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.
The
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
Read more @ timthomas.net
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