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johnd at 07:30 AM on 29 March 2010The human fingerprint in global warming
The frequency of cold and warm days and nights will be a function of cloud cover as much as anything, especially cold or warm nights. -
Tom Dayton at 07:21 AM on 29 March 2010The human fingerprint in global warming
If anyone wants more details on Martin Hedberg's comment that the relationship between CO2 and temperature is indeed robust, see the Skeptical Science argument There’s no correlation between CO2 and temperature. -
Martin Hedberg at 07:05 AM on 29 March 2010The human fingerprint in global warming
manacker (max) notes that temperature does not always increase as CO2 increases. He concludes therefore that the relationship between CO2 and temperature "is not so robust". Well, CO2 is not the only thing affecting planetary radiative balance and hence air temperature. Worth mentioning is for example other greenhouse gases, the albedo and that energy is absorbed by oceans as well as ice and land (not only the air gets warmer). You also have to include feedbacks and complexity. Neither this nor variations in temperature diminishes the impact CO2 has upon the greenhouse effect, radiative balance and temperature. The link he refers to analyses part of the climate with statistics, not physics. The author asks for more investigations, but he does not bother to read/understand even part of all the information at hand. No scientist of today claims that CO2 is the only thing affecting temperature, yet a lot of people believe scientist think so. Why? /Martin -
Tom Dayton at 07:03 AM on 29 March 2010The human fingerprint in global warming
Anyone reading the blog post that manacker linked to should be careful not take it at face value. It is merely a story on the San FranciscoExaminer.com blog by writer Tom Fuller, about scientist Bart Verheggen's suggestions about the statistics. The scientist Verheggen himself vigorously objected to Fuller's interpretation of his work, in a comment on Fuller's comments thread:Tom, Thanks for highlighting the discussion at my blog. However, I think you’re overstating the significance. I replied to the quote you cited from whbabcock: “No, that’s not what this thread is about. It’s about a few things: Whether the temperature data contain a unit root, and what the consequences would be for how to analyze the time series. You would be correct with your inference if AGW was only based on (perhaps spurious?) correlation, but it’s not. It’s based on physics and a myriad of observations.” VS’ argument does not contradict “the theory” (of radiative transfer; or of AGW for that matter). It may mean that statistical significance in one (!) part of the observations is not as easily established as I’ve thought in my (admitted) statistical naievity. It has no bearing on physics based GCM studies about expected future climate changes. See also ourchangingclimate.wordpress.com/2010/03/18/the-relevance-of-rooting-for-a-unit-root/
That last link is to a page where the scientist Verheggen further debunks the overinterpretation and incorrect interpretation by commenter VS and by the San Francisco Examiner's Tom Fuller. Commenter VS's incorrect interpretation has been further debunked in extreme detail by the professional time-series statistician specialist Tamino in his post Not a Random Walk. -
shargash at 06:50 AM on 29 March 2010The human fingerprint in global warming
John, isn't the greater warming at the poles than the equator also a fingerprint of the greenhouse effect? -
shargash at 06:49 AM on 29 March 2010The human fingerprint in global warming
manacker @9 said, "The observed multi-decadal warming and cooling cycles cannot be explained by the gradually and smoothly increasing CO2 levels" That is true of atmospheric temperatures. However, the heat content of the oceans is much great than the atmosphere. We can see the effect in El Nino/La Nina years, but much of the ocean still cannot be measured directly. For that reason, the satellite measurements are much more reliable than atmospheric temperatures. -
manacker at 06:28 AM on 29 March 2010The human fingerprint in global warming
Atmospheric CO2 increase since the 19th century is likely to be at least partly a result of increased human CO2 emissions, as this article concludes. However, the correlation between atmospheric CO2 and “global temperature” is not so robust. The observed multi-decadal warming and cooling cycles cannot be explained by the gradually and smoothly increasing CO2 levels; the current global cooling despite record increase in CO2 is also hard to explain. A robust statistical analysis of the CO2 / temperature correlation, as proposed by pro-AGW scientists, Bart Verheggen, may shed more light on this apparent dilemma. http://www.examiner.com/x-9111-Environmental-Policy-Examiner~y2010m3d24-Global-warming-Bigger-than-Climategate-more-important-than-Copenhagenits-statistical-analysis Max -
Tom Dayton at 05:23 AM on 29 March 2010We're heading into an ice age
Scinan, I should clarify that the risk of a shutdown of the North Atlantic Current (thereby causing a Northern Hemisphere ice age) is not an ice age threat that is reduced by increased CO2. To the contrary, the threat is increased by increased CO2. The point of my previous comment was that the probability of it happening is very small. -
Tom Dayton at 05:15 AM on 29 March 2010We're heading into an ice age
Scinan, I think you are talking about the North Atlantic Current portion of the thermohaline circulation. It has happened in the distant past, so reasonably it has been speculated to be a risk in the near future. More research has revealed that it is extremely unlikely to happen in the near future. More information can be found by clicking on the links in the "More Information" section at the bottom of the page by climatologist Stefan Rahmstorf, "The Day After Tomorrow: Some Comments on the Movie." (For the quickest route to the information, skip straight to the bottom of that page.) -
Tom Dayton at 04:41 AM on 29 March 2010The human fingerprint in global warming
nerndt asked "Wouldn't increased water vapor cause the same effects as warming the troposphere and cooling the stratosphere?" CO2 is well mixed through the atmosphere, so additional CO2 will end up in the stratosphere as well as lower. The extra below the stratosphere prevents infrared radiation from reaching the stratosphere, thereby keeping the stratosphere cooler. You're right that water vapor below the stratosphere has that same insulating effect. But the extra CO2 in the stratosphere radiates more IR out toward space (as well as in all other directions), thereby cooling the stratosphere more. Water vapor, in contrast, is not well mixed in the atmosphere. It is sparse in the stratosphere, so water vapor's increase in the atmosphere as a whole does not cause a proportionately large increase in the stratosphere. So increasing water vapor in the atmosphere as a whole does not increase that stratospheric radiative cooling of the stratosphere that does come from CO2 increase. That's to the best of my knowledge, but somebody please correct me if I'm wrong. -
chparadise at 04:31 AM on 29 March 2010The human fingerprint in global warming
GFW beat me to the punch on explaining that the vast excess of oxygen relative to CO2 means that we haven't reduced our oxygen levels dangerously. One of your best posts yet John. The measured carbon isotope ratio graph alone is worth it in debates, and it's a result that makes perfect sense once one thinks of the theory. -
We're heading into an ice age
OK. I have said enough and I still do not know why I am listed as "N/A" But I have one last thought. What are the circumstances that would make more CO2 good? How do we know they arr not in the future? Best to ask many questions and be ready for anything that happens. Have a Swiss army knife of solutions - not one solution for one problem and only that one solution. Warmer will be inconvenient. Ice will kill most of us - if not all. I am gary4books.Response: The N/A is a website glitch, still trying to figure it out. -
Martin Hedberg at 03:56 AM on 29 March 2010The human fingerprint in global warming
Great post. I'd like to add (Or are they to indirect?): 1. Ocean acidification. 2. Is there a fingerprint of the C13/C12 in biomass? /Martin -
nerndt at 03:54 AM on 29 March 2010The human fingerprint in global warming
Good article, but a question: Why was the water vapor filtered out in the "Spectrum of greenhouse radiation" figure 5 when it dwarfs all other gases (95% for H2O versus 5% for all other gases combined)? Also, wouldn't increased water vapor cause the same effects as warming the troposphere and cooling the stratosphere? I always like to look for the obvious reasons for large effects before pursuing minor ones. Am I missing something? -
Steve L at 03:48 AM on 29 March 2010The human fingerprint in global warming
Actually, the production of CO2 and consequent reduction of atmospheric oxygen has been balanced a bit by loss of O2 from the oceans into the atmosphere. I typed "per meg" into google and found this website by Michael Bender at Princeton, and here's part of the explanation (scroll down past the nice figures to the bottom): "1 per meg = 0.001‰ using the standard stable isotope terminology. For reference, the amplitude of the annual cycle ranges up to about 80 per meg, and the annual decrease is about 20 per meg /yr. An error of 2.5 per meg in the rate of O2/N2 change translates to an uncertainty of ~ 1 Gt C/yr in the partitioning of CO2 sequestration." -
AstroHypercane2010 at 03:03 AM on 29 March 2010What CO2 level would cause the Greenland ice sheet to collapse?
The soot that lands over Greenland from forest fires, industrial activity and burning methane clathrates and methane releases from permafrost fires in peatbogs and fires in tropical rainforests will likely speed up the melting of the ice, along with the melting of the Arctic ice cap which will leave more open ocean in the north of Greenland. When the ice melts, the greatest sea level rise will be seen near Nova Scotia, Boston and New York. At the current rate the sea level rise globally from Greenland will be 0.8 cm by 2020. -
shawnhet at 03:00 AM on 29 March 2010Is the science settled?
Riccardo, I'm not sure what your point is, but obviously it is possible to increase WV in the atmosphere by *evaporation*, even if the temperature stays the same. However, since I am talking about condensation I am not talking about the temperature continually staying the same , but rather what happens when warm air cools. My point, is that when warm air of a given RH *cools* more water vapor is condensed than when cooler air of the same RH cools by the same amount. I will point you to the following page where you can do appropriate calculations to test this out. Relative humidity calculator Cheers, :) -
GFW at 02:44 AM on 29 March 2010The human fingerprint in global warming
John R., It's pretty much straight up math for a decent estimate on the O2. We've increased CO2 by ~105ppm. But roughly an equal amount has been taken up by sinks, mostly the ocean. So we've used enough O2 to make ~210ppm CO2. So, one would figure we've decreased the concentration of O2 by ~210 ppm. John's chart shows a decrease of between 300 and 400, so I'm in the ballpark. Maybe John can explain the difference. It could be some other usage of O2, or a difference between ppm (by volume) and ppm (by count). As for how that affects the total amount of O2 available, not much. Air is 21% oxygen, so that's 210,000 ppm (again, not completely sure if that's by volume or count). So decrease relative to original value is 0.1 to 0.2 percent. -
Steve L at 02:34 AM on 29 March 2010The human fingerprint in global warming
Agreeing with John Russell, the right axis on Fig 3 could use some explanation. Additionally, Fig 6a has a caption but Fig 6b doesn't (maybe self-explanatory). Your "eg" should be "ie" when explaining isotopes, no? Okay, enough nit-picking. Question: for anticipated solar effects on daytime and nighttime temperatures, are these model-based? I wonder if greater solar heating wouldn't generate more water vapour (greenhouse gas) that might then have the same apparent effect as expected for CO2. Could a similar comparison be done for winter versus summar, and would that also provide a useful fingerprint? Finally, I wonder if solar and CO2 forcings are simply predicted to have different effects spatially rather than temporally. Hmm, I notice that I'm asking questions about what models would predict under various scenarios. I always thought I agreed with your first statements about empirical results! -
John Russell at 01:52 AM on 29 March 2010The human fingerprint in global warming
Brilliant post, John. One question from a non-scientist. I always used to worry that burning of all this fossil fuel would reduce the concentration of oxygen in the atmosphere. When I mentioned it to those more knowledgeable than me, they'd give me a metaphorical pat on the head and say, "there, there, don't worry". I'm sure they're right and we shouldn't be concerned -- about this, anyway -- but can you put those falling oxygen levels into context for us, in terms of relative concentrations in the atmosphere? It's the first time I've seen a scientist mention this particular anthropogenic effect. -
Riccardo at 19:45 PM on 28 March 2010Is the science settled?
shawnhet, it all started here: "if we assumed that cloudiness increases proportionally with concentration of WV this would be a negative feedback." What i was pointing out is that, as a zeroth order aproximation, if relative humidity is constant on increasing temperature (the only possible way to increase WV concentration) you do not get this feedback, no matter what the actual concentration is. Conversely, it's true that you get an effect on precipitations and, maybe more important, on the energy flow through the system due to the increased latent heat exchange. This is the reason why in a warmer world we expect wet regions to become wetter and dry region dryer, and more frequent extreme events. -
shawnhet at 17:33 PM on 28 March 2010Is the science settled?
Riccardo #107, I don't understand what you are trying to say. If we hold the RH constant but increase the temperature, then we have increased the absolute concentration of WV molecules in the air, right? Then, when the water in the air condenses more water molecules are available to form clouds and raindrops etc... than when the air was cooler. There are *not*, for instance, the same amount of water molecules available to form clouds and raindrops when the RH is 60% and temperature is 10C and cools to 9C as when the RH is 60%, temperature is 20C and cools to 19C. Chris, #108 yes, you're right. I mistyped in my previous post. The feedback on the warming has already happened, there may be further warming in the pipeline from the forcing that will then be fed back upon. Cheers, :) -
Doug Bostrom at 17:01 PM on 28 March 2010It's freaking cold!
Muoncounter, it's a case of wheels within wheels. Not only are crypto-Communists trying to end our Western lifestyle by confiscating our incandescent lightbulbs, but at the same time the ruthless Canadians are extracting petroleum from tar sands, selling it to us Americans and then tricking us into burning it specifically so more C02 will be released, driving up temperatures, thus pushing the syrup industry north of the Canada-US border. They'll be selling that syrup back to us hungry, desperate pancake eaters later. It's a dastardly plot, one of many. -
Doug Bostrom at 16:49 PM on 28 March 2010Empirical evidence that humans are causing global warming
climateresponse at 16:22 PM on 22 March, 2010 Well, here you are, after all, at a site dedicated to explanation of the topic. No need to complain! Your concern has also already been answered by a very excellent writer and scientist by the name of Dr. Spencer Weart. Do take a few hours to read his book The Discovery of Global Warming, published free to read on the Web. -
Tom Dayton at 16:21 PM on 28 March 2010Empirical evidence that humans are causing global warming
gallopingcamel, one reason people get frustrated with you is that you ask questions or (more usually) make strong assertions without bothering to look for the answers with the tools built into this Skeptical Science site. In this case, you could have clicked the "View All Arguments..." link under the thermometer at the top left of every page, or typed "hockey stick" into the Search field. Either method quickly would have led you to the page Hockey stick is broken, where you would find a bunch of hockey stick graphs made from data having nothing to do with tree rings. -
Tom Dayton at 16:12 PM on 28 March 2010What CO2 level would cause the Greenland ice sheet to collapse?
gallopingcamel, I'm glad you seem to mostly trust Dr. Archer's judgment about how long it takes for increased CO2 level to fall back to its original value. But note that I did not write "residence time" of CO2, nor "lifetime," because those terms are ambiguous and have been misinterpreted by "skeptics." What matters to climate is how long it takes for the total number of CO2 molecules in the air to drop from its increased value back to its pre-increased value. During that process, some individual molecules that were added by the initial event A (e.g., oil burning) are removed very quickly, but they are immediately replaced by other molecules from sources other than event A (e.g., the ocean). When the latter swap happens, the total number of molecules does not decrease. So the "residence time" or "lifetime" of an individual CO2 molecule is irrelevant. What matters is how long it takes for the total number of molecules to decrease. That was explained briefly by Chemist Dr. Doug Mackie in his Question and Answer 1. More details and references (including links to full text for most of them) are in the caption of the Global Warming Art image Carbon Dioxide Residence Time. What the IPCC "and others" actually wrote is not "35 to 100 years assumed by the IPCC and others" as you asserted. Their actual statements are extracted as snippets, with links to the full texts, by Dr. Lisa Moore in her post Greenhouse Gases: How Long Do They Last?" Additional links to papers by Archer, Solomon, and others are on the Global Warming Links page here on the Skeptical Science site, in the Links for "CO2 Has a Short Residence Time" page. (That skeptic argument does not yet have its own, full page, so it appears only in the Links page, which you can get to via the "Links" link in the blue horizontal bar at the top of every Skeptical Science page.) -
gallopingcamel at 16:04 PM on 28 March 2010Empirical evidence that humans are causing global warming
There is little doubt that the burning of fossil fuels by humanity has caused concentrations of CO2 in Earth's atmosphere to rise. The graph of CO2 concentration versus time is a "Hockey Stick". Can anyone show that global temperatures over the last 1,000 years correlate with the CO2 concentrations without using tree ring temperature proxies? -
gallopingcamel at 15:08 PM on 28 March 2010What CO2 level would cause the Greenland ice sheet to collapse?
Phillipe (#67) and Tom (#68). Thanks for the links. I have not been able to access the Berger & Loutre paper so I have been reading everything I can find from David Archer and his various collaborators. Considering the huge implications of these papers it is surprising that there has been so little coverage in the media. If these folks are right: 1. It is already too late to prevent the Greenland ice cap from collapsing. 2. Mankind has the power to delay the next Ice Age almost indefinitely. From my perspective this is wonderful news. Maybe that is why it has been ignored by the "Main Stream Media". One of the worst things about being a sceptic is that you distrust the good news as much as the bad news, so I tried to pick holes in Archer's case. Archer's main planks are: A. CO2 is a dominant driver of global temperature. B. CO2 residence time in the atmosphere is orders of magnitude longer than the 35 to 100 years assumed by the IPCC and others. It is not likely that we will agree on point "A" so let's look at the CO2 residence time issue. Archer is a chemist so it was no surprise that I cannot fault his arguments relating to ocean absorption and the weathering of silicate rocks. The complexities of the various models he mentions (CLIMBER etc.) are way beyond my pay grade. Hopefully, Berenyi Peter will be able to comment. There appears to be a carbon sequestration process with a short time constant that Archer & Co may have overlooked. As my lab shares a car park with the folks running the relevant experiment, I forgot about chasing gamma rays for long enough to find out what my tree hugger colleagues have been up to. Take a look at the FACE experiment: http://c-h2oecology.env.duke.edu/site/face.html My hypothesis is that plant growth will accelerate as CO2 concentrations rise, thereby sequestering carbon rapidly (decadal time constant). If true, this effect would overwhelm the longer term processes described by Archer et al. Unfortunately (from my point of view), the FACE experiment is inconclusive. The trees exposed to CO2 at levels predicted for 2050 did grow very rapidly for several years but then slowed down owing to lack of nutrients (primarily nitrogen). Your comments would be appreciated. -
muoncounter at 08:19 AM on 28 March 2010It's freaking cold!
On the lighter side: Since we had such a cold winter in North America, there was an uptick in the 'all that snow means global warming doesn't exist' hysteria. Bet we won't here much about this: "It’s on the calendar. It’s widely advertised. This year, everybody knows about it but the trees. And they are the central characters in Vermont’s annual maple syrup open house this weekend, when tourists descend on the state to watch trees being tapped and sap being boiled. Sugaring season ended early for many syrup farmers in southern Vermont, sabotaged by unseasonably warm weather." NY Times, 27 March 2010 (http://www.nytimes.com/2010/03/27/us/27syrup.html) -
chris at 07:21 AM on 28 March 2010Is the science settled?
shawnhet at 06:42 AM on 28 March, 2010 My point was directed to folks who might think that WV feedback on the forcing we've already added was yet to happen. That's not quite right. As you indicate we've had the water vapour feedback on the warming that's occurred so far. However since we haven't yet had all the warming that will eventually result from the forcing we've already added, there is still some water vapour feedback to accrue as a reult of the current forcing. -
Riccardo at 07:15 AM on 28 March 2010Is the science settled?
shawnhet, if you hold the temperature fixed there's no way to vary WV. In all practical purposes we can safely assume that it is in equilibrium at the given temperature. If you let the temperature increase, instead, you'll get increasing concentration at constant relative humidity. -
shawnhet at 06:42 AM on 28 March 2010Is the science settled?
Riccardo:it's not concentration that matters for your argument, it's relative humidity. Also consider that precipitations counteract, or limit, cloudiness increase; the water in the rain drops must come from somewhere after all. Well, obviously, the RH and the concentration of WV are inseparable at a given temperature, but IAC, it is the concentration that matters. It takes a certain number of water molecules to make a raindrop or a cloud. Thus, if everything else is equal we would expect more raindrops and clouds to form when air @ (for example) 15C cools by 1C, than if air @ 14C cools by 1C. Your second sentence is possibly true, but I was going for the least possible assumptions in my "model" namely that everything stayed *proportionally* the same when WV went up. It is possible that increasing WV will cause precipitation to take place quicker which could potentially reduce the amount of clouds. This issue is too complicated for me to get into right now, but it is interesting. Ned:That's right; if we eliminated all other forcings then the water vapor feedback would settle at its new equilibrium level very rapidly. However, it seems likely that we'll keep emitting CO2 for some time now. The water vapor feedback will continue to increase as long as CO2 continues to increase, and it will persist as long as CO2-induced warming persists (i.e., thousands of years). I don't really disagree here, except maybe about the thousands of years. My point was directed to folks who might think that WV feedback on the forcing we've already added was yet to happen. Cheers, :) -
Riccardo at 05:44 AM on 28 March 2010How we know global warming is happening, Part 2
TH, Hansen's reference is a generic assesment: "We conclude that there has been no reduction in the global warming trend of 0.15-0.20°C/decade that began in the late 1970s.". I'd suggest to quantify the "divergence" you keep linking from woodfortrees if you do not trust my number for GISS and their own number for UAH. -
Riccardo at 05:31 AM on 28 March 2010What CO2 level would cause the Greenland ice sheet to collapse?
and i would add that accelerated ice loss appears to be spreading throughout northwest Greenland as a whole. -
How we know global warming is happening, Part 2
Hansen is currently quoting 0.15-0.20 http://data.giss.nasa.gov/gistemp/paper/gistemp2010_draft0319.pdf The graph I linked shows the divergence between GISS and UAH. -
chris at 05:13 AM on 28 March 2010Is the science settled?
fydijkstra at 23:02 PM on 27 March, 2010 Setting aside the “hot spot” point that has already been addressed ( Riccardo and Tom Dayton), there are a couple more points that could be addressed in your post fydijkstra: There is no high confidence that the warming in the last 30 years is exceptional, nor that anything in the pattern of warming in the last 200 years is closely related to the concentration of CO2. Since paleoanalysis indicates that the warming of the last 30 years is exceptional at least in the context of the last millennium and likely last two millennia, I don’t think your statement is supported by the science (see e.g. overlay of paleotemp reconstructions, and more recent paleoanalysis). Likewise, the pattern of warming is entirely consistent with the expected effects of enhanced [CO2], both in degree (see chris) and its nature (polar amplification; tropospheric warming associated with stratospheric cooling etc.). The relationship of the global temperature with patterns of ocean circulation is much better than the relationship with the CO2 concentration. That’s certainly incorrect. Analysis of ocean current contributions to 20th century warming indicates that these have made close to zero net contribution to warming over the last 100 years, and in fact reinforce the dominant role of enhanced greenhouse gas contributions ( Swanson et al (2009)[*] There is a lot of evidence, that the climate is much more complicated than could follow from the high understanding of heat trapping by CO2 alone. I don’t think anyone would say otherwise. That doesn’t negate the fact that we have rather high scientific certainty that raised greenhouse gas levels have dominated the Earth’s surface temperature rise during the last 100-150 years. [*]K.L. Swanson et al. (2009) Long term natural variability and 20th century climate change Proc. Natl. Acad. Sci. USA 106, 16120-16130(from the conclusions): This analysis indicates that natural contributions (largely ocean circulation variability) have had a significant effect on 20th century temperature variability. However the nett contribution to overall 20th century warming is close to zero. Essentially ocean circulation variability made a positive contribution to early (1900-1940) 20th century warming, a negative contribution to mid 20th century warming and a positive contribution to late 20th century warming. Once the natural variability is removed the externally forced (greenhouse) contribution is manifest as a continuous accelerating warming.
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Riccardo at 04:53 AM on 28 March 2010How we know global warming is happening, Part 2
TH, you quote strange number. UAH offically quote 0.132 °C/decade while from GISS annual averages I get 0.16 °C/decade. And UAH is only one of the two satelite datasets, RSS giving 0.15 °C/decade. I do not see much of a difference considering that they do not even measure the same thing. -
chris at 04:16 AM on 28 March 2010Is the science settled?
Whoops….two of my links don’t work. They should be: R. Knutti and G. C. Hegerl (2008) The equilibrium sensitivity of the Earth's temperature to radiation changes Nature Geoscience 1, 735-743 Knutti and Hegerl (2008) and: Murphy DM et al. (2009) An observationally based energy balance for the Earth since 1950 J. Geophys. Res.114 art. #D17107 Murphy et al. (2009) -
How we know global warming is happening, Part 2
Riccardo, GISS (1.7) does show a steeper trend than UAH (1.2) since 1979. -
Riccardo at 03:42 AM on 28 March 2010How we know global warming is happening, Part 2
TH, so you should deduce that my comment did not refer to your latest but to the second latest comment. More explicitly, i find a bit weird that "none of the mainstream skeptic sites dispute the satellite lower troposphere record" while questioning surface temperature datasets. -
Philippe Chantreau at 03:37 AM on 28 March 2010Guest post: scrutinising the 31,000 scientists in the OISM Petition Project
The "guesses" of the modelers are educated by the underlying physics, chemistry etc. There is not that much variation among these guesses. Parameterizations are bounded and so are the changes resulting from different parameterizations. There is indeed a variety of GCMs, that's why IPCC takes into consideration the results of many runs for many models. However, GCMs do not show wildly different results and indeed allow to form projections within reasonable limits, so the end result is usable. It seems you're trying the tired old "skeptic" argument "we don't know for sure so we don't know at all," or something in that same vein. Disappointing from someone better informed and better reasoning than the run of the Watts' mill type of "skeptic." -
chris at 03:16 AM on 28 March 2010Is the science settled?
Ken Lambert at 14:39 PM on 27 March, 2010 Yes O.K. Ken, but you’re really addressing a quantitative accounting that (a) we’re yet not in a position to determine (see discussion by Trenberth in the article under discussion), and (b) doesn’t really add or subtract from our present understanding of the Earth’s surface temperature sensitivity to enhanced radiative forcing. Of course, we could come up with some numbers that would accommodate a 3oC surface temperature rise (say) in response to a radiative forcing from doubling atmospheric [CO2] . But that wouldn’t take us any further than our present understanding of climate sensitivity which comes from many analyses of paleotemp/CO2 relationships (in which equilibrium responses are more likely to apply), and analyses of transient responses (e.g. to volcanic forcing). These indicate an Earth surface temperature response near 3 oC, with a range of 2-6 oC that is reasonably well-bounded at the low end and poorly bounded at the top end (see Knutti and Hegerl (2008) , for a review). On long timescales that allow slow ocean/ice sheet feedbacks the climate sensitivity might be larger than this (e.g. Lunt DJ et al. (2010)). Inspection of the temperature evolution/atmospheric CO2 levels during the last 150-odd years helps us quite a bit but also illustrates the nature of the uncertainty. So since the mid-19th century we’ve had around 0.8-0.9 oC of surface temperature rise, and [CO2] levels have gone up from around 286 ppm to 386 ppm. How might this translate into a climate sensitivity? Detailed quantitition is problematic since (i) the aerosol forcing which has certainly opposed some of the greenhouse-induced warming is poorly defined, and (ii) the climate-response time, particularly the very slow equilibration of the oceans to enhanced forcing, is not known with certainty. So we don’t know (i) if that 0.85 oC of warming (1850ish to now) would by 1 oC or 1.2oC or what, if we were to remove our atmospheric aerosols, and (ii) whether the current level of forcing will give us another 0.3 oC or 0.6 oC or what, of warming once the climate system has re-equilibrated with the forcing at some time in the future (multi-decadal timescale). We also have to account for non-greenhouse gas contributions to warming (v. likely these are small), and the fact that non-CO2 greenhouse contributions (methane, tropospheric ozone, N2O) need to be factored in. Can we say anything helpful? Well yes, we can look at the expected warming from various climate sensitivities [***] to determine the surface temperature rise expected from the enhanced [CO2]. For a climate sensitivity of 2oC (of surface warming per doubling of atmospheric [CO2]) this is around 0.85 oC at equilibrium (for a [CO2] increase from 286-386 ppm), and for a 3 oC sensitivity, ~1.25 oC of surface warming. Since we’ve already had 0.85 oC of warming without taking account the aerosol effect and the climate response time), it’s very unlikely that the climate sensitivity can be lower than 2 oC of warming per doubling of atmospheric CO2. A similar conclusion was recently obtained from an obviously (!) much more detailed analysis of the Earth’s energy balance since 1950 (Murphy et al. (2010) -------------------------------------------- [***] delta T = (ln([CO2]final/[CO2]start))*s/ln(2) where deltaT is the surface temperature change expected from a change in [CO2] from [CO2]start to [CO2]final in ppm, and s is the climate sensitivity in oC. -
How we know global warming is happening, Part 2
Riccardo, I was referring to GISS and you responded with a comment that skeptics don't trust ground based measurements - when obviously I do. -
Riccardo at 02:58 AM on 28 March 2010How we know global warming is happening, Part 2
TH, there are not that many temperature datasets, you can take for granted that we all know them. P.S. a secular linear trend can be acceptable only when there's nothing more we can say on the parameter under study. This is definitely not the case for temperature and related forcings. -
Tom Dayton at 02:33 AM on 28 March 2010Is the science settled?
fydijkstra, regarding the "hot spot", see the Skeptical Science argument "There’s no tropospheric hot spot." Be sure to read the comment by jshore. -
mspelto at 02:11 AM on 28 March 2010What CO2 level would cause the Greenland ice sheet to collapse?
It is worth noting the sensitivity of northern Greenland to the 400 ppm scenario. Think about the Petermann Glacier as an indicator of this. http://glacierchange.wordpress.com/2010/03/27/petermann-glacier/ -
Scinan at 00:47 AM on 28 March 2010Antarctica is gaining ice
I find it interesting too that as the land ice moves toward the sea, not only will there be less land ice, but it occurs to me that there is likely to be much more sea ice as well. Rather like a migration. Moves from here to there. At some point as that nasty green line becomes vertical, there is going to be a whole grunch of sea ice as no land ice remains to jump in. -
How we know global warming is happening, Part 2
Riccardo, GISS is the surface record from Dr. Hansen. It shows 0.8C warming for the last 130 years, or 0.6C per century. http://www.woodfortrees.org/plot/gistemp/from:1880/plot/gistemp/from:1880/trend -
Ned at 00:30 AM on 28 March 2010Is the science settled?
shawnhet writes: WV feedback(whatever its magnitude or sign for all but the very recent *atmospheric* temperature changes has *already happened*. There may be some other temperature changes coming along "in the pipeline", but WV adjusts to a temperature change in a period of months or so. That's right; if we eliminated all other forcings then the water vapor feedback would settle at its new equilibrium level very rapidly. However, it seems likely that we'll keep emitting CO2 for some time now. The water vapor feedback will continue to increase as long as CO2 continues to increase, and it will persist as long as CO2-induced warming persists (i.e., thousands of years). -
s1ck at 00:04 AM on 28 March 2010Is the science settled?
fydijkstra, I think a frequent logical misconception is to somehow separate a natural greenhouse warming from AGW (although this manoeuvre may well be justified or even indispensable in a political debate). Just consider a noisy recorder output from any experimental measurement. If you take the baseline for granted, you can of zoom into the graph, focus on the noisy ups and downs and complain that nobody will ever be able to predict the future. Zoom out to get the whole picture. The more GHGs, the warmer. That's it. BTW, Ned, chris, the discussion on WV, feedbacks that run away and those that won't was highly instructive.
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