Study: no acceleration in global warming, climate sensitivity to CO2 too high

New research yields old result: Climate warming slow, steady. Observed value is half that of CMIP5 climate models.

HUNTSVILLE, Ala. (Nov. 29, 2017) — The rate at which Earth’s atmosphere is warming has not significantly accelerated over the past 23 years, according to research at The University of Alabama in Huntsville (UAH).

If you take away the transient cooling in 1983 and 1992 caused by two major volcanic eruptions in the preceding years, the remaining underlying warming trend in the bottom eight kilometers (almost five miles) of the atmosphere was 0.096 C (about 0.17° Fahrenheit) per decade between January 1979 and June 2017.

That was unexpectedly close to the 0.09 C warming trend found when similar research was published in 1994 with only 15 years of data, said Dr. John Christy, director of UAH’s Earth System Science Center.

This work might also indirectly affirm recent research showing the atmosphere is less sensitive to the warming effects of rising levels of carbon dioxide and other greenhouse gases than global climate models have suggested.

“We indicated 23 years ago — in our 1994 Nature article — that climate models had the atmosphere’s sensitivity to CO2 much too high,” said Christy, the lead author in the study, which has been accepted for publication in the 2017 fourth quarter edition of the Asia- Pacific Journal of Atmospheric Sciences and is available online. “This recent paper bolsters that conclusion.”

Mathematically removing the natural but transient climatic effects of volcanoes and El Niño/La Niña Pacific Ocean warming and cooling events leaves an underlying climate trend, all or some part of which might be attributed to human causes — including enhanced greenhouse forcing caused by rising levels of CO2 and other manmade greenhouse gases in the atmosphere.

At present, however, there is no accepted tool or technique for confidently estimating how much of the warming in the past 38+ years might be due to natural causes.

“For the purposes of this research, we assumed the climate was stable during that time, that the natural climate trend would have been zero,” Christy said. “If the natural trend was zero, then the climate models say the atmosphere is more than twice as sensitive to CO2 as the data might suggest.

“Of course, if the natural trend was greater than zero — if the natural climate was warming even a little bit — then the models have the atmospheric sensitivity to CO2 even further out of whack than that.”

The paper also describes a new index for determining the sensitivity of the climate system to extra greenhouse gases. Previously, indexes of this kind were based on the surface temperature.

Christy and UAH’s Dr. Richard McNider created a new index: the Tropospheric Transient Climate Response, which is based on the bulk atmosphere. That is a more representative quantity for any impact of increased greenhouse gases.

“The idea behind this index is to determine what the temperature increase will be by the decade when anthropogenic greenhouse gas forcing — which is dominated by CO2 — doubles what it was in about 1880,” Christy said. “We should reach that level — about 560 ppm of CO2 — in the latter half of this century.

“From our observations we calculated that value as 1.1 C (almost 2° Fahrenheit), while climate models estimate that value as 2.3 C (about 4.1° F),” Christy said. “Again, this indicates the real atmosphere is less sensitive to CO2 than what has been forecast by climate models. This suggests the climate models need to be retooled to better reflect conditions in the actual climate, while policies based on previous climate model output and predictions might need to be reconsidered.”

In their research, which was supported by the U.S. Department of Energy, Christy and McNider found the climatic effects of El Niño/La Niña warming and cooling events in the eastern equatorial Pacific Ocean largely cancelled each other out over the study period.

That left the El Chichon and Pinatubo volcanic eruptions in 1982 and 1991 as the remaining major natural perturbations to the climate trend, although that had as much to do with the timing of the eruptions as it did with the cooling caused by the nearly global distribution of volcanic ash in the upper atmosphere.

“Those eruptions happened relatively early in our study period, which pushed down temperatures in the first part of the dataset, which caused the overall record to show an exaggerated warming trend,” Christy said. “While volcanic eruptions are natural events, it was the timing of these that had such a noticeable effect on the trend. If the same eruptions had happened near the more recent end of the dataset, they could have pushed the overall trend into negative numbers, or a long-term cooling.

“By taking them out of the equation, we leave behind only that part of the climate influenced by nature’s long-term changes and human influences.”

Other researchers have tried to calculate the climate’s sensitivity using temperature data collected at the Earth’s surface. But that data lacks complete global coverage, especially over the oceans. Changes in the character of the land surface near thermometers (such as paving and urban growth) and changes in the thermometer instruments over time also add uncertainty to the data.

“Additionally, surface temperatures used for tracking climate change use the average of daily maximum and minimum temperatures,” said McNider, a distinguished professor emeritus at UAH. “Those minimum nighttime temperatures reflect only the temperature of a shallow layer of air near the surface and not temperatures in the deep layer of the atmosphere.”

Using satellite instruments to collect temperature data from the bulk atmosphere is a better and more robust tool for detecting the addition of energy related to extra greenhouse gases in the atmosphere, Christy said.

The unadjusted climate trend in the deep troposphere from January 1979 to June 2017 was +0.155 C (about 0.279° F) per decade.¹ After adjusting for the volcanoes and other less significant effects, including the Atlantic multi-decadal oscillation and the Pacific decadal oscillation, the trend drops to 0.096 C per decade — or about 0.364 C (0.66° F) total since December 1978.

Christy and McNider suggest two other possible explanations for the discrepancies between climate model forecasts and reality:

• The transfer of heat energy between the atmosphere and the ocean isn’t well understood, including the roles of wind, currents and ocean conditions. If more heat is transferred to the oceans than is accounted for by the models, that “is a negative atmospheric feedback, at least on shorter time scales.”
• Heat the models suggest should be staying in the atmosphere might instead be expelled more readily through the atmosphere into space, or is being more rapidly mixed into the oceans. In either case, that heat would not be available for warming the atmosphere.

“Also, if the atmosphere isn’t accumulating heat at the rate forecast by the models, then the theoretical positive climate feedbacks which were expected to amplify the CO2 effect won’t be as large,” McNider said. “For instance, one of the major climate feedbacks built into the models is increased water vapor. It was hypothesized that if CO2 warmed the atmosphere, the amount of water vapor — itself a powerful greenhouse gas — in the atmosphere should increase.

“The water feedback built into the models, however, depends first on warming in the deep layer of the atmosphere,” he said. “The lack of warming there means this feedback will be much less.”

As part of an ongoing joint project between UAH, NOAA and NASA, Christy and Dr. Roy Spencer, an ESSC principal scientist, use data gathered by advanced microwave sounding units on NOAA and NASA satellites to get accurate temperature readings for almost all regions of the Earth. This includes remote desert, ocean and rain forest areas where reliable climate data are not otherwise available.

Since the first microwave sounding unit was launched into orbit in November 1978, satellite-based instruments have measured the temperature of the atmosphere from the surface up to an altitude of about eight kilometers above sea level.

This is an especially important region of the atmosphere because climate models have forecast the deep layer of the lower atmosphere is the area where CO2-influenced warming should occur first and by the greatest amounts.

Once the monthly temperature data are collected and processed, they are archived for immediate access by atmospheric scientists in the U.S. and abroad.

The complete version 6 lower troposphere dataset is available here: http://www.nsstc.uah.edu/data/msu/v6.0/tlt/uahncdc_lt_6.0.txt

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(1) The 0.155 C per decade trend reported here differs from the 0.13 C per decade trend recently reported in the Global Temperature Report, which is published monthly by UAH’s Earth System Science Center. The research reported here was done using an earlier version of the satellite microwave sounding unit dataset. That dataset was revised and updated, and the revisions published (Spencer et al., APJAS 2017) while the research reported here was under peer review.

The new 2017 paper: 2017_Christy_McNider (PDF)

The 1994 paper: 1994_ChristyMcNIder