Global Science Report: Another Indication of Lukewarming

In March 1990, NASA’s Roy Spencer and University of Alabama-Huntsville’s (UAH) John Christy dropped quite a bomb when they published the first record of lower atmospheric temperatures sensed by satellites' microwave sounding units (MSUs). While they only had ten years of data, it was crystal clear there was no significant warming trend.

It was subsequently discovered by Frank Wentz of Remote Sensing Systems (RSS), a Santa Rosa (CA) consultancy, that the orbits of the sensing satellites successively decay (i.e., become lower) and this results in a spurious but slight cooling trend. Using a record ending in 1995, Wentz showed a slight warming trend of 0.07⁰C/decade, about half of what was being observed by surface thermometers. 

In 1994, Christy and another UAH scientist, Richard McNider, attempted to remove “natural” climate change from the satellite data by backing out El Niño/La Niña fluctuations and the cooling associated with two big volcanoes in 1983 and 1991. They arrived at a warming trend of 0.09⁰C/decade after their removal.

Over the years, Spencer and Christy slightly revised their record repeatedly, and its latest iteration shows a total warming trend of 0.13⁰C/decade, which includes natural variability. But it is noteworthy that this is biased upward by very warm readings near the end of the record, thanks to the 2015–16 El Niño.

Recently, Christy and McNider carried out a similar analysis to what they did in 1994 and found removing the volcanoes and natural sea surface temperature changes resulted in a warming trend nominally the same as their 1994 finding, at 0.10⁰C/decade—far, far beneath the 0.2–0.3⁰C/decade predicted for the current era by the models in the latest (2013) report of the UN’s Intergovernmental Panel on Climate Change.

Much as Christy and McNider said in 1994, it appears that the sensitivity of temperature to carbon dioxide changes in those models is just too high.

Here’s the illustration at the heart of the paper:

Because the print is so small in the figure legend, we’ll paraphrase it here. The top plot (red) is the temperature of the lower troposphere (“TLT”), from the surface to about eight kilometers in altitude.  The blue plot is the “natural” sea surface temperature (SST) component, now a combination of El Niño and other known oscillations, such as the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). The middle black plot is the raw satellite data minus the oceanic oscillations, and the bottom one adjusts that for the two big volcanoes in 1983 and 1992. 

The new Christy and McNider paper also calculates the “transient sensitivity” of temperature to increasing carbon dioxide. The transient sensitivity is the temperature change observed at the time that atmospheric carbon dioxide doubles from its preindustrial background. Given observed rates of increase, this should occur sometime around 2070. The sensitivity works out to 1.1⁰C, which is slightly below half of the average transient sensitivity of all the climate models in the latest (2013) report of the UN’s Intergovernmental Panel on Climate Change.

This is another indication that if business-as-usual continues, including a continued transition from coal to natural gas for electrical generation, the world will easily meet the Paris Accord target of total anthropogenerated warming of less than 2.0⁰C by the year 2100.

Note that this is based on the satellite-sensed lower atmospheric temperatures. Our next post will compare them to the reanalysis data described in our last Global Science Report.