Patterns of evidence that have accumulated in recent decades allow researchers to distinguish between natural and human-caused climate change, leading to today’s scientific belief that the world is warming at an unprecedented rate, according to a climate scientist who spoke last month at Livermore’s Bankhead Theater.
The patterns are often called “climate fingerprints” an analogy to fingerprints at a crime scene, according to the scientist, Dr. Ben Santer of Lawrence Livermore National laboratory.
Scientists have found that “different things that can influence climate have different impacts on the climate system…like each one of us has slightly different whorls and ridges on our fingers,” he said in his talk, which was part of the Rae Dorough Speaker Series.
Santer is one of the best-known climate scientists in the U.S. He is a member of the National Academy of Sciences and has won a number of scientific awards for his work.
He has also been the subject of personal attacks and threats going back to 1995, when he played a leading role in an international consensus report that concluded that “the balance of evidence shows a discernible human influence on global climate.”
The claim was modest by the standards of today, when evidence points overwhelmingly to a dominant human effect on climate, but in 1995 the attacks that followed were anything but modest.
Some in Congress suggested that Santer should be fired. The Wall Street Journal ran letters denouncing him and refused to publish letters from his professional colleagues supporting him. One critic called for his prosecution at the World Court.
Among the lessons he learned in those days, he said at the Bankhead, was that, “If you’re a scientist, you have to defend the work that you do.”
A Long History
The scientific effort to explain why and how the Earth’s atmosphere retains much of its heat rather than losing it all to space goes back to the first decades of the 19th century, Santer said.
He cited the work of Joseph Fourier in France, Svante Arrhenius in Sweden and John Tyndall in England as helping to establish a scientific explanation of how atmospheric gases like carbon dioxide trap heat.
In recent decades, there has been good “broadbrush” agreement between the expected effects of the warming climate and the actual changes observed on Earth, he said.
As examples, he mentioned the disappearance of sea ice, “particularly in the Arctic”; more moisture in a warmer atmosphere; declining volume of glaciers and ice sheets; and increases in the heat content of the uppermost ocean.
These are strongly suggestive but do not rise to the standard of statistical proof that climate change is human-caused, he said. Santer’s own work depends on rigorous statistical analyses.
Also lending evidence to the picture of a warming climate are the so-called “proxy” studies of natural features that preserve contemporary atmospheric conditions like temperature, moisture and carbon dioxide content.
Scientists sample and study ice cores, tree rings and geologic sediments to generate thousands of years of paleoclimate history to compare and contrast with modern climate trends.
For example, Santer showed a slide portraying some 800,000 years of carbon dioxide concentrations inferred from proxy samples. During this time, the greenhouse gas fluctuated in the atmosphere at levels of roughly 180 to 280 parts per million.
In the past two centuries, he said, carbon dioxide concentrations “have moved into uncharted territory…where we have not been in at least the last million years.”
He had just returned from a carbon dioxide monitoring station on the Canary Islands. Even there, out in the Atlantic Ocean and far from pollution sources, evidence of human influence on the climate was clear.
“The sobering reality for all of us is that we are now well over 400 parts per million,” he said.
The predicted range by 2100 is 550 to 900 parts per million “depending on what mix of energy we use in the 21st century.”
The climate is affected by both natural and human factors, Santer said. “Climate change is not an either-or proposition, all human or all-natural. It’s both.”
Volcanic eruptions and changes in the sun’s output are examples of natural events that influenced climate. Cutting down forests and burning fossil fuels that generate greenhouse gases are examples of human ones.
These and many other influences occur and mix in an ever-changing atmosphere that is made more complex by what Santer called a “zoo” of regularly occurring cycles that may have different causes and last for years, decades or longer.
These have names like South Atlantic El Niño oscillation and Atlantic multi-decadal oscillation natural internal; they also include cycles in solar output and in the Earth’s orbit.
Climate fingerprinting tries to separate and identify the different effects “in a rigorous statistical way,” Santer said.
“That’s one of the things we [climate scientists] do – we study these natural oscillations of climate. These sorts of things happened before we were on the planet and will continue to happen for millennia to come.”
The precision and thoroughness of the effort has improved dramatically over the past four decades with satellite based sensors that can monitor the atmosphere around the clock.
For example, landmark predictions by Manabe and Wetherald at Princeton a decade prior to the satellite era suggested that atmospheric warming patterns would be identifiably different if caused by the accumulation of greenhouse gases than if caused by increased heat output from the sun.
Specifically, Santer said, if warming were caused by the sun, the entire atmosphere would warm, whereas if greenhouse gases were responsible, the lower atmosphere would warm while the upper atmosphere would cool.
The predicted layered heating could not be tested in the absence of satellites and weather balloons rugged enough to rise into the stratosphere.
Once they were available, the prediction was found to be correct and today is regarded as part of the scientific refutation of claims that Earth is warming because of increased solar output.
“We and others have done this kind of fingerprinting not only with temperature but with atmospheric circulation, with ocean heat content, with the amount of water vapor in the atmosphere, with rainfall, with many many different things,” Santer said.
“What we found is that there is this internal and physical consistency to the story the climate system is telling us, and the story is that natural causes can’t explain the changes being independently measured and monitored for decades now.
“Only human influence can do that.”