Scientists declared 2016 the warmest year on record last week, mostly blaming man-made global warming. It was the third straight record-breaking year.

A majority of Americans — 61 percent — say they are “very” or “somewhat” worried about global warming, according to a national survey conducted by the Yale Program on Climate Change Communication shortly after the presidential election in November.

But the same Yale program found last year that about 1 in 5 Americans was doubtful or dismissive of global warming and tended to oppose climate action.

President Donald Trump’s nominee for secretary of state, Exxon Mobil CEO Rex Tillerson, expressed doubt during testimony before a Senate committee this month about greenhouse-gas predictions. Trump has called global warming a “Chinese hoax.”

As for consensus, studies show that more than 97 percent of scientists agree that humans are responsible for ongoing climate change.

So how do experts convince doubters? The Dispatch asked a group of scientists to share their most-compelling evidence. Here are their responses:

Michael Mann, Department of Meteorology, Penn State University

In response to the very popular denialist talking point that "climate changes all the time," if I had to boil it down to one point, it would be this:

Yes, natural factors influence climate. But natural factors, such as volcanoes and changing solar intensity, have actually been nudging the climate in the direction of slight cooling over the past half century. It is only the human factor — i.e., the burning of fossil fuels and resulting increase in the “greenhouse effect”— that can explain the warming.

Glenn Branch, deputy director of the National Center for Science Education

I have to confess to a degree of skepticism about the idea that there's a silver bullet in countering climate-change denial. This is in part due to the variety of forms that climate-change denial takes. All that said, there is something that is close to a one-size-fits-all argument to counter climate-change denial.

The evidence here is not scientific but more sociological.

And it is that the overwhelming consensus — over 97 percent — of the relevant scientific community is that recent climate change is a real phenomenon driven by humans. That evidence is supported by multiple studies conducted by different researchers in different ways ... but the results are the same. The evidence supporting human-caused global warming has produced a clear scientific consensus.

Lonnie Thompson, Department of Earth Sciences, Byrd Polar Research Center, Ohio State University

Our research concentrates on the record in the:

• Ice and the loss of glaciers globally

• Unprecedented carbon-dioxide levels in our atmosphere as documented by the 800,000-year history archived in the bubbles of trapped air in ice cores

• Sea-level rise

• Global ocean warming

• Increase in the extreme events

• Increases globally in losses from global insurance companies

• The fact that the only way we can explain the changes observed in climate over the last 50 years is by adding the human influences

• 20th century extinctions

• The spread of tropical diseases to higher latitudes and altitudes

Probably the most convincing argument is of all the scientists who have studied climate change over the years, no one has yet come up with a better scientific explanation for what we are observing in the real world.

Benjamin Santer, Model Diagnosis and Intercomparison Program, Lawrence Livermore National Laboratory

In my opinion, the most convincing piece of evidence for a "discernible human influence" on global climate is the very distinctive pattern of warming of the lower atmosphere (the troposphere) and cooling of the upper atmosphere (the stratosphere).

We know of no natural causes that can produce such changes. This pattern of tropospheric warming and stratospheric cooling is a clear 'fingerprint' of human-caused changes in greenhouse gases … and it was predicted 50 years ago by Suki Manabe and his colleagues at the Geophysical Fluid Dynamics Lab in Princeton using only very sparse temperature measurements from weather balloons.

Subsequent satellite observations confirmed professor Manabe’s prediction.

Richard Alley, Department of Geoscience, Penn State University

You might think of the evidence for human-caused global warming/climate change from fossil-fuel CO2 as a four-legged stool, with physics, models, data and history.

Start with physics. We know of no way that it is possible to raise CO2 in the atmosphere without turning up the Earth’s thermostat — the physics that gives us such high confidence is used successfully in many other ways every day, with no serious questions. In all of these implementations, CO2 causes warming, and nobody has found any way to build a physically realistic model in which that does not occur. Turn off the warmth from CO2, and models tend to turn the Earth into a giant frozen snowball.

And this applies in history. Climate has always changed, and that has always had large impacts on living things. Many causes of climate changes exist, and we see their influence in the past. The understanding that works for recent events also works far back in the past, with more CO2 bringing more warmth.

With this four-legged stool, we can go to the public and say that human release of CO2, especially from burning fossil fuels, is changing the climate, that the changes will continue to grow if we continue to burn fossil fuels and release the CO2, and that this will have large impacts on us.

But this doesn't rest on one bit, or one idea, or one piece of evidence — it rests on the interwoven evidence of science, from fundamental understanding, implemented in models and painstakingly tested against recent data and Earth’s long history, leading to predictions that are repeatedly proving to be accurate.

mrenault@dispatch.com

@MarionRenault