Monday, January 18, 2016

A Response to the “Data or Dogma?” Hearing

By Benjamin D. Santer, Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, CA.
and Carl Mears, Remote Sensing Systems, Santa Rosa, CA.

Senator Ted Cruz opens the hearing. (Credit: Click to Enlarge.
On December 8, 2015, Senator Ted Cruz – the chairman of the Senate subcommittee on Space, Science, and Competitiveness –convened a hearing entitled “Data or Dogma?”  The stated purpose of this event was to promote “…open inquiry in the debate over the magnitude of human impact on Earth’s climate”.  In the course of the hearing, the chairman and several expert witnesses claimed that satellite temperature data falsify both “apocalyptic models” and findings of human effects on climate by “alarmist” scientists.  Such accusations are serious but baseless.  The hearing was more political theatrics than a deep dive into climate science.  

Satellite-derived temperature data were a key item of evidence at the hearing.  One of the witnesses for the majority side of the Senate subcommittee showed the changes (over roughly the last 35 years) in satellite- and weather balloon-based measurements of the temperature of the mid-troposphere (TMT), a layer of the atmosphere extending from the Earth’s surface to roughly 18 km.  Satellite TMT measurements are available from late 1978 to present.  Observed TMT data were compared with TMT estimates from a large number of model simulations.  This comparison was ‘Exhibit A’ for the majority side of the subcommittee.

Senator Cruz used Exhibit A as the underpinning for the following chain of arguments:  1) Satellite TMT data do not show any significant warming over the last 18 years, and are more reliable than temperature measurements at Earth’s surface; 2) The apparent “pause” in tropospheric warming is independently corroborated by weather balloon temperatures; 3) Climate models show pronounced TMT increases over the “pause” period; and 4) The mismatch between modeled and observed tropospheric warming in the early 21st century has only one possible explanation – computer models are a factor of three too sensitive to human-caused changes in greenhouse gases (GHGs).  Based on this chain of reasoning, Senator Cruz concluded that satellite data falsify all climate models, that the planet is not warming, and that humans do not impact climate.

This logic is wrong.  First, satellites do not provide direct measurements of atmospheric temperature: they are not thermometers in space.  The satellite TMT data plotted in Exhibit A were obtained from so-called Microwave Sounding Units (MSUs), which measure the microwave emissions of oxygen molecules from broad atmospheric layers. Converting this information to estimates of temperature trends has substantial uncertainties.  The major uncertainties arise because the satellite TMT record is based on measurements made by over 10 different satellites, most of which experience orbital decay and orbital drift over their lifetimes.  These orbital changes affect the measurements of microwave emissions, primarily due to gradual shifts in the time of day at which measurements are made.  As the scientific literature clearly documents, the adjustments for such shifts in measurement time are large, and involve many subjective decisions.  Further adjustments to the raw data are necessary for drifts in the on-board calibration of the microwave measurements, and for the transition between earlier and more sophisticated versions of the MSUs.

In navigating through this large labyrinth of necessary adjustments to the raw data, different plausible adjustment choices lead to a wide range of satellite TMT trends.  This uncertainty has been extensively studied in the scientific literature, but was completely ignored in the discussion of Exhibit A by Senator Cruz and by witnesses for the majority side of the subcommittee.  The majority side was also silent on the history of satellite temperature datasets.  For example there was no mention of the fact that one group’s analysis of satellite temperature data – an analysis indicating cooling of the global troposphere – was repeatedly found to be incorrect by other research groups.
In summary, the finding that human activities have had a discernible influence on global climate is not falsified by the supposedly “hard data” in Senator Cruz’s Exhibit A.  The satellite data and weather balloon temperatures are not nearly as “hard” as they were portrayed in the hearing.  Nor is a very large model error in the climate sensitivity to human-caused GHG increases the only or the most plausible explanation for the warming rate differences in Exhibit A.  Indeed, when the observational temperature datasets in Exhibit A are examined over their full record lengths – and not just over the last 18 years – they provide strong, consistent scientific evidence of human effects on climate.  So do many other independent observations of changes in temperature, the hydrological cycle, atmospheric circulation, and the cryosphere.

Climate policy should be formulated on the basis of both the best-available scientific information and the best-possible analysis and interpretation.  Sadly, neither was on display at the Senate hearing on “Data or Dogma?”  There was no attempt to provide an accurate assessment of uncertainties in satellite data, or to give a complete and balanced analysis of the reasons for short-term differences between modeled and observed warming rates.  Political theater trumped true “open inquiry”.

Read more at A Response to the “Data or Dogma?” Hearing

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