WUWT readers may recall a recent essay titled ‘What global warming really looks like’ – Michael Oppenheimer FAIL where I pointed out the desperate and ridiculous claims by Michael Oppenheimer to try to link the recent Colorado forest fires to
global warming climate change climate disruption. He did a miserable job with his claims, and the data from the National Interagency Fire Center didn’t support his claims at all.
It seems Mr. Oppenheimer is again in the news, this time being smacked down due to another example of some shonky science where he and two other Princeton researchers tried to prove that imagined climate driven crop issues in Mexico were creating climate refugees. Here’s the Princeton press release from July 26, 2010. It immediately became a darling story of climate media bloviators from the BBC to Scientific American to Yale 360 who wrote:
Rising temperatures and reduced crop yields in Mexico could force as many as 6.7 million Mexicans to emigrate to the United States over the next 70 years, according to a new study. Researchers from Princeton University, led by atmospheric scientist Michael Oppenheimer, made that projection after studying historical patterns of emigration, climate change, and crop yields in Mexico between 1995 and 2005. Oppenheimer and his colleagues concluded that for every 10 percent reduction in crop yield, an additional 2 percent of Mexicans aged 15 to 65 could emigrate to the United States.
Well, it turns out Oppenheimer’s paper is complete junk, and when the error is corrected by less excitable researchers without an agenda, they find no evidence of a causal link.
Here’s the paper that started it all:
Linkages among climate change, crop yields and Mexico–US cross-border migration (Full PDF here)
Shuaizhang Fenga, Alan B. Kruegera, and Michael Oppenheimer
Edited* by Stephen H. Schneider, Stanford University, Stanford, CA, and approved June 24, 2010 (received for review March 3, 2010)
Climate change is expected to cause mass human migration, including immigration across international borders. This study quantitatively examines the linkages among variations in climate, agricultural yields, and people’s migration responses by using an instrumental variables approach. Our method allows us to identify the relationship between crop yields and migration without explicitly controlling for all other confounding factors. Using state-level data from Mexico, we find a significant effect of climate-driven changes in crop yields on the rate ofemigration to the United States.
The estimated semielasticity of emigration with respect to crop yields is approximately −0.2, i.e., a 10% reduction in crop yields would lead an additional2%of the population toemigrate. We then use the estimated semielasticity to explore the potential magnitude
of future emigration. Depending on the warming scenarios used and adaptation levels assumed, with other factors held constant, by approximately the year 2080, 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. Although the results cannot be mechanically extrapolated to other areas and time periods, our findings are significant from a global perspective given that many regions, especially developing countries, are expected to experience significant
declines in agricultural yields as a result of projected warming.
And here’s the rebuttal paper:
Unobserved time effects confound the identification of climate change impacts (Full PDF here)
Maximilian Auffhammera, and Jeffrey R. Vincent
A recent study by Feng et al. [Feng S, Krueger A, Oppenheimer M (2010) Proc Natl Acad Sci USA 107:14257–14262] in PNAS reported statistical evidence of a weather-driven causal effect of crop yields on human migration from Mexico to the United States. We show that this conclusion is based on a different statistical model than the one stated in the paper. When we correct for this mistake, there is no evidence of a causal link.
After the model in the work by Feng et al. (1) is correctly estimated using their data, the statistical evidence suggesting a causal relationship between weather-driven crop yields and emigration from Mexico to the United States disappears. The statistical evidence in the work by Feng et al. (1) is based on an incompletely controlled before-and-after comparison of emigration rates and crop yields across states. Any omitted factor
positively (negatively) correlated with yields and negatively (positively) correlated with emigration rates over time affecting all states confounds this estimated effect. The results in the work by Feng et al. (1), therefore, cannot be given a causal interpretation,because a variety of factors changed during this period. According to the work by Feng et al. (1), these factors included “the effect of NAFTA, the Peso crisis, and changes in US border controls such as increased border enforcement after 2001” (ref. 1, p. 14258); regarding the second factor, after 1994, “the Peso depreciated considerably against the US dollar, doubling the real wage rate earned by emigrants” (1). The work by Feng et al. (1) also points to the “reform of the land tenure system and the opening of Mexico’s economy
through liberalized trade and deregulation of markets” (1), which further impoverished small farmers and rural landholders.
These factors would confound a pure before/after comparison, and therefore, they make it absolutely essential to control for them through time effects. Results from correct estimation of the model suggest that, for the sample used in the work by Feng et al. (1), these factors, and not weather, were responsible for the change in emigration rates between the two periods.
As Sheldon Cooper would say: “Bazinga!”
Dr. Roger Pielke Jr. Sliced this paper up when it first came out, calling it “silly science”. He wrote on Jul27, 2010:
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.
I wonder if we’ll see this in Retraction Watch soon. It sure deserves to be retracted. This is truly junk climate science.
[UPDATE] I trust that Anthony will not object to my pointing out that I posted an extensive rebuttal of Oppenheimer’s nonsense, entitled “Border Transgressions“, no less than two years ago when it first came out in 2010. As a result, I am overjoyed that my views have been upheld. I’m also proud that WUWT, as usual, was on the case from the start.
UPDATE2: Apologies to Willis for my oversight in not including his excellent essay. I believe this one was published directly prior to our change of authorship rules, and somehow I missed it. – Anthony