Wednesday, January 16, 2013

Paper finds Midwest US droughts were less extreme during 20th century & linked to solar activity

A paper published in Geophysical Research Letters finds Midwest US droughts were less frequent and less extreme during the 20th century in comparison to the past 3100 years.The authors find the precipitation proxy "record of the past ∼3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly, especially prior to 300 AD. Drought variability was anomalously low during the 20th century; ∼90% of the variability values during the last 3100 years were greater than the 20th-century average."  The authors also find droughts were linked to the natural changes in the Pacific Decadal Oscillation [PDO] and to solar activity. 
Both graphs show the year 2000 at the left side. The West US drought index in the top graph shows a  significant decrease of West US drought severity over the past 2000 years. The bottom graph horizontal line shows mean Midwest US drought variability over the 20th century was near the lowest levels of the past 3100 years.
d18O is a proxy for precipitation and is correlated to changes over the past 3,100 years in solar activity [shown by the d14C proxy]

  1. Jian Tian et al



Abstract: Paleorecords are key for evaluating the long-term patterns and controls of drought. We analyzed calcite in annually laminated sediments from a Minnesota lake for oxygen-isotopic composition (δ18O). The δ18O [precipitation proxy] record of the past ∼3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly, especially prior to 300 AD. Drought variability was anomalously low during the 20th century; ∼90% of the variability values during the last 3100 years were greater than the 20th-century average. δ18O [the precipitation proxy] is strongly correlated with the index of the Pacific Decadal Oscillation (PDO) during the past 100 years, and periodicities of the late-Holocene δ18O record are similar to those of the PDO. Furthermore, time series of δ18O and atmospheric Δ14C [the solar activity proxy] are generally coherent after 700 AD. Both the Pacific climate and solar irradiance probably played a role in drought occurrence, but their effects were non-stationary through the late Holocene.

4 comments:

  1. http://scienceandpublicpolicy.org/images/stories/papers/originals/drought_trends_asia.pdf

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  2. http://www.ncdc.noaa.gov/paleo/drought/drght_data.html

    The Last 500 Years
    Droughts of the last four centuries are well documented in paleoclimatic proxies such as historical documents and tree rings. Spatial patterns of drought for every year since 1700 have been generated from a gridded network of tree-ring reconstructions and are featured in this section. Highlighted in this section are those periods with droughts that appear to have been more severe than any we have experienced in the 20th century.


    The Last 2000 Years
    A number of tree-ring records exist for the last two millennia which suggest that 20th century droughts may be mild when evaluated in the context of this longer time frame. The evidence from long tree-ring records is augmented with paleodrought records from other proxies, such as lake sediments that reflect changes in salinity and precipitation/evaporation balances.

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  3. http://www.nipccreport.org/articles/2013/feb/5feb2013a1.html

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  4. http://www.nipccreport.org/articles/2013/feb/6feb2013a2.html

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