Climate change is one of the biggest challenges facing the U.S., and the world overall, according to the scientific and national security communities. Yet the issue was virtually shut out by the three presidential debates.

In the debate on Wednesday night, the debate moderator, Fox News' Chris Wallace, chose to bring up the national debt, the crisis in the besieged Syrian city of Aleppo and abortion, among other issues, while ignoring climate.

The only time it was uttered was early in the debate when Democratic nominee Hillary Clinton raised it of her own initiative, mentioning during an answer that climate change "is a real issue" that needs to be addressed.

Climate advocates and experts were disappointed this issue was shut out of these contests. Climate change was not asked about at the only vice presidential debate, either.

So, four debates; four shout-outs by moderators to deficit scolds; not one question about climate change. It's really disgraceful. — Paul Krugman (@paulkrugman) October 20, 2016

Remember, of course, that the debates were held in what is virtually certain to be the warmest calendar year on record since thermometer records began in 1880.

Debate concludes with no Qs on climate change, though 2016 is expected to be the hottest year on record https://t.co/B6YrXZq0k3 #DebateNight — Climate Nexus (@ClimateNexus) October 20, 2016

It's sad that the closest we came to a climate question was Ken Bone's inquiry about preserving jobs in fossil fuel industries 😐 https://t.co/zrDdG1mN2g — Gabrielle Canon (@GabrielleCanon) October 20, 2016

Three debates

Zero climate questions — Jason Kowalski (@JasonK350) October 20, 2016

Had Wallace asked about the issue, he would have found out that Clinton has a plan for cutting emissions of planet-warming greenhouse gases by more than 80 percent by 2050.

Republican nominee Donald Trump, on the other hand, has maintained that the issue is a hoax, and favors pulling the U.S. out of the Paris Agreement on Climate Change. His climate stance has been condemned by numerous prominent scientists. (At the first debate, Trump denied he's a climate denier in response to an economics question.)

After tonight’s debate, Trump will have escaped 3 debates without having to reveal that he does not believe in climate change. #Mediafail — Touré (@Toure) October 20, 2016

Bonus: President Obama and Leonardo DiCaprio warn of climate change dangers