://snowdog.larc.nasa.gov/jin/getocnlut.html

    Direct measurements (SHEBA):
    http://data.eol.ucar.edu/codiac/projs?SHEBA

    An old non-paywalled paper, clear and instructive: Cogley 1979
    http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1979)107%3C0775%3ATAOWAA%3E2.0.CO%3B2

    Some history:
    http://www.aip.org/history/climate/simple.htm

  41. 41
    Neil says:

    Does the model shown include feedback? I seem to recall an article on the Science of Doom giving the effect of doubling CO2 emissions to equate to a global mean temp rise of approximately 1C without any feedback taken into consideration? This model shows a 2.5C rise by 2150. Curious to understand the difference eg:-
    1) Feedback
    2) This is not just the effects of C02
    3) Based on current comsumption CO2 will more than double by 2150

    Based on what I have read today the basic science of increases in CO2 warming the earth is well understood. It is just the feedback mechanisms that are less well known?

    I personnally think it would be useful to split these out in any analysis?

    ;-)

  42. 42

    32 (wili),

    I believe “masking” is an inaccurate term. It implies that the warming has happened, but we can’t see it. Rather, it’s simply that the planet takes time to warm (there’s a huge, huge volume of water involved, and that water is in constant motion).

    The analogy I use is that CO2 is like a thermostat in a house. Adding CO2 is like turning up the thermostat to 100 degrees F, and then breaking the switch so that it can’t be turned back down. Your house will not instantly reach 100 degrees. It will take considerable time for your furnace to heat all of the air in the house, but there’s still nothing you can do to stop it. Sitting around after the first hour and saying “gee, it’s only 85 degrees now, I must not have turned the thermostat up as high as I thought” is just wishful thinking.

    To use another analogy, adding CO2 is like adding a blanket. You’re not instantly warm the moment you put on the extra blanket. It takes time to heat. You may actually get too warm, eventually, and want to kick off that extra blanket (except the CO2 blanket is stuck to you with super glue). And we’re constantly adding CO2, so we’re constantly adding more and more (super-glued) blankets.

    The bottom line is that, depending on climate sensitivity, current CO2 levels equate to anywhere from 1 to 1.5 C warming, and we’ve only seen about 0.5 to date (more if this year’s temperatures represent a step change, but it’s obviously too early to tell). That doesn’t mean that warming stopped at 0.5 C, or that climate sensitivity is lower than anticipated. It just means that we’ll have to wait to see the other 0.5 to 1.5 C manifest itself in measurable temperature changes.

    That’s a large part of the danger of GHGs. “What you see is what you get” isn’t true. “What you see now is only a small part of what you are going to get” is more accurate.

  43. 43

    32 (wili),

    How likely is it that it is much less (presumably not very) or much more.

    The answer to that question is the complex, heavily debated issue of “climate sensitivity.” Currently, multiple lines of evidence (past temperature changes implied by proxies, computer models, the physics, etc.) point to a wide range of values, with the most commonly cited being somewhere around 3C per doubling of CO2. That’s the temperature increase you’re going to get, no matter how long it takes for the planet to warm to that point.

    Putting a probability on those estimates is (to me) impossible [although one way to attempt to do so is to take the results of 20+ GCMs and plot them as a potential probability distribution]. It’s like arguing the probability of a particular team winning the Super Bowl next year. No matter how knowledgeable you are, or how strong a case you make, there’s just not enough information to be precise, and even if there were, on any given Sunday… Of course, as time goes on and we acquire more knowledge (events unfold, old theories are disproved, new evidence is found, new ideas are embraced) we can narrow that range, and increase confidence.

    So now it’s a race between knowledge and physics. Current score: physics 23, knowledge 7 with an unknown amount of time left on the game clock.

  44. 44

    41 (Neil),

    The answer to your question is 1 (positive feedbacks, which are not just the direct effects of CO2). The primary positive feedback is water vapor. As the temperature rises due to CO2, specific humidity rises (i.e. more H2O in the atmosphere), and H2O is itself a strong GHG, which raises the temperature, which raises the specific humidity fractionally more. The next main feedbacks are albedo changes (mainly from shrinking ice cover) and increased CO2 release from various sources (such as the conversion of forests to savanna as a result of regional climate change). And then each positive feedback in turn elicits more of the same (although less powerful — otherwise you get a runaway increase) positive feedbacks.

    This is balanced by some (weaker) negative feedbacks such as increased albedo due to more cloud cover due to greater humidity.

    This information is readily available, although I don’t know the best, cohesive source to which to direct you. Perhaps someone else can provide a link.

  45. 45
    votenotokyoto says:

    Let’s assume it is all true and our greenhouse gas emmissions are going to cause the Earth to warm by more then 2C if we maintain our current emmissions. Overall isn’t that a good thing for the Earth? I know there will be some losers but there will be more winners than losers.

    If you doubt this consider the potential effect of 2C temperature fall.

  46. 46

    voteno 45,

    No, there will be far, far more losers. Our agriculture will collapse. No food.

  47. 47
    Paul A says:

    @votenotokyoto #45

    I would say sea level rise is a big negative for everybody. John Cook covered the subject a while ago by listing the positive and negatives impacts of global warming from peer reviewed studies. There were far more negatives than positives:

    http://www.skepticalscience.com/Peer-reviewed-impacts-of-global-warming.html

  48. 48
    Leonard Evens says:

    “Let’s assume it is all true and our greenhouse gas emmissions are going to cause the Earth to warm by more then 2C if we maintain our current emmissions. Overall isn’t that a good thing for the Earth? I know there will be some losers but there will be more winners than losers.”

    How do you know that there will be more winners than losers. Also, how do you know that the net gain to the winners will offset the loss to the losers. Will there be enough of a net gain for the winners to compensate the losers and still come out ahead.

    Do you think you and the people close to you are likely to be winners? Would you have the same opinion if it were pretty clear you were going to be losers?

  49. 49

    45 (votenotokyoto),

    2C is a global average change. Regional changes will be much more dramatic, and may be accompanied by major precipitation changes even in areas that see much smaller temperature changes. This will mean the conversion or expansion of some ecosystems, for instance deserts (SW U.S.A.), or transitions from forest to savanna (Amazon), etc. This will also mean major changes in water availability (for crop irrigation as well as human consumption), which is already an issue in many areas due to poor resource management and unrestrained or ill advised population growth.

    In addition, it’s overly simplistic to think things like “we’ll just move our agriculture further north.” For example, the start, end, and length of the growing season varies with latitude, so some crops will not be productive further north just because it’s warmer. I remember seeing one study that talked about much of Canada having inappropriate top soil for agriculture, itself having been greatly scarred by the advance and retreat of glaciers, which carved away the topsoil and deposited it in the American Midwest, making it more fertile. Recreating the top soil in parts of Canada and the northern U.S. is something that could take thousands of years to generate naturally.

  50. 50
    Leonard Evens says:

    Neil asks
    “Does the model shown include feedback?”

    and then he states his personal opinion:

    “Based on what I have read today the basic science of increases in CO2 warming the earth is well understood. It is just the feedback mechanisms that are less well known?”

    A good source for an explanation of the feedbacks is “Global Warming, Understanding the Forecast”, by David Archer. You can see this as a set of video lectures at
    geoflop.uchicago.edu/forecast/docs/lectures.html
    The presentation is for non-science majors, so it should be possible for anyone with a reasonable background in basic science to floow. Of course, it isn’t comprehensive. to understand things on that level, one would have to do the equivalent of getting a graduate degree in climate science, which would require at least three years of solid work, assuming a strong background in the relevant physics and mathematics.

    You will discover, if you tstudy the matter, that the feedbacks are reasonably well undersood, and that there are multiple lines of evidence suggesting that the IPCC’s range of possibilities is right. It doesn’t just depend on computer models. There have been only a very few serious attempts to justify a net climate sensitivity of about 1 deg K, most notably Lindzen’s Iris Effect. But these have not been supported by the evidence. Of course, it is possible that Lindzen is right and everyone else who has looked into the matter is just being stupid, or worse, lying for various nefarious reasons. But, seriously, how likely is that?


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