Michael Mann is Distinguished Professor of Meteorology at Penn State University and was recognized in 2007, with other IPCC authors, for contributing to the award of the 2007 Nobel Peace Prize for his work as a lead author on the "Observed Climate Variability and Change"chapter of the Intergovernmental Panel on Climate Change (IPCC) Third Scientific Assessment Report. Mann contributed this article to LiveScience's Expert Voices: Op-Ed & Insights.
It happens every six years or so: The Intergovernmental Panel on Climate Change (IPCC) publishes its assessment of the current state of scientific understanding regarding human-caused climate change. That assessment is based on contributions from thousands of experts around the world through an exhaustive review of the peer-reviewed scientific literature and a rigorous, several-years-long review process. Meanwhile, in the lead-up to publication, fossil-fuel industry front groups and their paid advocates gear up to attack and malign the report, and to mislead and confuse the public about its sobering message.
So in the weeks leading up to the release of the IPCC Fifth Assessment scientific report, professional climate-change deniers and their willing abettors and enablers have done their best to distort what the report actually says about the genuine scientific evidence and the reality of the climate-change threat. [FAQ: IPCC's Upcoming Climate Change Report Explained]
This time, however, climate-change deniers seem divided in their preferred contrarian narrative. Some would have us believe that the IPCC has downgraded the strength of the evidence and the degree of threat. Career fossil-fuel-industry apologist Bjorn Lomborg, in Rupert Murdoch's "The Australian," wrote on Sept. 16: "UN's mild climate change message will be lost in alarmist translation." On the other hand, serial climate disinformer Judith Curry, in a commentary for the same outlet five days later, announced, "Consensus distorts the climate picture."
So, make up your mind, critics: Is it a "mild message" or a "distorted picture?" Consistency, they might well respond, is simply the "hobgoblin of little minds" after all — but in reality, that's only if you ignore the foolishness.
Indeed, claims that members of the IPCC have downgraded their scientific confidence have been plentiful among the usual purveyors of climate-change misinformation: Fox News, the editorial pages of the Wall Street Journal and various conservative tabloids in the United States, Canada, Germany and Australia. Fox News even sought to mislead its viewers with a bait and switch, focusing attention instead on a deceptive, similarly named report that calls itself the Nongovernmental International Panel on Climate Change (NIPCC), which simply regurgitates standard shopworn denialist myths and erroneous talking points. That non-peer-reviewed report was published by the discredited industry front group known as the Heartland Institute in the lead-up to the publication of the actual IPCC report, presumably to divert attention from the actual scientific evidence.
In reality, the IPCC has strengthened the degree of certainty that fossil-fuel burning and other human activities are responsible for the warming of the globe seen over the past half century, raising their confidence from "very likely" in the previous report to "extremely likely" in the current one. The IPCC expresses similar levels of certainty that the Earth is experiencing the impacts of that warming in the form of melting ice, rising global sea levels and various forms of extreme weather. [Climate Scientists: IPCC Report Must Communicate Consensus]
What about the converse claim, promoted by critics, that the IPCC has exaggerated the evidence?
Well, if anything, the opposite appears closer to the truth. In many respects, the IPCC has been overly conservative in its assessment of the science. The new report, for example, slightly reduces the lower end of the estimated uncertainty range for a quantity know as the equilibrium climate sensitivity — the amount of warming scientists expect in response to a doubling of carbon dioxide (CO2) concentrations relative to preindustrial levels (concentrations that will be seen mid-century, given business-as-usual emissions).
The IPCC reports a likely range of 1.5 to 4.5 degrees Celsius (roughly 3 to 8 degrees Fahrenheit) for this quantity, the lower end having been dropped from 2.0 degrees C in the fourth IPCC assessment. The lowering is based on one narrow line of evidence: the slowing of surface warming during the past decade.
Yet there are numerous explanations of the slowing of warming (unaccounted for effects of volcanic eruptions and natural variability in the amount of heat buried in the ocean) that do not imply a lower sensitivity of the climate to greenhouse gases. Moreover, other lines of evidence contradict an equilibrium climate sensitivity lower than 2 degrees C. It is incompatible, for example, with paleoclimate evidence from the past ice age, or the conditions that prevailed during the time of the dinosaurs. (See this piece I co-authored earlier this year for the Australian Broadcasting Corp. for a more detailed discussion of the matter.)
The IPCC's treatment of global sea-level rise is similarly conservative — arguably, overly so. The report gives an upper limit of roughly 1 meter (3 feet) of sea-level rise by the end of the century under business-as-usual carbon emissions. However, there is credible peer-reviewed scientific work, based on so-called "semi-empirical" approaches that predict nearly twice that amount — i.e., nearly 6 feet (2 m) of global sea-level rise this century. These latter approaches are given short thrift in the new IPCC report; instead, the authors of the relevant chapter favor dynamical modeling approaches that have their own potential shortcomings (underestimating, for example, the potential contribution of ice-sheet melting to sea-level rise this century).
As some readers may know, the conclusion that modern warming is unique in a long-term context came to prominence with the temperature reconstruction that my co-authors and I published in the late 1990s. The resulting "Hockey Stick" curve, which demonstrates that the modern warming spike is without precedent for at least the past 1,000 years, took on iconic significance when it was prominently displayed in the "Summary for Policy Makers" of the 2001 Third IPCC Assessment report. Thus, the "Hockey Stick" curve, as I describe in my recent book, "The Hockey Stick and the Climate Wars," became a focal point of the attacks by industry-funded climate-change deniers.
So, it might not come as a surprise that one of the most egregious misrepresentations of the IPCC's latest report involves the Hockey Stick and conclusions about the uniqueness of modern warming. [4 Things to Know About the IPCC's Climate Change Report]
An urban legend seems to be circulating around the echo chamber of climate-change denial, including contrarian blogs and fringe right-wing news sites. The claim is that the IPCC has "dropped" or "trashed" the Hockey Stick conclusion regarding the unprecedented nature of recent warmth.
A good rule of thumb is that the more insistent climate-change deniers are about any particular talking point, the greater the likelihood is that the opposite of what they are claiming actually holds. The IPCC has, in fact, actually strengthened its conclusions regarding the exceptional nature of modern warmth in the new report. A highlighted box in the "Summary for Policy Makers" states the following (emphasis mine):
In the northern Hemisphere, the period 1983-2012 was likely the warmest 30-year period of the last 1400 years (medium confidence).
The original 1999 Hockey Stick study (and the 2001 Third IPCC Assessment report) concluded that recent Northern Hemisphere average warmth was likely unprecedented for only the past 1,000 years. The 2007 IPCC Fourth Assessment extended that conclusion back further, over the past 1,300 years (and it raised the confidence to "very likely" for the past 400 years). The new, Fifth IPCC Assessment has now extended the conclusion back over the past 1,400 years. By any honest reading, the IPCC has thus now substantially strengthened and extended the original 1999 Hockey Stick conclusions.
Only in the "up is down, black is white" bizarro world of climate-change denial could one pretend that the IPCC has failed to confirm the original Hockey Stick conclusions, let alone contradict them. [How Words Affect Climate Change Perception]
The stronger conclusions in the new IPCC report result from the fact that there is now a veritable hockey league of reconstructions that not only confirm, but extend, the original Hockey Stick conclusions. This recent RealClimate piece summarizes some of the relevant recent work in this area, including a study published by the international PAGES 2k team in the journal Nature Geoscience just months ago. This team of 78 regional experts from more than 60 institutions representing 24 countries, working with the most extensive paleoclimate data set yet, produced the most comprehensive Northern Hemisphere temperature reconstruction to date. One would be hard-pressed, however, to distinguish their new series from the decade-and-a-half-old Hockey Stick reconstruction of Mann, Bradley and Hughes.
Conclusions about unprecedented recent warmth apply to the average temperature over the Northern Hemisphere. Individual regions typically depart substantially from the average. Thus, while most regions were cooler than present during the medieval era, some were as warm, or potentially even warmer, than the late-20th-century average. These regional anomalies result from changes in atmospheric wind patterns associated with phenomena such as El Niño and the so-called North Atlantic Oscillation. [U.S. Will Warm Dramatically By 2084, NASA Model Shows (Video)]
Colleagues and I, quoting from the abstract of our own article in the journal Science a few years ago (emphasis mine), stated:
Global temperatures are known to have varied over the past 1,500 years, but the spatial patterns have remained poorly defined. We used a global climate proxy network to reconstruct surface-temperature patterns over this interval. The medieval period [A.D. 950-1250] is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally.
These conclusions from our own recent work are accurately represented by the associated discussion in the "Summary for Policy Makers" of the new IPCC report (emphasis mine):
Continental-scale surface-temperature reconstructions show, with high confidence, multidecadal periods during the Medieval Climate Anomaly (year 950-1250) that were, in some regions, as warm as in the late 20th century. These regional warm periods did not occur as coherently across regions as the warming in the late 20th century (high confidence).
However, never underestimate the inventiveness of climate-change deniers. Where there's a will, there is, indeed, a way: A meme now circulating throughout the denialosphere is that the IPCC's conclusions about regional warmth contradict our findings, despite the fact that those conclusions are substantially based on our findings.
One could be excused for wondering if climate-change deniers have lost all sense of irony.
The most egregious example of this latest contortion of logic found its way into the purportedly "mainstream" Daily Mail, courtesy of columnist David Rose, who admittedly has a bit of a reputation for misrepresenting climate scientists and climate science. Rose wrote in his column on Sep. 14, "As recently as October 2012, in an earlier draft of this report, the IPCC was adamant that the world is warmer than at any time for at least 1,300 years. Their new inclusion of the 'Medieval Warm Period' — long before the Industrial Revolution and its associated fossil-fuel burning — is a concession that its earlier statement is highly questionable."
The most charitable interpretation is that Rose simply didn't actually read or even skim the final draft of the report, despite writing about it at length. For, if he had, he would be aware that the final draft of the report comes to the strongest conclusion yet about the unprecedented nature of recent warmth, extending the original Hockey Stick conclusion farther back than ever before — to the last 1,400 years.
Moreover, he would be aware that the existence of regional medieval warmth rivaling that of the late 20th century does not contradict that conclusion — indeed, it is the regional heterogeneity of that warmth, as established in ours and other studies, that leads the IPCC report to conclude that current levels of hemispheric average warmth are unprecedented for at least 1,400 years.
The lesson here, perhaps, is that no misrepresentation or smear is too egregious for professional climate-change deniers. No doubt, we will continue to see misdirection, cherry-picking, half truths and outright falsehoods from them in the months ahead as the various IPCC working groups report their conclusions.
Don't be fooled by the smoke and mirrors and the Rube Goldberg contraptions. The true take-home message of the latest IPCC report is crystal clear: Climate change is real and caused by humans, and it continues unabated. We will see far more dangerous and potentially irreversible impacts in the decades ahead if we do not choose to reduce global carbon emissions. There has never been a greater urgency to act than there is now.
The latest IPCC report is simply an exclamation mark on that already-clear conclusion.
Mann is author of two books, "The Hockey Stick and the Climate Wars: Dispatches from the Front Lines" (Columbia University Press, 2012), which will soon to be available in paperback with an update and a new guest foreword by Bill Nye "The Science Guy", and "Dire Predictions: Understanding Global Warming" (DK Publishing, 2008). You can follow him on Twitter: @MichaelEMann. The viesws expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on LiveScience.
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Michael E. Mann is presidential distinguished professor and director of the Center for Science, Sustainability and the Media at the University of Pennsylvania. He is author of the new book "Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis." (PublicAffairs, 2023) and "The New Climate War: The Fight to Take Back Our Planet" (PublicAffairs, 2021).