Skiers ski on a thin layer of snow in Villard-de-Lans, central-eastern France, on Dec. 27. (Jean Pierre Clatot/Agence France-Presse via Getty Images)

LAST WINTER, bitter cold on the East Coast prompted Sen. James M. Inhofe (R-Okla.) to take a snowball onto the Senate floor in mockery of climate scientists. This winter, the weather is so warm that there are not two snowflakes in the Washington area for Mr. Inhofe to scrape together.

Just as last winter’s cold did not disprove global warming, this winter’s warmth does not, in itself, establish that humans are raising Earth’s average temperature. Rather, it is the long-term trend that matters — and that is concerning. The past decade was warmer than the previous one, which was warmer than the one before that, and so on. So far, this decade looks sure to fit into that trend.

We may, too, be getting a taste of what a warming world feels like. According to a 2014 paper, one potential effect is a doubling of the number of strong El Niño years. That’s the phenomenon that has perturbed the jet stream and pushed this winter’s temperatures up. Though the El Niño variation predated human influence, it, among many other natural cycles, probably will not be immune to it. And even if this year’s El Niño cannot be blamed squarely on climate change, it nevertheless feels like a fitting end to a remarkable year in the policy and politics of global warming.

For those following the climate debate, 2015 brought some inspiring highs — and disheartening lows. President Obama’s Environmental Protection Agency finalized landmark carbon dioxide regulations that promise to slash greenhouse gas emissions from U.S. power plants. This policy played a crucial role in galvanizing the international climate effort, which culminated last month in a huge international climate deal. The Paris agreement will not cut emissions enough to avoid serious risks. But it will get the world a good chunk of the way there, and it represents the beginning of a process in which all major emitters will be expected to step up.

The very same EPA rules also inspired a race among Republicans to embarrass themselves. A standard critique, such as that of Sen. Marco Rubio (Fla.), was to argue that climate regulations will uselessly harm the economy; that exaggerates the potential costs without acknowledging the unprecedented global agreement the measure helped to produce. Sen. Ted Cruz (Tex.) went further, holding a hearing on climate science with a witness list that was a funhouse-mirror image of the scientific community: short on those who accept mainstream climate science, packed with climate critics.

Mr. Cruz insisted that there has been a “pause” in global warming since 1998, a date critics choose as their starting point because it was another El Niño year marked by very high temperatures. Like Mr. Inhofe and his snowball, Mr. Cruz’s point ignored the long-term nature of the warming trend. It also might be flatly wrong: A June paper in the top-flight journal Science found that the warming “pause” reflected biases in temperature data rather than a significant plateau in real temperature rise.

Alas, this finding also led to a nasty GOP reaction. House Science, Space and Technology Committee Chairman Lamar Smith (Tex.) subpoenaed emails relating to the study, which came from the National Oceanic and Atmospheric Administration. Congressional oversight of federal spending is important. That’s no excuse to conduct fishing expeditions designed to personally discredit scientists and undermine peer-reviewed research with lines from informal emails.

With the events of the past year in mind, the presidential candidates — and American voters — must ask themselves: Do they want to build on 2015’s climate progress, or do they want more of last year’s climate buffoonery?