Fred Millar talks fast. He's sitting at the witness stand to tell the six-person jury what he knows about crude oil trains.

"The way the federal government measures the impact is in what they call 'societal damage,'" Millar, a 37-year hazardous materials and homeland security expert, explains. "And in their federal regulatory documents, they use the term 'societal damage': how much it might cost a community, or society, if you have a serious accident."

Millar flew out from D.C. for this trial, here at the squat, dun-colored Snohomish District courthouse, which sits across a strip of Lynnwood car dealerships and budget motels. The trial, which started on Monday, is not an everyday occurrence. It's one-of-a-kind, according to supporters of Abby Brockway, Michael Lapointe, Patrick Mazza, Jackie Minchew, and Liz Spoerri—the five local climate activists charged with criminal trespass and blocking a train. They call themselves the Delta 5, after the BNSF Delta rail yard in Everett.

The Snohomish case is the first time, supporters say, that an unusual legal defense has been allowed by a judge in a US climate disobedience trial. The defendants' lawyers are using a "necessity" defense, a legal maneuver arguing that the harm of whatever was being protested exceeds the harm caused by the civil disobedience itself.

In this case, the defendants are arguing that the harm caused by crude-oil train traffic and climate change is greater than the harm caused by the eight hours they spent locked to a structure blocking the tracks.

If they're acquitted, the jury will have agreed: that business-as-usual fossil fuel dependency is a real and imminent threat. A big enough threat, in fact, that it was necessary for the defendants to do what they did.

The bald and bespectacled Millar ticks off the crude-oil train safety issues in his Southern twang: aging rail cars, speeds at which an accident would result in them being damaged, the volatility of the materials, the crumbling infrastructure, the railroad industry's resistance to regulation, the lack of regulation, the lack of public information, the frequency of accidents, the societal damage. And then he turns to Lac-Mégantic, Quebec, where 47 were killed in a catastrophic crude-oil train derailment in 2013. This, he says, wouldn't even qualify as a "worst-case" scenario; it was at night, with few people working, in a small town.

"There was only one thing open in town, and that was a little music cafe because two groups were having birthday parties with their friends," Millar says.

He continues: "When the people who went out to smoke [cigarettes] heard the huge crash, they ran up the hill, they reported, and then they had to look back and see all their friends burn up... And what they described was rivers of fire. And there have been academic studies about that."

The aim of all this, according to Tim DeChristopher, a climate activist who founded a nonprofit dedicated to promoting the climate necessity defense, is to show "how urgent the climate crisis is and how much our government is failing to adequately address that crisis."

DeChristopher, who spent 21 months in prison himself after blocking bids on Utah oil and gas leases, believes an acquittal of the Delta 5 would help undermine the government's moral authority on climate change.

Which is why, earlier in the day, defense lawyers brought in Richard Gammon, a professor emeritus of chemistry at the University of Washington, and asked him, point-blank, if he thought the government was doing enough to address the realities of climate change.

"No, the federal government hasn't done enough," Gammon told the jury. He summed up his thoughts on state and county governments, too: "I think there's much more that can be done at all levels."

Defendant Patrick Mazza says a successful climate necessity defense could pave the way for all sorts of future social justice causes. "It sets up a new precedent for citizens to have their voice heard where other legal channels are blocked."

But what kind of can of worms does that open? If a necessity defense relies on a jury's judgment of what kind of civil disobedience is necessary, what kind of civil disobedience would be considered necessary?

"Certain communities will be sympathetic with other points of view," Bob Goldsmith, the defense lawyer for Abby Brockway, tells me. Goldsmith tried another necessity defense case back in the '70s for a nuclear power-plant protest—and helped get the defendant acquitted.

So I ask him a different question: Does the necessity defense rely on the jury's sympathies?

He pauses. "I don't know."

The Delta 5 trial continues, and closing arguments are expected today. Check back for more updates.

Here's part two and the final verdict of the trial.