Thursday, February 12, 2015

New paper claims AGW pushed the "Western US toward the driest period in 1,000 years"

A modeling study published in Science Advances claims global warming has pushed the "Western US toward the driest period in 1,000 years" and "the U.S. Southwest and Great Plains will face persistent drought worse than anything seen in times ancient or modern, with the drying conditions "driven primarily" by human-induced global warming."

However, the tree-ring proxy data in the paper shows that at the end of the record in ~2002, soil moisture of the central plains was considerably above the average of the past millennium, and peaked around ~1930, a relatively warm period in the US. The proxy record also shows many periods of drought during the Little Ice Age and that the 20th century was relatively wet period in comparison to the past millennium. 

For the US Southwest, the proxy data also shows a soil moisture peak around ~1930. If warming is a cause of decreased soil moisture as the paper claims, the proxy data would be expected to show the opposite pattern to that observed. Although the end of the Southwest proxy record in ~2002, conditions were relatively dry, but not as dry as at least 3 other periods during the Little Ice Age. Many other paleoclimate studies have found both droughts and floods were more common during the Little Ice Age in comparison to the 20th century. 

Thus, the claim that AGW has "pushed the Western US toward the driest period in 1,000 years" is not supported by the proxy data shown in the paper. In addition, the modeling claim that AGW will cause "unprecedented risk of drought in the 21st century" is entirely based upon overheated climate models which have been falsified at confidence levels exceeding 98%. As shown below, the models did not reproduce the peaks in soil moisture around ~1930 or the peak around ~2000 in the central plains, further evidence that the modeling assumptions are incorrect and the claim of unprecedented drought not supported by observations. 

1000 year drought history based on tree rings shown in brown (higher values represent higher soil moisture). Green, red, blue lines are projections from (falsified) climate models. 

Warming pushes Western US toward driest period in 1,000 years: Unprecedented Risk of Drought in 21st Century

Date: February 12, 2015

Summary: During the second half of the 21st century, the U.S. Southwest and Great Plains will face persistent drought worse than anything seen in times ancient or modern, with the drying conditions "driven primarily" by human-induced global warming, a new study predicts.

The research says the drying would surpass in severity any of the decades-long "megadroughts" that occurred much earlier during the past 1,000 years -- one of which has been tied by some researchers to the decline of the Anasazi or Ancient Pueblo Peoples in the Colorado Plateau in the late 13th century. Many studies have already predicted that the Southwest could dry due to global warming, but this is the first to say that such drying could exceed the worst conditions of the distant past. The impacts today would be devastating, given the region's much larger population and use of resources.

"We are the first to do this kind of quantitative comparison between the projections and the distant past, and the story is a bit bleak," said Jason E. Smerdon, a co-author and climate scientist at the Lamont-Doherty Earth Observatory, part of the Earth Institute at Columbia University. "Even when selecting for the worst megadrought-dominated period, the 21st century projections make the megadroughts seem like quaint walks through the Garden of Eden."

"The surprising thing to us was really how consistent the response was over these regions, nearly regardless of what model we used or what soil moisture metric we looked at," said lead author Benjamin I. Cook of the NASA Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory. "It all showed this really, really significant drying."

The new study, "Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains," will be featured in the inaugural edition of the new online journal Science Advances, produced by the American Association for the Advancement of Science, which also publishes the leading journal Science.

Today, 11 of the past 14 years have been drought years in much of the American West, including California, Nevada, New Mexico and Arizona and across the Southern Plains to Texas and Oklahoma, according to the U.S. Drought Monitor, a collaboration of U.S. government agencies.

The current drought directly affects more than64 million people in the Southwest and Southern Plains, according to NASA, and many more are indirectly affected because of the impacts on agricultural regions.

Shrinking water supplies have forced western states to impose water use restrictions; aquifers are being drawn down to unsustainable levels, and major surface reservoirs such as Lake Mead and Lake Powell are at historically low levels. This winter's snowpack in the Sierras, a major water source for Los Angeles and other cities, is less than a quarter of what authorities call a "normal" level, according to a February report from the Los Angeles Department of Water and Power. California water officials last year cut off the flow of water from the northern part of the state to the south, forcing farmers in the Central Valley to leave hundreds of thousands of acres unplanted.

"Changes in precipitation, temperature and drought, and the consequences it has for our society -- which is critically dependent on our freshwater resources for food, electricity and industry -- are likely to be the most immediate climate impacts we experience as a result of greenhouse gas emissions," said Kevin Anchukaitis, a climate researcher at the Woods Hole Oceanographic Institution. Anchukaitis said the findings "require us to think rather immediately about how we could and would adapt."

Much of our knowledge about past droughts comes from extensive study of tree rings conducted by Lamont-Doherty scientist Edward Cook (Benjamin's father) and others, who in 2009 created the North American Drought Atlas. The atlas recreates the history of drought over the previous 2,005 years, based on hundreds of tree-ring chronologies, gleaned in turn from tens of thousands of tree samples across the United States, Mexico and parts of Canada.

For the current study, researchers used data from the atlas to represent past climate, and applied three different measures for drought -- two soil moisture measurements at varying depths, and a version of the Palmer Drought Severity Index, which gauges precipitation and evaporation and transpiration -- the net input of water into the land. While some have questioned how accurately the Palmer drought index truly reflects soil moisture, the researchers found it matched well with other measures, and that it "provides a bridge between the [climate] models and drought in observations," Cook said.

The researchers applied 17 different climate models to analyze the future impact of rising average temperatures on the regions. And, they compared two different global warming scenarios -- one with "business as usual," projecting a continued rise in emissions of the greenhouse gases that contribute to global warming; and a second scenario in which emissions are moderated.

By most of those measures, they came to the same conclusions.

"The results … are extremely unfavorable for the continuation of agricultural and water resource management as they are currently practiced in the Great Plains and southwestern United States," said David Stahle, professor in the Department of Geosciences at the University of Arkansas and director of the Tree-Ring Laboratory there. Stahle was not involved in the study, though he worked on the North American Drought Atlas.

Smerdon said he and his colleagues are confident in their results. The effects of CO2on higher average temperature and the subsequent connection to drying in the Southwest and Great Plains emerge as a "strong signal" across the majority of the models, regardless of the drought metrics that are used, he said. And, he added, they are consistent with many previous studies.

Anchukaitis said the paper "provides an elegant and convincing connection" between reconstructions of past climate and the models pointing to the risk of future drought.

Toby R. Ault of Cornell University is a co-author of the study. Funding was provided by the NASA Modeling, Analysis and Prediction Program, NASA Strategic Science, and the U.S. National Science Foundation.

Story Source:

The above story is based on materials provided by The Earth Institute at Columbia University. Note: Materials may be edited for content and length.

Journal Reference:
Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon. Unprecedented 21st century drought risk in the American Southwest and Central Plains. Science Advances, 12 February 2015 DOI: 10.1126/sciadv.1400082


  1. The Sahara desert, about the size of the continental US, used to be lush with trees and grasses, filled with rivers and lakes (and fish), and teeming with large mammals and human populations with livestock for thousands of years, or from about 10,500 years ago to about 5,500 years ago. And yet, prior to 10,500 years ago, the Sahara was just as much of a desert as it is now. Natural climate change, driven by solar forcing of a change in monsoon patterns, abruptly turned the vast Sahara from a desert into a savanna (wet/dry climate) within a span of a few hundred years (!). Then, about 5,500 years ago, the solar-driven monsoon patterns changed suddenly again, causing the desertification that exists today and the migration/destruction of ancient civilizations.
    California is currently in its 4th year of drought conditions. The same area that is now California had a 240-year-long drought that started in the 9th century (~850 AD). And then there was another drought that lasted 180 years beginning in the 12th century (1140 to 1320 AD). There were 50-year droughts in the 1300s to 1600s period. Not only that, but scientists have linked severe California droughts to cooler sea surface temperatures in the tropical Pacific (driven by ENSO cycles).

    And yet despite these well-established geological determinations, the anthropocentric we-did-it alarmists among us think that since there's a drought in California right now, we need to take responsibility for it. Thus, we need a carbon tax.

    Late Holocene climate in western North America was punctuated by periods of extended aridity called megadroughts. These droughts have been linked to cool eastern tropical Pacific sea surface temperatures (SSTs). Here, we show both short-term and long-term climate variability over the last 1,500 y from annual band thickness and stable isotope speleothem data. Several megadroughts are evident, including a multicentury one, AD 1350–1650, herein referred to as Super Drought, which corresponds to the coldest period of the Little Ice Age. Synchronicity between southwestern North American, Chinese, and West African monsoon precipitation suggests the megadroughts were hemispheric in scale. Northern Hemisphere monsoon strength over the last millennium is positively correlated with Northern Hemisphere temperature and North Atlantic SST. The megadroughts are associated with cooler than average SST and Northern Hemisphere temperatures. Furthermore, the megadroughts, including the Super Drought, coincide with solar insolation [radiation] minima, suggesting that solar forcing of sea surface and atmospheric temperatures may generate variations in the strength of Northern Hemisphere monsoons. Our findings seem to suggest stronger (wetter) Northern Hemisphere monsoons with increased warming.
    Eastern China temperatures varied with the solar activity, showing higher temperatures under stronger solar irradiation, which produced stronger summer monsoons. During Maunder, Dalton and 1900 sunspot minima [colder climates], more severe drought events occurred, indicating a weakening of the summer monsoon when solar activity decreased on decadal timescales. On an interannual timescale, dry conditions in the study area prevailed under El Niño conditions, which is also supported by the spectrum analysis. Hence, our record illustrates the linkage of Asian summer monsoon precipitation to solar irradiation and ENSO: wetter conditions in the study area under stronger summer monsoon during warm periods, and vice versa. During cold periods, the Walker Circulation will shift toward the central Pacific under El Niño conditions, resulting in a further weakening of Asian summer monsoons.
    We conclude that a significant component of century-scale variability in Yucatan droughts is explained by solar forcing.
    These coherencies corroborate strong visual correlations and provide convincing evidence for solar forcing of east-central North American droughts and strengthen the case for solar modulation of mid-continent climates.
    In this study, the nature and causes for observed regional precipitation trends during 1977–2006 are diagnosed. ... [R]elationships between SST and rainfall change are generally not symptomatic of human-induced emissions of greenhouse gases (GHGs) and aerosols. ... The pattern of rainfall trends occurring in response to such external radiative forcing [described in climate models] also departs significantly from observations, especially a simulated increase in rainfall over the tropical Pacific and southeastern Australia that are opposite in sign to the actual drying in these areas.
    Discussion: The persistent presence of these solar cycles and their connections with monsoon records over a wide range of regions highlight the dominated solar control of the monsoon at centennial timescales. The persistence of these different periodicities also indicates that the influence of the low-frequency solar activity on the AM [Asian Monsoon cycle] is independent of other climate backgrounds, such as ice volumes, orbital configurations, and concentrations of major greenhouse gases.

  3. IPCC summarizing statements from AR5 (2013):
    “In summary, the current assessment concludes that there is not enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought or dryness(lack of rainfall) since the middle of the 20th century."
    Recent drought in 1993–2008 was still within the frame of natural climate variability based on the 306 yr PDSI reconstruction. The dry and wet phases of Lingkong Mountain were in accordance with changes in the summer Asian-Pacific oscillation and sunspot numbers, they also showed strong similarity to other tree-ring based moisture indexes in large areas in and around the CLP, indicating the moisture variability in the CLP [Chinese Loess Plateau] was almost synchronous and closely related with large-scale land–ocean–atmospheric circulation and solar activity.
    [M]uch of the Northern Hemisphere's terrestrial surface has become less arid over the last 50 years. However, whether the decrease in pan evaporation is a phenomenon limited to the Northern Hemisphere has until now been unknown because there have been no reports from the Southern Hemisphere. Here, we report a decrease in pan evaporation rate over the last 30 years across Australia of the same magnitude as the Northern Hemisphere trends (approximately −4 mm a−2). The results show that the terrestrial surface in Australia has, on average, become less arid over the recent past, just like much of the Northern Hemisphere.
    Droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the country over the last century.
    Tree-ring records show that the twentieth century has been moist from the perspective of the last millennium and free of long and severe droughts that were abundant in previous centuries. The recent drought, forced by reduced precipitation and with reduced evaporation, has no signature of model-projected anthropogenic climate change.
    [D]rought conditions over the period of instrumental records (since 1895) do not exhibit the full range of variability, severity, or duration of droughts during the last millennium. Thirteen decadal to multidecadal droughts (i.e., ≥10 years) occurred during the last millennium – the longest lasting sixty-one years and centered on the late twelfth century.
    Extreme droughts of greater intensity than that of the 1930s were more frequent before AD 1200. This high frequency of extreme droughts persisted for centuries, and was most pronounced during AD 200–370, AD 700–850 and AD 1000–1200. We suggest that before AD 1200, the atmospheric circulation anomalies that produce drought today were more frequent and persistent.
    [I]t is not possible to conclude that drought conditions in general have become more severe or frequent.The period analysed and the selection of stations strongly influenced the regional pattern. For most stations, no significant changes were detected.
    Little change in global drought over the past 60 years
    Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years.