Here are three statements that shouldn’t be controversial: climate change is real; human activities are a major factor contributing to it; and the path we are on will lead to massive social, economic, and human suffering here in the United States and around the world. Yet whenever the subject comes up, in venues large (like the Intergovernmental Panel on Climate Change’s 5th Report, which synthesizes 9,000 studies prepared by more than 800 authors from 40 countries) and small (a blog post last week by Erin Rogers of our Environment Program on the need to focus on carbon pollution rather than adaptation), some people, for whatever reason, want to continue a debate that should be regarded as settled.

In our case, critics challenged the scientific consensus on climate change, questioned our motives, and tried to change the subject. One pointed to the PBS NewsHour (which we support) as being unwilling to air the views of skeptics, and intimated that the Hewlett Foundation is responsible for silencing dissent on that program. But even if we were telling the NewsHour what to air (we’re not, of course—our grant to them is for general operating support, and long predates our work on climate change), what of the thousands of other news outlets, governments, and scientific academies around the world that have reached the same conclusion about the irrefutable reality of climate change?

Ours is just one small blog, of course, and it has attracted only a few critical comments. On a far larger scale, critics of climate science have seized on a finding by the IPCC as evidence that the threat posed by global warming is overstated if not downright fabricated. In particular, critics trill that the panel’s 5th report indicates that the Earth’s temperature has not increased as much as predicted by the 4th report released six years ago, and in fact, has slowed in recent years—a trend the skeptics say the IPCC cannot adequately explain.  

This is, if you will pardon the pun, just so much hot air. In fact, the 5th Report differs from the 4th in precisely the ways we should expect (and want) from good science, relying on continued work and refinements in collecting data to produce better, more accurate models. More important, the IPCC has an explanation for the slowdown in rising temperatures that makes quite good sense.

It is incontrovertible fact that the last three decades are the hottest on record. But the emission of CO2 is still only one effect among many influencing temperature in a very complex system. What distinguishes CO2 is that it is unidirectional—it only increases warming—while other influences can cut in either direction, the result being fluctuations of various sorts over the short-term. There is an 11-year solar cycle that influences temperatures differently at different times; El Nino and La Nina effects from ocean currents similarly add to or counteract warming; cloud formations and precipitation have varying impacts; and so forth. Taken together, what these produce, and what we should expect, is precisely what we have seen: a consistent rise in temperatures over a long period, with shorter term fluctuations and a lot of variability.

What such fluctuations do not do is call into question the fundamental conclusion that climate change is happening. Indeed, by funding research on these fluctuations, the Hewlett Foundation and its partners have helped refine and sharpen our methods of evaluation and analysis. But even as we have learned more about variability, we have seen the level of scientific and statistical certainty about the basic fact of climate change—and the human contributions to it—grow steadily. We are now more certain than ever.

Is there absolute consensus? Of course not. There are a handful of scientists who continue to doubt. There remain scientists out there who doubt the connection between smoking and cancer too. But what sensible person would start smoking today? With smoking, the evidence and consensus at some point became so overwhelming that it would have been imprudent not to act. And climate change is well past that threshold.

Bear in mind that climate science consists of more than just measuring temperature. It encompasses multiple fields—from measuring ocean acidity and ocean currents, to monitoring changes in plant and animal life, studying tree rings (dendrochronology), tracking the seas’ rise and their ice melting, and more. Each is a separate specialty, comprised of different people using different techniques, working independently in different institutions, funded by different sources, their work reviewed by different cohorts of peers, and published in different journals.

Yet their conclusions all point in the same direction.   

Imagine a criminal trial in which the case against the defendant is based on eyewitness identification. That can be good evidence, but taken alone there are lots of reasons for doubt and lots of holes that can be punched in the state’s case. Or imagine a trial in which the case against the defendant is based solely on fingerprints, or solely on DNA, or solely on circumstantial evidence. In each instance, it’s decent enough evidence, but there would be good reason to doubt if that was all there was.  

Now, what if you had all these forms of evidence—an eyewitness, fingerprints, DNA, and circumstantial evidence—and all pointed toward the defendant's guilt? At that point, there really is not much room for reasonable doubt. This is the situation with climate change.

My friends and colleagues will tell you that there’s nothing I like more than a good argument. We welcome conversation about the best way to respond to climate change or what our priorities should be in addressing it. There is plenty of room for liberals and conservatives to disagree about what we should do about climate change without denying the conclusions of an overwhelming consensus of scientists working in many disciplines, peddling conspiracy theories, or trying to sow unreasonable doubt where reason and common sense (not to mention concern for our children and grandchildren) tell us to act. There is still time to do something about global warming; it’s delay we can no longer afford.

The time for action to reduce greenhouse gas emissions is now.