Everything you want to know about California’s drought (except when it will end).

Summary: For decades scientists warned us that the American southwest had experienced long brutal droughts. We did nothing to prepare. Now we reap the consequences, as California experiences the worst droughts of the past century. With no end in sight, we must mobilize to mitigate the damage. Which might prove severe. This can become a blessing, if we learn from it. While we bicker about the climate change in 2100, which depends on innumerable and unknowable factors (how much coal will we burn?), scientists warn us that we’re unprepared for the inevitable occurrence of past extreme weather.  Katrina and Sandy were normal, as is this drought. More exciting weather is coming.

Click here to see the updated version of this post as of 14 March 2015!

“We don’t even plan for the past.”
— Steven Mosher (member of Berkeley Earth; bio here), a comment posted at Climate Etc.

Preparing for Extreme Weather
From the NCAR-Wyoming Supercomputing Center

Contents

  1. The California drought: it’s bad
  2. Climate Science gives us worse news
  3. About this & past California droughts
  4. Useful Sources of Information
  5. For More Information
  6. The Hydro-Illogical Cycle

(1)  The California drought: it’s bad.

It’s bad, with no end in sight.

  1. California Facing Worst Drought on Record“, NOAA, 20 January 2014.
  2. California breaks drought record as 58% of state hits driest level“, Los Angeles Times, 31 July 2014.
  3. Groundwater: California’s big unknown“, NOAA, 27 August 2014 — We’re in trouble if that starts to run out.

How bad is it? Let’s look at the past year (the California “water year” runs from October to September). The average is 23″; 1924 was the driest year at 9″; the last 12 months was 12″. Six of past 8 years were dry. The new “water year” started in October a little dry: 0.7″ vs average of 1.2″. But not as bad as 1917 – 1934: a 17 year-long period with only one year of ab0ve-average rainfall (including the record low of 1924)! Click all images to expand.

California: Annual precipitation
From the California Climate Tracker website. Click to expand.

Do we all share the pain? Not equally. California is like that.

California precipitation: Percent Normal by region
California Climate Tracker website Click to expand.

But precipitation is just one factor that makes a drought. Temperature also has a large effect on the soil. Hence the creation of sophisticated indexes to measure droughts, such as the Palmer Drought Severity Index and the Standardised Precipitation-Evapotranspiration Index (SPEI). Rainfall in California has been low, but not unusually so (as climate goes). But it’s warm, and so the drought indexes are at their lows for the past century.  Some ignored this factor, as in this article at Watts Up with That, which opens with a map of the Palmer Index for California, then ignores that — concluding that the experts cited in the above articles are wrong, and describing this as the worst California drought on record is a “wildly incorrect statement that seems focused on creating public panic.”

Graph of SPEI California
From Mashable, 14 August 2014. Click to expand.

Now for the very bad news.  This year has been bad, but drought’s impact increases over time. And this drought is in its third year (see this graphic from the US Drought Monitor showing the increasing severity). Here is the 36 month SPEI back to 1895, from NOAA, 13 November 2014:

The dryness and heat has been especially intense over California. The last 36 months (November 2011-October 2014) has been the driest and warmest such 36-month period on record. This combination has resulted in the most extreme 36-month SPEI on record. By this measure, the current drought is the most severe drought for California in the last 120 years.

SPEI for California 1895-2014
From NOAA website, 13 November 2014. Click to expand.

(2)  Climate science research gives us worse news.

As usual here, I let scientists speak for themselves. These are articles are an antiode to the confident assertions of climate activists that blame all extreme weather on climate change, such as “The Crisis Over California’s Water“, Joshua Frank, Counterpunch, 21 November 2014 (citing 2 scientists’ theory does not make it fact, especially for historically common phenomena; also note the implication that all climate change is anthropogenic). Red emphasis added to the interesting parts.

(a) Long-Term Aridity Changes in the Western United States“, Edward R. Cook et al, Science, 5 November 2004 — A mildly stated conclusion:

If the Z-C modeling results hold up, it is plausible that continued warming over the tropical Pacific, whether natural or anthropogenically forced, will promote the development of persistent drought-inducing La Nina–like conditions. Should this situation occur, especially in tandem with midcontinental drying over North America, the epoch of unprecedented aridity revealed in the DAI reconstruction might truly be a harbinger of things to come in the West.

History of drought in western USA
Cook et al, Science, 5 November 2004. Click to expand.

The Colorado River Compact, allocating the water, was negotiated in 1921 — near the wettest time during the past 1,200 years.

(b) North American drought: Reconstructions, causes, and consequences“, Edward R. Cook et al, Earth-Science Reviews, March 2007 — In this later paper, Cook et al give us the long-suspected bitter news. Excerpt from “conclusions”:

These reconstructions, many of which cover the past 1000 years, have revealed the occurrence of a number of unprecedented megadroughts over the past millennium that clearly exceed any found in the instrumental records since about AD 1850, including an epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period. In terms of duration, these past megadroughts dwarf the famous droughts of the 20th century, such as the Dust Bowl drought of the 1930s, the southern Great Plains drought of the 1950s, and the current one in the West that began in 1999 and still lingers on as of this writing in 2005.

… The extraordinary duration of past North American megadroughts is difficult to explain, but climate models strongly point to tropical Pacific Ocean SSTs {sea surface temperatures} as a prime player in determining how much precipitation falls over large parts of North America.

(c)  Unfortunately, climate models are not yet able to give reliable forecasts for regional precipitation (they do neither well):  “The Key Role of Heavy Precipitation Events in Climate Model Disagreements of Future Annual Precipitation Changes in California“, David W. Pierce et al, Journal of Climate, August 2013 — Open copy here. “Climate model simulations disagree on whether future precipitation will increase or decrease over California, which has impeded efforts to anticipate and adapt to human-induced climate change. This disagreement is explored in terms of daily precipitation frequency & intensity.”

(3)  Articles about this and past California droughts.

Here are excellent explanations written for a general audience.

(a)  Past droughts, an example — and warning:

  1. California drought: Past dry periods have lasted more than 200 years, scientists say“, San Jose Mercury News, 25 January 2014 — “Researchers have documented multiple droughts in California that lasted 10 or 20 years in a row during the past 1,000 years.”
  2. Could the California drought last 200 years?“, National Geographic, 13 February 2014 — “Clues from the past suggest the ocean’s temperature may be a driver.”

(b)  Water management in California

  1. Drought in the United States: Causes and Issues for Congress“,  Congressional Research Service, 12 August 2012 — What do you guess Congress did with these recommendations?
  2. California faces growing water management Challenges“, Public Policy Institute of California, January 2014.
  3. The drying of the West“, The Economist, 22 February 2014 — “Drought is forcing westerners to consider wasting less water.”
  4. It Takes How Much Water to Grow an Almond?!“, Mother Jones, 24 February 2014 — “Why California’s drought is a disaster for your favorite fruits, vegetables, and nuts.”
  5. The Dust Bowl Returns”, Blain Roberts and Ethan J. Kytle (Assoc Profs of History at CSU – Fresno), op-ed in the New York Times, 9 February 2013 — “How long can we continue to grow a third of the nation’s fruit and vegetables?”

(c)  Analysis of the drought

  1. Science linking drought to global warming in dispute“, New York Times, 16 February 2014 — “It all adds up across the Southwest to an increasingly stressed water system. That’s what they might as well get ready for.”
  2. A climate analyst clarifies the science behind California’s extreme drought“, Andy Revkin, New York Times, 6 March 2014.
  3. Clarifying the discussion about California drought and climate change“, Peter Gleick (President, Pacific Institute), Science Blogs, 6 March 2014.
  4. California Drought Is Worst Since at Least 1895, Data Shows“, Mashable, 14 August 2014.

(4)  Useful Sources of Information.

The Internet provides a wealth of information about climate. NOAA especially provides wonderful toys.

  1. Make your own climate map, showing precipitation or one of the drought indexes.
  2. Make your own animated climate map, showing the evolution of the drought over time.
  3. NOAA’s Seasonal Drought Outlook at the Climate Prediction Center.
  4. The U.S. Drought Portal — A wealth of information about past and present droughts in USA, and their impacts.
  5. US Drought Monitor — U Nebraska – Lincoln and Federal Agencies — Ditto as above.
  6. The California Climate Tracker — make graphs and maps of California climate data. By the Desert Research Institute.
  7. Westmap — make graphs and maps of climate date. By the Desert Research Institute
  8. Paleoclimate Drought Resources – “What paleoclimatology tells us about drought, from the World Data Center for Paleoclimatology”.

Truth Will Make You Free

(5)  For More Information.

See these posts:

  1. Have we prepared for normal climate change and non-extreme weather?
  2. Let’s prepare for past climate instead of bickering about predictions of climate change.
  3. Droughts are coming. Are we ready for the past to repeat?
  4. Everything you wanted to know about California’s drought (except when it will end).

(6)  The Hydro-Illogical Cycle

From the SPEI website.

The Hydro-illogical cycle
From the SPEI website

21 thoughts on “Everything you want to know about California’s drought (except when it will end).

  1. I wouldn’t say we’re doing no planning; there was just now a state ballot measure for a water projects bond:
    http://ballotpedia.org/California_Proposition_1,_Water_Bond_(2014)
    I’m not an expert on the subject, so I can’t say how much these projects will actually help, but hopefully $7 billion of infrastructure spending will do more than nothing.

    I think the best long-term solution is to transition to less water-intensive agricultural methods, which I understand are now extremely inefficient and use the vast majority of the state’s available water. I remember one source saying 7% of the state’s water is used to grow alfalfa for horses.

  2. One has to wonder if the population growth and ag growth over the past few decades, together with the ultra heavy reliance on the state’s strategic water supply underground, is not already past the point of no return. Would love to see some articles on that subject. Regardless, and supposing that it is, and with all the competing demands, this may end up looking like an Indy race I saw one time where three cars were all side-by-side coming around turn #4 at the race’s start to get the green flag, and as they headed into turn #1 to take the pole position, neither driver would back down, and do you know what happened? You got it. All three ended up in the wall with tires and fiberglass everywhere. We will only get what we want and deserve. Unselfishness will fix the day…always, and selfishness will ruin the day. Let us humble ourselves down, me included and especially so, and realize there is a God, that we are his children, and that he will hear our sincere prayers. I know he will. I just hope we are not compelled to be humble. As we forget him, he may be slow to answer those prayers. We can do this!

    1. Jerry,

      I too would like to see modeling of California’s water use with different patterns of agriculture. Or we can wait, as water shortages will force re-structuring of agriculture even without rationalizing the way we provide farmers with cheap water.

      “One has to wonder if the population growth and ag growth over the past few decades, together with the ultra heavy reliance on the state’s strategic water supply underground, is not already past the point of no return.”

      What is “past the point of no return”? The State has lots of water; a more rational farming program could still produce larger crops. As for California’s economy: “{A}griculture and related manufacturing account for nearly 4/5’s of all business and residential water use—but make up just 2% of state GDP and 4% of all jobs.” From a report by the Public Policy Institute of California, based on a September 2011 conference at Stanford.

    2. Good morning-
      The point of no return, of course, is when the airplane cannot return to its original point of departure without running out of fuel, so it forges on with the intention of making its destination…and if it doesn’t, it will go into triage mode, or crash – like the Indy cars did.

      Question/Observation: “the State has lots of water”…okay,this is a good place to start your next blog. Take your point and do all the research on that topic. From what I have read (Google articles only…tons of them), I’m not so sure.

      Frankly, if I were to make another comparison, it would be to compare the drought deficit to the national (or many State, city, consumer, company) debt. One might say, we have lots of money too, just like lots do water! The answer to both is yes…we have lots of both indeed. But Fromm supply and demand perspective, demand has so outstripped demand for water and spending that these huge deficits have been paid for by the underground acquifers and the national debt. And both are not slowing down (usage, or reliance) to get the job done.

      And by that I mean the underground water supply (not to mention the 100+ year old water pipes in CA that must also be replaced or else…) is clearly not going to supply the 30 million increase in the state’s population and ag industries whose population is only increasing, in the future. It will not sustain the historical uses in the future without an Indy wreck (what I am saying is that there will be a wreck, but will it be one car and minor, or all three?).

      As for the debt picture…can it possibly be repaid or are we really insolvent? Has there not already been a wreck and society will not admit it? All three cars into the wall at some point?

      I am just saying that we are clearly past the point of no return on some of these trends. There will be consequences for both, and others too. I just wonder if the country hasn’t lost so much of their grandparent’s morality to avoid the wrecks that are coming. I am 100% positive that goodness will prevail, however. There is a solution to every problem. Thank you.

    3. Jerry,

      You are reading people that feed your fear. That is how Americans are ruled, so you have lots of company.

      The rains come every year, the rivers flow. California is not dependent on irreplaceable acquifers (unlike much of the Midwest; for those areas farming will take a big step down in future years).

      Perhaps it’s population will increase by 30million, mostly from immigration. Those long-term forecasts have not proven accurate in the past, but perhaps this one is accurate. Saying that there is insufficient water for those numbers is not accurate, since most of the water goes to ag — much of which is incredibly cost-ineffective.

      “The point of no return … is when the airplane cannot return to its original point of departure without running out of fuel”

      By that definition western civilization passed that point centuries ago. Changes in technology, religion, political philosophy, and economic systems, plus population growth — all these have created irreversible changes, one after another after another. The arrow of time is a distinguishing characteristic of western civ.

  3. Some good points there…the world does go on, and I am trying to be open. Take the point about the 100+ year old water pipes in Los Angeles. One article said it would take the entire budget for 10 years to replace the pipes that must be replaced. There is a simple example of “past the point of no return”. If that is true, there will be a wreck, right? Or, like you said, we will find another way around the old pipes, like new above-ground pipes, etc. I see your point, and like I said earlier, there is a solution to any problem. But one of society’s problem is that we won’t “talk” about what needs to be done because we done want to face up with reality…it might hurt too much. We are all too “short-term”, and our becoming who we are will have consequences, right? Like, what will happen to LA when a large quake rolls through and tears apart the old pipes? That is all I am saying. We get (consequence-wise) what we want…what or who we have become. Decisions matter.

    By the way, thank you for your useful responses. I appreciate having a reasonable discussion, and your points are well needed and taken. Jerry Staker

    1. Jerry,

      “One article said it would take the entire budget for 10 years to replace the pipes that must be replaced. There is a simple example of “past the point of no return”.”

      The population of LA in 1910 was roughly 320 thousand. It’s roughly 18 million today (metro area) – growth of 56x. US real GDP per capita has grown from roughly $5,000 to almost $50,000. So we can roughly size the growth in LA’s real GDP (income) over the past century as the close order of 500x to 600x (for LA probably the latter).

      Plus construction projects that were large and difficult in 1914 are far easier with modern tech (the Model T car was rolled out in 1908).

      Replacing anything built a century ago in LA is feasible today. The very opposite of “point of no return”.

  4. Google up the article I referenced. The politicians would not run on the need to do the pipe repair work so it got pushed off year after year. Now, they estimate it will cost the same as 10 years’ worth of the entire budget (city or county, I can’t remember). So, the years keep rolling by…and we had a 20 million gallon leak recently that flooded a college campus and caused extensive…and that pipe break out of the blue was only 90 years old. And they say about 25% of all the pipes need to be replaced. So, you say it is feasible? How? The city or county which ever it was, does not have the money, it would take 10 years to replace it with all the budget. Debt the answer? Can it really be funded when the politicians have avoided the necessary taxes in the past? I see that as being past the point of no return. It will be a train wreck with triage remedies that didn’t need to be. That is my point. The national debt is the same thing. There is no way on earth 1) we could repay the debt (without collapsing the economy), and it just keeps growing. The demographics don’t support it. And he pending entitlements…all this is unfounded too. There will be a train wreck on that point too. We are insolvent today as a country already and have been living off a credit card, true? We are well past the point of no return and the consequences will follow one day, just like the three Indy cars did in my first example. Actually, I see a time coming where debt will be wiped out because of failed restructurings and default, which wipes out he wealth of those who own the bonds, and inflation takes the rest of the cake, then we start all over. So yes life will go on. But, the consequences of a lot. Of these things didn’t need to happen, that’s all. Also, with the moral bankruptcy of society today, I wouldn’t be surprised if God doesn’t compel us, his children, to be more humble these days. Your thoughts on that angle too? Again, thank you for the discussion. Jerry

    1. Jerry,

      Yes, the papers print doomster articles. That was the very first sentence I said in my first reply to you. It is clickbait, almost always disprovable by simple logic and facts.

      I have been reading these for fifty years, about in aggregate hundreds of ills. I have read reports of others going back to the late 19th century. The world is always doomed. These forecasts are almost never based on actual analysis, and seldom true.

      In college it was overpopulation and water and air pollution. Now the developed world worries about falling population (as fertility has collapsed), and has made fantastic progress on pollution.

      More to the point, since 2003 I have had scores of conversations like this in comments with people terrified about some doom. Since so much time has passed, many of these forecasts have been proven false.

      To mention just one, from 2005-2008 there were the peak oil doomsters, since oil production had already peaked. DDOOM DOOM DOOM. Now oil prices are in free fall due to production over demand, and some producers face bankruptcy.

      The latest is about the “inevitable” ebola epidemic in America and Europe.

      What I have learned after spending vast effort is that people infected by the doomster bug cannot be cured by any resources of fact or logic at my disposal. So you can believe whatever doomster fantasies you like, and the rest of us will drive to the future.

  5. I see your point, and agree that facts are facts. Do you have the time to help put the facts together on the water pipe situation so there is clarity?

    Also, can the national debt ever be repaid without a restructuring, or rolling it over? I don’t think so. To inflate it away is also a wreck.

    Nothing dooms day about that. The U.S. can’t even live within its means for a single year! Those are facts and to dismiss it as being illogical or doomsday talk is unreasonable.

    Thanks, Jerry Staker

    1. Jerry,

      The water-doom claim is too silly to bother with, esp for LA where the average age of the infrastructure is young (as cities go, since LA’s rapid growth was in the 1930s, accelerating thru WW2 and after). It’s not worth discussion, esp since you ignored my correction to your absurd claim about “100 year old” infrastructure, switching to new claims.

      NYC allowed its infrastructure to rot since the 1930s. Metro-North and PATH had even electrical equipment from the 1930s and 1940s. Much of its physical infrastructure had reived ex little attention since the 1920s (e.g., the water system, the ParkAvenue tunnel into Grand Central). The reconstruction began in the 1980s, continues today — without any whining about doom doom doom.

      As I said from experience, doomsters are chaff in comments. As each absurd claim is debunked, you’ll come up with more.

      And no, the U.S. Federal debt is not unsustainably large. It’s not even large vs those most of our peers by any meaningful metrics. Nor is there any need to “pay it off”.

  6. I had assumed this was a reasonable blog, or that a discussion could be held without belittling the guest. I had a reasonable request to see if you (whoever you are) would take one minute and research that article on LA’s old pipes to help me understand the facts as they may be. I don’t need to worry about things from a doomsday perspective because I am well-prepared from a physical, mental, emotional, and spiritual perspective. All four such tires have lots of tread and are equally inflated and care for. But I do like to be in the know. And I listen to both sides of the day’s arguments to get the facts. I wish you had time to help me with the facts on the water pipes, at least from your perspective. But if you are not willing, that’s fine, and we will pass on it.

    I apologize if I missed something you were pointing out. I thought I had acknowledged what you said but apparently didn’t do so. If you are willing, kindly point out the piece you thought I missed and I will address the same.

    From a principle perspective, debts should be repaid. With the untold entitlement commitments, our debts are and will be unsustainable. That’s a wreck, and the consequences will follow. Any sane and reasonable person knows that. That’s all I am talking about.

    Let me give you some feedback. First, I am new to this blog, and my so-called blogging experience is extremely limited. I don’t appreciate my opinions and comments to get to the truth of things being too “silly to bother with”. For that reason, I will not engage you further if I wasting your time. Just let me know if I am bothering you. Once again, I am just trying to be sincere and maybe I don’t know who all these things work, but I do feel bad that my comments are considered as being silly. Just saying…Jerry. Staker

    1. Jerry,

      The world is burning, with serious problems blighting the lives of billions. People getting shot for carrying toy guns. People fighting poverty. Debates about stripping health care from the working poor. We’re in 13th year of a Long War, perhaps losing. And in other lands there are problems even more serious.

      Worries that LA — one of the richest cities in human history — cannot afford to maintain its water pipes are difficult to take seriously.

      That the US, by some measures one of the world’s richest nations, having by many measures one of the lowest debt loads among our peers (lower than we & the UK had in the past by many metrics), has a govt debt problem that cannot be handled is also difficult to take seriously. Debts need to be paid off in the sense of individual loans. A nation’s debt, or that of a business, or even family, do not. A person can carry a mortgage their entire life (re-financing it, for example) — so long as they have enough income. Nobody says they’re doomed.

      I went several rounds with the LA water thing, starting with your claim about the “hundred year old infrastructure”. With no signs of being heard. Then you brought up the debt, starting a new round. Worse, experience shows that (generalizing) people raising this as an insolvable problem are as open to data and analysis as blocks of wood.

      I no longer have the patience for this, as experience shows (with no counter-examples found to date) that these discussions can go on for dozens of rounds without the slightest effect (look at the 2005-2008 threads on peak oil to see irreplaceable hours wasted). Doomsters believe what they believe. So I go a few rounds, identify this as a doomster thread, and give up. Life is short.

      We stereotype for reasons, although they’re not always correct. You might be the exception (there are always exceptions). If so, I apologize.

  7. Australia’s big drought was not unusual. Other droughts blamed on CO2-caused global warming are also probably natural events.

    Antarctic ice cores tell 1000-year Australian drought story“, Australian government’s Department of the Environment, 16 December 2014:

    Dr Tessa Vance from the Antarctic Climate & Ecosystems Cooperative Research Centre said the study, just published in the journal Geophysical Research Letters, could help improve catchment management strategies in drought-affected areas of Queensland and New South Wales.

    “Up until now we had no clear way of knowing whether the prolonged drought Australia experienced recently was a historical anomaly,” said Dr. Vance. “The study shows that the Millennium Drought was far from an exceptional event for eastern Australia during the past thousand years.” “Droughts lasting longer than five years are in fact a normal part of long-term climate variability, and should therefore be factored into catchment management.” “We now have a much clearer picture of the multi-decade cycles in Australian rainfall patterns, and therefore a more reliable means of predicting future trends.”

    The ice core used for this study was drilled by the Australian Antarctic Division at a remote site on Law Dome, 100 km southeast of Australia’s Casey station.

    Explaining the findings, Dr Vance said the ice core analysis had significantly enhanced our understanding of a relatively poorly understood phenomenon known as the Interdecadal Pacific Oscillation (IPO). The IPO describes a roughly 25-year cycle in the sea surface temperature, wind and other factors in the Pacific Ocean. The IPO’s positive phase is closely linked with longer and more severe droughts in the United States and Australia. The risk of droughts occurring in Australia is higher during the IPO’s positive phase.

    Dr Vance said that until now, there was no reliable means of measuring long term trends in the IPO index prior to the start of instrumental measurement about a century ago. “We know that rainfall in eastern Australia is extremely variable, but the relatively short instrumental climate records have made it difficult to judge whether long droughts are unusual or result of a long-term cycle.” “We can now look back and see a very clear pattern of rise and fall in Australia’s rainfall going back a thousand years.” “We hope this work will provide policy makers and planners with better estimates of the likelihood of long droughts, as well as their duration, in order to better manage water resources.” “This is particularly important given that projected future warming is likely to result in reductions in river flows, potentially exacerbating the impact of future mega-droughts.”

    Since the late 1960s there has been a 15-20% decline in winter rainfall in south-west Western Australia, and a 10% increase in snowfall at Law Dome.

     

  8. Exploring the causes of the California drought

    A Link Between the Hiatus in Global Warming and North American Drought“, Thomas L. Delwort (NOAA) et al, 27th Conference on Climate Variability and Change, American Meteorological Society, 5 January 2015 — Abstract:

    Portions of western North America have experienced prolonged, severe drought over the last decade, with significant impacts across a range of sectors. This drought has occurred at the same time as the so-called global warming hiatus – a decade long period with little increase in global mean surface temperature.

    Using simulations with medium and high-resolution climate models, as well as observational analyses, we find a strong link between the hiatus and recent decadal-scale drought over North America, with the connection arising from the impact of anomalously strong easterly winds on the tropical Pacific Ocean. These anomalous easterly winds induce enhanced upwelling of cold sub-surface water in the eastern Pacific. This relatively cold water creates a broad area of negative sea surface temperature anomalies over the eastern Pacific. These SST anomalies, in turn, induce a global scale atmospheric response, including atmospheric ridging in the mid troposphere extending from the central North Pacific through the western and southern U.S. This ridging systematically reduces the likelihood of precipitation over western North America.

    In our simulations the increased tropical easterly winds, in association with changing radiative forcing, increase the likelihood of a decade with a 15% or more reduction in precipitation from 3% to 46%, with most of the increased likelihood coming from the enhanced winds.

    If the enhanced tropical easterly winds are the result of natural variability, this implies that the recent decadal drought over portions of western North America is also a result of natural climate variability. The drought should recede, at least temporarily, if the persistent easterly winds in the tropical Pacific return to conditions observed in prior decades. It is critical to better understand the physical processes that have led to the sustained increase in tropical easterly winds over the last decade or so. Such understanding may help to quantify the likelihood of continued drought.

  9. More about the large natural fluctuations in rainfall:

    Long-term oscillations in rainfall extremes in a 268 year daily time series“, Marco Marani and Stefano Zanetti, Water Resources Research (an AGU journal), in press — Abstract:

    We analyze long-term fluctuations of rainfall extremes in 268 years of daily observations (Padova, Italy, 1725-2006), to our knowledge the longest existing instrumental time series of its kind. We identify multidecadal oscillations in extremes estimated by fitting the GEV distribution, with approximate periodicities of about 17-21 years, 30-38 years, 49-68 years, 85-94 years, and 145-172 years.

    The amplitudes of these oscillations far exceed the changes associated with the observed trend in intensity. This finding implies that, even if climatic trends are absent or negligible, rainfall and its extremes exhibit an apparent non-stationarity if analyzed over time intervals shorter than the longest periodicity in the data (about 170 years for the case analyzed here). These results suggest that, because long-term periodicities may likely be present elsewhere, in the absence of observational time series with length comparable to such periodicities (possibly exceeding one century), past observations cannot be considered to be representative of future extremes.

    We also find that observed fluctuations in extreme events in Padova are linked to the North Atlantic Oscillation: increases in the NAO Index are on average associated with an intensification of daily extreme rainfall events. This link with the NAO global pattern is highly suggestive of implications of general relevance: long-term fluctuations in rainfall extremes connected with large-scale oscillating atmospheric patterns are likely to be widely present, and undermine the very basic idea of using a single stationary distribution to infer future extremes from past observations.

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