27 July 2010

Silly Science

A new paper is out in a journal getting a reputation for silly science that predicts that climate change will lead to a massive influx of Mexicans across the border to the United States. Here is how the LA Times breathlessly opened its news story on the PNAS paper:

Climbing temperatures are expected to raise sea levels and increase droughts, floods, heat waves and wildfires.

Now, scientists are predicting another consequence of climate change: mass migration to the United States.

Between 1.4 million and 6.7 million Mexicans could migrate to the U.S. by 2080 as climate change reduces crop yields and agricultural production in Mexico, according to a study published online this week in the Proceedings of the National Academy of Sciences. The number could amount to 10% of the current population of Mexicans ages 15 to 65.

A reporter emailed me an embargoed copy last week asking for my reactions. Here is how I responded (and I pulled no punches):
To be blunt, the paper is guesswork piled on top of "what ifs" built on a foundation of tenuous assumptions. The authors seem to want to have things both ways -- they readily acknowledge the many and important limitations of their study, but then go on to assert that "it is nevertheless instructive to predict future migrant flows for Mexico using the estimates at hand to assess the possible magnitude of climate change–related emigration." It can't be both -- if the paper has many important limitations, then this means that that it is not particularly instructive. With respect to predicting immigration in 2080 (!), admitting limitations is no serious flaw.

To use this paper as a prediction of anything would be a mistake. It is a tentative sensitivity study of the effects of one variable on another, where the relationship between the two is itself questionable but more importantly, dependent upon many other far more important factors. The authors admit this when they write, "It is important to note that our projections should be interpreted in a ceteris paribus manner, as many other factors besides climate could potentially influence migration from Mexico to the United States." but then right after they assert, "Our projections are informative,nevertheless, in quantifying the potential magnitude of impacts of climate change on out-migration." It is almost as if the paper is written to be misinterpreted.

Climate change is real and worthy of our attention. Putting forward research claims that cannot be supported by the underlying analysis will not help the credibility of the climate science community. Even with the voluminous caveats in the paper, to conclude that "climate change is estimated to induce 1.4 to 6.7 million adult Mexicans (or 2% to 10% of the current population aged 15–65 y) to emigrate as a result of declines in agricultural productivity alone" is just not credible. The paper reflects a common pattern in the climate impacts literature of trying to pin negative outcomes on climate change using overly simplistic methods and ignoring those factors other than climate which have far more effect.
One of the paper's authors, Michael Oppenheimer, a Princeton professor and lead author of the forthcoming IPCC report on extremes explains his motivation with the paper:
Our primary objectives were, No. 1, to give policymakers something to think about and, No. 2, to give researchers a spur to start answering some of the more complicated questions
One of the climate impacts scholars whose work was relied on in the PNAS paper was critical:
Diana Liverman, a University of Arizona climate researcher, criticized the new study for basing its forecasts in part on research that she worked on in the early 1990s that looked at crop yields in only two central Mexico sites.

In reply, Oppenheimer said the Princeton study found similar results in a second crop-yield study, and the crop reductions predicted for Mexico are typical of what has been predicted for other countries in that latitude.

Liverman said that while she believes climate change could cause widespread migration, she has seen no study documenting it. Having studied the problems of Mexican farmers for two decades, she said she has found that a bad economy, the government's withdrawal of agricultural subsidies and the North American Free Trade Agreement have caused problems far greater than climate change.

Nature also has a set of critical reactions. The LA Times article recovered from its breathless opening with a well-buried lede:

Philip Martin, an expert in agricultural economics at UC Davis, said that he hadn't read the study but that making estimates based solely on climate change was virtually impossible.

"It is just awfully hard to separate climate change from the many, many other factors that affect people's decisions whether to stay in agriculture or move," he said.

In silly science however, nothing is impossible.


  1. This sounds like an attempt to capitalize on conservative xenophobia in order to get them to be more supportive of climate change legislation.

  2. I don't know what it is with economists these days. Nick Stern used to be a smart guy. Michael Greenstone is really clever. Alan Krueger may win a Nobel Prize. Paul Krugman deserves three. Yet, confront them with climate change, and they start talking utter rubbish.

    The silly PNAS paper makes three mistakes. First, it confuses decadal weather variability with climate change. Second, it fails to control for other determinants of migration that may well be correlated with weather during the sample. Third, they extrapolate beyond belief.

    To belabor the third point, their largest yield change is -48% between now and 2080. If technological progress would bring about a 1% yield increase per year, then the two effects cancel each other out. 1% may be too low, -48% is probably too high. Based on their logic, we'll all be moving to Mexico so.

  3. "The silly PNAS paper makes three mistakes"

    add another oops..


    Total mexican labor forces 46.2 million
    Percentage involved in agriculture 13.7%.

    46.2 * 13.7% = 6.3 million agricultural workers.

    Projecting more then 100% of the Mexican Agricultural labor force emigrating due to 'tough times on the farm' seems somewhat unrealistic.

  4. Shouldn't the precautionary principle apply here ? Why not bring the Mexicans to the USA now, and let them get adjusted ? By 2080, their offspring will be Americans. Culture shock is a terrible thing.

    The LA Times certainly understands the mindset of its 'liberal' readership. There is obviously no tactic too low for these 'scientists'. I await the eugenics scare stories with interest.

  5. “It is a tentative sensitivity study of the effects of one variable on another, where the relationship between the two is itself questionable but more importantly, dependent upon many other far more important factors.”

    This is precisely the problem I have with almost every single socioeconomic study done in the name of Climate Change. Prof. Pielke, I know you've touched on this in the past, and probably in your books as well. Would you mind speculating why there are so many of these sub-par studies getting both funding and media coverage? I don't mind the science being settled with regard to the 2-6 degree response to CO2, but bristle when it is followed up with sketchier claims like the one above or even the Stern economic analysis. How many people are actually publishing studies that you would respect? How many of these are reaching the media?

    A few of the examples which have been most egregious: Malaria studies showing increased range of mosquitoes due to climate change when the effects are swamped by humans maintaining water infrastructure. One study that I was given was basically a computer model that based increased malaria on the increased human usage of exposed water tanks (used to be on Discovery here [http://dsc.discovery.com/news/2009/01/27/mosquito-disease-warming.html] but they've removed the page). There's also the typical "Climate change might have an effect" attribution regularly seen, as here [http://allafrica.com/stories/200901261750.html].

    Then there’s the WHO "World Health Report 2002" which breaks down mortality according to many factors — nutrition, violence, disease, etc. It claims that the increased number of people dying from climate change in 2005 was 154,000! How did they determine this? It turns out this number was the error term between the 1960-1990 average using the calculated regression terms and the actual mortality in 2005. Absolutely no direct contribution of course, and no admission that an error in their model could drastically affect that number (say degradation of hygienic infrastructure).

    "Climate change was estimated to be responsible in 2000 for approximately 2.4% of worldwide diarrhoea, 6% of malaria in some middle income countries and 7% of dengue fever in some industrialized countries. In total, the attributable mortality was 154 000 (0.3%) deaths and the attributable burden was 5.5 million (0.4%) DALYs."

  6. Quote, Stephen Schneider, "On the one hand, as scientists we are ethically bound to the scientific method. On the other hand, we are not just scientists but human beings as well. To do that we need to get some broad based support, to capture the public’s imagination. That, of course, means getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have. Each of us has to decide what the right balance is between being effective and being honest."

    What can be more effectively scary to Americans than the notion of millions of Mexicans escaping to the US.

    Lets be honest, this is just racist propaganda.

    The fact that PNAS has published such a paper should make Americans wince with embarrassment.

  7. PNAS seems to have less and less to do with real science and more and more to do with post normal science.

  8. AS the 1st poster suggested this is just an obvious attempt by the alarmists to rope in more support for their climate scam by playing on people's legitimate concerns about massive illegal immigration.

    We have one set of alarmists, Whitley Streiber & Art Bell, suggesting in their film that Americans will have to migrate to Mexico due to "climate change", and we have these guys suggesting that the Mexicans will all migrate to the USA. Maybe the 2 respective phenomenona will cancel each other out.

  9. Well, de mortuis nil nisi bonum, but possibly this spate of silliness may dry up somewhat now one of its enabling factors is no longer with us

    This seems to be a very lame attempt to get conservatives on board the carbon tax train by playing to another conservative issue (concerns about immigration). I doubt it will work. Conservatives ideas on securing the border seldom involve concerns about how pleasant Mexico is for its inhabitants.

    The level of political naivety of some obviously very smart academics astonishes me.

  10. Things caused by global warming:


    Currently at 784.

  11. Well, let's look at those "critical letters" in Nature (follow the link above). One of them says
    ....IMO, the biggest finding is that Mexico will be in serious trouble if it does not take steps to adapt to climate change. In fact, it appears that if Mexico refuses to adapt at all, then the estimate of 10% of Mexico's population being displaced will prove to be a serious under-estimate. (BTW, I am assuming that the 1.4 – 6.7 million emigration figure is for the best case scenario instead of the worst case one.)....

    another one says

    This study is interesting, however, incomplete. It is not as simple as climate change influences migration, but also, how climate change, US foreign policies, and Mexican –US agreements affect the lives of the farmers and farming conditions in Mexico (ex., The North American free trade agreement that mostly benefits Canada and the US and leaves Mexican farmers without the possibility to compete, therefore impoverish and forced to migrate. ) It would be interesting to see what the statistics would say if such agreements would lead fair deals for Mexico in this case, maybe the migration would be given within states of Mexico and not across the border.

    but, of course the paper does talk about that and a third

    I'm not a climate scientist or political scientist but I do think that various aspects are true at least from a layman's point of view.
    First because the crops grow in an environment that's being grossly neglected and willfully ignored which with a diminished forest and diminishing water supply to provide protection from droughts and other growing issues which because of less indigenous foliage the plant eating insects that no longer have the forest to feed, will eat the crops......

    Really critical reactions.

  12. -12-Joshua Halpern

    You are getting your politics confused with your science. Human-caused climate change is real, and requires attention to both adaptation and mitigation. But it does not mean that bad science gets a free pass.

  13. Willis Eschenbach has posted an analysis of this paper on Watts Up With That that efficiently and cleanly relegate its results to the garbage bin, bin.

  14. Eli,

    From CBSnews
    'Oppenheimer himself freely acknowledges:
    the fudgy nature of predicting climate change’s effects, and that while the numbers make for a sexy headline, you shouldn’t take them too seriously.'

    'Fudgeing' numbers to make 'sexy headlines' is called 'politics', not 'science'.

    All politicians 'fudge numbers' to make 'sexy headlines' . Of course the vast majority of the public believes politicians are habitual liars incapable of telling the truth.

    Here is 'trust' poll by profession.

    82% of people polled believe politicians are liars, 72% believe journalists are liars.

    I would speculate because politicians and journalists like 'sexy headlines' unsupported by fact.

  15. You don't need climate change to make Mexicans migrate. In a recent survey 30% of Mexicans expressed a desire to emigrate to the USA. The climate change thing invites people to write papers with predictions. After all nobody is is going to call you on it in 2080!

  16. Malcolm, there's really no need to evoke Stephen Schneider here. But since you did, you might as well use the full quote and tell the full story: http://climatesight.org/2009/04/12/the-schneider-quote/.

  17. rjtklein: "there's really no need to evoke Stephen Schneider here."

    The full story doesn't change the context of the quote or how it's generally used. He finishes with:

    "Each of us has to decide what the right balance is between being effective and being honest. I hope that means being both."

    There should be no balance issue! Scientists *have* to be honest, or forced to be honest through regulation ie the pharmaceutical industry, or science cannot be trusted to be used where its most needed!

    However, Climategate shed light on the balance between honesty and effectiveness. Glaciergate shed light on the balance between honesty and effectiveness. Amazongate shed light on the balance between honesty and effectiveness. Malaria, immigration, floods, hurricanes, blacklists, and to top it off, hostile reactions to people like Dr Curry highlighting problems in climate science.

    I'm afraid the balance is clearly in favour of effectiveness. Schneiders interview merely summarises why and that is why he is quoted.

  18. DaveJR

    Everything in this thread confirms my firm belief that scientists are a scpetic's best friends.

    The trick of being a con man is to be smarter than your victims. The best way to get caught is to mistakenly believe you are.

    To discuss the 'science' in this and the reply thread is to totally miss the point. I could invent a host of even more offensive and scary future scenarios.