Climate change: an explanation of the evidence

Op-ed: An explanation of climate change and why it really is man made.

EPA Administrator Scott Pruitt unleashed a furor this month when he said: "I would not agree that [carbon dioxide] is a primary contributor to the global warming that we see. We don't know that yet, we need to continue the debate and the review and analysis."

This is essentially the same answer Mr. Pruitt gave in his confirmation hearing, one that was echoed by Rick Perry, Rex Tillerson and Ryan Zinke during their confirmation hearings as secretaries of Energy, State and the Interior. It's clearly an orchestrated strategy: Agree that climate change is not a hoax — thus avoiding the stigma associated with denying what is increasingly obvious — but cast doubt about the causes in order to avoid addressing the problem. If global warming might be natural, why would we disrupt our fossil fuel-economy?

In fairness, President Trump's Cabinet is not alone. According to a Pew Research Center poll, only 48 percent of Americans believe that global climate change is mostly due to human activity. More surprising, only 28 percent know that almost all climate scientists say that climate change is human-caused based on the massive amount of evidence. But you should not be expected to take the experts' word for it blindly. On something so important you deserve an explanation of this evidence, one that a non-scientist can understand. Let me try.

Warming of the atmosphere and the oceans at the surface of Earth during about the last 100 years is an unequivocal fact. 2016 was the warmest year in the instrumental record, and 16 of the 17 warmest years have occurred since 2000. It has been more than 100,000 years since the planet has been this warm. The melting of glaciers and polar ice sheets, shrinking of sea-ice coverage, increasing heat content of the oceans, and sea-level rise are further evidence of the warming.

It is also a fact that the concentrations of several naturally occurring gases have increased in the atmosphere beginning in the 19th century: carbon dioxide by 44 percent, methane (the dominant component of natural gas) by 150 percent, and nitrous oxide (the laughing gas your dentist uses to ease your anxiety) by 21 percent. These are called greenhouse gases because, unlike the nitrogen and oxygen comprising most of the atmosphere, they absorb and emit infrared radiation from the sun, keeping that energy from returning to space and thus warming the lower atmosphere. Scientists first discovered this greenhouse effect in 1824 and by 1896 had predicted the planet would warm as emissions of carbon dioxide grew. So, the global warming we have observed is totally unsurprising. Carbon dioxide concentrations now exceed 400 parts per million, the highest levels in four million years, and the added carbon has the chemical fingerprint of long-buried fossil fuels.

Why are scientists so cocksure that the increase in greenhouse gases is the primary contributor to global warming? Aren't there also natural causes that could be causing warming, such as the planetary adjustments that initiate or end ice ages, or maybe the cause is increased intensity of the sun?

Earth's climate has been in a relatively warm state for the past several thousand years following the last ice age, allowing human civilization to flourish. During the 19th century there was actually a slow cooling trend, perhaps indicating the planet was heading to another ice age in a few thousand years. But, the warming over the past decades has come to an already warm world and at a faster rate and in the opposite direction than typical of this phase of the planetary cycle. Also, measured solar radiation has actually declined slightly since the 1980s, producing a cooling, not a warming, influence. At the same time that the lower atmosphere has been warming, the lower stratosphere above it has been cooling — a clear sign that the insulating greenhouse effect is responsible for these changes.

So, how much of the observed global warming is due to human greenhouse gas emissions? An international team of scientists brought the evidence together and produced a best estimate of 110 percent, with carbon dioxide itself responsible for 70 of the warming. How could the overall effect be more than 100 percent? Well, humans have also had some cooling influences, such as by increasing fine particles in the atmosphere and cutting down dark forests. In aggregate, the human effects on global warming have been 170 times greater than the natural forces.

Sorry, Administrator Pruitt, it isn't even a close call. And this information and more is still available on EPA's website.

Donald Boesch is president of the University of Maryland Center for Environmental Science; his email is boesch@umces.edu.

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