Arguments
One satellite data set is underestimating global warming
Posted on 25 March 2015 by John Abraham
A very important study was just published in the Journal of Climate a few days ago. This paper, in my mind, makes a major step toward reconciling differences in satellite temperature records of the mid-troposphere region. As before, it is found that the scientists (and politicians) who have cast doubt on global warming in the past are shown to be outliers because of bias in their results.
The publication, authored by Stephen Po-Chedley and colleagues from the University of Washington, discusses some major sources of error in satellite records. For instance, after satellites are launched, they scan the Earth’s atmosphere and calibrate the atmospheric measurements using a warm target onboard the satellite and cold space. The accuracy with which the atmospheric measurements are calibrated can influence the inferred temperature of the atmosphere (called the warm-target bias). Additionally, over the years, multiple satellites have been launched and the selection of which satellite data are used can play a role. Finally, biases can occur because the satellite orbits drift during their lifetime and the influence of diurnal temperature variation can affect the global temperature trends.
It is known that there is a problem because there are multiple groups that create satellite temperature records. For instance, NOAA, Remote Sensing Systems (RSS), and the University of Alabama Huntsville (UAH). The problem is, their results don’t agree with each other. In particular, the UAH team, led by Dr. John Christy and Dr. Roy Spencer (who have discounted the importance and occurrence of climate change for years) present results that differ quite a bit from the others. In fact, in the current paper, it is stated that “Despite using the same basic radiometer measurements, tropical TMT trend differences between these groups differ by a factor of three.”
An important aspect to this issue is that for many reasons, it is expected that the tropospheric temperatures in the tropics will warm more than surface temperatures. This is called “tropospheric amplification.” According to two satellite groups, there is in fact such amplification. According to the UAH team, there is no amplification. The presence or absence of amplification is often used by some skeptics to discount the importance of global warming.
Why are there differences? Well that’s part of what this paper tries to answer.
Each of the teams try to deal with and correct for various satellite errors. For instance, sometimes the diurnal cycle effect is removed using temperatures from climate models. The RSS and NOAA teams “apply a drift correction based on the diurnal cycle from a GCM (global climate model) whereas UAH produces a microwave sounding unit mid-tropospheric temperature diurnal correction based on temperature comparisons between three co-orbiting satellites ... UAH does not yet correct the diurnal drift for satellites carrying Advanced Microwave Sounding Units because they attempt to use these satellites during periods when the diurnal drift is small.”
The present paper presents a calibration scheme that allowed them to obtain a diurnal correction from the satellite measurements themselves, in particular, by solving for a common diurnal cycle correction using temperatures from all available satellites.
As the authors state in the paper, their new results agree with the two groups that show more warming. They disagree with UAH. As the authors state,
In general, our trends corrected with a GCM and trends corrected with our observationally derived diurnal cycle correction are similar to trends from NOAA and RSS ... the UAH ocean trend is notably lower than trends from the other datasets.
So, how do the trends compare? Well the lowest trend, in degrees Celsius heating per decade are from UAH and they equal 0.029 for the 1979–2012 period for the mid-troposphere region between 20° South and 20° North. The new results are almost 4 times higher at 0.114°C per decade. The results using a diurnal correction from a climate model are in close agreement with the new findings (0.124°C per decade). As additional support, the NOAA and RSS values are also close to the corrected results. The simple fact is, UAH is an outlier.
They also discovered that the results from RSS, NOAA, and the new study all show tropical amplification and are in agreement with the expected amplification from climate models. They state, “There is no significant discrepancy between observations and models for lapse rate change between the surface and the full troposphere.”
To summarize the amplification factor, the new study obtains a value of 1.4. If the diurnal cycle is eliminated using climate models, the result is 1.49. According to NOAA and RSS, the values are 1.31 and 1.10, respectively. Again, UAH is the outlier with an amplification factor of 0.56.
I wrote to Stephen Po-Chedley and asked for a summary. He told me,
Very interesting. I hope other groups set out to replicate the results. If it holds up it will definitely help advance the science, but will also be the second time a major flaw in Spencer's work is caught by others.
Actually, I think there have been several major corrections made in the UAH dataset that were the result of outside examination - there's a list of them here. Accounting for orbital decay, diurnal corrections, errors in the tropics, etc.
My bad. If these results hold then it will be another major error in the list of UAH errors that have been corrected by outside observers. Something that skeptics should approve of, undoubtedly.
One looking at the Trend Calculator will wonder if the "RSS" and "UAH" values are swapped in this discussion ???
Something does not make sense. Also there is Spencer's own discussion of RSS problems. The modeled atmosphere profile is not clearly discussed, so people often believe that the "satellite" data is less fallible than other measures. Resulting satellite derived temperature values all seem to read low over time compared to surface measurements.
This is a very valuable start, but it is only a start.
bjchip @4, the satellite data comes in various channels, each of which measures data from different altitudes, as shown below. As you can see, the instruments cannot pick out a given altitude uniquely, but absorb radiation from a broad range of altitudes, but more strongly at some altitudes than others:
The channel used for the study, the TMT channel, is the lowest actual instrumental record in the satelite data. The TLT channel is an artificial channel made by deriving data from the TMT and TTS (?) channels, and using the later to eliminate temperature information from above the tropopause in the former. Both UAH and RSS use different methods to do this. The SKS trend calculator uses the TLT channel data. The paper, in contrast, compared the TMT data. In doing so they avoided confounding their study with differences introduced by the different methods used by the two groups in calculating the TLT channel.
Sure, accounting for diurnal drift is important but is there any account for Dr. Roy Spencer's belief that "Earth and its ecosystems—created by God’s intelligent design and infinite power and sustained by His faithful providence —are robust, resilient, self-regulating, and self-correcting, admirably suited for human flourishing, and displaying His glory. Earth’s climate system is no exception. Recent global warming is one of many natural cycles of warming and cooling in geologic history"? (Spencer, Roy W. (signatory and advisor), An Evangelical Declaration on Global Warming, Cornwall Alliance, May 1, 2009) This could certainly influence analysis of satellite topospheric temperature data.
bjchip, the data discussed in the paper by Stephen Po-Chedley et al. is only looking at the narrow tropical band, 20 degrees north and south of the equator. The SkS calcualtor is looking at global coverage (or at least most of the globe).
Roy Spencer also happens to be a "young earth" creationist who believes that the earth is only 6,000 years old. In essence, he doesn't believe in geologic history.
rocketeer - it would seem the Cornwall Alliance theology conveniently ignores anything concerning divine consequences for human greed and disrespectful destruction of divine creation.
villabolo - Roy Spencer is not a "young earth creationist". I believe that he is a proponent of "intelligent design" however. (eg see here )
If UAH (0.139C/dec) underestimates global warming, then RSS (0.122C/dec) underestimates it by even more.
Thanks to both of you (Tom and Ian). I understand the issue now .
Still, there is apparently a different problem with RSS that has been publicized and even discussed by Spencer... and the Trend calculator is going to leave people wondering about this (if they are using it as intended). Since I "knew" that RSS was giving me lower readings than UAH based on the calculator and that it had a problem I also read with that bias making it harder to see the forest... too many trees.
How very interesting and informative.
Please note that the datasets being compared here are for TMT, middle troposphere, and are NOT the TLT lower troposphere data generally used as a proxy for surface temperatures.
The issue identified here thus has no effect on surface temperature trends as measured from satellite (where UAH's trend is higher than RSS's trend). It does, however, affect the tropical troposheric "hot spot" at 10 km, which Dr. Roy has implied (based in part on his UAH TMT data) does not exist.