Meanwhile today while CO2 is increasing, the Antarctic ice cap is also increasing.
Bill Illis writes about it:
Ice sheets formed in Antarctica about 35 million years ago when CO2 was about 1,200 ppm. Ice sheets also formed in Antarctica about 350 to 290 million years ago when CO2 was about 350 ppm. Ice sheets also formed in Antarctica about 450 to 430 million years ago when CO2 was about 4,500 ppm. The more common denominator is when continental drift places Antarctica at the south pole.
Below, Antarctica today.
New data illuminates Antarctic ice cap formation
From a Bristol University Press release issued 13 September 2009
A paper published in Nature
New carbon dioxide data confirm that formation of the Antarctic ice-cap some 33.5 million years ago was due to declining carbon dioxide in the atmosphere.
A team of scientists from Bristol, Cardiff and Texas A&M universities braved the lions and hyenas of a small East African village to extract microfossils from rocks which have revealed the level of CO2 in the Earth’s atmosphere at the time of the formation of the ice-cap.
Geologists have long speculated that the formation of the Antarctic ice-cap was caused by a gradually diminishing natural greenhouse effect. The study’s findings, published in Nature online, confirm that atmospheric CO2 started to decline about 34 million years ago, during the period known to geologists as the Eocene – Oligocene climate transition, and that the ice sheet began to form about 33.5 million years ago when CO2 in the atmosphere reached a tipping point of around 760 parts per million (by volume).
The new findings will add to the debate around rising CO2 levels in the Earth’s atmosphere as the world’s attention turns to the UN Climate Conference in Copenhagen which opens later this year.
Dr Gavin Foster from the University of Bristol and a co-author on the paper said: “By using a rather unique set of samples from Tanzania and a new analytical technique that I developed, we have, for the first time, been able to reconstruct the concentration of CO2 across the Eocene-Oligocene boundary – the time period about 33.5 million years ago when ice sheets first started to grow on Eastern Antarctica. “
Professor Paul Pearson from Cardiff University’s School of Earth and Ocean Sciences, who led the mission to the remote East Africa village of Stakishari said: “About 34 million years ago the Earth experienced a mysterious cooling trend. Glaciers and small ice sheets developed in Antarctica, sea levels fell and temperate forests began to displace tropical-type vegetation in many areas.
“The period culminated in the rapid development of a continental-scale ice sheet on Antarctica, which has been there ever since. We therefore set out to establish whether there was a substantial decline in atmospheric carbon dioxide levels as the Antarctic ice sheet began to grow.”
Co-author Dr Bridget Wade from Texas A&M University Department of Geology and Geophysics added: “This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago.
“Our study is the first to provide a direct link between the establishment of an ice sheet on Antarctica and atmospheric carbon dioxide levels and therefore confirms the relationship between carbon dioxide levels in the atmosphere and global climate.”
The team mapped large expanses of bush and wilderness and pieced together the underlying local rock formations using occasional outcrops of rocks and stream beds. Eventually they discovered sediments of the right age near a traditional African village called Stakishari. By assembling a drilling rig and extracting hundreds of meters of samples from under the ground they were able to obtain exactly the piece of Earth’s history they had been searching for.
Further information:
The paper:Atmospheric carbon dioxide through the Eocene–Oligocene climate transition. Paul N. Pearson, Gavin L. Foster & Bridget S. Wade. Nature online, Sunday 13th September.
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@ur momisugly P Wilson (13:06:04) :
However, AGW theorists don’t even bother to look into the analysis put forth by, lets face it, peer reviewed analyses using data sets that don’t lend much support to AGW theory.
Although that may be the perception because there are very few journal articles that appear to question AGW, it is not true. The very few scientists that do publish arguing for non-AGW reasons for global warming get MUCH attention by the scientific community. Here are a few examples that come right to mind:
Lindzen’s Tropical Heat Vent (Iris) paper
Svensmark’s Cosmic Rays papers
McClean, de Freitas, and Carter’s recent paper about ENSO. Of course, this paper was thoroughly vetted by experts and pretty much destroyed. It received A LOT of attention and these three are very well known skeptics.
There seems to be a preponderance of people posting on WUWT that believe there is a conspiracy by international scientists to promote AGW by hiding data or purposely misrepresenting data. Science is a culture of skepticism and trying to prove the prevailing wisdom is incorrect. What scientist doesn’t want to be the next Galileo, Darwin, or Einstein? Please do not respond with “but they only get money if they toe the party line”. There is quite a bit of money out there to support every scientist who wishes to refute AGW and we see that money being spread around.
The low probability conspiracies are far more interesting than the highly probable boring explanation so humans tend to look for conspiracies. Examples include: the Trade Towers were demolished by the US and it was not Bin Laden, TWA Flight 800 was shot down by a US missile, the JFK grassy knoll shooter, FDR knew Pearl Harbor was to be attacked but didn’t say anything, etc., etc., etc. These are all extremely unlikely such as the notion that an overwhelming majority of international climate experts have all agreed to conspire to delude the billions of folks on the planet and just a very small % of them – the Plimers, Singers, Moncktons, etc. – are trying to save us all from this mass hoax.
Common sense and a sense of probability should reveal the silliness in this notion.
The first person to show proof of what IS causing the unprecedented modern day global warming and that it is NOT greenhouse gas emissions by humans is the next Nobel Science winner. Sounds like a pretty good incentive for somebody to be anti-AGW.
This disagreement is in the term unprecedented. greenhouse gases create a milder overall climate although they are feedbacks and not forcings. Between ENSO, PDO’s solar forcing and orbital disposition, , I’m afraid that one ends up with a far more boring and monotonic – along with the DATA that corresponds to these phenomena. It gives us a picture of a generally tolerable long term climate that cools and warms. Nothing in the last 30 years have been exceptional, and as a period, is something slightly below average warming.
So I suggest tht these predictions of catastrophe, hysteria and doom make it all seem rather less boring than it is. Naturally, if all the blame is put onto 3% of a trave gas, tehen this indeed puts humans back at the centre of the universe, with the guilt on their shoulders, and one can only financially exploit such an ocuraence by vilifying it.
However given the spectroscopic absorbtion of c02 – something which I have studied – and its ability to become excited by certain values of heat, it is a rather marginal affair. So is water vapour, though much more potent. Both are feedbacks and depend on climate forcings which are not constant.
I’m not sure that Veizer, Bob Tisdale, Bob carter, Jaworowski and Segalstad among quite a few scientists could be dismissed as irrelevant or silly. Indeed, the Catholic Church destroyed Galileo, if that is a term you find appropriate, although centuries later of course he was right.
Certainly they’re not selling their integrity to the highest bidder
Hmm. Sorry Scott Mandia, we’re being devilled down the path of rhetoric. Back to the Science and data!
Take caillon’s graphs on this link:
http://icebubbles.ucsd.edu/Publications/CaillonTermIII.pdf
it emphatically shows that c02 has never amplified temperature, despite claims to the contrary.
So the first question is what unprecendented global warming?
Kindest Regards
P Wilson,
“it emphatically shows that c02 has never amplified temperature, despite claims to the contrary”.
Nicolas Caillon’s 2003 Science paper says exactly the opposite. Did you read it? Here’ the conclusion:
“Finally, the situation at Termination III differs from the recent anthropogenic CO 2 increase. As recently noted by Kump, we should distinguish between internal influences (such as the deglacial CO2 increase) and external influences (such as the anthropogenic CO2 increase) on the climate system. Although the recent CO 2 increase has clearly been imposed first, as a result of anthropogenic activities, it naturally takes, at Termination III, some time for CO2 to outgas from the ocean once it starts to react to a climate change that is first felt in the atmosphere. The sequence of events during this Termination is fully consistent with CO2 participating in the latter ~4200 years of the warming. The radiative forcing due to CO2 may serve as an amplifier of initial orbital forcing, which is then further amplified by fast atmospheric feedbacks that are also at work for the present-
day and future climate.”
Smokey,
“Finally, you again dodged answering my 13:11:59 post. Are every one of those statements completely false?”
I dodged nothing – you didn’t address a question to me.
But since you now ask:
“Historically CO2 and temperature have never been linked nor have they been since accurate records have started.”
False: see for instance the Science paper referenced above.
“There has been no sudden warming that would portend catastrophic climate change coming.”
False. “Portend” means to give a sign of the future. Is such a catastrophic change inevitable? No. It depends on our actions.
“There may have been recent warming which is easily within the range of historic variability and no evidence that it has been caused by anthropogenic CO2.”
False. We might even agree here as you said earlier “I’ve been consistent in saying there may be a *very* slight effect from CO2”.
“Political motivation behind the AGW Industry to the general public who are otherwise informed only by AGW propaganda which is at times hysterical and patently false.”
This statement is confused. I’d agree that “at times hysterical and patently false” statements are given out concerning AGW, and of course there are political motivations involved, on all sides. But the implication that AGW is just an industry is false.
Perhaps you might now respond to the many questions I have addressed to you that so far remain unanswered? But I won’t hold my breath.
@ur momisugly P Wilson (18:55:47) :
CO2 is a forcing mechanism. Water vapor is a feedback response.
My main two points were that the skeptics that do publish get a lot of attention and that believing in conspiracies is silly.
TomP
Another quick one at this stage-manipulating a 7inch screen asus is not easy when you are away!
I suspect that you did not read my earlier post when I made 17 (properly considered and relevant) links. That you have now done so is to your credit so hopefully you will not go off at surreal tangents again 🙂
In that earlier series of Links I not only mentioned Hansen but also cited a link showing the disagreement to the Essex/Mcitrick papers-as follows;
“Link 10 this refers to the fuss about McKitrick’s paper which was hotly refuted by various people as it queried the very core of AGW data.
http://www.sourcewatch.org/index.php?title=Talk:Ross_McKitrick
Link 11 this technical interrogation of the calculations from Climate Audit
http://www.climateaudit.org/?p=2015
So we have several sets of parallel discussions whereby the meaning or worth of a single global temperature is queried in the first place, and the reliability of the information gathered is contested. This revolves mainly around changes in weather station locations, numbers, methodology and general consistency, and therefore the overall reliability of the information derived.”
To ignore or just dismiss these papers as irrelevant made me suspect that you and Scott (to a lesser extent Joel who knows us better) just assume we are illogical ‘deniers’ who have never read a scientific paper in their lives. I wrote a piece on the difference between a sceptic and denier (small d and non perjorative) which I will post again at some point-we are all not the same by any means.
Please realise most sceptics are perfectly reasonable people who have thought deeply about their position but just disagree with the computer generated scenarios. The world has been this way before climatically and it does not help when warmists just disregard the many various papers and historic records on the subject.
For my part I look forward to the reply from Essex and what Mcitricks reactions might be would also be entertaining 🙂
best regards
tonyb
The graph shows no amplification. Here is another:
http://icecap.us/images/uploads/CO2,Temperaturesandiceages-f.pdf
at any 2 points of equal c02 concentration the temperature is higher and when temperature is falling co2 is higher. This is consistent to temperature leading c02. Given that 270ppm has the same effect as 400ppm, due to its absorbtion band, that the 270ppm isn’t an objective measure of global c02, and that downturns in temperature correspond with rising c02 (the lag factor) there is no evidence to say that c02 is a forcing or a salient feedback.
I did say *it emphatically shows that c02 has never amplified temperature, despite claims to the contrary.”
I’ve read the whole of the IPCC 4th assessment, particularly chapter 2, and the crucial calculations required to justify this mechanism of AGW by c02 are absent. Not there.
So given that there is now some 387ppm c02 in the atmosphere, we should be 4-5C warmer globally that we are if the forcing mechanism is correct, and this temperature should monotonously increase year after year. The Met office on their information pages say “C02 is now the dominant climatic factor”. So next year, we presume that the global temperature will suddenly shoot up by over 6C to catch up for the last 10 years of average-downward temperatures.
Incidentally, the Met Office, who claim to be world leaders in climate prediction, said in early 2007 that on the basis of c02 emissions, 2007 would be the hottest year on record.
Scott Mandia. “Silly” is not a scientific concept.
Tom P. Here is a temperature reconstruction using reliable proxies of the last 2000 years.
http://www.ncasi.org/publications/Detail.aspx?id=3025
And Scott Mandia. How could water vapor be a feedback and co2 a response? Oceans have a very high heat capacity which means that longwave radiation cannot penetrate them. Air doesn’t have the heat capacity of oceans. It follows that only solar radiation has enough energy to cause a water vapour feedback. On the other hand, oceans can heat the atmosphere and expel vast amounts of c02 during warmisng phases. Its this sort of rewriting the laws of physics that has corrupted climate science.
Even wikipedia, not the most rigorous source, claimed that the holocene climatic optimum was warmer than the 20th century. Looking at the graph, so was the MWP
http://en.wikipedia.org/wiki/File:Holocene_Temperature_Variations.png
Tony B, P Wilson,
I’m am familiar with all your supposed objections to AGW you cite and nearly all have well grounded refutations in the literature, or don’t actually say what you imagine they do.
For instance if you read to the end of the paper Loehle issued a correction saying that in fact the MWP was not significantly warmer than today.
The Essex paper was an exception as it was published in the mainstream literature. I was rather curious as to the point they were trying to make.
Through discussions with the authors, and checks with others who really know their thermodynamics, there are indeed some fatal problems with this paper along the lines I outlined and a fully justified (though rather unexpected way) to calculate average global temperatures. This may well lead to a publication in its own right, so thanks for bringing the paper to my attention.
If you claim to be intelligent sceptics, please at least be sceptical about the data you cite. Claiming that there have been no record highs in the last forty years was just nonsense, as is claiming that a 25% increase in CO2 is predicted by anyone to cause a 4-5 C warming (hint – that’s an estimate of the doubling temperature).
At least come up with some stylish nonsense, like the Essex paper!
P Wilson:
I am not sure why you think heat capacity has anything to do with how much the IR radiation will penetrate the oceans. As for what happens to IR hitting oceans, there are only 3 possibilities: it can be transmitted through (down to the bottom), reflected, or absorbed. It can’t just magically disappear. It doesn’t get transmitted very far, to say the least. And, I don’t believe that most of it gets reflected (but if you believe otherwise, I would be interested to see the source of your knowledge) . So that leaves absorption. The fact that the absorption is so strong that the IR doesn’t get far before being absorbed does not mean that it does not get absorbed and the heat transferred by conduction and convection away from the thin layer where it is absorbed.
It is rich irony that someone who tells me that the Stefan-Boltzmann Eqn doesn’t work at all for objects around room temperature is complaining about others rewriting the laws of physics!!!
That reconstruction has been strongly critiqued and it was not even published in a recognized peer-reviewed journal of any stature in the field. “Energy and Environment” is a journal received by only a handful of libraries around the world. If you want your work to be taken seriously by other scientists, you don’t publish there. It is for those who know that their work would not make it into a real scientific journal and whose main audience is not their fellow scientists but rather the larger public and particularly the echo chamber for the “skeptic” point-of-view.
Scott can comment further on this but I think it may be true that the climate is, as near as we can determine, about at the level of the climatic optimum and, almost all scientists believe, will soon surpass it. As for the MWP, I don’t see where you get the impression that it was warmer. The top graph doesn’t extend the line up to the current temperature but rather just marks it with the “2004” label because it is difficult to resolve the 20th century rise on a graph of the last 12,000 years…The line would be essentially vertical, reflecting the rapid rate of increase.
P Wilson:
Actually, their prediction at the beginning of 2007 was that there was a 60% chance that 2007 would set a new record. And, this was not on the basis of CO2 emissions but rather a variety of factors “such as solar effects, El Niño, greenhouse gases concentrations and other multi-decadal influences.” For short term forecasts like this, the most important factor is undoubtably the ENSO (El Niño).
As it turned out, the moderate strength El Niño that they were strongly basing their forecast on quickly weakened and I believe turned into a moderately strong La Nina by year’s end (if not sooner), which resulted in a significant drop in the temperature anomaly over the year. They predicted the anomaly would be 0.54 C and noted that their mean forecast error over the previous 7 years had been 0.06 C. The first two months actually averaged 0.576 C but then after that it fell and the yearly average turned out to be 0.405 C…So, their error was 0.135 C and it was not one of their better years of forecasting. (In fact, it might have been their worst…I don’t know.)
The forecasting of the El Niño / La Niña is actually a lot more difficult than forecasting trends in response to changes in forcings since the former is very sensitive to initial conditions.
TomP and Joel
The stylish nonsense was my stylish nonsense, the comment about record temperatures was not. You are lumping us all together again (although P Wilson seems very bright in his own right)
Joel
I see that you are a fan of DR Mann of ‘The MWP is an outdated concept’ fame 🙂
I suspect this very good thread is about to die. I think it falls off the edge of the world which as we all know is flat. See you on another thread. Keep us informed on the Essex paper TomP
tonyb
@ur momisugly P Wilson:
One cannot use a graph that has an X scale of thousands of years to compare to the climate change time scale of one to two centuries. Your graphs are essentially wiping out any trends that are 100 years old or less.
These graphs are also a bit moot because humans were not around pumping in off the chart CO2 levels. Pre-Industrial Revolution CO2 levels ranged between 190 ppm and 300 ppm. Today they are rapidly approaching 400 ppm. CO2 concentrations are known accurately for the past 650,000 years. As of August 2009 CO2 is 386 ppm which took about 100 years to increase. For comparison, it took over 5,000 years for an 80 ppm rise after the last glacial maximum.
As Joel said the 4-5 C increase is for a doubling of CO2 and were are not there yet but more is on the way of course.
There is certainly more water vapor in the atmosphere than other greenhouse gases and water vapor is a much more efficient greenhouse gas than carbon dioxide. Water vapor molecules typically spend about 10 days in the atmosphere while CO2 remains for hundreds to many thousands of years. Because water vapor is short-lived it cannot be a climate change forcing mechanism but CO2 certainly is.
A climate forcing mechanism such as CO2 is one that will cause a change in climate. A feedback mechanism is one in which the forced change is either amplified (positive feedback) or dampened (negative feedback). Water vapor is a very important positive feedback mechanism. As more greenhouse gases are emitted by humans, global air temperatures are rising. When the air gets warmer, the saturation vapor pressure of water increases. That means that more water vapor can be present in warmer air. Because the average relative humidity of the climate is conserved, a warmer climate means that there will be more water vapor in the air. In turn, this causes a greater greenhouse effect which amplifies the initial warming caused by increasing industrial greenhouse gases. Bottom line: water vapor cannot cause global warming but it certainly will increase the warming.
Scott Mandia (12:41:25) :
I think if we are doing comparisons on a geologic time scales, we have to take into account the era we are looking at… So the earth has been in an ice age for 2.5odd million years(major glaciation, technically it could be argued the earth has been in an ice age for tens o millions o years) being punctuated with brief interglacials, driven by the malkovich cycles. The previous interglacial s, and this one initially have been extremely chaotic! With climate fluctuations OVER 100times the magnitude of recent trends http://www.pnas.org/content/97/4/1331.full (this should be pretty common knowledge, its been thoroughly documented, and was big news at the time of the discovery)
the holocene maximum (not 2000ybp, the bronze age) Was significantly warmer than today, for a period of several thousand years(still not as warm as past interglacial s, with lower GHG levels)… which certainly brings into question the assertion that water vapor cannot force/or maintain an extended climate shift. It may require a trigger, but as you have stated, we have accurate records for GHGs.
Now if we look further back in the geologic record across the various hot house, ice house periods. There is a correlation, with low co2 and a chaotic climate. With the hot house periods being essentially stable for 100s o millions o years, and the ice house periods being chaotic violent climate shifts between glaciation/interglacial periods.
But anyway. This interglacial is longer lived than the previous ones we have reconstructions for(possibly for the last 2.5million years). So why exactly are we expecting it to behave the same atmospherically and climatically? And why would we want it too? We know that atmospheric co2 levels of 300ppm havnt stopped past glaciations(and i believe a mile thick ice sheet over Europe, Eurasia, Canada and North America would be slightly more adverse too life than the slow warming we are experiencing comparatively to the business as usual glacial interglacial cycles)
TomP
You made a comment above that appeared to be aimed at me
“For instance if you read to the end of the paper Loehle issued a correction saying that in fact the MWP was not significantly warmer than today.”
I have not made any reference to Loehle so I do not know why you have brought it up . You are right in saying he made a correction but the context was that whilst the mwp was not “significantly” warmer it was still warmer.
http://www.worldclimatereport.com/index.php/2008/02/11/a-2000-year-global-temperature-record/
If we had had been citing the numerous studies that explore the idea of a warmer mwp I would have referenced Soon and Baliunas 2003, demonocal and Box et al (arctic) as they are all worth reading. Some papers can be a little dull of course, Having read dozens of studies of the MWP i feel I know more about Fish proxies, boreholes and moss, than is good for me 🙂
I prefer more interesting proxies such as those from Lamb or ‘live’ history that comes from studying the Vikings and examining the background to the climate diaries of the Byzantine empire, an interesting sweep of 1000 years of history that encompassed every type of climate possible.
tonyb
Joel. From biogenic silica concentrations preserved in lacustrine sediments in North America, to elevation and soil periglacial and glacial processes in the Italian pennines, the studies, peer reviewed at that, all state tha ttemperatures from 700 to 1300 were “higher than present” and in winter “0.9C greater than late 20th century warmth. There are a lot of these papersfrom across the northern and southern hemisphere, that go beyond ring tree studies, which have severe limitations with temperature correlation.
As for the Met Office here in th eUK, they have made so many seasonal and yearly forecast failures that its a matter of routine. In their most recent warning of this winter just gone, they stated that the” trend of milder than average winters continues”, though said it without any probability at all. That is, 100% certainty.
to their credit, they have archives of their premonitions.
however, given that they state that c02 is now the most dominant factor, and that it is 60% likely that 2007 will be the hottest year on record, 60% is greater than 51%. They could say “It is 60% likely that the MWP was cooler today on the basis of our information about it”. Judging by this, one ought to take it in reverse? they would do themselves some scientific justice in recanting the carbon dioxide thesis in future predictions. In truth however, the Met office are abysmal at predicting weather and climate.
of course, if they had said it is 40% likely that 2007 will be the hottest year on record… So if their margin of error is so high, then how can we believe lesser instiutions than the Met Office, which works together with the Hadley Centre?
Scott: C02 is only a proxy of climate. I’m sure you’re aware by now of direct air measurements of c02 from the 19th century, and inferred measurements of c02 from further back in relation to isotopes found in various bio matter over the centuries. Why ar these either suppressed or ignored by lobbyists and activists?
Given that the energy of longwave re-radiation is so weak, and that air is apoor conductor, in addition to the fact that oceans have a high heat capacity, there is no argument for it forcing a tmperature change in oceans. The sui has enough difficulty in heating oceans due to its heat capacity. Thats why asphalt and beaches are warm when waves are cooler. Physically there is no argument for c02 transferring heat into oceans as the sort of radiation it intercepts is too minimal. It would be like trying to melt a candle with a 2 watt lamp torch. Once the sun has heated oceans however, then it stores a lot of heat
Tom P.
How would you rebut Huang, S. and Pollack, H.N. 1997. Late Quaternary temperature changes seen in world-wide continental heat flow measurements. ? and the many other studies of the construction along these lines of the last 1000-2000 years?
tree ring data alone is unreliable as aproxy of past temperatures. At higher temps, droughts can occur and trees receive less water, and its too easy to infer that it is because the temperature is so cool that trees don’t show the healthy growth that comes with warmer temperatures.
how do you explain grape records as far as Hadrians wall? Decomposed organic matter under Greenland ice sheets dated to 975AD, and an inferred warmer temperature than today from ice cores in Greenland during the period?
These data pieces are by no means limiterd to the northern hemisphere.
TonyB:
Irregardless of how things work out for TomP on the more fundamental points that he has raised, my point still stands: that the authors failed to show the relevance to determining trends in average global temperature and, in fact, their own data shows that the trend is quite resilient to changes in how you average as long as you don’t do something totally ridiculous. This isn’t hard to verify as their data (and code if you have the software package they used, which I don’t) is available for download. (I will give them credit for practicing what they preach in terms of making their data and code available.)
TonyB and P Wilson: By the way, in regards to the MWP, I have shied away from the details in this area of research since it is not one of the more interesting climate questions to me personally. However, I think you are missing the main point of the Mann et al. work, which was not to argue that there were not many areas in the world that had comparable…or sometimes even greater…warmth in a broadly defined several hundred year period that was referred to as the “Medieval Warm Period”. It is rather that these warm periods tended to occur at different times during that broad period in the different places (they were asynchronous) whereas today they are occurring more uniformly throughout the world (they are synchronous). Hence, when you compute the average temperature during that period, what you get is a broad diffuse bump whereas today they add together to give a sharper, higher spike.
Let’s take a simplistic analogy: Imagine that we have a week where an isolated hot bubble of air moves across the northern tier of the U.S., so it is 95 F in Seattle on Monday in Helena, Montana on Tuesday, in Minneapolis on Wednesday, in Detroit on Thursday, and in Boston on Friday. And, let us suppose that on the other days, the temperature is 75 F (e.g., so in Seattle, it is 75 F on every day but Monday, in Minneapolis, it is 75 F on everyday but Wednesday, and so forth). Then you would get an average temperature of 79 F for the (5-day) week across those 5 cities. Now suppose you had another week where it was 85 F in all 5 of those cities for all 5 days. Then you would get an average temperature of 85 F across those cities for that week.
So, the second week would have a higher average temperature even though you could look at the first week and say, “Look…in all 5 of those cities there were days where it got up to 95 F…That must have been one hot week!” This is roughly what was being done before Mann et al studied the question. Mann et al were the first ones to really come along and try to compute the average temperature over the whole globe with sufficient time resolution to see how it varied. Since then, several other groups have confirmed Mann et al.’s basic results (although some show a more pronounced Little Ice Age than Mann et al found). And, yes, there are lots of detailed questions about temperature proxies, bristlecone pines, CO2 fertilization, and what-have-you, and scientists will continue to refine their techniques for doing these proxy studies. But, just pointing to random places in the world and saying there was a period of significant warmth there sometime during the period from 800-1300 is not enough to establish that the world as a whole (or hemisphere as a whole) was warmer anytime during the MWP than it is today.
Mann et al. (2008) used a multiple proxy database consisting of a diverse (1,209) set of annually (1,158) and decadally (51) resolved proxy series including tree-ring, marine sediment, speleothem (mineral deposits), lacustrine (lake deposits), ice core, coral, and historical documentary series. All 1,209 series were available back to at least A.D. 1800, 460 extend back to A.D. 1600, 177 back to A.D. 1400, 59 back to A.D. 1000, 36 back to A.D. 500, and 25 back to year 1 B.C. This proxy database represents a significant extension of the database used in related earlier studies.
Mann et al. (2008) is a free download here:
http://www.pnas.org/content/early/2008/09/02/0805721105.abstract