March 13, 2017 at 3:07 PM
The controversy continues over Environmental Protection Agency Administrator Scott Pruitt's statement Thursday questioning the most important scientific conclusion about climate change — namely, that it is driven by human beings.
"I think that measuring with precision human activity on the climate is something very challenging to do, and there's tremendous disagreement about the degree of impact, so no, I would not agree that it's a primary contributor to the global warming that we see," Pruitt said on CNBC — triggering a wide range of rebukes in the process.
On Monday, the American Meteorological Society, a key scientific organization whose members have considerable expertise in studying the climate and weather, wrote a letter to Pruitt strongly critiquing the remarks.
"The world's 7 billion people are causing climate to change and our emissions of carbon dioxide and other greenhouse gases are the primary cause," wrote the group's executive director, Keith Seitter. "This is a conclusion based on the comprehensive assessment of scientific evidence. It is based on multiple independent lines of evidence that have been affirmed by thousands of independent scientists and numerous scientific institutions around the world. We are not familiar with any scientific institution with relevant subject matter expertise that has reached a different conclusion."
The Environmental Protection Agency did not immediately respond to a request for comment on the American Meteorological Society's letter.
Not long afterward, a group of 30 U.S. scientists who share an expertise in climate change also wrote to Pruitt, with a similar message. As they put it:
… human beings are changing the Earth's climate. This key conclusion follows from the basic laws of physics. Just as there is no escaping gravity when one steps off a cliff, there is no escaping the warming that follows when we add extra carbon dioxide and other greenhouse gases to the atmosphere.
The signatories included Nobel Laureate Mario Molina, Princeton climate scientist Michael Oppenheimer, the National Center for Atmospheric Research's Kevin Trenberth and the Carnegie Institution's Ken Caldeira, among many others.
Climate scientists on Twitter have also taken it upon themselves to set the record straight:
Essentially all warming from ~1950 is human caused, & CO2 rise is 70 percent of total forcing in that period. So, 70 percent. Precise enough? https://t.co/M52rcpe6XD
— Gavin Schmidt (@ClimateOfGavin) March 13, 2017
So why are scientists so upset about Pruitt's words?
In brief: Scientists are confident that human beings are driving global warming because that is the explanation for the current temperature trends that we see — the planet is already about 1 degree Celsius warmer than it was in preindustrial times — that they consider to be best supported by the preponderance of the evidence.
That evidence includes a physical understanding of how carbon dioxide levels in the atmosphere affect the planet's temperature; evidence from past climates (in which carbon dioxide has served as a key planetary knob controlling temperature); and a stark record of ever rising concentrations of the atmospheric gas that have been predictably accompanied by rising temperatures.
The conclusion is also supported by the lack of any other major factor — such as changes to the sun, the activity of volcanoes, or natural cycles like El Nino — that appears capable of explaining changes to the Earth's temperature. Finally, it is backed by the results of multiple computer simulations that represent scientists' most sophisticated mathematical understandings of how the Earth's system — binding together the atmosphere, the oceans and land surfaces — works.
Accordingly, the U.N.'s Intergovernmental Panel on Climate Change — the top scientific authority on the matter, though it is often rated as rather conservative by scientists — has steadily increased the level of confidence it places in this central conclusion.
In 1995, the IPCC merely said that "the balance of evidence suggests that there is a discernible human influence on global climate." By 2001, though, the group had concluded it was "likely" that humans were driving global warming. By 2007, the IPCC said it was "very likely." By 2013, it was saying it was "extremely likely."
There continue to be think tanks and advocates who say otherwise — but their case becomes harder and harder to make. No wonder, then, that Pruitt's remarks triggered such a strong response.
"We understand and accept that individuals and institutions both public and private can reach differing conclusions on the decisions and actions to be taken in the face of this reality," the American Meteorological Society letter finished. "That's the nature of the political process in a democratic society. But mischaracterizing the science is not the best starting point for a constructive dialogue."
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