Saturday, August 18, 2012

Migration and Climate Exposure from 1900 to 2010

It is Saturday night so it is time to calculate some weighted means.   The United States consists of roughly 3,000 counties.  For each of these counties, I have data on its average February temperature in the middle of the 20th century (so take the 1950 to 1980 annual average February temperature) and time average this. Let's call this "tmean2" and I have the average temperature for each county over the same years in August.  Call this "tmean8".

I then conduct the following thought experiment.  Given the spatial distribution of the population across counties in the year 1900, what was the average temperature exposure for the U.S population?  So, I'm weighting the county average February and August Temperature by the year 1900 distribution of people across U.S counties. Intuitively, more people lived in Detroit in those days and fewer lived in Los Angeles. As you can see below, the average American lived in a place that was 34.1 degrees F. in winter and 73.8 in Summer and 22.8% of people lived in a coastal county.

summ tmean2 tmean8 coast [w=pop1900]

    Variable |     Obs      Weight        Mean   Std. Dev.       Min        Max
      tmean2 |    2818    75438942     34.1235   9.595696   9.107881   69.19515
      tmean8 |    2818    75438942    73.81865   4.546925   60.25134   91.89582
       coast |    2818    75438942    .2284033   .4198783          0          1

Now, I repeat this exact same exercise using the same climate data but now I use the 2010 calendar year population distribution. We know the population has moved from the Rust Belt to the Sun Belt.

. summ tmean2 tmean8 coast [w=census2010pop]

    Variable |     Obs      Weight        Mean   Std. Dev.       Min        Max
      tmean2 |    3110   304169234    40.06854   12.17356   9.107881   69.19515
      tmean8 |    3110   304169234     75.0742   5.515168   60.25134   91.89582
       coast |    3111   306665669    .3432595   .4748736          0          1

Note that the exposure to warmer winters has increased by 6 degrees! Exposure to warmer summers has gone up 1.2 degrees and coastal living has increased by over 10 percentage points.  These composition shifts hint at what migration could do to "average climate exposure" if places such as Las Vegas and Phoenix are "too hot".   But, note the great demand to be in warmer winter places!  Perhaps in the future more households will telecommute and live Northern cities in summer and Southern Cities in winter.   How is that for an adaptation strategy?

It looking at these calculations, it is important to note that there is no "climate change" here. I never shift the spatial distribution of climate conditions. Taking the climate distribution as fixed, I reweight these data using variation in how many people live in each of these counties.  A more sophisticated exercise would also allow for the shifting climate distribution.

A long time ago, Mike Cragg and I talked about these issues in this 1999 RSUE paper.  

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