The data is out for both RSS and UAH, and I’m presenting them both here. Click for full sized graphs.
RSS from Remote Sensing Systems of Santa Rosa, CA. RSS data here (RSS Data Version 3.2)
UAH from Dr. Roy Spencer, University of Alabama, Huntsville. Reference: UAH lower troposphere data
Since Dr. Spencer released the April UAH data first on his own blog, I’ll give him the honor of explaining the data and possible reason for divergence of the two data sets.UAH Data
YR MON GLOBE NH SH TROPICS
2009 1 0.304 0.443 0.165 -0.036
2009 2 0.347 0.678 0.016 0.051
2009 3 0.206 0.310 0.103 -0.149
2009 4 0.091 0.126 0.055 -0.010
Once again there is a rather large discrepancy between our monthly anomaly (+0.09 deg. C.) and that produced by Remote Sensing Systems (RSS, +0.20 deg. C). We (John Christy and I) believe the difference is due to some combination of three factors:
1) we calculate the anomalies from a wider latitude band, 84S to 84N whereas RSS stops at 70S, and Antarctica was cooler than average in April (so UAH picks it up).
2) The monthly anomaly is relative to the 1979-1998 base period, which for RSS had a colder mean period relative to April 2009 (i.e. their early Aprils in the 1979-1998 period were colder than ours.)
3) RSS is still using a NOAA satellite whose orbit continues to decay, leading to a sizeable diurnal drift adjustment. We are using AMSU data from only NASA’s Aqua satellite, whose orbit is maintained, and so no diurnal drift adjustment is needed. The largest diurnal effects occur during Northern Hemisphere spring, and I personally believe this is the largest contributor to the discrepancy between UAH and RSS.
UPDATE: Basil Copeland writes in comments.
And for those who are unhappy with either linear or 4 order polynomial trends, may I suggest Hodrick-Prescott smoothing?
I like to also keep track of the USA48 UAH anomalies:
The USA48 series appears flatter than the global series. That’s an illusion created by the differences in scale. The global series is not as volatile as the USA48, because it averages out all kinds of regional variation in climate around the globe. The scope of this averaging can be seen by plotting the two together, on the same scale:
The “Average Decadal Change Rate” shown on the chart is calculated as 120 times the average 1st difference of the smoothed trend lines, a number that should be fairly immune to any claims of cherry picking.
Frankly, I was surprised. E.g., on its own, the USA48 chart looks flatter. But it isn’t, really. In fact, it is steeper. Before anyone concludes that the high rate of growth for USA48 somehow demonstrates AGW, do keep in mind that during most of this time frame, the PDO was in a warm phase, and that the PDO warm phase has a strong influence on continental US temperatures.