The authors find wet conditions are associated with more frequent El Ninos, and droughts with periods having fewer El Ninos. Although some climate scientists such as Kevin Trenberth have claimed AGW will cause more frequent El Ninos [and thus less drought], the El Nino reconstruction in this paper shows the frequency of El Ninos in the past 3000 years has been persistently much higher and much lower than the present for periods lasting several hundred years each.
Bottom line is that this paper demolishes claims that current California drought conditions are relatively "extreme" or linked to man-made CO2, as well as claims that man-made CO2 controls the frequency of natural El Ninos.
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| Horizontal axis is calendar years before the present [BP] over the past 3000 years [most recent history at left side of graph], vertical axis is a proxy of wetness with higher values indicating more precipitation and less drought. Green line shows recent drought area (%) is at some of the lowest levels of past 1,100 years. |
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| Horizontal axis is calendar years before the present [BP] over the past 3000 years, bottom graph shows a proxy of wetness with higher values indicating more precipitation and less drought, most recent values at some of the highest wetness/lowest drought levels of the past 3000 years. Middle graph shows frequency of El Ninos in recent past is about in the middle of the range of extremes over the past 3000 years. |
Highlights
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- 3000 year decadal resolution record from Zaca Lake, CA.
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- Winter season moisture source and run-off reconstruction.
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- Pluvial or drought conditions associated with variable moisture sources.
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- Record extends knowledge of drought beyond tree ring reconstructions.
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- Evidence for persistent coupling between El Niño and run-off over 3000 years.
Abstract
Change in water availability is of great concern in the coastal southwest United States (CSWUS). Reconstructing the history of water pre–1800 AD requires the use of proxy data. Lakes provide long-lived, high-resolution terrestrial archives of past hydrologic change, and their sediments contain a variety of proxies. This study presents geochemical and sedimentological data from Zaca Lake, CA (Santa Barbara County) used to reconstruct a 3000 year history of winter season moisture source (δDwax) and catchment run-off (125–2000 μm sand) at decadal resolution. Here we show that winter season moisture source and run-off are highly variable over the past 3000 years; superimposed are regime shifts between wetter or drier conditions that persist on average over multiple centuries. Moisture source and run-off do not consistently covary indicating multiple atmospheric circulation modes where wetter/drier conditions prevail. Grain-size analysis reveals two intervals of multi-century drought with less run-off that pre-date the “epic droughts” as identified by Cook et al. (2004). A well-defined wet period with more run-off is identified during the Little Ice Age. Notably, the grain size data show strong coherence with western North American percent drought area indices for the past 1000 years. As a result, our data extend the history of drought and pluvials back to 3000 calendar years BP in the CSWUS [coastal southwest United States]. Comparison to tropical Pacific proxies confirms the long-term relationship between El Niño and enhanced run-off in the CSWUS [coastal southwest United States]. Our results demonstrate the long-term importance of the tropical Pacific to the CSWUS [coastal southwest United States] winter season hydroclimate.
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