Much has been made of the differences between the UAH and RSS satellite data products for the lower troposphere layer average temperature anomalies. But the vast majority of the commentary on this issue is ignorant of the underlying data issues. Below is a plot of the differences between the two (note: I downloaded RSS from KNMI as the non-anomaly data, then anomalized to the 1981-2010 annual cycle to match UAH. I did this because UAH does not cut out some of the high latitude southern hemisphere data in their reported global averages but RSS does, whereas the KNMI non-anomalized data for RSS has the same spatial coverage. However, the differences are minimal as far as I can tell. I also rounded to 2 decimal places to match.)
Also present are the averages of 11 point and 13 point centered averages. I have highlighted two periods which are of interest. How did I select the dates? It wasn’t by looking at the discontinuities themselves. Rather, from my readings of various papers by John Christy, I saw that the transition from NOAA-11 to NOAA-12 was of particular interest in the differences of trends between the two satellites. So I identified the date at which NOAA-12 became operational; September 1991, and I identified the date at which the next satellite after NOAA-12, NOAA-14 came on line; April 1995. This would be the period during which the effect of the NOAA-11 and NOAA-12 transition should be most apparent, and indeed we see a continuous warming of RSS relative to UAH during that period. The later period, on the other hand, was identified as the period during which UAH was making use of AQUA as a data backbone. That was from August of 2002 to the end of 2009. At the time, non-AQUA satellites were diurnally drifting warm, as such they needed cooling corrections applied to them: the fact that RSS cooled during this period relative to UAH strongly indicates that RSS’s diurnal drift adjustment (which was not necessary for AQUA) is excessive.
Clarifying note: RSS also makes use of AQUA, however, it does not treat it the same way as UAH does. UAH treated AQUA as superior for assessing the trend over the period to other satellites (hence “backbone”) whereas RSS treated it as equal to the other satellites after applying their diurnal adjustment.
So we can be pretty confident that, at lest during that period, RSS is cooling excessively. This suggests that RSS should probably also be wrong about the earlier, warm shift, too, since it would also arise in that manner from excessive corrections by RSS.
But just to be sure, can we check the data against something else? For example, during a short term period, surface temps and LT’s roughly move together, albeit with different magnitudes. The answer might be yes. Herein, I will use GISS surface temperature data (downloaded from KNMI, 2500 km smoothing, anomalized to 1981-2010 mean, values from December 1978-November 2013). First, let’s detrend all the data: we only are interested in the spurious shift over a short period, not making data all agree in their long term trends, since their long term trends agreeing is a hypothesis we wish to be able to test. Next, let’s remove seasonal noise by taking the averages of 11 point and 13 point centered averages. That looks like this:
Blue is the average of the two satellite datasets: that way, we aren’t assuming either is superior to the other. Red is GISS Next, we estimate the tropospheric amplification factor by linear regression: the best fit slope is about 1.337. We use that factor to multiply GISS detrended anomalies, both smoothed and unsmoothed. Now, we compare those, by taking differences, to the UAH and RSS detrended anomalies (smoothed and unsmoothed) over the period of interest involving the spurious shift with NOAA-12:
Red and dark red are RSS-GISS, blue and black UAH-GISS, green and purple are RSS-UAH. Both satellite datasets warming relative to GISS over the period of interest, but RSS definitely warms more. The differences of the smoothed endpoints for RSS-GISS, UAH-GISS, and RSS-UAH, are ~0.0787 K, ~0.0121 K, and ~0.0622 K, respectively. The linear trends are ~0.0389 K/yr, ~0.0216 K/yr, and ~0.0160 K/yr, respectively. GISS appears to confirm that RSS warms spuriously during this period, and even suggests the possibility that UAH warms spuriously over this period, too.
If I correct for RSS’s spurious shift, the differences now look like this:
Notice that now, before the AQUA period, there is essentially no difference between UAH and RSS in trend terms. And remember: RSS would be expected to be wrong over this period and UAH right, due to the way in which they differently handle the AQUA data, which doesn’t need diurnal drift correction. So if I correct RSS for the drift over the AQUA period (and generously assume that there was no additional drift before or after) The differences now look like this:
What we see is that two simple corrections, which are based on a combination of independent data and understanding of the underlying satellites, removes a lot of the distinctive features of the differences between the two datasets. However, it appears that the trend difference between the two is largely unchanged, because these two errors mostly balance one another out. RSS is still cooling relative to UAH over the entire dataset, and it is hard to determine the origin of the remaining discrepancies. If we return to the NOAA-12 discrepancy, UAH looked like it might have a slight warm bias, too. If I correct for that, it will bring the UAH trend down closer to RSS’s corrected trend. However, this doesn’t account for the whole remaining discrepancy, which remains about .01 K/decade of RSS cooling relative to UAH. Another possibility is that I need to extend the AQUA correction forwards and backwards in time a bit, since the satellites that drifted during that period in RSS probably drifted before and after, to. If I extend it backwards to the start of NOAA-15 in December of 1998, and forward to the present, the difference in long term trends reverses, and now RSS warms slightly relative to UAH. It looks like we have a plausible explanation for the UAH-RSS divergences, and slight variations in those adjustments for the differences can switch which warms more or less relative to the other. Here is our final estimate for both datasets adjusted for spurious shifts and trends:
Red is RSS and blue is UAH, with corrections applied to both for shifts relative to GISS during the NOAA-12 step, and RSS corrected for spurious cooling since NOAA-15.