It did take scientists a while, although not two centuries, to see the heliocentric model’s merit. Astronomers quietly adopted Copernicus’s calculations soon after they were published, but without at first accepting the heliocentric premise on which they were based. As young, open-minded astronomers replaced their elders, a paradigm shift toward the modern view began. By the time of Johannes Kepler’s recognition of simple elliptical orbits in 1609 (see the article by Owen Gingerich in PHYSICS TODAY, September 2011, page 50 ) and Galileo’s observations the following year, many top astronomers had converted to the Copernican view.

The revelations from Galileo’s telescope (lunar craters, migrating sunspots, planetary moons, and more), though spectacular, didn’t directly validate the heliocentric model. Instead, their most important effect was to challenge the preconceived notions that prevented the model’s acceptance: that the heavens were perfect, that all celestial objects orbited Earth, that Scripture fully described the universe (exemplified by Dante Alighieri’s conception of a geocentric divine arrangement, shown in figure 1).² Once those errors were revealed, the mind reopened to new possibilities. Modern educators have recently realized that a similar process is important in teaching physics in the classroom: Identifying and revealing incorrect intuitions—based on, say, friction-dominated systems—is sometimes necessary before students will truly assimilate an understanding of more general validity, such as Newton’s laws of motion. (See the article by Edward Redish and Richard Steinberg in PHYSICS TODAY, January 1999, page 24 .)

More astute critics such as Tycho Brahe had a legitimate objection to the Copernican theory: If Earth is moving, one should see evidence of parallax in the shifting of the stars over the course of a terrestrial orbit, and Tycho could find none. But stars in Galileo’s telescope remained point-like even under strong magnification, which suggested that they were very distant indeed, and that the parallax would therefore be unobservably small; Galileo’s observations thereby removed Tycho’s objection. (Parallax was eventually observed in 1838.)

Despite the power of the new theory and its observational successes, many people, even in the scientific community, could not relinquish the idea that the universe was built around them. Their belief was so strong that some scientists simply refused to look through Galileo’s telescope, and others invented ridiculous explanations for what it showed.² Compromise models became popular; Tycho himself proposed that the planets orbit the Sun but maintained that the Sun and its entourage all orbit Earth. Over time such crutches fell by the wayside; Copernicus’s view was generally accepted among scientists by the late 17th century and among the public by the late 18th century.²

The progression of the global warming idea so far has been quite similar to that of Copernicanism. The idea that changes in atmospheric greenhouse gas concentrations can and do cause significant climate changes (a notion for which I will use the shorthand term “greenhouse warming”) was proposed qualitatively in 1864 by renowned physicist John Tyndall, when he discovered carbon dioxide’s opacity to IR radiation. In 1896 Nobel laureate Svante Arrhenius quantitatively predicted the warming to be caused in the future by coal burning; the prediction was tested and promoted by steam engineer Guy Callendar in the late 1930s. At first few could accept that humans were capable of influencing the climate of an entire planet, but over time, and with more calculations, scientists found the possibility increasingly difficult to dismiss.

As with Copernicanism, astute observers found legitimate objections. The 15-micron absorption of atmospheric CO₂ was already largely saturated, which some argued would prevent additional CO₂ from having any effect. The ocean, with its large carbon-storing capacity, seemed poised to soak up most of the human emissions. By the 1970s, however, those objections had deflated in the face of contrary evidence,³ and a growing number of papers on climate were noting the likelihood of future warming.⁴

Many who are unwilling to accept the full brunt of greenhouse warming have embraced a more comforting compromise reminiscent of the Tychonic system: that CO₂ has some role in climate but its importance is being exaggerated. But accepting a nonzero warming effect puts one on a slippery slope: Once acknowledged, the effect must be quantified, and every legitimate method for doing so yields a significant magnitude. As the evidence sinks in, we can expect a continued, if slow, drift to full acceptance. It took both Copernicanism and greenhouse warming roughly a century to go from initial proposal to broad acceptance by the relevant scientific communities. It remains to be seen how long it will take greenhouse warming to achieve a clear public consensus; one hopes it will not take another century.

Inconvenient scientific claims also show parallels in their political progression. In the decades before Galileo began his fervent promotion of Copernicanism, the Catholic Church took an admirably philosophical view of the idea. As late as 1615, Cardinal Robert Bellarmine acknowledged that “we should . . . rather admit that we did not understand [Scripture] than declare an opinion to be false which is proved to be true.” But the very next year he officially declared Copernicanism to be false, stating that there was no evidence to support it, despite Galileo’s observations and Kepler’s calculations.² Institutional imperatives had forced a full rejection of Copernicanism, which had become threatening precisely because of the mounting evidence.

Even Albert Einstein was not immune to political backlash. His theory of general relativity, excerpted on the notebook page in figure 2, undermined our most fundamental notions of absolute space and time, a revolution that Max Planck avowed “can only be compared with that brought about by the introduction of the Copernican world system.”⁵ Though the theory predicted the anomalous perihelion shift of Mercury’s orbit, it was still regarded as provisional in the years following its publication in 1916.

When observation, by Arthur Eddington and others, of a rare solar eclipse in 1919 confirmed the bending of light, it was widely hailed and turned Einstein into a celebrity. Elated, he was finally satisfied that his theory was verified. But the following year he wrote to his mathematician collaborator Marcel Grossmann:

Instead of quelling the debate, the confirmation of the theory and acclaim for its author had sparked an organized opposition dedicated to discrediting both theory and author. Part of the backlash came from a minority of scientists who apparently either felt sidelined or could not understand the theory. The driving force was probably professional jealousy,⁶but scientific opposition was greatly amplified by the anti-Semitism of the interwar period and was exploited by political and culture warriors. The same forces, together with status quo economic interests, have amplified the views of climate contrarians.⁷

The historical backlashes shed some light on a paradox of the current climate debate: As evidence continues to accumulate confirming longstanding warming predictions and showing how sensitive climate has been throughout Earth’s history, why does climate skepticism seem to be growing rather than shrinking? All three provocative ideas—heliocentricity, relativity, and greenhouse warming—have been, in Kuhn’s words, “destructive of an entire fabric of thought,” and have shattered notions that make us feel safe.² That kind of change can turn people away from reason and toward emotion, especially when the ideas are pressed on them with great force.⁸

The agitations of modern greenhouse proponents appear to have provoked an antiscience backlash similar to the one against Galileo. In the space of only two years, almost as fast as Bellarmine changed his position on Copernicanism, leading moderates have been squeezed out of the main conservative political parties in both the US and Australia and replaced by hard-line rejecters of climate science. In Australia, climate policy was the leading issue behind the backlash; in the US it was one of many contributing factors. Because the Catholic Church of Galileo’s day had generally been a supporter of science and open inquiry, the condemnation of Copernicanism as it grew scientifically solid shocked many devout Catholics.² Likewise, modern conservative political parties have until recently been friends of science, including climate and environmental studies. In the 1970s Republicans and Democrats in Congress were equally concerned about climate change, and as recently as 2004 leading Republicans were—at least in public—enthusiastic in their support of science. Their recent rejections of climate science have probably shocked many supporters. In both cases the backlash seems to have come when leaders were pushed to act on the basis of new evidence. (Figure 3 further illustrates the connection between economic incentives and rejection of climate science.)

The ugly nature of the current climate debate, with its increasingly frequent characterization of scientists as opportunists, totalitarians, or downright criminals, is also, unfortunately, not new. Copernicus (posthumously) and his prominent followers through Isaac Newton were all accused of being heretics or atheists. Einstein was derided by his political opponents through the 1920s and 1930s as a Communist—despite his dim view of the Soviet Union—or simply as a fraud. When a group of American women tried to prevent him from entering the US because of his supposed Communism, he quipped, “Never before have I experienced from the fair sex such energetic rejection of all advances, or if I have, then certainly never from so many at once.”⁹ At one point Einstein stopped giving public lectures out of fear for his personal safety, also now a worry for some greenhouse warming proponents.