Ben Santer, WaPo: I’m a climate scientist. And I’m not letting trickle-down ignorance win.

How to fight the Trump administration's darkness

Fact Checkers Glenn Kessler and Michelle Lee examine several of President Trump's claims from his speech announcing the U.S. withdrawal from the Paris climate accord on Thursday. (Video: Meg Kelly/Photo: Jabin Botsford/The Washington Post)

by Ben Santer, The Washington Post, July 5, 2017

I’ve been a mountaineer for most of my life. Mountains are in my blood. In my early 20s, while climbing in France, I fell into a crevasse on the Milieu Glacier, at the start of the normal route on the Aiguille d’Argentiere. Remarkably, I was unhurt. From the grip of the banded ice, I saw a thin slit of blue sky 120 feet above me. The math was simple: Climb 120 feet. If I reached that slit of blue sky, I would live. If I didn’t, I’d freeze to death in the cold and dark.

Now, more than 40 years later, it feels like I’m in a different kind of darkness — the darkness of the Trump administration’s scientific ignorance. This is just as real as the darkness of the Milieu Glacier’s interior and just as life-threatening. This time, I’m not alone. The consequences of this ignorance affect every person on the planet.

Imagine, if you will, that you spend your entire professional life trying to do one thing to the best of your ability. In my case, that one thing is to study the nature and causes of climate change. You put in a long apprenticeship. You spend years learning about the climate system, computer models of climate and climate observations. You start filling a tool kit with the statistical and mathematical methods you’ll need for analyzing complex data sets. You are taught how electrical engineers detect signals embedded in noisy data. You apply those engineering insights to the detection of a human-caused warming signal buried in the natural “noise” of Earth’s climate. Eventually, you learn that human activities are warming Earth’s surface, and you publish this finding in peer-reviewed literature.

You participate in rigorous national and international assessments of climate science. You try to put aside all personal filters, to be objective, to accommodate a diversity of scientific opinions held by your peers, by industry stakeholders, and by governments. These assessments are like nothing you’ve ever done before: They are peer review on steroids, eating up years of your life.

The bottom-line finding of the assessments is cautious at first. In 1995, the conclusion is this: “The balance of evidence suggests a discernible human influence on global climate.” These 12 words are part of a chapter on which you are first author. The 12 words change your life. You spend years defending the “discernible human influence” conclusion. You encounter valid scientific criticism. You also encounter nonscientific criticism from powerful forces of unreason, who harbor no personal animus toward you but don’t like what you’ve learned and published — it’s bad for their business.

Your peers are your fiercest critics. They are constantly kicking the tires. Show us that your “discernible human influence” results aren’t due to changes in the sun, or volcanic activity, or internal cycles in the climate system. Show us that your results aren’t due to some combination of these natural factors. Convince us that detection of a human fingerprint isn’t sensitive to uncertainties in models, data, or the statistical methods in your tool kit. Explain the causes of each and every wiggle in temperature records. Respond to every claim contradicting your findings.

So you jump through hoops. You do due diligence. You go down every blind alley, every rabbit hole. Over time, the evidence for a discernible human influence on global climate becomes overwhelming. The evidence is internally and physically consistent. It’s in climate measurements made from the ground, from weather balloons, and from space — measurements of dozens of different climate variables made by hundreds of different research groups around the world. You write more papers, examine more uncertainties, and participate in more scientific assessments. You tell others what you’ve done, what you’ve learned, and what the climatic “shape of things to come” might look like if we do nothing to reduce emissions of heat-trapping greenhouse gases. You speak not only to your scientific peers but also to a wide variety of audiences, some of which are skeptical about you and everything you do. You enter the public arena and make yourself accountable.

After decades of seeking to advance scientific understanding, reality suddenly shifts, and you are back in the cold darkness of ignorance. The ignorance starts at the top, with President Trump. It starts with untruths and alternative facts. The untruth that climate change is a “hoax” engineered by the Chinese. The alternative fact that “nobody really knows” whether climate change is real. These untruths and alternative facts are repeated again and again. They serve as talking points for other members of the administration. From the Environmental Protection Agency administrator, who has spent his career fighting climate change science, we learn the alternative fact that satellite data shows “a leveling off of warming ” over the past two decades. The energy secretary tells us the fairy tale that climate change is primarily due to “ocean waters and this environment that we live in.” Ignorance trickles down from the president to members of his administration, eventually filtering into the public’s consciousness.

Getting out of this metaphorical darkness is going to be tough. The administration is powerful. It has access to media megaphones and bully pulpits. It can abrogate international climate agreements. It can weaken national legislation designed to protect our air and water. It can challenge climate science and tell us that more than three decades of scientific understanding and rigorous assessments are all worthless. It can question the integrity and motives of climate scientists. It can halt satellite missions and impair our ability to monitor Earth’s climate from space. It can shut down websites hosting real facts on the science of climate change. It can deny, delay, defund, distort, dismantle. It can fiddle while the planet burns.

I have to believe that even in this darkness, though, there is still a thin slit of blue sky. My optimism comes from a gut-level belief in the decency and intelligence of the people of this country. Most Americans have an investment in the future — in our children and grandchildren, and in the planet that is our only home. Most Americans care about these investments in the future; we want to protect them from harm. That is our prime directive. Most of us understand that to fulfill this directive, we can’t ignore the reality of a warming planet, rising seas, retreating snow and ice, and changes in the severity and frequency of droughts and floods. We can’t ignore the reality that human actions are part of the climate change problem and that human actions must be part of the solution. Ignoring reality is not a viable survival strategy.

Trump has referred to a cloud hanging over his administration. The primary cloud I see is the self-created cloud of willful ignorance on the science of climate change. That cloud is a clear and present threat to the lives, livelihoods, and health of every person on the planet, now and in the future. This cloud could be easily lifted by the president himself.

For my own part, I don’t intend to spend the rest of my life in darkness or silently accepting trickle-down ignorance. I didn’t climb out of a crevasse on the Milieu Glacier for that.

https://www.washingtonpost.com/news/posteverything/wp/2017/07/05/im-a-climate-scientist-and-im-not-letting-trickle-down-ignorance-win/

Ben Santer: Trump and the Tide of History

Today our government ceded moral, ethical, economic, and political leadership to other, more enlightened countries.

Credit: NCDOT Communications Flickr 

by Ben Santer, Scientific American, June 1, 2017

In two of the most famous lines in Shakespeare’s “Julius Caesar,” Brutus notes the importance of watching the ebb and flow of the tide of history, and acting at the right time:

"There is a tide in the affairs of men.

Which, taken at the flood, leads on to fortune;

Omitted, all the voyage of their life

Is bound in shallows and in miseries."

Today—on June 1st, 2017—President Donald J. Trump withdrew the United States from the Paris Climate Agreement. Today the United States missed the rising tide that will carry almost other countries of this world towards a more secure and sustainable future. Today, a President who ran on the promise to “Make America great again” left America diminished, isolated in the world, distrusted by our closest friends and allies. Today, the American government ignored the will of most of its own people, stepped off the world stage, and kicked the can of human-caused climate change down the road. Today our government ceded moral, ethical, economic, and political leadership to other, more enlightened countries.

Today was a sad day if you are a climate scientist, and you’ve spent most of your adult life studying the nature and causes of climate change. Today, the overwhelming scientific evidence of a human-caused global warming signal was ignored. Today, the risks of dangerous human interference in the climate system were ignored. Today, science and rationality lost; willful ignorance and narrow self-interest won.

Today was a sad day if you believe—as I do—that the United States is part of a community of nations and cannot ignore the global problems threatening all of us. It was a sad day if you believe that it’s unfair to burden future generations with dangerous climate change they did nothing to incur. It was a sad day for Americans who believe—as I do—that U.S. jobs will not be created by turning back the clock and embracing antiquated energy-production sources. We cannot make coal great again; nor can we make the horse and buggy great again.  

This day is significant. A century from now, future historians will look at today’s decision by President Trump. They will study how this decision changed the arc of history, and changed the trajectory of earth’s climate system. They will see who stood for a sustainable climate future, and who stood for short-term profit and political self-interest. The future historians will know how it all turned out—whether things ended poorly for much of humanity, or whether the U.S. “Parexit” decision was a temporary aberration, and quickly led to more enlightened U.S. leadership.

Our President owns casinos, hotels, and golf courses. He does not own our atmosphere and our oceans. Those are part of the global commons. The health of that global commons concerns all of us. With today’s decision to leave the Paris Climate Agreement, the President has seriously jeopardized the health of the atmosphere and oceans we share with other citizens of this planet. Tomorrow begins the hard work of ensuring that the United States is not “bound in shallows and in miseries,” rejoins the family of nations, and is lifted on the rising tide of a sustainable energy and climate future.     

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

https://blogs.scientificamerican.com/observations/trump-and-the-tide-of-history/

WaPo: Scientists just published an entire study refuting Scott Pruitt on climate change

Environmental Protection Agency Administrator Scott Pruitt speaks with coal miners in Sycamore, Pa., in April. (Justin Merriman/Getty Images)

by Chris Mooney, The Washington Post, May 24, 2017

In a sign of growing tensions between scientists and the Trump administration, researchers published a scientific paper Wednesday that was conceived and written as an explicit refutation to an assertion by Environmental Protection Agency Administrator Scott Pruitt about climate change.

The study, in the journal Nature Scientific Reports, sets up a direct test of a claim by Pruitt, made in written Senate comments following his confirmation hearing, that “over the past two decades satellite data indicates there has been a leveling off of warming.”

After reviewing temperature trends contained in three satellite data sets going back to 1979, the paper concludes that the data sets show a global warming trend — and that Pruitt was incorrect.

“Satellite temperature measurements do not support the claim of a ‘leveling off of warming’ over the past two decades,” write the authors, led by Benjamin Santer of Lawrence Livermore National Laboratory. Santer co-authored the study with three Livermore colleagues and scientists from MIT, the University of Washington in Seattle, and Remote Sensing Systems, which keeps one of the three satellite temperature data sets.

“In my opinion, when incorrect science is elevated to the level of formal congressional testimony and makes its way into the official congressional record, climate scientists have some responsibility to test specific claims that were made, determine whether those claims are correct or not, and publish their results,” said Santer in an interview, when asked about the framing of the research.

The study wades into an ongoing and highly fraught debate over how to interpret the temperature records of the planet’s lower atmosphere, or troposphere, provided by polar orbiting satellites.

Such data have often been cited by climate change doubters so as to suggest that there is no global warming trend, or that global warming has recently slowed down, and therefore to contradict thermometer-based measurements taken at the planet’s surface (which show a clear warming trend).

But the new study finds that all of the three satellite data sets — kept by Remote Sensing Systems, the Center for Satellite Applications and Research at the National Oceanic and Atmospheric Administration, and the University of Alabama at Huntsville — show a long-term warming trend in the middle-to-upper part of the troposphere. After correcting for a cooling-down of the stratosphere (the layer above the troposphere), the paper finds that the trend is roughly 0.36 degrees Fahrenheit per decade for the first two data sets, and 0.26 degrees Fahrenheit per decade for the third.

The study further examined whether any shorter temperature trend in these data sets could be described as a “leveling off,” as Pruitt had put it. It did so by examining 20-year periods in the data sets and comparing those with the predictions of climate simulations that reflected the natural variations of the climate but excluded human-caused greenhouse gas emissions. These models were thus meant to represent what the climate would do on its own if humans were not altering it.

The study finds warming trends for all the 20-year periods, including the “last two decades” referred to by Pruitt, although it acknowledges that the trend is somewhat lower over these later periods. But it attributes this to natural climate variations, including a very strong El Nino event in 1997 and 1998 that caused dramatic warmth around the beginning of the 20-year window that ends in the present.

Even in these periods that saw somewhat less warming, the study finds that it was still far more warming than would be without human perturbations of the climate. “The probability that internal variability could produce warming exceeding that observed over the last 20 years is only 1.6 %, 3.1 %, and 6.3% (respectively)” in the three data sets, the authors find.

“Pruitt is not correct in saying that warming has leveled off,” Santer said. “It hasn’t in any of the satellite data sets, and indeed, in older and newer versions of the three satellite data sets, we judge the most recent warming to be statistically significant — to be larger than the warming that our current model-based estimates tells us that we should see due to internal variability alone.”

The EPA did not immediately respond to a request for comment.

“Another solid piece of work by Santer et al. that demonstrates multi-decadal satellite-derived global tropospheric temperatures are increasing far more than we would expect from natural causes,” said Thomas Karl, a longtime climate researcher who formerly headed NOAA’s National Centers for Environmental Information.  “Other satellite instruments, which measure temperatures closer to where we live, work, and grow our food show at least as much, or more warming, in recent decades.”

Gavin Schmidt, who heads the Goddard Institute for Space Studies at NASA, said by email that when it comes to measurements of the Earth’s troposphere by satellite, “the trends over the whole period are clear.”

“This doesn’t however imply that (a) there aren’t still issues with the satellite retrievals (there may well be), and (b) that models did a perfect job over this time period,” Schmidt cautioned.

John Christy, a researcher at the University of Alabama at Huntsville who keeps that data set and whose work has been often cited by climate change “skeptics,” agreed there is a warming trend in the satellite data overall but said that climate models predict that it should be larger. “The datasets are still significantly cooler than the model average,” he said by email.

Christy also argued that the other two data sets, which are warmer than his, are “outliers regarding the magnitude.”

“I wouldn’t get too excited about this study,” Christy said.

But it is not as though a scientific study refuting one of his statements to the Senate holds much risk for Pruitt, said Sarah Binder, a senior fellow at the Brookings Institution and a political scientist at George Washington University.

“It’s significant in the sense that it shows the limits of the confirmation process, especially when the president’s party controls the Senate and senators can no longer filibuster nominees. In other words, it’s possible to float factually inaccurate statements and yet not ding your chances of confirmation,” Binder said. “Of course, the climate change issue is highly partisan: Republicans tend to disagree with a general scientific consensus that the earth is warming. So the idea that a Republican EPA nominee might give [a] factually contested statement on climate change and not pay a price is not terribly surprising.”

In the end, Santer argued, scientists should fact-check politicians even if they’re at a disadvantage when it comes to how long it takes to do so.

“These claims were made in the U.S. Senate, in a confirmation,” said Santer. “It takes time however to set the record straight, to do due diligence, to do the research necessary to address the claims. And one would hope that the scientific response receives at least some token amount of attention, and that the original incorrect claim does not dominate the public discourse on these critically important issues.”

https://www.washingtonpost.com/news/energy-environment/wp/2017/05/24/scientists-just-published-an-entire-study-refuting-scott-pruitt-on-climate-change/

Peter Sinclair video: Paris Climate Agreement - A Good Start

by Peter Sinclair, Yale Climate Communications, February 10, 2016

I was at the American Geophysical Union Fall Meeting in San Francisco when news of the Paris agreement came through, so had the chance to collect fresh reactions from some of the best climate experts on the planet.

Generally, hopeful reactions, mixed with a warning.  And, Jeff Goodell of Rolling Stone on the difference between “should” and “shall.”

Link:  http://climatecrocks.com/2016/02/10/new-vid-scientists-on-the-paris-agreement/

Ben Santer & Carl Mears: A Response to Ted Cruz's “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), Skeptical Science, January 17, 2016

[PDF available here:  http://skepticalscience.com/docs/Santer_Mears_Data_Dogma.pdf]

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” (ref. 1). 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 [a] 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 (ref. 2). 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 (refs. 2–4)[b]. Converting this information to estimates of temperature trends has substantial uncertainties [c]. 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 (ref. 5) and orbital drift (refs. 6–8) 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 [d], and involve many subjective decisions (refs. 2–4, 6–8). Further adjustments to the raw data are necessary for drifts in the on-board calibration of the microwave measurements (refs. 9, 10), and for the transition between earlier and more sophisticated versions of the MSUs [e].

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 (refs. 2–10). 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 (refs. 2–15). 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 (refs. 2, 3, 5–10).

Such corrective work is ongoing. Satellite estimates of atmospheric temperature change are still a work in progress (refs. 2, 3, 8), and the range of estimates produced by different groups remains large.[f] The same is true of weather balloon atmospheric temperature measurements (refs. 2, 11–13, 15–17)[g]. Surface thermometer records also have well-studied uncertainties (refs. 2, 19, 20), but the estimated surface warming of roughly 0.9 °C since 1880 has been independently confirmed by multiple research groups (refs. 2, 15, 19, 20).

The hearing also failed to do justice to the complex issue of how to interpret differences between observed and model-simulated tropospheric warming over the last 18 years. Senator Cruz offered only one possible interpretation of these differences – the existence of large, fundamental errors in model physics (refs. 2, 21). In addition to this possibility, there are at least three other plausible explanations for the warming rate differences shown in Exhibit A: errors in the human (refs. 22–25), volcanic (refs. 26–30), and solar influences (refs. 24, 31) used as input to the model simulations; errors in the observations (discussed above) (refs. 2–20); and different sequences of internal climate variability in the simulations and observations (refs. 23, 24, 30, 32–36). We refer to these four explanations as “model physics errors,” “model input errors,” “observational errors,” and “different variability sequences.” They are not mutually exclusive. There is hard scientific evidence that all four of these factors are in play (refs. 2–20, 22–36).

“Model input errors” and “different variability sequences” require a little further explanation. Let’s assume that some higher extraterrestrial intelligence provided humanity with two valuable gifts: a perfect climate model, which captured all of the important physics in the real-world climate system, and a perfect observing system, which reliably measured atmospheric temperature changes over the last 18 years. Even with such benign alien intervention, temperature trends in the perfect model and perfect observations would diverge if there were errors in the inputs to the model simulations [h], or if the purely random sequences of internal climate oscillations did not “line up” in the simulations and in reality (refs. 23, 24, 30, 32–36).

In short, “all models are too sensitive to CO₂” is not the only valid explanation [i] for the model–data differences in Exhibit A (refs. 2, 11, 13, 18, 22–24, 26, 30, 32–38). Dozens of peer-reviewed scientific studies show that the other three explanations presented here (“model input errors,” “observational errors,” and “different variability sequences”) are the primary reasons for most or all of the warming rate differences in Exhibit A [j].  

But what if climate models really were a factor of three or more too sensitive to human-caused GHG increases, as claimed by the majority side of the subcommittee? The telltale signatures of such a serious climate sensitivity error would be evident in many different comparisons with observations, and not just over the last 18 years. We’d expect to see the imprint of this large error in comparisons with observed surface temperature changes over the 20th century (refs. 37–42), and in comparisons with the observed cooling after large volcanic eruptions (refs. 30, 43, 44). We don’t. There are many cases where observed changes are actually larger than the model expectations (refs. 41, 42), not smaller.

In assessing climate change and its causes, examining one individual 18-year period is poor statistical practice, and of limited usefulness. Analysts would not look at the record of stock trading on a particular day to gain reliable insights into long-term structural changes in the Dow Jones index. Looking at behavior over decades – or at the statistics of trading on all individual days – provides far greater diagnostic power. In the same way, climate scientists study changes over decades or longer (refs. 39–42, 45), or examine all possible trends of a particular length (refs. 23, 38, 46–48). Both strategies reduce the impact of large, year-to-year natural climate variability [k] on trend estimates. The message from this body of work? Don’t cherry-pick; look at all the evidence, not just the carefully selected evidence that supports a particular point of view.

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 data sets 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 (refs. 41, 42, 48) – as do many other independent observations of changes in temperature, the hydrological cycle, atmospheric circulation, and the cryosphere (refs. 41, 42).

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.”

Climate change is a serious issue, demanding serious attention from our elected representatives in Washington. The American public deserves no less.  

Acknowledgments

We gratefully acknowledge the comments and valuable suggestions from Professor Susan Solomon (M.I.T.) and Dr. Mike MacCracken (The Climate Institute). 

Footnotes

1. Prof. John Christy from the University of Alabama at Huntsville.
2. MSU estimates of the temperature of tropospheric layers also receive a small contribution from the temperature at Earth’s surface.
3. This conversion process relies on an atmospheric radiation model to invert the observations of outgoing, temperature-dependent microwave emissions from oxygen molecules. Since oxygen molecules are present at all altitudes, the microwave flux that reaches the satellite is an integral of emissions from thick layers of the atmosphere.  
4. At the end of the hearing, Senator Cruz questioned the reliability of thermometer measurements of land and ocean surface temperature and highlighted the large adjustments to “raw” surface temperature measurements (adjustments which are necessary because of such factors as changes over time in thermometers and measurement practices). He did not mention that the surface temperature adjustments are typically much smaller than the adjustments to “raw” MSU data (refs. 2, 3, 8).
5. This transition occurred in 1998, at the beginning of the 18-year “no significant warming” period highlighted by Senator Cruz.
6. For example, over the longer 1979–2014 analysis period, tropospheric warming is a robust feature in all observational TMT datasets. For shorter, noisier periods (such as 1996–2014), the sign of the TMT trend is sensitive to dataset construction uncertainties.
7. Disappointingly, Exhibit A neglects to show at least one weather balloon temperature data set with substantial tropospheric warming over the last 18 years (18).
8. Such as leaving out volcanic cooling influences that the real world experienced (refs. 23, 24, 26–30).
9. The model results shown in Exhibit A are from so-called “historical climate change” simulations. These simulations involve changes in a number of different human and natural influences (e.g., human-caused changes in GHG levels and particulate pollution, and natural changes in solar and volcanic activity). They are not simulations with changes in GHG levels only, so it is incorrect to interpret the model-versus-observed differences in Exhibit A solely in terms of model sensitivity to GHG increases.   
10. Another incorrect claim made at the hearing was that the mainstream scientific community had failed to show the kind of model–data comparisons presented in Exhibit A. Results similar to those in Exhibit A have been presented in many other peer-reviewed publications (refs. 2, 13, 18, 23, 24, 30, 32, 35, 38, 46, 47).
11. Such as the variability associated with unusually large El Niño and La Niña events, which yield unusually warm or cool global-mean temperatures, respectively. The El Niño event during the winter of 1997 and spring of 1998 was likely the largest of the 20th century and produced a large warming “spike” in surface and tropospheric temperatures.

References

1. https://www.commerce.senate.gov/public/index.cfm/2015/12/data-or-dogma-promoting-open-inquiry-in-the-debate-over-the-magnitude-of-human-impact-on-earth-s-climate.
2. T. R. Karl, S. J. Hassol, C. D. Miller, and W. L. Murray (Eds.): Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC, USA, 164 pp. (2006).
3. C. Mears, F. J. Wentz, P. Thorne, and D. Bernie, Assessing uncertainty in estimates of atmospheric temperature changes from MSU and AMSU using a Monte-Carlo technique, J. Geophys. Res., 116, D08112, doi: 10.1029/2010JD014954 (2011).
4. J. R. Christy, W.  B. Norris, R.W. Spencer, and J. J. Hnilo, Tropospheric temperature change since 1979 from tropical radiosonde and satellite measurements. J. Geophys. Res., 112, D06102, doi: 10.1029/2005JD006881 (2007).
5. F. J. Wentz and M. Schabel, 1998: Effects of orbital decay on satellite-derived lower-tropospheric temperature trends. Nature, 394, 661 (1998).
6. C. A. Mears and F. W. Wentz, The effect of diurnal correction on satellite-derived lower tropospheric temperature. Science, 309, 1548 (2005).
7. C.-Z. Zou et al., Recalibration of microwave sounding unit for climate studies using simultaneous nadir overpasses. J. Geophys. Res., 111, D19114, doi: 10.1029/2005JD006798 (2006).
8. S. Po-Chedley, T. J. Thorsen and Q. Fu, Removing diurnal cycle contamination in satellite-derived tropospheric temperatures: Understanding tropical tropospheric trend discrepancies. J. Clim., 28, 2274 (2015).
9. C. A. Mears, M. C. Schabel, and F. W. Wentz, A reanalysis of the MSU channel 2 tropospheric temperature record. J. Clim., 16, 3650 (2003).
10. S. Po-Chedley and Q. Fu, A bias in the mid-tropospheric channel warm target factor on the NOAA-9 Microwave Sounding Unit. J. Atmos. Oceanic Technol., 29, 646 (2012).
11. P. W. Thorne, J. R. Lanzante, T. C. Peterson, and D. J. Seidel, K.P. Shine KP, Tropospheric temperature trends: History of an ongoing controversy. Wiley Inter. Rev., 2, 66 (2011).
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