The Committee on Science, Space & Technology of the US House of Representatives conducts regular evidence hearings on various science topics. On Wednesday 29th March, there is a hearing on “Climate science: assumptions, policy implications, and the scientific method”. The following letter, summarising the scientific findings of Fyfe et al. (2016) and Karl et al. (2015), has been submitted as evidence to this hearing.
The broader context is that the Committee Chairman, Mr. Lamar Smith, has previously discussed the findings of Fyfe et al. (of which I was a co-author), claiming: “A new peer-reviewed study, published in the journal Nature, confirms the halt in global warming”. This statement is incorrect, and motivated the clarification on what Fyfe et al. actually says.
Dear Mr. Smith,
We are coauthors of the Fyfe et al. paper published in 2016 in Nature Climate Change . You recently referenced this paper at a Subcommittee hearing on March 16, 2016 . We are writing to clarify what the Fyfe et al. paper actually finds and claims. We also want to ensure that the conclusions of the Fyfe et al. paper are not misconstrued as a criticism of Thomas Karl, of the Karl et al. paper published in Science in 2015 , or of the valuable research that Dr. Karl and his team have performed over many years.
Thomas Karl is a first-rate climate scientist. He served NOAA with distinction for decades. Dr. Karl and his colleagues at the National Center for Environmental Information (NCEI) developed rigorous scientific methods for estimating global changes in land and ocean surface temperatures. This is a critically important area of climate science.
NCEI made its surface temperature data sets freely available to the climate science community. This helped scientists around the world to conduct research on the size, rate, and causes of long-term temperature change, and helped to improve our knowledge of natural climate variability. NCEI temperature data are also key yardsticks for evaluating the performance of computer models of the climate system.
Science is dynamic, not static. All surface temperature data sets have evolved over time, as scientists found better ways of accounting for the effects of changes in measurement systems, measuring practices, and the geographical coverage of observations. Similar improvements have occurred in measurements of the heat content of the world’s oceans, and in satellite estimates [4,5,6] of temperature change in Earth’s atmosphere. The evolution of observed temperature data sets is a normal, on-going scientific process. It is not evidence of questionable behavior.
In their 2015 Science paper, Karl et al. identified changes in three different aspects of surface temperature measurement systems. These observing system changes must be addressed in order to reliably estimate the true, climate-related temperature signals in the data. After accounting for the evolution of the measuring system, Karl et al. concluded that the rate of surface warming in the first 15 years of the 21st century was “at least as great as (in) the last half of the 20th century”.
Fyfe et al. acknowledged the “high scientific value” of the work performed by Dr. Thomas Karl and his colleagues. We stand by our statement. It is of great benefit to understand how observational temperature data are affected by changing measurement systems. Karl et al. deserve credit for focusing attention on this issue, and for inspiring important research on the further improvement of surface temperature datasets .
While Karl et al. focused on developing a better understanding of temperature observations, Fyfe et al. summarized and synthesized scientific understanding of decadal changes in warming arising from natural variability of the climate system. The emphasis in the Fyfe et al. paper was on studying internal variability (caused by phenomena like El Niños, La Niñas , and the Interdecadal Pacific Oscillation [9-12]) and on assessing the effects of natural external changes in volcanic aerosols  and the Sun’s energy output .
Fyfe et al. found that the rate of temperature increase in the early 21st century was slower than during the latter part of the 20th century. Reduced warming was apparent in both surface observations and in satellite measurements of the temperature of Earth’s lower atmosphere (the troposphere).
The bottom line is that Karl et al. and Fyfe et al. reached different conclusions regarding the warming rate in the early 21st century. This was largely due to different justifiable choices the two sets of authors made about the timescales and periods of interest. The Karl et al. finding – that the recent rate of surface warming is larger than in previous data sets – is supported by an independent study of surface temperature measurements . Other sources of information support the Fyfe et al. finding of a reduced rate of surface warming in the early 21st century. These sources include independent satellite estimates of tropospheric temperature change, physical understanding of the waxing and waning of different “modes” of internal variability, and measurements of the changes over time in volcanic aerosols and the Sun.
All of the factors studied by Karl et al. and Fyfe et al. (changing observing systems, internal variability, and natural variations in the Sun and volcanoes) affect temperature records, and affect our interpretation of the size and significance of decade-to-decade changes in warming rate. The scientific challenge is to reliably quantify the contribution of each factor to short-term changes in warming rate .
Finally, we would like to emphasize that Karl et al. and Fyfe et al. agree on the most important scientific points. We agree that human influence on climate is real, is large, and is ongoing. We agree that this influence is primarily due to fossil fuel burning, and to the resulting human-caused changes in atmospheric levels of heat-trapping greenhouse gases . We agree that human-caused changes in greenhouse gases should lead – and do lead – to global-scale warming of Earth’s atmosphere, oceans, and land surface . We agree that we have identified large global warming signals in the observed surface temperature changes from the late 19th century to the present , in the satellite atmospheric temperature data that have featured prominently in recent Congressional hearings [19,20], and in ocean heat content measurements .
And we agree with Karl et al. that on top of the underlying global-scale warming trend over the past 150 years, we should see – and do see – natural, decade-to-decade ups and downs caused by internal variability, volcanic activity, and changes in the Sun’s energy output. These decade-to-decade fluctuations in warming are not a scientific surprise. They have been discussed at length in every national and international assessment of climate science. Sometimes the “ups” act in the same direction as human influences, leading to accelerated warming. Sometimes the “downs” lead to a short-term decrease in warming. Our disagreement with Karl et al. about the size of the most recent short-term fluctuation does not call into question the reality of long-term human-caused warming.
Ben Santer, Matthew England, Ed Hawkins, Michael Mann, Gerald Meehl, Yu Kosaka, Shang-Ping Xie
The co-authors of the Fyfe et al. paper, who are Canadian Government scientists (John Fyfe, Greg Flato, Nathan Gillett & Neil Swart), felt that it would not be appropriate for them to communicate directly to elected officials in the U.S. pursuing an inquiry. However, they did write a supporting note to Ben Santer affirming their scientific support for the statements made in the letter written by himself and the other co-authors of the Fyfe et al. paper.
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 Karl, T.R., A. Arguez, B. Huang, J.H. Lawrimore, J.R. McMahon, M.J. Menne, T.C. Peterson, R.S. Vose, and H.-M. Zhang, 2015: Possible artifacts of data biases in the recent global surface warming hiatus. Science, 348, 1469-1472.
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 The basis for our understanding of human-induced changes in climate stretches back to the 1850s, when carbon dioxide was first identified as a greenhouse gas. It is not a new development. Similarly, our observational understanding of large-scale temperature change dates back to the 1930s, when it was first shown that global land areas were warming (see ref. 17).
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