Anthony Watts has published one of his most foolish posts ever, courtesy of Erl Happ. If this is the level of “science” from fake skeptics, then God help us all. But rather than rip the arguments to shreds (taking candy from a baby), I’d like to examine some of the data used, which mostly come from the NCAR/NCEP reanalysis data set.
The NCEP reanalysis data set is the output of computer models which take as input a vast array of meteorological data, including temperature, pressure, wind speed and direction, and many others, and simulate the weather in order to estimate a wide variety of data over the entire globe. As such it provides world-wide coverage of a great many interesting quantities. All quantities are provided at monthly time resolution, and at a variety of levels of the atmosphere where appropriate.
These are computer model output rather than directly observed data, but they are based on (and therefore constrained by) a great quantity of direct observations. The NCEP data are also internally consistent, based on the same model for the entire period from January 1948 to the present, which makes them suitable for examining climate change. Let’s take a close-up view of just one of the available variables, the estimated surface air temperature.
One of the striking facts is that the size of the seasonal temperature cycle is dramatically different bewteen the two hemispheres:
Note that the northern hemisphere has a much larger seasonal cycle than the southern. This is almost surely due to the greater thermal inertia of the southern hemisphere, which in turn is due to the greater proportion of ocean to land in that hemisphere. We can also see this in a “boxplot” of monthly temperatures by month of the year. For the northern hemisphere we see a range from winter to summer of over 12 deg.C:
But for the southern hemisphere, the annual range is only a bit over 6 deg.C, about half that of the north:
We can of course transform temperature to temperature anomaly (the difference between temperature at a given time, and its average for the same time of year) to remove the annual cycle. This allows an interesting comparison between the overall temperature changes in the two hemispheres (smoothed temperature anomaly):
Over the period from 1948 to the present, the two hemispheres have warmed by about the same amount. However, during the 1948-1975 period the southern hemisphere warmed while the northen cooled, making global average temperature roughly stable. This is consistent with the idea that northern cooling was due to sulfate aerosol emissions; those emissions were predominantly a northern-hemisphere phenomenon, so the north cooled while the south continued to warm due to increasing greenhouse-gas concentrations. Since 1975 both hemispheres have warmed, but the northern has warmed considerably faster than the southern.
Another interesting point is changes in the size of the annual cycle. We can estimate this with a wavelet analysis, which yields the time-varying amplitude (actually semi-amplitude, which is simply half the amplitude) of the annual cycle (I’ve plotted semi-amplitude anomaly so they’ll be roughly on the same scale):
There’s only slight indication of a decrease in the amplitude in the north, but great reduction of the amplitude in the south. This tells us that in the southern hemisphere, winter has warmed much faster than summer, but in the northern hemisphere the two seasons have warmed at nearly the same rate. The annual cycle has actually increased globally, because the offsetting influence of the southern hemisphere has been much reduced while the northern influence has remained strong.
I also extracted data for each 10-degree latitude band, 18 in all from the south pole to the north. Here are smoothed temperature anomalies for each, with each latitude band offset by half a degree so that the lines go from the southernmost latitude band (90S to 80S) at the bottom to the northernmost (80N to 90N) at the top:
Both poles have shown markedly faster warming than the global average since about 1990. A close-up on the latitude bands of the polar regions (both north and south) shows this is detail, and reveals that the north polar regions have warmed faster than the southern, although both poles appear to have warmed rapidly over the last two decades:
If we look at the equatorial region, we see that not only was the 1948-1975 cooling absent from the southern hemisphere, it was also absent from the equatorial region at least as far north as latitude 20N:
The mid-century cooling is confined to regions north of latitude 20N, but is clearly present in the northern hemisphere extratropics, which again is consistent with mid-century cooling being due to industrial emissions of sulfate aerosols:
There are more data fields in the NCAR/NCEP reanalysis data set, including temperature at multiple atmospheric levels, humidity (both relative and absolute), and many others. In future posts we’ll take a look at some of the interesting changes that have occurred in some of those other quantities.