Illustration: Aïda Amer/Axios

Climate change wasn't front and center in last night's Democratic primary debate, but the topic produced some noteworthy moments.

Why it matters: It was the last debate before the Iowa caucuses, and multiple polls have shown that climate is among voters' priorities there.

A brief but telling scuffle broke out when a climate question arrived near the end.Amy Klobuchar, asked why she doesn't support calls to ban fracking, said natural gas is a "transition fuel" on the path to carbon neutrality by midcentury.

The other side: Bernie Sanders shook his head as she spoke. His plan includes a fracking ban and is generally far more aggressive than Klobuchar's.

  • Klobuchar also touted carbon pricing, which has lost cachet on the left and isn't in Sanders' plan.

Quick take: It highlighted the divide between moderate and left Democrats over the role of natural gas in the energy mix.

  • A standing reminder that a fracking ban is DOA in Congress (though a Democratic president could restrict fracking federal lands), and getting to net-zero emissions by 2050, let alone sooner, would be immensely difficult.

The big picture: Several moments captured the way climate is now stitched into the fabric of Democrats' discussion of lots of topics — and why Sanders' approach excites young activists.

Take the moment when Sanders cited the absence of climate provisions to explain his opposition to the USMCA trade deal.

  • When his USMCA answer veered into a mini-speech on climate, The Des Moines Register's Brianne Pfannenstiel interjected, "We're going to get to climate change, but I'd like to stay on trade."
  • Sanders replied, "They are the same in this issue."

My thought bubble: That view of climate as the mother of all cross-cutting topics is a reason Sanders is so popular in the Green New Deal camp.

More climate highlights

1. Candidates knew their audience as they made the case that their plans would help Iowa farmers. For instance, after frontrunner Joe Biden said, "We're the only country in the world that's ever taken great crisis and turned it into great opportunity," he added:

  • "And one of the ways to do it is with farmers here in Iowa, by making them the first group in the world to get to net-zero emissions by paying them for planting and absorbing carbon in their fields."

2. The heat from Australia's fires has made its way into the U.S. contest. Sanders, Tom Steyer and Pete Buttigieg all cited the fires. "This is no longer theoretical and this is no longer off in the future," Buttigieg said of the effects of climate change.

3. Biden didn't emphasize climate as much as Sanders and Steyer. He stuck to his message of citing his past work (including green energy spending in the 2009 stimulus) and job opportunities from building out climate-related infrastructure.

  • But, but, but: Biden gaffed a bit when he touted "12 billion gallons of gasoline — barrels of gasoline to be saved immediately" by reinstating mileage rules implemented when he was VP.
  • He's presumably referring to 2012 estimates that standards imposed through 2025 would save 12 billion barrels of oil over the life of the covered vehicles.

Go deeper: 4 takeaways from the 7th Democratic debate

Go deeper

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Joe Biden speaks Friday about "The Biden Plan to Beat COVID-19," at The Queen theater in Wilmington, Del. Photo: Drew Angerer/Getty Images

This is one of the bigger signs of trouble for President Trump that we've seen in a poll: Of the final debate's seven topics, Joe Biden won or tied on all seven when viewers in a massive Axios-SurveyMonkey sample were asked who they trusted more to handle the issue.

Why it matters: In a time of unprecedented colliding crises for the nation, the polling considered Biden to be vastly more competent.

Bryan Walsh, author of Future
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Entomologists in Washington state on Thursday discovered the first Asian giant hornet nest in the U.S.

Why it matters: You may know this insect species by its nom de guerre: "the murder hornet." While the threat they pose to humans has been overstated, the invading hornets could decimate local honeybee populations if they establish themselves.