How certain are scientists that climate change is caused by human activity? As the Friday release of a report from the Intergovernmental Panel on Climate Change suggests: as certain as they are that cigarettes cause cancer. So why do we keep smoking?

The report, the fifth issued by the body over the course of several decades, offers new depth and a new level of certainty, but of the kind you might have when you're checking to see if you have your wallet. You tap your pocket, it's there, but you give a peek anyway just to make sure. We knew climate change was happening as the result of human activity, as we have since the first report in 1990. But we keep taking more detailed peeks, with more and more evidence. Photos of the wallet in your pocket, sworn testimony from 18 Jesuit priests affirming it's there, wearing pants with see-through pockets. That's the stage we're at now on climate change. One reporter for the Associated Press described the existing certainty as follows:

Top scientists from a variety of fields say they are about as certain that global warming is a real, man-made threat as they are that cigarettes kill.

They are as sure about climate change as they are about the age of the universe. They say they are more certain about climate change than they are that vitamins make you healthy or that dioxin in Superfund sites is dangerous.

So how is America responding to this new report? Here is a good example, picking on CNN's enthusiastic "New Day" program. (To be fair, it's on-air coverage was better.)

First of all, this isn't "new evidence." Quite the opposite: it's a collation of the existing scientific data, reviewed and compiled by other experts. It is old evidence, the World Book encyclopedia of the evidence that's out there.

But the aggravating part of the tweet is the first part. "Do you believe global warming is a man-made problem?" the show asks, as we do with regularity. "Scientists who studied for years and hew to rigorous methodology and systematically identifying and eliminating doubt before making assumptions have determined with enormous certainty that climate change is real. Fine — but do you agree, morning television viewer?" If you're curious how they respond, take a look at the replies to that tweet.

That response has been cultivated, carefully and deliberately for years, by those who would rather that the world not undertake the steps needed to address climate change — reducing the production and burning of fossil fuels like coal and oil. And it is here that the analogy between climate change and smoking grows ever stronger.

Do you believe smoking causes cancer? Of course you do. It has been demonstrated repeatedly. We don't poll on that because it is established science. But it took decades to get to that point, thanks to tobacco companies.

The University of California at San Francisco has an archive of documents from tobacco companies and advocacy organizations spanning the middle of the 20th century — a period in which science had proven that smoking caused cancer, but the companies that sold cigarettes still denied it. Compare these two documents, one from an industry advocacy group in July 1960, the other from a tobacco company in March 1961.


Tobacco companies knew that cigarettes killed. They discussed how to reduce the cancer-causing agents, even as they and their allies did their best to assure the public that there was nothing to fear. The priority was the fiscal bottom-line. Once the truth came out, smoking declined and restrictions were put in place — and public health improved. Now, that's assumed fact.

What's really amazing once you extend that analogy to climate change, though, is that fossil fuel companies acknowledge their role in climate change publicly. Here, for example, ExxonMobil's statement on the issue. "Our strategy to reduce greenhouse gas emissions is focused on increasing energy efficiency in the short term, implementing proven emission-reducing technologies in the near and medium term, and developing breakthrough, game-changing technologies for the long term." That's how ExxonMobil wants to reduce climate change. This is like the company that produced the second document above sending out a mailer explaining how it would reduce the cancer-causing agents in its cigarettes and fund anti-smoking campaigns. Granted, Exxon keeps selling gasoline (and isn't eager to stop or raise its prices) but at least it admits it.

Despite science and fossil fuel companies agreeing that climate change 1) is a problem 2) caused by fossil fuel consumption that 3) needs to be addressed, the public — and Congress — is obstinate about inaction. Earlier this year, we looked at the trend in polls asking about "belief" in climate change over time. Here's what the trend in responses looks like since 2008. Asked if climate change is happening, the blue trend line is people who say "yes;" the red line, "no."

That article considered opinion in the aftermath of Hurricane Sandy, a storm whose worst effects — the deaths and massive power outage — were linked directly to climate change-fueled sea level rise. But because the winter was pretty cold, people's "belief" in the fact of climate change had waned. And without broad public pressure, Congress hasn't acted.

As part of its report, the IPCC created a "summary for policymakers," recognizing that it was those who could create the carbon markets or emissions-reduction policies that it needed to convince at this point. Again, tobacco is instructive: people still smoke, but we've improved public health by substantially reducing the number of smokers. Stop checking your pockets. Your wallet is right there in your pocket, we promise. Now do something.

Photo: A man cleaning his Sandy-damaged home stops for a cigarette break. (AP)