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“If You See Something, Say Something,” is the headline on a Sunday Op-Ed article by Michael E. Mann, the Penn State climate scientist who, after years of attacks from groups fighting restrictions on greenhouse gases, has become a prominent climate and political campaigner, as well.

The piece appropriately defends the right of scientists to be citizens, fighting disinformation and pressing for action — a theme explored here starting with a 2008 contribution from Richard Somerville, a longtime climate scientist at the Scripps Institution of Oceanography.

This section is particularly valuable:

In fact, there is broad agreement among climate scientists not only that climate change is real (a survey and a review of the scientific literature published say about 97 percent agree), but that we must respond to the dangers of a warming planet. If one is looking for real differences among mainstream scientists, they can be found on two fronts: the precise implications of those higher temperatures, and which technologies and policies offer the best solution to reducing, on a global scale, the emission of greenhouse gases. For example, should we go full-bore on nuclear power? Invest in and deploy renewable energy — wind, solar and geothermal — on a huge scale? Price carbon emissions through cap-and-trade legislation or by imposing a carbon tax? Until the public fully understands the danger of our present trajectory, those debates are likely to continue to founder.

There’s a troubling section, however, in which Mann creates a flawed dichotomy, hailing a paper by James Hansen and Jeffrey Sachs of Columbia University (and others) pressing for deep carbon cuts and criticizing a peer,* Ken Caldeira of the Carnegie Institution, for complaining that the paper failed the Stephen Schneider / Gavin Schmidt test for distinguishing between the “is” of science and the “ought” determined by individual feelings about the state of the world and how to shape it.

I asked Caldeira if he’d like to elaborate on his views and you can read his thoughts below.

If Mann had wanted to point to an opposite end to the spectrum of ways in which scientists can contribute to public discourse on global warming science and risks, a better choice (in my view) would have been Susan Solomon’s handling of the rollout of the 2007 science report from the Intergovernmental Panel on Climate Change. In a 2007 profile of Solomon, I included this passage from the Paris news conference:

When a reporter asked Dr. Solomon “to sum up what kind of urgency this sort of report should convey to policy makers,” she gave the furthest thing from a convenient sound bite. “I can only give you something that’s going to disappoint you, sir, and that is that it’s my personal scientific approach to say it’s not my role to try to communicate what should be done,” Dr. Solomon said. “I believe that is a societal choice. I believe science is one input to that choice, and I also believe that science can best serve society by refraining from going beyond its expertise. “In my view, that’s what the I.P.C.C. also is all about, namely not trying to make policy-prescriptive statements, but policy-relevant statements.”

Climate scientists, like all of us, come in all shapes and sizes and demeanors. I agree with Mann that it’s unwise for scientists to avoid the public debate over drivers of climate risk and options for reducing it. But I agree with Caldeira (and Gavin Schmidt and the departed Steve Schneider) that it’s counterproductive to blur lines between observations based on science and values-based views on solutions.

Here’s Caldeira’s note:

The issue of going beyond expertise is an important one. There is a disease wherein one develops expertise in one area and then feels free to pontificate on other areas about which one knows nothing. This is an affliction of many senior scientists, common even among Nobel Prize winners, and an affliction to which I have not been immune. If someone is speaking with great confidence while uttering pure hogwash, this does tend to reduce confidence in the utterances of the scientist. So, there is a cost to science and to our personal credibility when scientists make poorly supported assertions in areas outside of their expertise. In any case, scientists should be clear when they are making an assertion that is an empirical fact and when they are simply expressing their values and political opinions. Human beings do have a responsibility to speak out on issues that we feel strongly about. One way to thread the needle is for climate scientists to speak out loudly and in detail about the areas we know something about — climate change and its consequences — but then speak with a greater degree of generality when coming to prescriptions about what exactly we should do. In other words, it is one thing to say (as a human being who happens to be a scientist) that we need to stop using the sky as a waste dump for our greenhouse gas pollution. It is another thing entirely to wegh in on specific policy instruments (taxes versus cap-and-trade versus regulations), specific energy technologies, and so on. It is fine for climate scientists to say (as human beings) that we need policies to rein in greenhouse gas emissions, that to do this we will need energy technologies with near-zero emissions, etc, and that we need to do all of this very soon. It disturbs me when anyone, including climate scientists, (1) fails to distinguish between matters of empirical fact and matters of values and political opinion, and (2) speaks with an air of authority on topics about which they are largely ignorant. I do not claim to be entirely innocent of either of these transgressions. Although I work to try to keep myself on the straight and narrow, I do sometimes succumb to temptation.

Postscript, 5:00 p.m. *| At the asterisk above, my characterization of Mann’s positions, as Mann and others have said on Twitter, was indeed too caricatured — although I maintain that his piece could easily be interpreted as very sympathetic to one approach and critical of the other.

Ken Caldeira offered this note in the comment thread:

Michael Mann and I largely agree on what needs to be done, and our primary differences relate to what we do in the role of ‘informed citizen’ and what we do in the role of ‘scientist’. I was thankful that he quoted me, airing alternate views in his Op-Ed piece. Michael Mann may or may not be critical of my viewpoint, but I see no evidence that he is critical of me as a person. Some of my best friends are people I strongly disagree with. A more difficult question is what a scientist should do when we feel strongly about something but have no special relevant expertise. For example, if I feel strongly that Obama should pardon Edward Snowden, should I make public statements on this matter? Would I be using my standing as a climate scientist to communicate about civil liberties and national security issues about which I am not expert? Is this bad? Is keeping quiet about injustice that I perceive a greater evil? In any case, it seems important for scientists to make clear that our political statements are in our roles as ordinary people, not in our role as climate scientists.

Postscript, 12:32 p.m. | Nick Kristof’s column today explores his readers’ leading candidate for “neglected topics,” which, of course, is climate change.