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Medieval warmth and English wine

Filed under: — gavin @ 12 July 2006

Never let it be said that we at RealClimate don’t work for our readers. Since a commenter mentioned the medieval vineyards in England, I’ve been engaged on a quixotic quest to discover the truth about the oft-cited, but seldom thought through, claim that the existence of said vineyards a thousand years ago implies that a ‘Medieval Warm Period‘ was obviously warmer than the current climate (and by implication that human-caused global warming is not occuring). This claim comes up pretty frequently, and examples come from many of the usual suspects e.g. Singer (2005), and Baliunas (in 2003). The basic idea is that i) vineyards are a good proxy for temperature, ii) there were vineyards in England in medieval times, iii) everyone knows you don’t get English wine these days, iv) therefore England was warmer back then, and v) therefore increasing greenhouse gases have no radiative effect. I’ll examine each of these propositions in turn (but I’ll admit the logic of the last step escapes me). I’ll use two principle sources, the excellent (and cheap) “Winelands of Britain” by geologist Richard C. Selley and the website of the English Wine Producers.

Are vineyards a good temperature proxy? While climate clearly does impact viticulture through the the amount of sunshine, rainfall amounts, the number of frost free days in the spring and fall, etc., there a number of confounding factors that make it less than ideal as a long term proxy. These range from changing agricultural practices, changing grape varieties, changing social factors and the wider trade environment. For instance, much early winemaking in England was conducted in Benedictine monasteries for religious purposes – changing rites and the treatment of the monasteries by the crown (Henry VIII in particular) clearly impacted wine production there. Societal factors range from the devastating (the Black Death) to the trivial (working class preferences for beer over wine). The wider trade environment is also a big factor i.e. how easy was it to get better, cheaper wine from the continent? The marriage of Eleanor of Aquitaine and the English King in 1152 apparently allowed better access to the vineyards of Bordeaux, and however good medieval English wine was, it probably wasn’t a match for that!

However, for the sake of argument, let’s assume that climate is actually the dominant control – so what does the history of English vineyards show?

The earliest documentation that is better than anecdotal is from the Domesday Book (1087) – an early census that the new Norman king commissioned to assess his new English dominions, including the size of farms, population etc. Being relatively ‘frenchified’, the Normans (who had originally come from Viking stock) were quite keen on wine drinking (rather than mead or ale) and so made special note of existing vineyards and where the many new vines were being planted. Sources differ a little on how many vineyards are included in the book: Selley quotes Unwin (J. Wine Research, 1990 (subscription)) who records 46 vineyards across Southern England (42 unambiguous sites, 4 less direct), but other claims (unsourced) range up to 52. Lamb’s 1977 book has a few more from other various sources and anecdotally there are more still, and so clearly this is a minimum number.

Of the Domesday vineyards, all appear to lie below a line from Ely (Cambridgeshire) to Gloucestershire. Since the Book covers all of England up to the river Tees (north of Yorkshire), there is therefore reason to think that there weren’t many vineyards north of that line. Lamb reports two vineyards to the north (Lincoln and Leeds, Yorkshire) at some point between 1000 and 1300 AD, and Selley even reports a Scottish vineyard operating in the 12th Century. However, it’s probably not sensible to rely too much on these single reports since they don’t necessarily come with evidence for successful or sustained wine production. Indeed, there is one lone vineyard reported in Derbyshire (further north than any Domesday vineyard) in the 16th Century when all other reports were restricted to the South-east of England.

Wine making never completely died out in England, there were always a few die-hard viticulturists willing to give it a go, but production clearly declined after the 13th Century, had a brief resurgence in the 17th and 18th Centuries, only to decline to historic lows in the 19th Century when only 8 vineyards are recorded. Contemporary popular sentiment towards English (and Welsh) wine can be well judged by a comment in ‘Punch’ (a satirical magazine) that the wine would require 4 people to drink it – one victim, two to hold him down, and one other to pour the wine down his throat.

Unremarked by most oenophiles though, English and Welsh wine production started to have a renaissance in the 1950s. By 1977, there were 124 reasonable-sized vineyards in production – more than at any other time over the previous millennium. This resurgence was also unremarked upon by Lamb, who wrote in that same year that the English climate (the average of 1921-1950 to be precise) remained about a degree too cold for wine production. Thus the myth of the non-existant English wine industry was born and thrust headlong into the climate change debate…

Since 1977, a further 200 or so vineyards have opened (currently 400 and counting) and they cover a much more extensive area than the recorded medieval vineyards, extending out to Cornwall, and up to Lancashire and Yorkshire where the (currently) most northerly commercial vineyard sits. So with the sole exception of one ‘rather improbably’ located 12th Century Scottish vineyard (and strictly speaking that doesn’t count, it not being in England ‘n’ all…), English vineyards have almost certainly exceeded the extent of medieval cultivation. And I hear (from normally reliable sources) they are actually producing a pretty decent selection of white wines.

So what should one conclude from this? Well, one shouldn’t be too dogmatic that English temperatures are now obviously above a medieval peak – the impact of confounding factors in wine production precludes such a clear conclusion (and I am pretty agnostic with regards to the rest of the evidence of whether northern Europe was warmer 1000 years than today). However, one can conclude that those who are using the medieval English vineyards as a ‘counter-proof’ to the idea of present day global warming are just blowing smoke (or possibly drinking too much Californian). If they are a good proxy, then England is warmer now, and if they are not…. well, why talk about them in this context at all?

There is a bigger issue of course. For the sake of argument, let’s accept that medieval times were as warm in England as they are today, and even that global temperatures were similar (that’s a much bigger leap, but no mind). What would that imply for our attribution of current climate changes to human causes? ……. Nothing. Nowt. Zero. Zip.

Why? Well, warm periods have occured in the past, and if not the medieval period, then probably the last interglacial (120,000 years ago), certainly the Pliocene (3 million years ago), without question the (Eocene 50 million years), and in particular the Paleocene-Eocene Thermal Maximum (55 million years ago), and so on. Current theories of climate change do not rely on whether today’s temperatures are ‘unprecedented’. Instead they examine the physical causes of climate change and match up what we know about their physical effects and time history and see which of the multiple drivers or combination can best explain the observations. For the last few decades, that is quite clearly the rise in greenhouse gases, punctuated by the occasional volcano and mitigated slightly by the concomittant rise in particulate pollution.

Understanding past climate changes are of course also very interesting – they provide test cases for climate models and can have profound implications for the history of human society. However, uncertainties (as recently outlined in the NAS report) increase as you go back in time, and that applies to our knowledge of the climate drivers as well as to temperatures. So much so that even a medieval period a couple of tenths of a degree warmer than today would still be consistent with what we know about solar forcing and climate sensitivity within the commonly accepted uncertainties.

My oenological research project has not then lead me to any profound insights into climate change in the past, but it has given me a little more respect for the dedication of my winemaking compatriots. So next time I’m in the area, I’ll drink to that!

81 Responses to “Medieval warmth and English wine”

  1. 1

    Winemaking is certainly a poor climate proxy. I have heard that wine is made in every one of the 50 states. I have personally drunk wine on the spot which was made in what are pehaps the most unexpected states — Alaska, Hawaii, and Arizona. As you might expect, they were not particularly good. But they were drinkable by modern standards, let alone medieval standards (when wine was less important as a gourmet experience than as a good way to get sterilized fluids, and also a good way to preserve fruit calories for winter consumption).

  2. 2
    pete best says:

    Here in the UK the growing season appears to be getting shorter due to their being more sunshine and heat. I have visited several vinyards in the south of england (below London) and they have told me this. There are even reports of red wine grapes now being grown in the south of the UK and they are suprisingly good. In addition to this a lot of the south of the UK (which is chalky like Champagne in France is) has been a target for grape growers from France and Calafornia. It would appear that UK sparking Wine is very good and has won some top awards in the last few years.

    therefore I would suggest that the UK is warming in realtion to grape growing. On another related matter is butterflies, where in Europe the UK is often the most northern range of their habitat. One particular butterfly, the comma has in recent decades extended its range from Middle England into northern England and even Scotland and has been successful in colonisation.

  3. 3
    Sean D says:

    I like Gavin’s synopsis of the twisted logic of people like Singer in the first paragraph. It is useful to bring out in the open the (il)logic behind these types of distracting and mostly irrelevant arguments about the existence of said vineyards. I’m surprised no one has suggested that global warming will be good because England will be able to produce wine.. …I’m sure someone will trot that one out soon enough.

  4. 4
    Don Baccus says:

    Re: #2 “Here in the UK the growing season appears to be getting shorter due to their being more sunshine and heat. I have visited several vinyards in the south of england (below London) and they have told me this.”

    In a fine coincidence, The Oregonian, my daily newspaper here in Portland, Oregon, ran a piece a couple of days ago on the future of winemaking in the western United States. Winemakers here in Oregon are reporting the same situation as you describe for the UK. Oregon made its mark on the wine map for its decent pinot noir. Winemakers are already making plans to switch to warmer-climate grapes like syrahs and the like. And attribute it to global warming.

    In fact, the article was based on a commissioned study which among other things predicts that many areas in California that now produce grapes for cheap table wine will become too hot to grow grapes by the end of the century. Again, attributed to global warming.

  5. 5
    Tom Fiddaman says:

    French grape harvests have been explored as a proxy too:

    Grape ripening as a past climate indicator
    Nature, Vol. 432, 18 November 2004.

    French records of grape-harvest dates in Burgundy were used to reconstruct spring-summer temperatures from 1370 to 2003 using a process-based phenology model developed for the Pinot Noir grape. Our results reveal that temperatures as high as those reached in the 1990s have occurred several times in Burgundy since 1370. However, the summer of 2003 appears to have been extraordinary, with temperatures that were probably higher than in any other year since 1370.

  6. 6
    Dano says:

    RE 4 (Baccus):

    Here in the Seattle area, winemakers are crowing over the news – Walla Walla and Yakima wine growers are very happy to hear that CA will be negatively affected by AGW.

    Another negative effect will be that, according to some UW scientists, it will be cloudier in Western WA due to AGW, as the marine layer will move inland more often (can’t hunt down a ref now).



  7. 7
    Marlowe Johnson says:

    Strangely enough the PNAS is also running a story on AGW and wine production

    “Future climate warming across the United States could reduce the areas suitable for premium wine production by up to 81% by the end of this century, severely damaging an important U.S. industry.”

    Now Gavin, is the timing of your post purely a coincidence or is there some kind all-out PR offensive by the wine industry to get us to stockpile our wine cellars? Be honest :).

  8. 8
    Rod Brick says:

    There is no (or at least little/uncertain) connection between climate temperatures and grape growing in medieval, but there is much joy in the Oregon vinewards over global warming: is this not having AND eating our cake?

  9. 9
    S Molnar says:

    Re #7: You messed up the link to the National Academy news page, but not to worry – the National Academy messed up the link from the news page to the article.

    A separate news account quoted one of the authors as saying that by 2100 Cape Cod will be a good wine-growing location. He obviously hasn’t been keeping up with Ice Sheets and Sea Level Rise, or he would know that Cape Cod will be underwater by then!

  10. 10
    Tom Cecere says:

    I think the logic trail is slightly mis-stated. There are another couple of links between iv) therefore England was warmer back then, and v) therefore increasing greenhouse gases have no radiative effect.

    Link iv.1: therefore current warmth is not unprecedented
    Link iv.2: therefore something other than man-made forcing can be responsible for this level of warming
    Link iv.3: therefore today’s warming is more likely caused by something natural than by man-made forcing.

    It’s still wrong, but most “skeptics” amongst the general public get stuck at iv.2…if it’s possible that natural causes are responsible, and it’s a hassle to remediate, then maybe we should just wait for the weather to change back again, no?

  11. 11
    William Hyde says:


    When you were in Halifax a decade ago we did our
    best to shelter you from Nova Scotia wine, but
    the industry does exist there, despite winters
    much colder than Yorkshire generally sees – or
    even experienced in the worst of the little ice
    age – and generally cool summers.

    The first favourable review of English wine that
    I recall was well back in the 1970s. Somewhat
    earlier Monty Python referred to a Welsh Claret,
    but not by implication favourably.

    Transportation issues alone would make the whole
    issue moot, even if England produced no wine
    at all today.

    The English wine argument is not quite as silly as
    the “vast fields of greenland wheat” argument I
    see from time to time, but it’s up there.

    William Hyde

    [Response: I remember it well. As I recall, I was ‘sheltered’ by being forced to consume copious amounts of Nova Scotian beer…. definitely did the trick. -gavin]

  12. 12
    Steve Sadlov says:

    Gavin I am surprised that you do not undestand why knowledge of past climate variability is important. Without knowledge of it, how can we do a good MSA of our current data collection network and past networks? Without knowledge of it, how can we assess the degree of current and projected abnormality versus which baselines, “control limits” and “spec limits?” If we don’t understand the innate “natural capability of the process” then we are hopeless in terms of understanding what might be deemed “Green”, “Yellow” and “Red” status. But of course, this is the true crux of the issue. To continue the Six Sigma analogy, there are three camps. One camp would be those who want to invest maximal effort “fixing” all root causes, whether or not they are capable of resulting in anything that is truly serious long term. The second camp, in which I reside, says, determine the Cpk, determine control and spec limits, and consider excursions beyond spec limits warnings and anything beyond the spec limits a disaster. Then of course there is the third camp, who would say “ship it” – I have to of course acknoledge that yes, there are actual real world despoilers out there who really would destroy the Earth. But to be in camp two, to me, is actually the best. Camp two is the most likely one to be good stewards while not being ideologically charged overreacting maniacs.

    [Response: Since I spend about half the time of my day job looking at natural variability in the pre-anthropogenic period, I have no idea why you think that I don’t understand its value – try reading some of my papers: Schmidt et al (2004) for instance. – gavin]

  13. 13
    Steve Sadlov says:

    RE: Gavin’s response to #12 – Well then, why do you spend so much time trying to downplay the MWP and when not doing that, trying to make the case that past variability really does not matter? You appear to be in camp one. I base this on: “There is a bigger issue of course. For the sake of argument, let’s accept that medieval times were as warm in England as they are today, and even that global temperatures were similar (that’s a much bigger leap, but no mind). What would that imply for our attribution of current climate changes to human causes? ……. Nothing. Nowt. Zero. Zip.” I beg to differ. I am not arguing AGW yea or nay. I am arguing the significance of AGW. Even assuming that ALL of whatever current and project abnormality is exclusively due to AGW (a suspect assertion) what does it mean? Are we Green, Yellow, or Red? He who can firstly determine the innate variability with sufficient confidence, and where we are and might be versus that variability with equally good confidence, and then tell us the status, will be a good candidate for a Nobel Prize.

    [Response: You don’t seem to be following the point. I have no preference for any particular medieval temperature – the record is what it is. Explaining what was going on then is of interest regardless of what actually happened. What I object to is the use of the medieval period as if it meant something for current change attribution. It doesn’t. And more importantly it doesn’t affect by an iota the projections for the future. To paraphrase your point, you seem to imply that the medieval period defines ‘natural’ variability. But ‘natural’ variability is made up of two components – internal variations and natural forcings (solar and volcanoes on this time scale). A globally warm medieval period could be a simple forced response to increased solar, in which case it doesn’t imply any larger intrinsic variability than already assumed, and since solar has been pretty much constant over the last 50 years, improvements to our understanding of solar forced climate changes are irrelevant for the last few decades. On the other hand if the medieval period was all intrinsic variability, that would be surprising, but similarly not much use for determining the forced response to increasing GHGs. Since it’s some balance of the two (with an uncertain split) it is extremely difficult to simply use medieval temperatures to conclude much of anything. – gavin]

  14. 14
    Eric says:

    Gavin, have you seen this link? It shows a pretty good
    correlation of wine quality with temperature on average. Also I
    remember from my wine tasting trip through Nova Scotia in the early
    80’s that the red wines were much worse than the whites, due mostly to
    climate. Perhaps there is historical data of the ratios of red to
    white which (disregarding cultural and taste preferences) could
    correlate to climate?

  15. 15
    BG says:

    Ok, so,

    If a forest catches fire on a hot day, everything burns down. Voila, its done. It was a NATURAL cause.

    Later, me and my buddies decide to have a party at another forest. Somebody throws a cigar on the leaves on the forest floor and we CREATE a fire. This is a MAN-MADE fire.

    Therefore, if there is a fire, it could be NATURAL, or MAN-MADE.

    Now, lets assume that wine does correlate with temperatures (positive correlation.)
    I think that Gavin’s point is:

    In the past these rises in temperature were NATURAL. Today, they could be (and lets face it, they are) MAN-MADE.

    Thus, you can provide all the evidence regarding wine and it means nothing in the context of the causes of todays warming. The same way that if me and my buddies were taken to court for lighting up the forest, we could provide ALL the evidence in the world that forests fires occur naturally, but the fact that they occur naturally does not prove that we did not cause that PARTICULAR fire.


  16. 16
    cwmagee says:

    Re 12, 13:
    I know what red and white are, but what are these “yellow” and “green” types of wine?

  17. 17
    Steve Sadlov says:

    Good debate Gavin, thanks. This is the type of constructive debate I enjoy. This rising tide lifts all boats.

  18. 18
    Arun says:

    I think the point being raised was that if the medieval warming by natural causes was not harmful, why would the modern warming by human causes be harmful?

  19. 19
    Wolfgang Flamme says:

    With winter temperatures well below 0°C regularly in a wine region here at Mainz, Germany the region has switched to growing much more red wine in the last decades. We are growing a fig tree in our garden fine, requiring no care at all. Our neighbours are growing a palm tree – they simply tie the leaves together in winter and shield it using a plastic bag. My uncle grows an olive tree for fun (several hundred years old) – frost-shielded by a tent in winter (which might not be neccessary since it has passed several years of travel and adaption).

    A (presumed) zero point something temperature increase just cannot account for all these observed changes over the last decades. Climate here ist still very(!) far from becoming mediterranean. So people, culture, tradition, lifestyle matter most. Nature following indeed looks like a very poor and unreliable indicator to me.

  20. 20

    There is the added component, of history, which surely must complement wine making in the UK. Any study assigning a medieval warming just on wine making is a bit tipsy… There are other periods when weather was remarked directly. For instance, when there was “no summer” after a volcanic eruption at about the 4th century CE. It would be good to read about contemporaneous historical evidence to make this subject complete.

  21. 21
    JohnLopresti says:

    I am glad to locate this picturesque discussion, having read of the PNAS report’s publication in our local Sonoma County, CA, Alexander Valley AVA ridgetop location’s nearby press outlet on the first page yesterday in a decent article by the viticulture writer. Even that media is skipping some of the science which will help correlate with climate change science. I look forward to exploring the ASEV site with respect to the enological view. For my own part, I will proffer here some rudimentary Vitis vinifera data, then complete the full download, such as it is, in chapter aggregates.

    Grape buds in order to produce viable and healthy leaves and clusters, need ultraviolet. Overcast skies in the UK would mitigate against opting to grow grapes.

    The ecclesiastical wines are a dual process product, often beginning with a low sugar grape from a short growing season, fortified with more grape sugar during fermentation, and then even cooked afterward with addition of more alcohol content. Typically dessert or sacramental wines may originate in various climatic regions, but also are possible to make by blending arts. Absent modern national governmental regulations, I would imagine all manner of vinification took place.

    Another aspect of suitability of a zone for viticulture, as the NAS report evidently mentions, is grapevines’ debilitation when there are excessive numbers of days warmer than about 88 degrees F, though there is a ceiling in the mid 90s beyond which leaf turgidity effects, wilt, and the like inflict damage on the vine. The Diffenbaugh report is saying if global warming produces 35 heat peak days during summer in CA premium viticultural lands, the wine will be inferior quality; that is right. Compare for example the wines of the midi in France, where excessive warmth lends itself better to bulk wine production rather than, as our host extolls, say, a Bordeaux. In this regard, this week I heard a scientist describing heat peaks 18 degrees warmer than currently during 2070-2100 in our region, a shift which would obliterate most premium grapes, absent an effort to grasp the hockeystick, soon.

    There are other seasonal parameters applicable to grapevines likely more readily available from the experts more knowledgeable in this than I, the excellent viticulture and and enology college at UC Davis; I am sure we will hear from them.

    UCD several decades ago developed a heat summation formula for suitability of coastal zones for wine growing, it is a complex calculation which counts number of days over 50 degrees Fahrenheit; and it intercalculates with the length of growing season from budbreak to first frost. There are horticultural workarounds, whereby we have microclimates which support vineyards in snowy climates. As the Diffenbaugh report also mentions, grapevine and fruit resistance to a host of fungal disease, mildews, and the like also affect suitability of a region for viticulture.

    I will give this some study and return, when I have a moment, if some of this agricultural enological view appears to inform the climatologic discussion more.
    John Lopresti
    Local CA viticulture writer:
    National picture US region reconfiguration

  22. 22
    Robin Johnson says:

    I’d like to add – who says that the Medieval Warm Period WASN’T man made?

    Human population in Northern Europe increased substantially from the 6th centuary onward due to the invention of the advanced plow and yoke in the “Dark Ages”. This was BEFORE any claimed warming. These vast numbers of additional humans cleared forests increasing CO2, mined and burned lots of peat which is a fossil fuel, and increased bovine herds substantially which raises methane levels. So maybe humans at least contributed to the Medieval Warm Period or made it warmer. One could then argue this warming was subsequently quashed by the Black Plague (1346 – 1380) and notably the medieval high was in 1370 just as reforestation was taking place. So I question the premise that the Medieval Warm period was definitely “natural”.

  23. 23

    Just to add a little modern day evidence, south facing chalky slopes along the downs and into the weald are defintiely producing high quality “champagnes”; try Nyetimber (already getting too pricey, I suspect) and the confusingly named range from Ridgeview. They are well to the north of traditional French winemaking areas doing similar things, but they benefit from proximity to the channel giving mild winters and modern winemaking techniques. That said, at an english winemaking event I attended last year (well, a wine drinking event really)there was unanimous belief in climate change as a factor in widening the envelope of the possible, with a number of producers eyeing the possibility of reds in a way they never would have ten years ago.

  24. 24
    pete best says:

    RE #22 Yep Nyetimber and Ridgeview now make award winning wines along with Breaky Bottom and a couple of others. UK sparklers are getting expensive but they do taste good, almost as good as a good champagne. Not all of the wine growing slopes are south facing either.

  25. 25
    Jose de Freitas / Portugal says:

    cwmagee asked: I know what red and white are, but what are these “yellow” and “green” types of wine?

    I can’t really speak for “yellow” but “green”, at least in Portugal, is a sparkling wine made with “green” grapes, meaning not yet completely ripened. It’s slightly less alcoholic than your average white or red (it’s between 11º and 12,50º), to be drunk very fresh. Generally a summer drink for seafood or late afternoon meals, doesn’t age very well (2-3years is what you normally get) and doesn’t travel very well either. It’s actually great, and comes mostly in “dry” flavors and “fruity” or slightly sweety taste. More confusingly, it can be made with either white grapes (very common) and red grapes (much more uncommon), giving rise to Red Green and White Green, which tends to befuddle foreigners. :-)

    I would suppose “yellow” is something along these lines too.


  26. 26
    tom root says:

    Re: comment #19
    “So people, culture, tradition, lifestyle matter most. Nature following indeed looks like a very poor and unreliable indicator to me.”

    It would seem that besides climate, micro-climate (of the type for grape growing) dictates whether or not suitable grapes can be grown in a particular location. As climate changes, so do the many micro climates in a region. So a hillside vineyard that was favorable for the past, say, 100 years may suddenly not produce many of the desired grapes. Growers skills at finding suitable micro climates, that will work for as many years as it takes to cultivate a vineyard, have increased remarkably (I presume) over the past 1000 years. Wolfgang’s conclusion seems to hold.

  27. 27
    chris says:

    Re: comment 22
    …and the suggestion that these periods (MWP) had significant man-made influence with respect to atmospheric CO2 production from forest and peat burning.
    The ice core record seems to rule out that suggestion I would have thought. I’m looking at data from Ethridge et al (1996) “Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn” [J. Geophys. Res. 101, 4115-4128 (1996)].

    The atmospheric CO2 levels were 279.4 ppm in 1006; 280.3 in 1046; 282.4 in 1096; 283.8 in 1146; 283.9 in 1196; 281.7 in 1246; 283.4 in 1327; 280.4 in 1387; 281.7 in 1446 etc. etc.

    In other words surely insignificant changes with respect to the Earths temperature…at least one would hope so or we’re in a whole lot of trouble!

    They do go down a bit (to the mid 270’s ppm) right through 1600 and 1700’s so these tiny changes do seem to correlate a bit with the warm and cool periods possibly. But changes in these levels might well be responses to other factors rather than being cause of the temperature variation.

    The other factor of course is the extent to which wine growing in England in the so-called MWP was an indication of a warmer Earth during this period or a small shift in the temperature distribution of the Earth due to ocean circulation patterns and suchlike. In other words not an indication of global warming at all!

  28. 28
    Lynn Vincentnathan says:

    Everyone knows the best wine grapes are grown in cool, not hot climates — so that Medieval period must have been a period of cooling :)

  29. 29
    Jim says:

    Thank you for this post, this is another claim that is endlessly repeated. A letter to the editor in today’s San Francisco Chronicle:

    Editor — So, now we are to fear that our beloved wine industry could be destroyed by climate change (“Now’s the time to cellar wine,” July 11)? Never mind that the dire predictions of scorched vineyards are based on models that climate scientists say “aren’t yet good enough to predict effects (of global warming) on future agriculture.”
    By the way, which Bush policy was it that caused temperature fluctuations in the 1500s to wipe out vineyards in England? Just wondering …


  30. 30
    Steve Sadlov says:

    # 21 – if temps above 85 are such a problem then what about places like Paso Robles, Calistoga, Livermore and misc Lake County locations, in terms of premium locations, and of course, the San Joaquin Valley in terms of 2nd tier? All of those places have routinely exceeded the century mark for days at a time, nearly every summer. And this is hardly anything new. This has been the case for many decades.

  31. 31
    Mike Jacobs says:

    According to Al Gore, by 2050, the Napa Valley, California’s premier wine growing area, will at best a saltwater estuary and at worst a deep water port. The wine growing regions will definitely be lost. As for me, I’ll have California ocean front property.

    [Response: Sorry to put the damper on your soon-to-be-beach-front home idea, but neither Al Gore, nor anyone else has suggested that sea level rise will be that fast. – gavin]

  32. 32
    Robin Johnson says:


    Yes. I know that the CO2 concentrations from 1006 to 1800 seem to be 280 +/-10. And so that is a valid point. But my understanding was that levels were lower in the 600 AD – 1000 AD time frame (when the bulk of the land clearing occurred). The additional methane has to be considered important as well. So I would think the CO2 ppm post land clearing is not a good proxy. But my real point is that it cannot be said that MWP was definitely not man-made until a natural explanation other than “natural variability” becomes the accepted scientific explanation. “Natural variability” is another way of saying “we don’t know”. And possibly we may never.

  33. 33
    cat black says:

    #30: I have lived in or around Calistoga and Lake County. It can become quite hot in those areas of an afternoon, but my experience is that this is almost always bracketed morning and evening by cooling breezes from the sea. As an indication, I’ve never lived with air conditioning in these areas, which in other parts of California is essentially impossible. I suspect (from observation only and no direct experience in the vineyards) that the grapes are able to integrate the heat over the day. Other places I have lived, like Butte County, also have a few novelty vineyards but the heat there is entirely more oppressive. Which is why I no longer choose to live there! I like “grape country” for similar reasons as the grapevines, I suspect.

  34. 34
    chris says:

    Robin – (re comments 22, 27, 32)

    that begs the question what the atmospheric CO2 levels (and methane too) were between 600 and 1000. I’d love to know! presumably the data is there somewhere??? [maybe the experts at this blog will tell us where to look!] So you may well be correct.

    however the implication is that either (i) that the CO2 levels would have had to be rather significantly below 270-ish ppm, or (ii) that the Earths temperature is really very sensitive to CO2 levels indeed. In the latter case that would indicate that we have a whole lot of warming in the pipeline from the raised levels that we’ve generated at the present time!

    At least in my researching there is a bit of a chasm between the fine-grained CO2 data that goes back to around 1000 AD and the ice core data that covers the 100’s of thousands of years before now [e.g. Siegenthaler et al. (2005) Science 310, 1313-1317 (2005)].

    I’m sure there’s something in your suggestion of land-clearances. But is it enough to cause significant climate change during these periods? I do think we need the CO2 levels to be sure. I’m still inclined to think that subtle changes in ocean circulation patterns, with resulting local effects on climate, are more likley to be responsible.

    For example, with all this talk on wine, has anyone mentioned the fact (apparently) that they were growing grapes in Greenland during the MWP? I can’t help thinking that some strengthening of the Gulf Stream, carrying greater volumes of warm water to Greenland and N.W. Europe, might be a better explanation of the MWP. But the atmospheric CO2 levels between 600-1000 might answer the question as to which is the dominant effect.

    Hopefully someone will point us in the right direction!

  35. 35
    Dano says:

    RE 30 (Sadlov):

    All plants have temps where they won’t photosynthesize any more but continue to live without adverse effects. Fruits, though, can be affected because metabolism shuts down, stopping translocation of sugars and starches to the fruit. This is the 35 peak heat days’ of which JLoP speaks (and the SJV heat that you imply reduces quality). The PNAS paper apparently implies that the peak heat days will increase, thus decreasing quality.

    More info on temps for growing is here.



  36. 36
    Stormy says:


    Saw you on Lou Dobbs. As Lou Dobbs said, “Time for solutions.” The three of you have to be very organized, each taking a piece of the answer.

    1. What effects are we in for–how can we mitigate those

    2. Is there a tipping point? How soon?

    3. What precisely can we do to stop further effects.

    Bring charts. Visuals will say more than words–and say it quicker.

    Smile. You’re on camera.

    [Response: Yeah, I know. These things can be tricky though – we were supposed to talk about wildfires and wine but another segment ran over and we got cut. I agree, we could have been more focussed though. Graphics would be nice but there is never enough time to them justice. We might get another chance soon though, so I’ll try and remember to smile… Thanks for watching! – gavin]

  37. 37
    Mark A. York says:

    RE: #30 Here’s the deal: sure it gets hot here in the summer, but the last few summers have been longer and hotter. i.e. it starts in mid-May and June gloom here in inland LA is less and less frequent in between 100+ heat waves. It’s holding true this year. It’s 104 today. To recap: starts earlier and lasts longer. That’s global warming in its simplest manifestation.

  38. 38

    I’m a chinese student.I am very concerned about the climate problem.
    I have a question:
    Do you think that one day we would fly to other planets to get on because of the bad weather on earth?

  39. 39
    Jaison Davis says:

    I think we all need to think about our mother nature as if we don’t do it right now then we will not be able to do it later. It’s our responsibility and we have to take care of it!

  40. 40

    The discussion from message 22 onwards has concentrated on the effects of deforestation to CO2 levels and of livestock to methane. I would like to ask if there could be a more clear link between local temperatures and deforestation through changes of humidity. Deforestation clearly affects local humidity and that through cloud formation etc. and my personal experience of life in Finland tells to expect 10 to 20 degrees colder nights during winter when the air is dry. Other variations are not so obvious and I expect there have been numerous studies about the local effects of humidity and deforestation to local climates as this is one of the pressing problems in many areas today.

  41. 41
    Eric says:

    Risto, here’s a link I posted in another thread: that has some abstracts that discuss vegetation-climate interaction (e.g. 3128110).

    It is an important topic in my opinion that will be resolved with much better modelling in the next decade or two. We ought to be able to model which vegetation changes yield the most bang for the buck, so to speak, for climate change mitigation.

  42. 42
    S Molnar says:

    Re 38: My answer is, No, we won’t solve the problem by going to other planets. If we can’t manage to put up a few windmills and solar panels, how are we going to succeed with massive interplanetary colonization? Apparently, Stephen Hawking disagrees – well, you can always judge a man by the quality of his opponents. (Oh, wait…)

  43. 43

    Re #38 and “Do you think that one day we would fly to other planets to get on because of the bad weather on earth? ”

    The problem is that there are no other habitable planets in the Solar system, so you would need some sort of cheap insterstellar travel. This is unlikely any time soon. In the meantime, most of the people on Earth will be unable to emigrate. It would be most efficient to fix the climate on Earth, rather than try to colonize other planets. In the very long run, of course, we will need to move to other planets when the Earth becomes uninhabitable (0.6-1.4 billion years from now).

  44. 44

    Thank’s Eric,

    this removed a contradiction from my mind as I remembered
    that deforestation is also used as an explanation for
    warming in certain areas. So it depends on your latitude.
    I quote the abstract …

    ” Pub No: 3128110
    Author: Snyder, Peter K.
    Date: 2004 Pages: 205

    …Using a coupled atmosphere-biosphere model (CCM3-IBIS),
    the influence of vegetation on climate at the regional,
    continental, and global scales is evaluated…
    …..For instance, the climate response to
    tropical deforestation is a surface warming due to a
    large reduction in latent cooling. In contrast, boreal
    deforestation causes a large surface temperature cooling
    due to a higher surface albedo…

    It was also shown that changes in the land surface
    resulting from land cover change affect the transport of
    energy throughout the three-dimensional atmosphere and
    can change the climate in adjacent regions or locations
    far removed from the surface forcing through modification
    of deep moist convection…”

  45. 45
    Jeffrey Davis says:

    Re: sea level rises swamping California wine growing areas

    As an adjunct to the movie we watched last night (“Eight Below”) I got out the National Georgraphic atlas to look at the area the movie was supposed to be about. On that map, there’s a small graphic which showed how much of Antarctica’s ice sits on bedrock and how much is on open water. I was stunned by how much of its ice rests on water. My question: does that ice actually rest on water or is it suspended above it like the arch of a bridge? That doesn’t seem likely. If the weight of Antarctic ice is actually already in the ocean then how much can its melting actually influence ocean water rises elsewhere?

  46. 46
    Hank Roberts says:

    Most of the icecap is ‘grounded’ — not floating because it’s far too thick and weighs too much. Compare the height at the surface to the depth of the bedrock and sea level. Compare a large block of ice in a shallow tray of water.

    Remember, the ocean also expands as the water warms; I think expansion due to warming in over the past century or so has been the explanation for sea level rise, before the ice started melting so fast in the past few years.

  47. 47
    tom root says:

    An article and comments about micro-climates and managing micro-climates from the very capable keepers of this site would be appreciated by myself, Risto, Doug and others. In a world with unpredictable climate changes, the technology for finding suitable micro-climates for any particular purpose might just be the next ‘big thing’.

    I remember the bumper sticker from 25 years ago which read: Think Global, Act Local. In other words, think about what is good for the whole planet and work toward that end in the local area.

  48. 48
    tom f says:

    I’m curious whether gavin has read _The Little Ice Age_, a recent book. Early on, the author discusses the MWP, particularly the evidence that French kings actually sought to block importation of English wine at this time.

    My questions for gavin are, have you read the book and is the author’s treatment of climate change sound?

    Secondly, the author specifically states that Europe during the MWP experienced summers roughly 1 degree C warmer than in the 2Oth C (p. 17). That does not necessarily contradict your statement that you are “pretty agnostic with regards to the rest of the evidence of whether northern Europe was warmer 1000 years than today”. But, do you disagree w/ his statement, and could you give me an idea of what counter-factuals lead you to agnosticism on whether Europe was, across all seasons, warmer 1K years ago than today?

  49. 49
    Stephan Harrison says:

    Re 48. Are you talking about the book by the late Jean Grove? As far as I remember, she discusses the MWP only in the context of Europe which doesn’t contribute to the argument concerning the global extent of the warming.

  50. 50
    tom f says:

    #49. The Little Ice Age: How Climate Made History, 1300-1850 (Paperback)
    by Brian M. Fagan, Basic Books, 2000