Many years later, I read an article that featured Wallace Broecker, the Columbia University scientist with revolutionary ideas about catastrophic climate change caused by abrupt slowdowns in ocean circulation. I was fascinated by his idea that the rapid onset of the Younger Dryas cold spell could have been caused by the collapse of an ice dam and a deluge of freshwater into the North Atlantic that shut off the Gulf Stream, stopping the flow of tropical heat to the northern continents and plunging them into ice-age conditions. He showed that there could be other causes of global catastrophes that don't involve impacts.

I was delighted when Broecker agreed to give the opening presentation at the American Geophysical Union (AGU) session I helped organize, but I was surprised to learn that he had abandoned his famous hypothesis about the cause of the Younger Dryas. He started his presentation by reminding everyone that he used to argue that it was triggered by the flood from the ice-age Lake Agassiz, but when he flew over the route the floodwaters should have followed, he saw no geomorphic evidence for a flood. He had changed his mind!

His primary objections to the impact hypothesis were the same as his objections to the flood he had previously championed as the explanation: lack of evidence and lack of uniqueness of the Younger Dryas. Abrupt changes in climate, both warming and cooling, have happened many times, and Broecker argues that the climate system is inherently unstable. Why should only one of a long sequence of changes have such an improbable and catastrophic trigger event—whether impact or flood—when the climate system has repeatedly undergone such changes all by itself?

In his 1987 CSICOP address, Carl Sagan said, "In science it often happens that scientists say, ‘You know that's a really good argument; my position is mistaken,' and then they actually change their minds and you never hear that old view from them again... . I cannot recall the last time something like that has happened in politics or religion."

Broecker's esteem among scientists was not diminished when he changed his mind. The Younger Dryas impact proponents would do well to follow his example.

Mark Boslough was co-organizer of the AGU Younger Dryas session in December. He is a physicist at Sandia National Laboratories and an adjunct professor at the University of New Mexico.

When a dog bites a man, that is not news, because it happens so often. But if a man bites a dog, that is news.

—John D. Bogart

As evidence for human-caused climate change has mounted, global warming denialists have responded by blaming the messengers. Climate researchers have endured abuse by bloggers, editorial writers, Fox News pundits, and radio talk show hosts who have called them liars and vilified them as frauds. The attacks had become increasingly vile as the past decade, the hottest in human history, came to an end. Angry activists have called for firings and criminal investigations, and some prominent scientists have received physical threats.

Politicians have also gotten into the act. In 2005, Sen. James Inhofe (R-OK) referred to global warming as the “greatest hoax ever perpetrated on the American people.” Rep. Joe Barton (R-TX) sent a harassing letter to Michael Mann (now a professor at Pennsylvania State University) and his coauthors of the famous “hockey stick” paleoclimate paper, demanding that they drop everything to provide him with extensive documentation about what he claimed were “methodological flaws and data errors” in their work.

Denialists have attempted to call the science into question by writing articles that include fabricated data. They’ve improperly graphed data using tricks to hide evidence that contradicts their beliefs. They chronically misrepresent the careful published work of scientists, distorting all logic and meaning in an organized misinformation campaign. To an uncritical media and gullible non-scientists, this ongoing conflict has had the intended effect: it gives the appearance of a scientific controversy and seems to contradict climate researchers who have stated that the scientific debate over the reality of human-caused climate change is over (statements that have been distorted by denialists to imply the ridiculous claim that in all respects “the science is settled”).

Science, however, has ground rules. Those who don’t follow the rules are entitled to their opinions but cannot legitimately claim to be participating in a scientific debate. One rule that must be followed for scientific results to be accepted is that they must be subjected to review and published in a scholarly scientific journal. This is a necessary but insufficient condition (nobody is compelled to accept the conclusions of a paper just because it has been refereed).

This rule is not intended to create a “high priesthood” of scientists or keep others from participating. On the contrary, the culture of science welcomes dissent and encourages contrarians to publish their ideas so they can be subjected to the same scrutiny that is applied to conventional thought.

Peer review is designed to screen out material that is demonstrably wrong, flawed, illogical, or fabricated. Non-specialists are not always able to quickly spot errors in a highly technical piece of work, so experts are recruited to make sure any mistakes are corrected and necessary documentation is provided before the science is published.

The first thing I do when I read an editorial or blog entry is check to see if the supposed science has been published in scientific literature. If not, I don’t see why I should bother to read what nobody could be bothered to put through scientific peer review. My reasoning is not that such material is necessarily wrong, but without any scientific review I have no assurance that anyone has checked to see if the equations are right, data sources correctly cited, figures properly attributed, or other workers’ conclusions fairly represented.

The global warming debate continues, at least among the science-challenged. The calculation of the mass of CO₂ produced from burning a gallon of gasoline was the subject of a recent vigorous disagreement on the letters page of our local newspaper. This is a question that a decent high school chemistry student should be able to answer, but the highly opinionated letter writers were not able to resolve their differences, despite the fact that reaction stoichiometry is indisputably settled science.

Likewise, a competent high school physics student understands how the so-called greenhouse effect works and that conservation of energy is also settled science. It has been known for over a hundred years that adding CO₂ to the atmosphere increases its infrared opacity, and when this happens, more energy from sunlight enters Earth’s atmosphere than escapes. The atmosphere must heat up on average. There is no scientific debate about this fact, and nobody has ever published a “zero-warming” theory to explain how it could be otherwise.

There is, however, a healthy, open, honest, and active scientific debate in the peer-reviewed scientific literature about the degree of climate change. The best scientific estimate of the amount of warming (when CO₂ levels double, which is likely to happen this century) is about 3°C. There are scientists who disagree—some think it’s higher and some lower—and have published the basis for their disagreement.

Having lost the scientific debate, denialists have now resorted to hacking into a computer system and stealing private correspondence to distract those who prefer controversy to science. To those of us in the scientific community, it came as no surprise that researchers who had endured personal attacks had trouble rising above the fray. But the harsh tone of some messages by Mann and others caught the attention of the voyeurs who read them precisely because they were in sharp contrast to the way scientists usually speak in public. The attempts to force editors not to publish papers critical of the scientists and suggestions to boycott journals were inappropriate and unsuccessful (journal editors resisted pressure and published the papers anyway). They also were not unusual—certainly not beneath those in the opposite camp. And even though the widely reported “trick” used to “hide the decline” was legitimate (using real temperatures instead of a faulty tree-ring proxy to represent the temperature record), it sounded like something denialists would do, so it was assumed to be crooked.

The very fact that Climategate was newsworthy is evidence that reporters hold scientists to a much higher standard than they hold denialists, even if they won’t admit it in their quest to report a controversy.

Perhaps the slick promotional billboards enticed you to make a short excursion from your planned trip. As you neared the site you would have seen a low ridge rising from the flat desert ahead. An earlier generation called the ridge “Coon Butte,” not realizing that it was actually the rim of a three-quarter-mile-wide crater.

When you stand on the rim, you look across an expansive circular cavity in solid rock that is so wide that it changes the wind patterns and attracts raptors that soar in the updrafts. This big hole truly should be listed as one of the natural wonders of the world.

What you may not know is that a century ago Meteor Crater was the subject of a great scientific controversy, and was a focal point for defining the scientific method and promoting scientific research at the dawn of twentieth-century American technological progress. One hundred years after that debate, Meteor Crater serves as a reminder of the importance of scientific knowledge and of the scientific method to our way of life.

In early 1896, the journal Science published an address that geologist Grove Karl Gilbert (1843-1918) had recently given to the Scientific Societies of Washington. Gilbert was the retiring president of the Geological Society of Washington and one of the top scientific thinkers of his time. He had also been chief geologist of the U.S. Geological Survey until Congress slashed the Agency’s budget in half, terminating his and others’ positions. His lecture was titled the “Origin of Hypotheses,” and was a description of the scientific method.

At the center of the scientific method, he said, is the hypothesis, or “the scientific guess.” Gilbert used the origin of Coon Butte to illustrate how this works. Four scientific guesses had been made at the time. The first came from a shepherd named Mathias Armijo, who found pieces of iron near the crater and reasoned that an explosion had hurled the metal out of the ground and formed the big hole (one does not have to be a scientist to think scientifically). Geologists who came to visit the site offered two scientific guesses involving two types of volcanic processes. A fourth hypothesis was the radical idea that a meteorite had hit the Earth.

Gilbert traveled to this then-remote part of the country and made measurements to test the various ideas. Because so little was known at the time about the physics of meteorite impacts, he predicted that such a cosmic collision would have left a very large piece of iron buried under the crater. His tests failed to find the predicted iron, so Gilbert rejected the impact idea. The small pieces of iron found on the surface by Armijo did prove to be meteorites but Gilbert concluded that they fell from the sky in an unrelated event (thereby also rejecting Armijo’s idea that they came out of the ground).

Of the two volcanic ideas, one predicted that volcanic rocks would be found in the crater. But the crater had none, so there was only one hypothesis left that had not been eliminated: some type of volcanic steam explosion.

That was the idea that Gilbert accepted as the correct explanation, even though he had arrived at the crater expecting to demonstrate that it was formed by an impact. He already supported the then-unpopular notion that such craters on the moon were formed by impacts, not volcanoes, but a good scientist does not allow personal feelings to get in the way of evidence. However, Gilbert was very careful to point out that there was much that was still not known about meteorites and impacts. He recognized that new facts might be discovered that would overturn his conclusion.

That is exactly what happened. We now understand that Gilbert overestimated the size of the meteorite that would be needed to pack enough punch to blast out such a big hole: Hypervelocity impacts are much more powerful than he realized. Furthermore, even a large iron meteorite will mostly vaporize in a giant explosion, leaving very few traces. Gilbert had made a mistake by assuming that the impact would leave a lot of buried iron.

It would be many years before a young scientist named Eugene Shoemaker and his colleagues from the U.S. Geological Survey would discover a rare new mineral in the rocks at the crater, a mineral that had been predicted to form from an impact. This discovery finally settled the controversy, and partially vindicated a shepherd’s original hunch that the hole was formed by some kind of colossal explosion involving iron.

The scientific process is sometimes slow, but it always involves making educated guesses that eventually lead to predictions that can be observed and put to a test. If the predictions turn out to be incorrect, the test is still successful as long as scientists learn enough to modify the theory, find a better one, or uncover mistaken assumptions. Unfortunately, even after the successes of twentieth-century science between Gilbert’s time and now, there are a lot of people who still don't like (or don't understand) the scientific form of reasoning.

In fact, modern science is now under attack from many directions. On the left are those who twist legitimate multiculturalism by going way beyond it to extreme relativism. They dogmatically assert that all ways of seeking knowledge are equally valid, but still insist that the scientific method is flawed because it originated in a time and place that causes them to view it as a Eurocentric, white male endeavor. Such thinking has encouraged proliferation of belief in pseudoscientific and unscientific ideas ranging from crystal healing to flying saucers. Even worse, it has turned some women and minorities away from careers in science, not only to their own detriment but to the detriment of society.

Science is also under attack from the religious right, whose literal interpretation of the Bible supersedes scientific evidence, logical reasoning, and common sense. In this fundamentalist view, any fact that is at odds with their own reading of the scriptures must be ignored. Unfortunately, this faction is not satisfied merely to reject science for itself, but it now has an active campaign to remove scientifically validated subjects (such as evolution) from the classroom and have them replaced by their own unscientific opinions (such as creationism).

Worst of all, science is now under attack by a budget-cutting Congress in Washington for whom dollars have measurable value but scientific knowledge does not. Members of Congress think that spending on basic science is like throwing money into a big hole in the ground. They do not realize that a dollar saved may be two dollars (or more) worth of knowledge lost.

Gilbert closed his late-nineteenth-century address by explaining that "fertility of invention implies a wide and varied knowledge of the causes of things,” and that deep understanding of nature through scientific research is essential. Gilbert told his audience that our “material, social, and intellectual condition” advances in lockstep with our “knowledge of natural laws.”

He concluded by comparing science to an investment: “Knowledge of nature is an account at [the] bank, where each dividend is added to the principal and the interest is ever compounded: And hence it is that human progress, founded on natural knowledge, advances with ever increasing speed!”

Since Gilbert spoke these words, our scientific bank account has led to inventions that his audience in Washington could not have imagined. Our investment has swollen with the advances we associate with modern living, with medical discoveries that have given us longer, healthier, happier lives, and with an unprecedented degree of national security.

We can thank Gilbert and his contemporaries for having the foresight to recognize 100 years ago the importance of this scientific bank account, and for making the effort to convince decision-makers to restore and increase funding for science. We should again ask those in Washington to pass along to future generations the American tradition of a strong investment in scientific knowledge, and trust in the scientific method. And we should remind them that research spending is money in the bank, not money in a hole.