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Increases in rainfall extremes linked to global warming

Rainfall extremes are increasing around the world, and the increase is linked to the warming of the atmosphere which has taken place since pre-industrial times. This is the conclusion of a recent study…

A warmer atmosphere holds more moisture, and that can lead to more extreme rainfall. AAP Image/Dave Hunt

Rainfall extremes are increasing around the world, and the increase is linked to the warming of the atmosphere which has taken place since pre-industrial times. This is the conclusion of a recent study which investigated extreme rainfall trends using data from 8326 weather-recording stations globally, some of which have records spanning more than a hundred years.

Of all the stations analysed, we found that two-thirds showed increasing trends over the course of the 20th and early 21st centuries. When we looked at the association between the intensity of rainfall extremes and a record of global mean near-surface atmospheric temperature, rainfall intensity was found to increase at a rate of between 5.9% and 7.7% for each degree, depending on the method of analysis.

This kind of change is precisely what can be expected if one assumes that the intensity of the most extreme rainfall events will scale with the capacity of the atmosphere to hold moisture. This is well known to increase with temperature at a rate of about 7% per degree.

Looking beyond globally averaged numbers, however, we also found distinct regional differences. The greatest increases occurred in the tropical belt; the smallest in the drier mid-latitudes where you will find most of the world’s deserts. In the higher latitudes, particularly in the northern hemisphere, the rate of change was close to the global average. Again, such changes seemed to be in quite close agreement with what global climate models say should happen as a result of global warming: a reassuring case of observations confirming theory.

The implications of this are likely to be significant for flood risk around the world. It’s true that 7% per degree doesn’t sound like much. But if we continue to follow the current trajectory of greenhouse gas emissions, we can probably expect in the order of three to five degrees of warming by the end of the 21st century. If the relationship between extreme rainfall and atmospheric temperature continues to hold, then this could mean as much as a 35% increase in extreme rainfall intensity on average globally.

What does this mean for the capacity of our infrastructure to handle current and future floods? Most flood-defence infrastructure, such as dams, levees, stormwater systems and coastal flood defences, has been designed to handle historical flood risk. If the risk of flooding increases, then such infrastructure will have increasing difficulty managing floods in the future. This would either lead to increased damage costs due to the flooding, or necessitate expensive infrastructure upgrades or resettlement of low-lying communities. Even the increase in extreme rainfall intensity observed thus far is likely to lead to substantial challenges for some existing infrastructure.

Nevertheless our analysis should not be interpreted as suggesting that the rapid increase in flood damage which has occurred over the past few decades is entirely attributable to climate change.

In fact, other changes such as deforestation, rapid urbanisation and an increase in the number of people living in flood plains are likely to account for the bulk of these changes. Furthermore, not all floods are caused by extreme rainfall events. Snow melt and storm surge also contribute to overall flood risk. Antecedent moisture – the wetness of the catchment prior to the flood-producing rainfall event – can also have a substantial influence on flood risk. In some parts of Australia this might even cause flood risk to decrease because of an expected increase in the number or severity of future droughts.

Despite all these caveats, our recent study contributes to the debate on how climate change will affect flood risk, by showing that the intensification of rainfall extremes is not just a projection made by climate models. Rather, it can already can be detected in our observational record.

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58 Comments sorted by

  1. Gerard Dean

    Managing Director

    Mr Westra

    Professor Tim Flannery the CEO of the Australian Climate Commission spent years telling us global warming and climate change meant that heavy, dam filling rains in Australia were a thing of the past. His opinion tipped the eastern seaboard states into building hugely expensive desalination plants so we could turn coal into water. The plants are now either unfinished or mothballed following strong dam filling rains.

    I thought the science was 'settled' on this years ago and then you go and contradict Professor Flannery.

    Who is right, you or Professor Flannery?

    Gerard Dean

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    1. takver takvera

      Journalist and Editor at Indymedia

      In reply to Gerard Dean

      I think you will find that BoM and CSIRO have been warning us for some time that south eastern Australia and the south west have clear long term climate trends of become drier. (See Climate variability and change in south-eastern Australia - csiro (2010)) But there will still be wetter years caused through natural weather variability and influenced by La Nina/ENSO cycles. This article draws upon the historical rainfall data that when we do get rain, it is likely to be more intense as average global temperature increases. So we will experience years of more intense drought in the southern parts of the continent interspersed with short periods of more intense flooding rain. We are likely to use those desal plants in coming years in the next extensive drought.

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    2. Greg Boyles

      Lanscaper and former medical scientist

      In reply to Gerard Dean

      WA is not getting dam filling rains, rather they are still at real risk of running out of water. Domestic water restrictions remain in place.

      In Victoria we have not had ANY significant rain for 3 months until the other day. Despite average to above average rainfall for a couple of years prior, this 3 months without rain has severely dried out gardens.This can only mean that the sub surface moisture reservoirs have not been adequately re-charged by the couple of years of above average rainfall…

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    3. Wade Macdonald

      Technician

      In reply to Greg Boyles

      Quote..."This is the second time that QLD has been devastated by floods in a few years and I can't see that this nation will be able to afford the cost of replacing lost infrastructure on this scale for a third time in coming years without out accruing massive deficits."

      Federal Government have the money but they send it off to other countries and the UN go figure.

      Secondly, regarding massive deficits...we already have one in most states and federally.

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    4. David Clerke

      Teacher

      In reply to Gerard Dean

      What we need is access to the original data and full analysis. The rest is speculation, this is what killed the Gerghis and Lewandowski papers before they were submitted and it was the refusal to release the real data which gave us Climategate.

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    5. Greg Boyles

      Lanscaper and former medical scientist

      In reply to Wade Macdonald

      "Federal Government have the money but they send it off to other countries and the UN go figure."

      Wade such flippant comments reveal that you really don't have any comprehension of the scale involved here.

      Big f'ing woop, the bid for the UN seat cost us $25 million over 5 years or there abouts. I personally don't care whether or not Australia has a seat on the UN.

      The estimated immediate cost of the 2010 - 2011 QLD floods was about $1 billion. There was a further hit to our GDP estimated at about $30 billion.

      $31 billion wade! Compared to that, $25 million is trivial small change!

      NO WE DON"T HAVE THE MONEY to replace massive amounts of flood damaged infrastructure and bare the flow on costs to our economy every few years like this!

      Unless you are prepared to pay increased taxes and flood levees to bail out QLD'ers. But then you/we are just as likely to be simultaneously dealing with economic ravages of drought.

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    6. Mike Swinbourne

      logged in via Facebook

      In reply to David Clerke

      Not so David.

      The thing that gave us 'climategate' was not a refusal to release data. It was deniers trawling through the personal emails of scientists, and cherry picking phrases that were then used out of context to support their ideological lies.

      "Climategate" was nothing more than a beat-up in the denier echochamber, and has been shown to be thus by a number of independent enquiries. But then, it had already been shown to be nothing more than lie by anyone who actually read the full emails, rather than the cherry picked phrases by themselves.

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    7. Mike Hansen

      Mr.

      In reply to Gerard Dean

      Time for you to get out an atlas Gerard. You appear to be geographically challenged.

      CSIRO has been predicting both. Higher rainfall over northern Australia and less in the south. Pretty much what has been happening.

      For example
      "In 1989, at a presentation to the Prime Minister’s Science Council, Dr Graeme Pearman of CSIRO summarised a scenario of climate change for Australia in 2030. He said there would be:
      * higher summer rainfall over northern Australia and extending further south.
      * possibly drier winters in southern Australia
      * more intense rainfall."

      https://theconversation.edu.au/droughts-and-flooding-rains-climate-change-models-predict-increases-in-both-5470

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    8. Bruce Tabor

      Research Scientist at CSIRO

      In reply to takver takvera

      As a long term keen observer of climate science, Tim Flannery's opinions have troubled me. All the evidence, including climate models, suggests the South West of Western Australia will experience a long term reduction in rainfall. However the climate models were always very ambiguous about south eastern Australia, eg:
      http://earthobservatory.nasa.gov/Features/GlobalWarming/page6.php
      It strikes me that far more care is needed by prominent people, such as Tim Flannery in declaring droughts to be due to climate change and I've heard prominent climate scientist privately express such concerns.

      For south eastern Australia the evidence suggests we will see greater extremes of weather. In simple terms, higher temperatures will produce greater evaporation. This can result in both more rain and dryer soils - floods and droughts could both be more severe. To the best of my ability to determine from the evidence, the long term rainfall trend in south eastern Australia is unclear.

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    9. Mike Hansen

      Mr.

      In reply to Bruce Tabor

      @Bruce Tabor

      There have been numerous articles at The Conversation from BOM and CSIRO climate scientists.

      Your claim "To the best of my ability to determine from the evidence, the long term rainfall trend in south eastern Australia is unclear." is at odds with Karl Braganza's point

      "A 10-20% loss of autumn and winter rainfall has occurred over both the southwest and southeast corners of the Australian continent. This is most significant in the southwest of Western Australia, where the changes have occurred since around 1970. In the southeast, similar rainfall reductions have been apparent since the mid 1990s. These changes are climatologically significant."

      Do you have a reference to support your claim. The one reference you give is not directly applicable.

      https://theconversation.edu.au/a-land-of-more-extreme-droughts-and-flooding-rains-5184
      https://theconversation.edu.au/droughts-and-flooding-rains-what-is-due-to-climate-change-6524

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    10. takver takvera

      Journalist and Editor at Indymedia

      In reply to Bruce Tabor

      From my reading the CSIRO and BoM reports there is a longterm trend for the south east to become drier, but it is much less prounounced than SW Western Australia. That link to the IPCC 2007 precipitation model map shows a reduction in winter precipitation in the south east. I believe I would not categorise it as 'unclear'

      Here is the BoM trend map for total annual rainfall 1970-2011
      http://www.bom.gov.au/cgi-bin/climate/change/trendmaps.cgi?map=rain&area=aus&season=0112&period=1970

      Note that this trend map covers a good part of the wettest two years on record for Australia in 2010-2011, and still there is a substantial rainfall deficit. So yes, greater extremes of weather in SE Australia, but also a longterm trend for a drier climate.

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    11. In reply to Gerard Dean

      Comment removed by moderator.

    12. In reply to Mike Hansen

      Comment removed by moderator.

    13. Bruce Tabor

      Research Scientist at CSIRO

      In reply to Mike Hansen

      Hi Mike,
      Both you and takver make good points and don't want to engage in a heated argument, nor do I want to be seen to contradict my colleagues at CSIRO - there is a certain level of tentativeness in that report - and I don't immediately have time to chance up references. I am a statistician and what I am looking at is what call the "level of evidence". Do all the lines of evidence point in the same direction or are they contradictory?

      For SW Western Australia (SWWA), all the evidence points…

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    14. Bruce Tabor

      Research Scientist at CSIRO

      In reply to takver takvera

      Thanks takver, I've partially answered this in my response to Mike Hansen. I'm a statistician and it is always dangerous to use extrapolation of existing trends - even long term trends - as your main evidence for change.

      I note that since 2011 we have had yet another year of this extraordinary rain fall. I wonder how those maps look now? The original document is here:
      http://www.bom.gov.au/climate/current/statements/scs38.pdf
      There's no question we've had an extended dry period and a long term…

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    15. Wade Macdonald

      Technician

      In reply to Greg Boyles

      Greg,

      Australia pays the UN in many ways including 10% of our carbon tax for example.

      I don't care about the 25 million for 1 UN council seat but roll everything else all together to the tune of hundreds of millions while local Queenslanders go without badly needed mitigative flood planning and basic resources this should cause us all some concern don't you agree?

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    16. takver takvera

      Journalist and Editor at Indymedia

      In reply to Bruce Tabor

      Bruce,
      this is slightly off the topic of extreme rainfall, but nevertheless an interesting diversion.
      You are right that the 2010 CSIRO assessment for the drying trend for south east Australia was more tentative. The reduction of rainfall over 20 year periods was on the order of 20% for SW western Australia and 10% for SE Australia. The drying trends in the long term observation data is there. Predicting the future is a little more complex, as you say, with different seasonal weather patterns and…

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    17. Greg Boyles

      Lanscaper and former medical scientist

      In reply to Wade Macdonald

      The carbon tax/ets has only a few months old and we haven't paid anything to the UN yet.

      But the 2010-2011 QLD, NSW and VIC floods, bushfires and cyclone damage has and is costing us dearly.

      And you are the one making the claim that the government has loads of cash to pay for all this replacement infrastructure etc, so why don't you prove it by coming up with some credible figures.

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    18. In reply to Mike Hansen

      Comment removed by moderator.

    19. Richard Koser

      Dude

      In reply to Wade Macdonald

      Wade, I don't know why you think the Australian carbon tax gets paid to the UN. I haven't heard that one before.

      If the Queensland government wasn't so keen to build Gina Rinehart a coal train track, and was prepared to charge companies a high price to dredge the Great Barrier Reef Marine Park, we'd have a lot more cash.

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  2. Gerard Dean

    Managing Director

    Mr Westra

    I note you are from the 'School of Civil, Environmental and Mining Engineering' at the University of Adelaide.

    Are they serious? I can see the links between the 3 fields of study but the contradictions are greater. Who wins the research places? Roads and bridges under civil, numbat habitat under environment or royalties under mining.

    I guess the University of Adelaide is following the US military idea of 'battle groups' where they form a group with infantry, fighter bombers, attack helicopters, artillery and tanks into one cohesive unit.

    Gerard Dean

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    1. In reply to Gerard Dean

      Comment removed by moderator.

  3. John Phillip
    John Phillip is a Friend of The Conversation.

    Grumpy Old Man

    Seth, how can the argument be made for a link between global warming and increased rainfall (your 1st paragraph) when the conclusion is made that there is an "expected increase in the number or severity of future droughts." Aren't the two scenarios mutually exclusive?

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    1. Les McNamara

      Researcher

      In reply to John Phillip

      It is possible to have more severe droughts and more severe flooding rains - just not at the same time and place...

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    2. Ryan Owens

      PhD Student

      In reply to John Phillip

      Your answer is in the title of the article. The increase is in rainfall extremes, not rainfall.

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    3. Mike Swinbourne

      logged in via Facebook

      In reply to John Phillip

      No John, they are not 'mutually exclusive'. But that much is obvious from both the article and the linked study. Can I recommend you read science papers on climate change such as the one provided. That way you might stop your predictable comments every time an article on climate change is presented.

      Both the article and the paper make it very plain that they are referring to individual extreme rainfall events - not the long term rainfall average which occurs over the course of a year or more. It is completely plausible that extreme rainfall events can increase in intensity causing flood events, but the long term average can actually be less rain.

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    4. Greg Boyles

      Lanscaper and former medical scientist

      In reply to Ryan Owens

      Again ignorant flippant remarks!

      The 'mere' extreme rainfall in QLD 2010-2011 cost us $1 billion in lost infrastructure and a further flow on hit to our GDP of about $30 billion.

      The current 'mere' extreme rainfall in QLD will cost us similarly.

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    5. Mike Hansen

      Mr.

      In reply to John Phillip

      CSIRO has been predicting both.

      Difficult concept for climate science deniers to get their head around but there you go.

      For example
      "In 1989, at a presentation to the Prime Minister’s Science Council, Dr Graeme Pearman of CSIRO summarised a scenario of climate change for Australia in 2030. He said there would be:
      * higher summer rainfall over northern Australia and extending further south.
      * possibly drier winters in southern Australia
      * more intense rainfall."

      https://theconversation.edu.au/droughts-and-flooding-rains-climate-change-models-predict-increases-in-both-5470

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    6. George Takacs

      Physicist

      In reply to Mike Swinbourne

      Mike,

      It is also true that you can have more droughts, more extreme rainfall events, and higher long term average rainfall. This is what I suspect has to be the case. On average, evaporation has to be balanced by precipitation. Warming of the ocean surface will lead to increased evaporation, hence long term average rainfall must increase.

      The problem is that it seems increasingly that we get long dry spells which end with a massive dumping of water, running off and eroding our soils.

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    7. Felix MacNeill

      Environmental Manager

      In reply to John Phillip

      No John, they are not mutually exclusive. The reasoning is quite simple and has been explained repeatedly to you and others.

      Firstly, it varies from region to region: some will become wetter and some drier - this is addressed in this article and has been covered extensively.

      Secondly, the co-existence of drought and flod is one of the most often observed facts of Australian weather. What was predicted and is being observed is that the droughts are tending to become longer and deeper and the rain, when it does come, tends to be more extreme, and therefore more likely to cause harm.

      To continue to repeat this line in the face of those explanations is either disingenuous or dishoneste. It really has no more meaning that arguing that, because it's daytime at the moment there's no such thing as night.

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    8. Felix MacNeill

      Environmental Manager

      In reply to Greg Boyles

      Greg, it might be wise to actually read a posting before attempting a rebuttal. Ryan does not use the word 'mere' anywhere. His post was simple, brief and to the point.

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    9. Greg Boyles

      Lanscaper and former medical scientist

      In reply to Felix MacNeill

      Well then sorry Ryan if it was not your intention to rationalist average rainfall versus extremes of rainfall.

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    10. David Arthur

      resistance gnome

      In reply to John Phillip

      John, with my right foot in the embers of a fire and my left foot on a block of dry ice, an economist will try to tell me that on average, my feet are comfortable.

      By the same token, a flood in one year followed by nine years of drought will produce a ten-year average of moderate conditions.

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    11. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Swinbourne

      Thank you Mike. I will continue to enjoy making my predictable comments and wait in anticipation of your predictable response. Cheers

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    12. Michel Stasse

      logged in via LinkedIn

      In reply to Les McNamara

      Well, obviously not the same time...! But the same place? ABSOLUTELY..... I live in Sub Tropical Qld, where it is normal to get a wet summer, and a dry winter. but the last twelve months has given us extremes the likes of which none of the locals who've lived here over 60 years ever recall happening before.

      Last January/February (2012) we got our entire annual average rainfall over those two months, we were cut off by two floods ten days apart. Not THAT unusual....... except that we literally…

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  4. Tim Traynor

    Rocket Surgeon

    Hooray for Victoria's desal plant then. :-|

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  5. Fred Pribac

    logged in via email @internode.on.net

    "This is the conclusion of a recent study which investigated extreme rainfall trends using data from 8326 weather-recording stations globally"

    Nice work.

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    1. David Bentley

      logged in via LinkedIn

      In reply to Fred Pribac

      Yes, and despite this being a piece of research based on data analysis and hard evidence, I don't think that will stop the uninformed commentry from those with no understanding of metorology or climate change science. These people don't seem to understand that changing weather patterns will impact different areas differently. Some areas will be drier, some areas will be wetter.....and the extremes in both instances will become more extreme. Capiche?

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    2. Wade Macdonald

      Technician

      In reply to David Bentley

      While I do agree that weather extremes would become more extreme under such circumstances we must also realise that past and current extremes are not measured using apples with apples in many instances.

      There are hundreds more reference sites for temperature measurement than what there was in 1910 for example.

      Hence the current claims of record heat will have some variability when comparing the old data with the new and more comprhensive set.

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    3. Wade Macdonald

      Technician

      In reply to Jane Rawson

      Thanks for that info Jane...

      However I must admit, the year I stated initially wasn't in reference to the BOM but more general. Historical data that has more variables due to lower comprehensiveness but more droughts encapsulated over a longer time frame as a comparison.

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  6. Comment removed by moderator.

  7. Spiro Vlachos

    AL

    The science is in! Once again!

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  8. Spiro Vlachos

    AL

    How about measuring the rainfall in Canberra in pre industrial times?

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    1. Mike Hansen

      Mr.

      In reply to Spiro Vlachos

      Spiro. What are you trying to say?

      Even people with a passing knowledge of science know that a warmer atmosphere can hold more water vapour.

      As Seth Westra explains "...the capacity of the atmosphere to hold moisture. This is well known to increase with temperature at a rate of about 7% per degree."

      So you would expect a warmer world to be a wetter world. Are you saying that this is wrong?

      Do you have anything at all? Or are we going to be repeatedly subjected to your ridiculous hand waving?

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  9. Dianna Arthur
    Dianna Arthur is a Friend of The Conversation.

    Environmentalist

    "Despite all these caveats, our recent study contributes to the debate on how climate change will affect flood risk, by showing that the intensification of rainfall extremes is not just a projection made by climate models. Rather, it can already can be detected in our observational record."

    In other words, we can rely on contemporary records, because climate models have been, unfortunately, proven to be true.

    Finally, Obama has stuck his head out and discussed quite extensively in his inauguration…

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  10. David Clerke

    Teacher

    Interesting how any comment pointing out that warmists have been reluctant (a euphemism) to release the actual data or even give enough information for it to b replicated gets removed as against community standards. Wonder why? A friend of mine, retired science lecturer, is in the warmist camp but refuses to even discuss historical data because his first post uni job was in Meteorology and he had to go around and check range gauges. Some were now on the side of walls (wind obviously making large difference in measured rainfall) some were under trees and others had even been built over. Without actual data no genuine peer review is possible. Nowadays real easy to publish any data you actually have, it is called the web.

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    1. Mike Hansen

      Mr.

      In reply to John Phillip

      It would help immensely John/John if you knew some climate science.

      From the article
      "The new results from the North Greenland Eemian Ice Drilling (NEEM) project, managed by Denmark's Centre for Ice and Climate, show that during the Eemian interglacial, the climate in North Greenland was about 8 degrees Celsius warmer than at present. The Eemian interglacial period began about 130,000 years ago and ended about 115,000 years ago. "

      [Note: that is 8 °C above the present-day annual average of roughly −25 °C. During last July's anomalous ice melt in Greenland it was estimated from satellite data that the temperature at high elevations over the Greenland ice sheet was a full 10°C (18°F) warmer than the daily average of the 2000′s decade.]

      So 130,000 to 115,000 years ago.

      Here is a graph of the "Hockey Stick" reconstruction.
      http://en.wikipedia.org/wiki/File:IPCC_2001_TAR_Figure_2.21.png

      Notice how it goes back 1000 years.

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