Another insane cold wave — not the infamous “polar vortex ” but its evil twin — is bringing sub-zero and single-digit temperatures to much of the nation. And global warming may be even more extreme, and potentially more catastrophic, than climate scientists had feared.

This is, of course, no contradiction. The rallying cry of the denialists — “It’s really cold outside, so global warming must be a crock!” — can be taken seriously only by those with a toddler’s limited conception of time and space. They forget that it’s winter, and apparently they don’t quite grasp that even when it’s cold in one part of the world, it can be hot in another.

Indeed, while the United States is having an unusually frigid month, Australia has been sweltering through record-breaking heat. Play had to be interrupted at the Australian Open tennis tournament when temperatures in Melbourne reached 109 degrees; one player said her plastic water bottle began to melt. The extreme heat came as officials reported that 2013 was the hottest year in Australia since record-keeping began more than a century ago.

On the global scale, 2013 was “merely” the fourth-warmest or seventh-warmest on record, depending on whether you believe the National Oceanic and Atmospheric Administration or NASA. The agencies take slightly different approaches in analyzing and extrapolating the available data, which accounts for the discrepancy, but they agree on the big picture: It’s getting hotter.

Nine of the 10 warmest years on record have occurred since 2002. Deniers who claim there has been a 15-year “pause” in global warming are cherry-picking the data to fit a pre-cooked conclusion: As a baseline they choose 1998, a year in which global temperatures took a huge, anomalous, one-time leap. If you treat 1998 as the statistical outlier that it obviously is, you see a steady and unbroken rise.

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Why is it getting warmer? There’s still just one explanation that fits the available facts: Since the Industrial Revolution, the large-scale burning of fossil fuels has increased the concentration of heat-trapping carbon dioxide in the atmosphere by an incredible 40 percent.

And according to an important new study, published Jan. 2 in the journal Nature, the eventual effect of human-generated carbon emissions could be greater than anticipated. Because of the impact of warming on cloud cover, the researchers calculated, average global temperatures could rise a full 7 degrees by the end of the century. This “would likely be catastrophic rather than simply dangerous,” the study’s lead author, Steven Sherwood of the University of New South Wales in Australia, told the Guardian newspaper. He said such a temperature increase “would make life difficult, if not impossible, in much of the tropics.” It would also guarantee the melting of so much polar ice that sea levels would rise dramatically, with dire implications for coastal cities around the world.

Is there uncertainty in these predictions? Of course. But human-induced global warming is the only explanation that fits the evidence. Until someone comes up with a better theory, it is foolish to wager that the near-unanimous consensus of climate scientists is totally wrong.

But, yes, we are fools. I’m not just talking about know-nothings on Capitol Hill such as Sen. James Inhofe (R-Okla.), who said that the cold spell this month “has to make everyone question — and I am going to tie this together — whether global warming was ever real.” I’m referring also to officials in the United States and around the world who accept the science and understand the peril but who will not take action because the economic and political costs are so high.

China, the world’s biggest emitter of greenhouse gases, is not about to shut down its economic growth and risk political instability. India is not about to abandon its quest to catch up with China. In African nations such as Nigeria and Kenya, which have burgeoning populations and high growth rates, the smokestacks are beginning to puff away. The United States, Europe and Japan will do what they can, at the margins, without surrendering the comforts that industrialization provides.

President Obama, who understands the science, should use his executive powers as best he can, not just to reduce carbon emissions but also to prepare the country for confronting the environmental, political and military hazards of a warmer world.

The day will come, I predict, when world leaders are willing, even desperate, to curb greenhouse gases. But by then, I’m beginning to fear, it will probably be too late.

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