Rainfall less variable in the warmer 20th century

From the Jackson School of Geosciences at the University of Texas at Austin, comes this suggestion that a warmer would is a more stable one, weather-wise:

Rainfall in South Pacific Was More Variable Before 20th Century

A new reconstruction of climate in the South Pacific during the past 446 years shows rainfall varied much more dramatically before the start of the 20th century than after. The finding, based on an analysis of a cave formation called a stalagmite from the island nation of Vanuatu, could force climate modelers to adjust their models. The models are adjusted to match the current levels of climate variability that are smaller now than they were in the recent past for this region.

“In this case, the present is not the key to the past, nor the future,” says Jud Partin, a research scientist associate at The University of Texas at Austin’s Institute for Geophysics who led the study. The institute is part of the Jackson School of Geosciences. “Instead, the past is the key to what may happen in the future.”

The researchers also discovered a roughly 50 year cycle of rainfall in Vanuatu, toggling between wet and dry periods. Vanuatu lies within the largest rain band in the southern hemisphere, the South Pacific Convergence Zone and its rainy season is from November to April. In the 20th century, rainfall during wet periods was about 7 feet per rainy season and during dry periods about 4 ½ feet per rainy season.

However, before the 20th century, the dry periods tended to be much drier, with rainfall as low as 1 foot per rainy season and wet periods that were still getting about 7 feet per rainy season. This means there were differences as large as 6 feet per rainy season between dry and wet periods.

“Without this record, you would not guess that this area could experience such large changes in rainfall,” says Partin.

While 20th century rainfall in Vanuatu experienced a smaller range from wet to dry periods than in the previous centuries, the biggest difference was during the dry periods. Dry periods in the 20th century were much wetter than dry periods in previous centuries. The researchers note that this overall wettening of Vanuatu is consistent with the hypothesis that anthropogenic climate change, caused by the emission of greenhouse gases, makes wet areas wetter and dry areas drier.

South Pacific Rainfall Reconstruction

Rainfall in the South Pacific (red) varied much more dramatically before the start of the 20th century than after. Rainfall values are first measured annually during the rainy season (November-April) and then averaged over each decade. The blue line indicates actual oxygen isotope values used to determine rainfall values.

The study was published online on September 6 in the journal Geology.

Stalagmites are rocky features that form on the floors of caves as water dripping from above deposits minerals over time. By analyzing the abundance of oxygen isotopes deposited in the minerals of one particular stalagmite, the scientists were able to reconstruct a history of rainfall going back 446 years. This is significant because rainfall measurements in this region are sparse and only span the past century. Decadal averages of oxygen isotopes increase and decrease in lockstep with rainfall. To convert oxygen isotope levels to actual rainfall values, the researchers calibrated the stalagmite data with actual rainfall measurements in Vanuatu from 1904 to 2003.

The stalagmite had a deposition rate about 100 times as high as typical stalagmites in the region, meaning much more material was deposited in a given year than elsewhere and therefore yielded a much higher resolution rainfall record than is typically possible. In the local dialect, known as Bislama, one would say of the stalagmite “Hem gudfala ston,” which means “This is a good stone.”

The 50-year cycle of rainfall in Vanuatu does not appear to be linked to any external forces, such as changes in solar intensity. No correlation was found with the sun’s regular 11-year cycle of intensity or the Little Ice Age, a multi-decade change in climate possibly caused by solar dimming.

Big Taurius Stalagmite from Taurius Cave on the island of Espiritu Santo, Vanuatu

The upper 25 cm (10 inches) or so of the stalagmite named Big Taurius from Taurius Cave on the island of Espiritu Santo, Vanuatu, which recorded large swings in rainfall over 446 years.

Instead, the researchers propose that the 50-year cycle, or Pacific Decadal Variability (PDV), arises from natural fluctuations in Earth’s climate. The PDV causes the South Pacific Convergence Zone to shift northeast and southwest over time. At times, the zone is over Vanuatu (corresponding to wet times) and at others, it is farther to the northeast (corresponding to dry times).

“This new result is part of a larger research program aimed at understanding climate changes in this important but understudied area of the tropical Pacific,” says co-author Terry Quinn, director and research professor at the Institute for Geophysics and professor in the Department of Geological Sciences.

Partin’s other co-authors at The University of Texas at Austin are Frederick Taylor, Charles Jackson and Christopher Maupin at the Institute for Geophysics and Jay Banner at the Department of Geological Sciences. Other co-authors are Chuan-Chou “River” Shen and Ke Lin at National Taiwan University; Julien Emile-Geay at the University of Southern California, Los Angeles; Daniel Sinclair at Rutgers University; and Chih-An Huh at Academia Sinica, Taiwan.

Funding for this research was provided by the National Science Foundation (award AGS-1003700) to Jud Partin, the Taiwan (Republic of China) National Science Council and National Taiwan University.

The University of Texas at Austin is committed to transparency and disclosure of all potential conflicts of interest of its researchers. The university has not identified any such potential of conflicts involving this study.

The study, “Multidecadal rainfall variability in South Pacific Convergence Zone as revealed by stalagmite geochemistry” is available at: http://geology.gsapubs.org/content/early/2013/09/06/G34718.1.abstract

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28 thoughts on “Rainfall less variable in the warmer 20th century

  1. Here, let me fix it for you. Rain may be less variable but it is more extreme due to climate change. See, now isn’t that better? 🙂

  2. The finding, based on an analysis of a cave formation called a stalagmite from the island nation of Vanuatu, could force climate modelers to adjust their models.
    Something needs to force the modelers to adjust their models to match reality. It should be obvious with a slight amount of thought as to why a warmer world is less variable. The CAGW conjecture demands the poles warm the most, reducing the planetary thermal range. This will cause milder weather systems, as their intensity is proportional to temperature gradients. Remove the gradients and things are calmer, duh.

  3. Typo :
    “comes this suggestion that a warmer would is a more stable one, weather-wise:”
    would s/b world

  4. The 50 year cycle thing is interesting. I thought the working assumptions about ENSO were more like 30 years. Islands are also downwind from large Asian population and industrial growth areas and 2 centuries of Australian land use changes so this data is not an easy thing to interpret

  5. Rainfall is driven by where the convergence zone sets up. Drier just means that it is not over Vanatu. Would be interesting to get this for all Islands in the tropics..

  6. “Dry periods in the 20th century were much wetter than dry periods in previous centuries. The researchers note that this overall wettening of Vanuatu is consistent with the hypothesis that anthropogenic climate change, caused by the emission of greenhouse gases, makes wet areas wetter and dry areas drier.”
    These two sentences do not make sense. How can the fact that dry periods are wetter be consistent with AGW when it should make dry periods drier? Was this peer reviewed?

  7. Is that data archived anywhere, Could be interesting.
    At least the last 150y looks almost perfect anti-correlation to SST: cooler is wetter.
    Pre 1800 looks more like 60y than 50 and later just less than 50. Spectral analysis would be interesting.
    data ??

  8. Objections being prepared for posting at SkS:
    1. [Gasp!] “Must have been funded by Big Oil…” /sarc
    (via the National Science Foundation? Not likely…)
    2. “There’s no evidence that this was a global phenomenon.”
    This is an interesting find. I hope the authors are able to continue to receive funding and keep their biases and preconceptions in check.
    “The researchers note that this overall wettening of Vanuatu is consistent with the hypothesis that anthropogenic climate change, caused by the emission of greenhouse gases, makes wet areas wetter and dry areas drier.” — Oh, please, grasping at straws once again?
    Interesting that the “…researchers also discovered a roughly 50 year cycle of rainfall in Vanuatu, toggling between wet and dry periods.” If memory serves correctly, PDO is a 60-year(ish) oscillation. Aren’t there others on that time scale as well, perhaps also an observed or theoretical solar “cycle” of about 60 years?

  9. Steveta_uk – The kind of morons who might confuse it with a stalactites. Not sure why they studying the floor rather than the ceiling though…

  10. Eve” These two sentences do not make sense. ”
    Come on Eve , try to keep up.
    It’s not just less variation, it’s EXTEMELY less variable than it used to be. That’s WEIRD.

  11. No correlation was found with the … Little Ice Age, a multi-decade change in climate possibly caused by solar dimming.
    What the F’ does that mean.
    Interesting data, but the paper is amateur hour.

  12. “Dry periods in the 20th century were much wetter than dry periods in previous centuries. The researchers note that this overall wettening of Vanuatu is consistent with the hypothesis that anthropogenic climate change, caused by the emission of greenhouse gases, makes wet areas wetter and dry areas drier.”
    <<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>
    In fact, the Sahara (a dry area) was wetter earlier in the Holocene, during the Climatic Optimum. Likewise the Gobi. The claim that a warmer world means a drier world is utterly refuted by studies of past climate conditions.

  13. leftturnandre says:
    The blue line indicates actual oxygen isotope values used to determine rainfall values.
    And we were told that oxygen isotopes in precipitation denote temperature as the many speleothem research papers tell, for instance: Vacco et al 2005
    http://www.sciencedirect.com/science/article/pii/S0033589405000827
    So what is it?
    =====
    That is the usual interpretation of d18O, it’s true. Maybe that accounts for my earlier observation about anti-correlation.
    That was the assumption of L. Thomson et al 2009 in their Gomez Dome proxy:
    http://climategrog.wordpress.com/?attachment_id=37
    Inquiring minds want the DATA.

  14. But, but, but……….a paper out this past June in the Geophysical Research said that Indian monsoons would become more variable. However, this was based on a climate model as opposed to actually looking at the evidence held in stalagmite. What will AR5 say?

    Abstract – 21 June 2013
    Enhanced future variability during India’s rainy season
    [1] The Indian summer monsoon shapes the livelihood of a large share of the world’s population. About 80% of annual precipitation over India occurs during the monsoon season from June through September. Next to its seasonal mean rainfall, the day-to-day variability is crucial for the risk of flooding, national water supply, and agricultural productivity. Here we show that the latest ensemble of climate model simulations, prepared for the AR-5 of the Intergovernmental Panel on Climate Change, consistently projects significant increases in day-to-day rainfall variability under unmitigated climate change. The relative increase by the period 2071–2100 with respect to the control period 1871–1900 ranges from 13% to 50% under the strongest scenario (Representative Concentration Pathways, RCP-8.5), in the 10 models with the most realistic monsoon climatology; and 13% to 85% when all the 20 models are considered. The spread across models reduces when variability increase per degree of global warming is considered, which is independent of the scenario in most models, and is 8% ± 4%/K on average. This consistent projection across 20 comprehensive climate models provides confidence in the results and suggests the necessity of profound adaptation measures in the case of unmitigated climate change.

  15. @Eve
    “How can the fact that dry periods are wetter be consistent with AGW when it should make dry periods drier? Was this peer reviewed?”
    Exactly. The phrase twisting needed to try to comply with every significant pronouncement of CAGW is long past the excruciating stage. It fits into the weather forecast for hotter, colder, wetter, drier, normaller, abnormaller, skewier, strangier, weirdier expectations of the Druidic tax-exempt public-trough-dwellers.
    Authors: The analytic work is fine – just leave out the genuflections.

  16. RHS says:
    September 10, 2013 at 8:30 am
    Steveta_uk – The kind of morons who might confuse it with a stalactites. Not sure why they studying the floor rather than the ceiling though…
    They forgot the ladder!

  17. Eve says:
    September 10, 2013 at 8:18 am
    …..How can the fact that dry periods are wetter be consistent with AGW when it should make dry periods drier? Was this peer reviewed?

    Co2 can cause dry areas to be dryer and dry areas to be wetter. Wet areas will become dryer and wetter at the same time. Eve, get with the program. :-). But first this:

    Media release received from the American Geophysical Union on October 29, 2012
    Global precipitation variability decreased from 1940 to 2009
    http://wattsupwiththat.com/2012/10/29/global-precipitation-variability-from-1940-to-2009-contradicts-models/

    And now expect wetdry.

    CO2 causes unchecked wetdry
    http://wattsupwiththat.com/2011/03/26/co2-causes-unchecked-wetdry/
    Counterintuitive, models wrong – rainfall more likely over drier soil
    http://wattsupwiththat.com/2012/09/12/counterintuitive-models-wrong-rainfall-more-likely-over-drier-soil/

  18. Does anyone have the original study? Because I see no obvious reason why a stalagmite would have constant growth over the centuries even if the rain is perfectly constant. Conditions on the surface are changing over time … why wouldn’t the stalagmite water change as well?
    Surely the amount of water making it to the stalagmite would not be expected to be stable over the centuries, would it?
    How have they established that the amount from the source is unchanged?
    w.

  19. Something seems amiss here. I thought stalagmite/-tite formation occured due to CO2 dissolving in H2O and forming carbonic acid. That acid dissolved some of the rock as it perculated through the soil and deposited the dissolved minerals after evaporation to form the aforementioned -mite/-tite structures. Caves maintain the yearly average temperature of a region, by design. The higher the average temp, the faster the evaporation of the carbonic acid thus the faster rate of structure growth – but with the rise of CO2 and an increase in rainfall, the structures should form slower, due to the diluted solutions. The same would seem true for the past as well, due to the cooler temperatures causing a slower rate of evaporation. This all just seems to be an oversimplified answer to a highly complex system (like all climatology). Also, I have to assume that these are young coral caverns (Vanuatu is volcanic – basalt and such) for the -mite/-tites to form – anyone know the reactive differences between young coral limestone and old bedrock limestone happens to be?

  20. Willis: If I recall my geology studies correctly, the water flowing through the substrata would pick up minerals along the way, but the transport time and the strata would seem to be key with regard to how “constant” the flow and mineral deposition rates are in any cavern. Shallow caves do have exceptional wet periods that may cause portions to flood (e.g., Mammoth Caves, in KY, and Marengo in Southern IN). Seeing the real paper (and the data) would be useful.
    Data Supplement: ftp://rock.geosociety.org/pub/reposit/2013/2013318.pdf

  21. A common observation here in south eastern Australia has been that generally, the second half of the 1900’s was wetter than the first half, with the droughts common around the mid 1800’s being worse than those around the turn of the century, about the time of Federation.
    Locally in south west Victoria, a local farmer observes that a lake adjoining his property had been generally dry during his time farming, whereas it was generally full during his fathers time, but generally dry during his grandfathers time.
    All anecdotal stuff, but gradually the experts will be finding more and more models that will prove that these things actually did happen in the real world.

  22. this overall wettening of Vanuatu is ↑ consistent with the hypothesis that anthropogenic climate change, caused by the emission of greenhouse gases, makes wet areas wetter and dry areas drier.

    The reverse, surely? Insert “not” at the arrow?

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