The most important aspect of climate change is this: It is a threat to human society.
That's still not how most people think about it, because of the way it has been coded as a boutique environmental issue, akin to wetlands preservation or protecting endangered species. But unchecked climate change will devastate human societies across the globe (in addition to wild animals and landscapes).
So when President Trump's recently appointed EPA chief Scott Pruitt responds to a question about carbon dioxide by saying that he "would not agree that it's a primary contributor to the global warming that we see," he's not just being a science denier. He's a clear and present danger to American national security, and humanity as a whole.
Now, Pruitt's statement is blinkered ignorance on the science too. You can ask huge organizations of climate scientists what they think about this, or you can do studies examining published research to get their views.
Or you can just crack open a science textbook and do a bit of simple reasoning.
Electromagnetic spectroscopy shows that carbon dioxide absorbs infrared light. The physics of heat predicts that if a concentration of an infrared absorber increases in the atmosphere, the equilibrium temperature should also increase, as infrared light that previously escaped to space was absorbed instead. (If you want to get fancy, quantum theory explains why molecules like carbon dioxide absorb infrared radiation and predicts with astounding accuracy exactly what wavelength is absorbed.) Finally, it turns out if you burn billions of tons of fossil fuels for decades on end, the concentration of carbon dioxide increases.
Indeed, when you conduct an experiment comparing a control greenhouse to one with more carbon dioxide, the latter does show an increased temperature (Mythbusters did this one time). And if you do all the meticulous physics and measurements, it still turns out carbon dioxide is responsible for something like two-thirds of total warming effects.
The Intergovernmental Panel on Climate Change, in its most recent summary for policymakers, writes that unchecked climate change will do things like "undermine food security," "reduce renewable surface water and groundwater resources in most dry subtropical regions," drive "increases in ill-health in many regions," sharply increase urban risk of "heat stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought, water scarcity, sea level rise, and storm surges," and lead to "major impacts on water availability and supply, food security, infrastructure, and agricultural incomes" in rural areas. (And note that the IPCC, in keeping with any huge collection of scientists, tends to understate things if anything.)
For the United States, that means more flooding, more drought (and more yanking back and forth between the two, like in California), more extreme weather, more heatwaves, loss of farmland, and loss of coastal areas to rising seas. And the most recent science is even more dire — a 2016 study doubled the estimate of sea level increase by 2100 to nearly two meters, and estimated a 15-meter rise by 2500 from Antarctic ice melt alone — which would put the former Miami about 100 miles out to sea. A new study of half a million square miles of permafrost found that about 10 percent (a piece the size of Alabama) are now melting and collapsing, very likely dumping millions of tons more of carbon dioxide into the atmosphere.
The hour is very late already. We are not even close to the right track for getting our emissions down fast enough to avoid disaster. If America does not implement some extremely aggressive climate policy within the next decade or so (as part of a concerted worldwide effort), we quite literally risk the destruction of the United States as an organized community.
Being a science denier is one thing. If Pruitt denied the heliocentric model of the solar system, it would be no less intellectually humiliating, but wouldn't mean much.
But he is now in charge of the agency charged with defending this nation against its most serious threat. Instead of organizing a response, he is stacking the EPA with more science deniers. Trump's first budget would slash EPA funding by a quarter, zeroing out many important climate and anti-pollution programs. This is unsurprising given Pruitt's former job as the attorney general of Oklahoma, where he made a name for himself suing the EPA over and over and over again trying to stop just those things.
It genuinely is akin to a fifth column — save not one loyal to a foreign power, but one to the oil and gas industry, with whom Pruitt has been working hand-in-glove for his entire career. To protect a few more quarters of fat profits for oil companies, he is apparently happy to risk national destruction.