sábado, 24 de maio de 2014

Dramatic Confirmation of Temperature Change for 1980-2010

I analyzed a new global gridded daily weather dataset to see what it would tell us about what climate change, particularly temperature change, we may have already witnessed during my lifetime. What I discovered surprised me very much. I was expecting that some places would show a 1-degree Celsius temperature increases over a 30-year period, with a few additional pixels showing up to a 2-degree increase. Instead, we see a lot of places with 1- and 2-degree increases, and even others with 3- and 4-degree temperature changes.
AgMerraDTMax_global_legendThe dataset I used, AgMERRA, was recently developed by Alex Ruane of NASA and Richard Goldberg of the University of Chicago. The pixels are a half-degree in size, and the data spans the period of 1980 to 2010. The data includes solar radiation, minimum temperature, maximum temperature, rainfall, and wind speed. My primary interest is in the monthly mean daily maximum temperature for the warmest month of the year (for brevity, “tmax”), since this may be the critical limiting factor for agriculture in the future under climate change.
My idea was simply to run a regression for each pixel on tmax using an intercept and the year as explanatory variables. The year parameter would be a measure of the year-to-year change in tmax. I would be able to tell the statistical significance at each point by looking at the z-statistic for the parameter from the regression. Being somewhat a skeptic, my guess was that only a few pixels — maybe less than 10 percent of them — would have temperature changes that would be significant.
Given there has been a global “pause” in warming of surface temperatures for the last decade, I considered cutting off the regression to 2000 or maybe 2004, but in the end decided that the best first pass at the data would be to use it all. If there were a pause, this would have the tendency to make the z-statistic less significant if we estimate it to 2010.
In the map of temperature changes presented here, we show the estimated 30-year change. That is, given the parameter estimated at each point, what did that imply for temperature changes over a 30-year period? By using the parameter times 30 to show change, we effectively smooth out the bumpiness of year-to-year variation. In the map, I only show results that are statistically significant at the 10 percent level. However, I noted that over 60 percent of the pixels that are significant at 10 percent are also significant at 1 percent.
The results are quite dramatic, showing how much change has already taken place. For me, this is a wake-up call. I have been thinking about climate change as something in the future, but something we get hints of now. But these fairly large temperature increases that have already happened in many parts of the world show that hundreds of millions of people are clearly dealing with climate change already. Temperature changes in this range are large enough that they are probably already adversely impacting crop yields.
For example, the map shows a fairly large area in Brazil that has a more than 3-degree Celsius rise in temperature over the 1980 to 2010 period. One of the reasons I took time to explore this data in the first place was to use it to prove that one of the climate models[1] I was using for my research was not believable, since it projected a 7 degree Celsius increase for the period from 2000 to 2050 in the same part of Brazil. In fact, the results that we see in the map suggest that the particular climate model I questioned is likely to be correct, or at least not terribly incorrect, at least for Brazil.
In the global scheme, we see large temperature increases in central Africa and the Horn, northern Canada, eastern Europe, western Asia, Mongolia, northeast China, and parts of Russia. And we actually note falling temperatures in northern Australia, New Guinea, and southern India. It is also important where temperature change has not occurred to a large extent, including the eastern and central United States, southern Canada, and much of Latin America, southern Africa, and south and southeast Asia. For my friends in New Zealand (not shown in the map), the dataset only covered the very northernmost portion of the country; and for my Alaskan friends (also not shown), only 2 small patches of temperature rise were noted.
I would like to emphasize that these are preliminary results that I have not yet reviewed thoroughly, and that no one else has reviewed either. Therefore, they are meant to stimulate discussion and to promote follow-up work. But if confirmed, they clearly say something to policymakers, to each person working in a field related to climate change, and to climate change skeptics.
– Timothy S. Thomas

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