Here are some key facts about humankind's impact on the earth's climate. Taken together they form a bedrock of understanding for which any attempt to dispute the global warming picture must account.
The founding insight can be traced back to a precise place and time: Stockholm, Dec. 11, 1895, when Svante Arrhenius stood before the Swedish Academy of Science to present his paper "On the Influence of Carbonic Acid upon the Temperature of the Ground." (Carbonic acid is now better known as carbon dioxide.) Arrhenius began by recalling how his predecessors had shown that the gas is transparent to visible light — the sun shines perfectly happily through all the CO2 between it and the earth's surface — but absorbs energy at longer wavelengths of light — infrared radiation, what we feel as heat.
Arrhenius then took this basic physical insight and used it to build a picture of a planetwide process. He showed that ... more carbon up there leads directly to more heat down here. He went on to discuss a possible link between CO2 levels and the ebb and flow of ice ages — and he even noted the possibility that burning coal or other fossil fuels might affect the carbon content of the atmosphere.
There it was: One hundred and twenty years ago physicists and chemists already knew that atmospheric CO2 molds global climate. There was and is no disagreement on this. Carbon dioxide is a greenhouse gas. It allows visible light to penetrate the atmosphere, and it acts as a blanket, keeping heat from radiating back out into space. This knowledge does not depend on any indirect measurements, assumptions, or elaborate numerical analysis. Rather, it emerges directly from the extremely well established basic understanding of the behavior of atoms and molecules.
The second piece of the puzzle is equally solid. We know how much carbon is in the atmosphere; we know that its concentration is going up; we know by how much. This isn't a case of argument-by-proxy, an attempt to reconstruct a record through pollen deposits or tree ring data or what have you (though such methods are powerful tools to extract information from the past). There is no question about these facts — because, more than 50 years ago, a guy climbed a tall mountain to find out.
In the mid 1950s, Charles Keeling ... built the first instrument that could accurately measure CO2 concentrations in atmospheric samples. Beginning in 1956, Keeling and his successors have measured atmospheric CO2 at an observatory high on the flanks of Mauna Loa, one of the two giant volcanoes that dominate the Big Island of Hawaii. There is nothing there to confound the work — no smokestacks, cars, anything. The graph that records what they've found over six decades is now called the Keeling Curve — and it is unequivocal.
One of the first things Keeling saw was a jigsaw trace tracking the change of the seasons. As plants grow in the land-rich Northern hemisphere in spring and summer, they grab CO2 out of the air. In winter, as leaves die and fall, some of that carbon gets released back into the atmosphere. As one of the obituaries that followed his death in 2005 put it, Keeling "had discovered that the earth itself was breathing."
But such small fluctuations can't hide the overall trajectory. When Keeling first began his measurements, carbon dioxide accounted for 310 parts per million of the atmosphere. Since then, each year has seen an increase, drawing a curve that is pretty close to a line pointing ever upward. As of April 13, 2016, the Mauna Loa observatory counted 408.70 parts per million of CO2.
That's just the way it is: a number that corresponds to a real quantity out there in nature ... it's not subject to debate. It's not an article of belief. We live on a planet that until recently sported 310 parts per million of carbon dioxide as a thermal blanket — and now has more than 400. Any debate about global climate begins from that unvarnished, unchallengeable reality.
The third beyond-dispute fact about climate change concerns who's responsible for that rise in atmospheric CO2.
Human society excretes a lot of carbon. The numbers are somewhat less precise than the Mauna Loa measurements — but they're still based on direct observation. A number of different agencies and research centers collect the various data sets on industrial activity, power generation, deforestation, and the like. In 2014, all that work put together tallied 35.9 billion tons of CO2 produced by burning of coal, oil, and gas, plus or minus a small variance. Land use changes added another 3.3 billion tons of the gas per year over the last decade, though here the uncertainty is larger — plus or minus 1.8 billion tons. (There are other greenhouse gases for which good estimates of human production exist — notably methane — but CO2 remains the single largest culprit in the climate change story.)
From Arrhenius's first musings about the impact of human action on climate, the key question was whether any possible carbon sinks — especially the oceans — could absorb both natural sources of CO2 (volcanoes, forest fires, and the like) and that released by everything people burn. Now we know — thanks to Keeling's observations — that the answer is no. The oceans do absorb some of the annual production of CO2 from both natural events and what we produce, but the way we live now creates an excess of carbon that overflows all such natural reservoirs.
These three facts: Atmospheric carbon dioxide regulates temperature at the earth's surface, its levels have been and are continuing to rise, and human beings are behind that increase — lead directly to a simple conclusion. All else being equal, human action is driving a global process that will create and likely already is leading to a warmer world.
Everything else isn't equal, of course. The global climate system is intricate, difficult to untangle, tricky to measure, and home to plenty of uncertainties. But here's the nub: Any claim that the world isn't getting hotter now and won't warm in the future can't rely on just one scrap of information or another. It has to make a bigger argument — some coherent account of why ever increasing amounts of carbon produced directly by human activity won't end up where at all our basic understanding of how nature works suggests it should.
So, when (a denier) argues that all of climate science is a hoax because one piece of information — squinted at just right — suggests a gap in the warming record, he's not thinking like a scientist. Instead, he's making a lawyer's case, pounding the table for the defense. That's fine work as rhetoric; we're trained through cultural understanding and uncounted hours of TV courtroom drama to see cases turn on each individual piece of evidence. "If the carbon don't fit, you must acquit" and all that.
But that's not how science works, not when studying climate or anything else. A century ago, Albert Einstein produced his General Theory of Relativity, a radical conception of gravity that displaced Isaac Newton's version. Yet Einstein's theory didn't erase all the successes the older idea had in explaining the motions of everything from the moons of Jupiter to Earth's tides. That's why one of the first calculations Einstein performed to test his new idea was to see if it could reproduce Newtonian results at the appropriate scales. Even the greatest discoveries don't invalidate older knowledge. Rather they frame such prior ideas within their newly emerging picture.
Much of contemporary science has accumulated into a deep understanding of the natural world that is inconvenient for (deniers). Willed ignorance is a disaster for climate policy in particular. It is worse as an approach to science in the public sphere. For centuries, human curiosity led us to the point where we know so much; it would be good — more, it may well be a matter of survival — to put all that knowledge to use.
Thomas Levenson is a professor of science writing at MIT and an Ideas columnist. His latest book is "The Hunt for Vulcan."