Most of Donald J. Trump's words and actions concerning climate change have pointed toward his unambiguous disbelief in it. As a presidential candidate and earlier, he repeatedly called global warming an “expensive hoax,” a Chinese plot to undermine U.S. manufacturing and “a very, very expensive tax.” As president, he has repeatedly suggested he will withdraw the U.S. from the Paris climate accord aimed at limiting carbon emissions. To head the Department of Energy and the Environmental Protection Agency, he picked Rick Perry and Scott Pruitt respectively, both of whom have questioned the role of carbon dioxide in climate change. Trump's nominees for secretary of agriculture, Sam Clovis, and director of NASA, Jim Bridenstine, have similar views, as do many other appointees to less prominent federal offices.

Yet during his first presidential debate with Democratic candidate Hilary Clinton, he denied calling climate change a Chinese hoax. And when reporters have directly asked White House press secretaries whether Trump denies global warming is real, they have skirted the question. The administration seems to want its official stance on the issue to be undeclared and skeptical.

Skepticism was the intellectual virtue that U.S. Senator James R. Inhofe (R–Okla.) said he was championing in November 2009, with the United Nations Global Warming Conference in Copenhagen fast approaching, when he took to the floor of the Senate and proclaimed it to be “The Year of the Skeptic.” Had the senator's speech marked a new commitment to dispassionate, rational inquiry, a respect for scientific thought and a well-grounded doubt in ghosts, astrology, creationism and homeopathy, it might have been cause for cheer. But Inhofe had a narrower definition of skeptic in mind: he meant “standing up and exposing the science, the costs and the hysteria behind global warming alarmism.”

Within the community of scientists and others concerned about anthropogenic climate change, those whom Inhofe called skeptics are more commonly termed contrarians, naysayers and denialists. Not everyone who questions climate change science fits that description, of course—some people are genuinely unaware of the facts or honestly disagree about their interpretation. What distinguishes the true naysayers is an unwavering dedication to denying the need for action on the problem, often with weak and long-disproved arguments about supposed weaknesses in the science behind global warming.

What follows is only a partial list of the contrarians' bad arguments and some brief rebuttals of them.

Claim 1: Anthropogenic carbon dioxide can't be changing climate, because CO2 is only a trace gas in the atmosphere and the amount produced by humans is dwarfed by the amount from volcanoes and other natural sources. Water vapor is by far the most important greenhouse gas, so changes in CO2 are irrelevant.

Although CO2 makes up only 0.04 percent of the atmosphere, that small number says nothing about its significance in climate dynamics. Even at that low concentration, CO2 absorbs infrared radiation and acts as a greenhouse gas, as physicist John Tyndall demonstrated in 1859. Chemist Svante Arrhenius went further in 1896 by estimating the impact of CO2 on the climate; after painstaking hand calculations, he concluded that doubling its concentration might cause almost six degrees Celsius of warming—an answer not much out of line with recent, far more rigorous computations.

Contrary to the contrarians, human activity is by far the largest contributor to the observed increase in atmospheric CO2. According to the Global Carbon Project, anthropogenic CO2 amounts to about 35 billion tons annually—more than 130 times as much as volcanoes produce. True, 95 percent of the releases of CO2 to the atmosphere are natural, but natural processes such as plant growth and absorption into the oceans pull the gas back out of the atmosphere and almost precisely offset them, leaving the human additions as a net surplus. Moreover, several sets of experimental measurements, including analyses of the shifting ratio of carbon isotopes in the air, further confirm that fossil-fuel burning and deforestation are the primary reasons that CO2 levels have risen 40 percent since 1832, from 284 parts per million (ppm) to more than 400 ppm—a remarkable jump to the highest levels seen in millions of years.

Contrarians frequently object that water vapor, not CO2, is the most abundant and powerful greenhouse gas; they insist that climate scientists routinely leave it out of their models. The latter is simply untrue: from Arrhenius on, climatologists have incorporated water vapor into their models. In fact, water vapor is why rising CO2 has such a big effect on climate. CO2 absorbs some wavelengths of infrared that water does not, so it independently adds heat to the atmosphere. As the temperature rises, more water vapor enters the atmosphere and multiplies CO2's greenhouse effect; the Intergovernmental Panel on Climate Change notes that water vapor may “approximately double the increase in the greenhouse effect due to the added CO2 alone.”

Nevertheless, within this dynamic, the CO2 remains the main driver (what climatologists call a “forcing”) of the greenhouse effect. As NASA climatologist Gavin Schmidt has explained, water vapor enters and leaves the atmosphere much more quickly than CO2 and tends to preserve a fairly constant level of relative humidity, which caps off its greenhouse effect. Climatologists therefore categorize water vapor as a feedback rather than a forcing factor. (Contrarians who don't see water vapor in climate models are looking for it in the wrong place.)

Because of CO2's inescapable greenhouse effect, contrarians holding out for a natural explanation for current global warming need to explain why, in their scenarios, CO2 is not compounding the problem.

Claim 2: The alleged “hockey stick” graph of temperatures over the past 1,600 years has been disproved. It doesn't even acknowledge the existence of a “medieval warm period” around A.D. 1000 that was hotter than today is. Therefore, global warming is a myth.

It is hard to know which is greater: contrarians' overstatement of the flaws in the historical temperature reconstruction from 1998 by Michael E. Mann and his colleagues or the ultimate insignificance of their argument to the case for climate change.

First, there is not simply one hockey-stick reconstruction of historical temperatures using one set of proxy data. Similar evidence for sharply increasing temperatures over the past couple of centuries has turned up independently while looking at ice cores, tree rings and other proxies for direct measurements, from many locations. Notwithstanding their differences, they corroborate that the planet has been getting sharply warmer.

A 2006 National Research Council review of the evidence concluded “with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries”—which is the section of the graph most relevant to current climate trends. The report placed less faith in the reconstructions back to A.D. 900, although it still viewed them as “plausible.” Medieval warm periods in Europe and Asia with temperatures comparable to those seen in the 20th century were therefore similarly plausible but might have been local phenomena: the report noted “the magnitude and geographic extent of the warmth are uncertain.” And a research paper by Mann and his colleagues seems to confirm that the Medieval Warm Period and the “Little Ice Age” between 1400 and 1700 were both caused by shifts in solar radiance and other natural factors that do not seem to be happening today.

After the NRC review was released, another analysis by four statisticians, called the Wegman report, which was not formally peer-reviewed, was more critical of the hockey-stick paper. But correction of the errors it pointed out did not substantially change the shape of the hockey-stick graph. In 2008 Mann and his colleagues issued an updated version of the temperature reconstruction that echoed their earlier findings.

But hypothetically, even if the hockey stick was busted ... What of it? The case for anthropogenic global warming originally came from studies of climate mechanics, not from reconstructions of past temperatures seeking a cause. Warnings about current warming trends came out years before Mann's hockey-stick graph. Even if the world were incontrovertibly warmer 1,000 years ago, it would not change the fact that the recent rapid rise in CO2 explains the current episode of warming more credibly than any natural factor does—and that no natural factor seems poised to offset further warming in the years ahead.

Claim 3: Global warming stopped in 1998; Earth has been cooling since then.

This contrarian argument might be the most obsolete and unintentionally hilarious. Here's how it goes: 1998 was the world's warmest year according to the U.K. Met Office Hadley Center's records; the following decade was cooler; therefore, the previous century's global warming trend is over, right?

Anyone with even a glancing familiarity with statistics should be able to spot the weaknesses of that argument. Given the extended duration of the warming trend, the expected (and observed) variations in the rate of increase and the range of uncertainties in the temperature measurements and forecasts, a decade's worth of mild interruption is too small a deviation to prove a break in the pattern, climatologists say.

If a lull in global warming had continued for another decade, would that have vindicated the contrarians' case? Not necessarily, because climate is complex. For instance, Mojib Latif of the Leibniz Institute of Marine Sciences in Germany and his colleagues published a paper in 2008 that suggested ocean circulation patterns might cause a period of cooling in parts of the Northern Hemisphere, even though the long-term pattern of warming remained in effect. Fundamentally, contrarians who have resisted the abundant evidence that supports warming should not be too quick to leap on evidence that only hints at the opposite.

In any case, the claim that a “warming pause” disproved ongoing climate change became completely academic when 1998 stopped being the warmest year on record. That title now goes to 2014. In fact, as Climate Central notes, “13 of the 15 hottest years on record have all occurred since 2000.”

Claim 4: The sun or cosmic rays are much more likely the real causes of global warming. After all, Mars is warming up, too.

Astronomical phenomena are obvious natural factors to consider when trying to understand climate, particularly the brightness of the sun and details of Earth's orbit because those seem to have been major drivers of the ice ages and other climate changes before the rise of industrial civilization. Climatologists, therefore, do take them into account in their models. But in defiance of the naysayers who want to chalk the recent warming up to natural cycles, there is insufficient evidence that enough extra solar energy is reaching our planet to account for the observed rise in global temperatures.

The IPCC has noted that between 1750 and 2005, the radiative forcing from the sun increased by 0.12 watt per square meter—less than a tenth of the net forcings from human activities (1.6 W/ m2). The largest uncertainty in that comparison comes from the estimated effects of aerosols in the atmosphere, which can variously shade Earth or warm it. Even granting the maximum uncertainties to these estimates, however, the increase in human influence on climate exceeds that of any solar variation.

Moreover, remember that the effect of CO2 and the other greenhouse gases is to amplify the sun's warming. Contrarians looking to pin global warming on the sun can't simply point to any trend in solar radiance: they also need to quantify its effect and explain why CO2 does not consequently become an even more powerful driver of climate change. (And is what weakens the greenhouse effect a necessary consequence of the rising solar influence or an ad hoc corollary added to give the desired result?)

Contrarians therefore gravitated toward work by Henrik Svensmark of the Technical University of Denmark, who argues that the sun's influence on cosmic rays needs to be considered. Cosmic rays entering the atmosphere help to seed the formation of aerosols and clouds that reflect sunlight. In Svensmark's theory, the high solar magnetic activity over the past 50 years has shielded Earth from cosmic rays and allowed exceptional heating, but now that the sun is more magnetically quiet again, global warming will reverse. Svensmark claims that, in his model, temperature changes correlate better with cosmic-ray levels and solar magnetic activity than with other greenhouse factors.

Svensmark's theory failed to persuade most climatologists, however, because of weaknesses in its evidence. In particular, there do not seem to be clear long-term trends in the cosmic-ray influxes or in the clouds that they are supposed to form, and his model does not explain (as greenhouse explanations do) some of the observed patterns in how the world is getting warmer (such as that more of the warming occurs at night). For now, at least, cosmic rays remain a less plausible culprit in climate change.


And the apparent warming seen on Mars? It is based on a very small base of measurements, so it may not represent a true trend. Too little is yet known about what governs the Martian climate to be sure, but a period when there was a darker surface might have increased the amount of absorbed sunlight and raised temperatures.

Claim 5: Climatologists conspire to hide the truth about global warming by locking away their data. Their so-called consensus on global warming is scientifically irrelevant because science isn't settled by popularity.

It is virtually impossible to disprove accusations of giant global conspiracies to those already convinced of them (can anyone prove that the Freemasons and the Roswell aliens aren't involved, too?). Let it therefore be noted that the magnitude of this hypothetical conspiracy would need to encompass many thousands of uncontroversial publications and respected scientists from around the world, stretching back through Arrhenius and Tyndall for almost 150 years. A conspiracy would have to be so powerful that it has co-opted the official positions of dozens of scientific organizations, including the U.S. National Academy of Sciences, the U.K.'s Royal Society, the American Association for the Advancement of Science, the American Geophysical Union, the American Institute of Physics and the American Meteorological Society.

If there were a massive conspiracy to defraud the world on climate (and to what end?), surely the thousands of e-mails and other files stolen from the University of East Anglia's Climatic Research Unit in England and distributed by hackers in 2009 would bear proof of it. None did. Most of the few statements from those e-mails that critics claimed as evidence of malfeasance had more innocent explanations that make sense in the context of scientists conversing privately and informally. If any of the scientists involved manipulated data dishonestly or thwarted Freedom of Information requests, it would have been deplorable; however, there is no evidence that happened. What is missing is any clear indication of a widespread attempt to falsify and coordinate findings on a scale that could hold together a global cabal or significantly distort the record on climate change.

Climatologists are often frustrated by accusations that they are hiding data or the details of their models because, as NASA's Schmidt points out, much of the relevant information is in public databases or otherwise accessible—a fact that contrarians conveniently ignore when insisting that scientists stonewall their requests. (And because nations differ in their rules on data confidentiality, scientists are not always at liberty to comply with some requests.) If contrarians want to deal a devastating blow to global warming theories, they should use the public data and develop their own credible models to demonstrate sound alternatives.

Yet that rarely occurs. In 2004 historian of science Naomi Oreskes published a well-known analysis of the peer-reviewed literature on global warming, “The Scientific Consensus on Climate Change.” Out of 928 papers whose abstracts she surveyed, she wrote, 75 percent explicitly or implicitly supported anthropogenic global warming, 25 percent were methodological or otherwise took no position on the subject—and none argued for purely natural explanations. Notwithstanding some attempts to debunk Oreskes's findings that eventually fell apart, her conclusion stands.

Oreskes's work does not mean that all climate scientists agree about climate change—obviously, some do not (although they are very much a minority). Rather the meaningful consensus is not among the scientists but within the science: the overwhelming predominance of evidence for greenhouse-driven global warming that cannot easily be overturned even by a few contrary studies.

Claim 6: Climatologists have a vested interest in raising the alarm because it brings them money and prestige.

If climate scientists are angling for more money by hyping fears of climate change, they are not doing so very effectively. According to the U.S. Government Accountability Office, between 1993 and 2014, federal spending on climate change research, technology, international assistance and adaptation rose from $2.4 billion to $11.6 billion. (An additional $26.1 billion was also allocated to climate change programs and activities by the economic stimulus package of the American Recovery and Reinvestment Act in 2009. Total federal nondefense spending on research in 2014 exceeded $65 billion.) Yet the scientific research share of that money fell sharply throughout that period: most of the budgeted money went to energy conservation projects and other technology programs. Climatologists' funding therefore stayed almost flat, whereas others, including those in industry, benefited handsomely. Surely the Freemasons could do better than that.

Claim 7: Technological fixes, such as inventing energy sources that don't produce CO2 or geoengineering the climate, would be more affordable, prudent ways to address climate change than reducing our carbon footprint.

Critics of standard policy responses to climate change have often seemed to imply that environmentalists are obsessed with regulatory reductions in CO2 emissions and uninterested in technological solutions. That interpretation is at best bizarre: such innovations in energy efficiency, conservation and production are exactly what caps or levies on CO2 are meant to encourage.

The relevant question is whether it is prudent for civilization to defer curbing or reducing its CO2 output before such technologies are ready and can be deployed at the needed scale. The most common conclusion is no. Remember that as long as CO2 levels are elevated, additional heat will be pumped into the atmosphere and oceans, extending and worsening the climate consequences. As NASA climatologist James Hansen has pointed out, even if current CO2 levels could be stabilized overnight, surface temperatures would continue to rise by 0.5 degree C over the next few decades because of absorbed heat being released from the ocean. The longer we wait for technology alone to reduce CO2, the faster we will need for those solutions to pull CO2 out of the air to minimize the warming problems. Minimizing the scope of the challenge by restricting the accumulation of CO2 only makes sense.

Moreover, climate change is not the only environmental crisis posed by elevated CO2: it also makes the oceans acidic, which could have irreversibly harmful effects on coral reefs and other marine life. Only the immediate mitigation of CO2 release can contain those losses.

Much has already been written on why schemes for geoengineering—altering Earth's climate systems by design—seem ill advised except as a desperate last-chance strategy for dealing with climate change. The more ambitious proposals involve largely untested technologies, so it is unclear how well they would achieve their desired purpose; even if they did curb warming, they might cause other significant environmental problems in the process. Methods that did not remove CO2 from the air would have to be maintained in perpetuity to prevent drastic rebound warming. And the governance of the geoengineering system could become a political minefield, with nations disagreeing about what the optimal climate settings should be. And of course, as with any of the other technological solutions, reducing the emission and accumulation of CO2 in the atmosphere first would only make any geoengineering solution easier.

All in all, counting on future technological developments to solve climate change rather than engaging with the problem straightforwardly by all available means, including regulatory ones, seems like the height of irresponsibility. But then again, responsible action on climate change is what the contrarians seem most interested in denying.