Democratic presidential hopefuls Sen. Cory Booker (N.J.), former vice president Joe Biden and Sen. Kamala D. Harris (Calif.) take the stage for a debate in Detroit. (Scott Olson/Photographer: Scott Olson/Getty)

In last week’s debates, the Democratic candidates repeatedly discussed how to respond to what an upcoming CNN forum calls the “climate crisis.” Over two days, the party’s presidential hopefuls sparred over whose policies would best address what the United Nations has called “the defining issue of our time.”

Scientific experts warn that rising temperatures, longer droughts and more extreme rainfall — all linked to the carbon released by burning fossil fuels — will soon affect everything from extreme weather to conflict and violence, from mass migration to imperiled physical and mental health. Even Wall Street is examining how climate change — and each community’s preparation for it, or lack thereof — will affect creditworthiness. Investors worry that coastal cities like Miami may be unable to pay their debts if rising seas damage infrastructure and hurt revenue from tourism.

What are Democrats promising?

Proposals vary. Some candidates, including Sens. Cory Booker of New Jersey, Kamala D. Harris of California and Bernie Sanders (I-Vt.), have backed the Green New Deal, congressional Democrats’ proposal to simultaneously address climate change and create jobs that will offset economic inequality. The Green New Deal relies on innovative technologies and carbon taxes to wean the U.S. off fossil fuels.

Others Democratic hopefuls, including Sen. Kirsten Gillibrand of New York, have gone further, calling for climate accountability. Gillibrand promises to “hold [corporate] polluters accountable for the damage they’ve caused the American people.” Former congressman Beto O’Rourke of Texas and Washington Gov. Jay Inslee promise to end tax breaks for fossil fuel companies. Inslee has made tackling climate change a top campaign priority and says, “like tobacco companies, [fossil fuel] industries have poisoned our air and polluted our planet for decades without recourse. They must answer for that.”

Others oppose penalizing the fossil fuel industry. Former Colorado governor John Hickenlooper — who worked as a geologist — prefers to “unleash market forces to help solve the emissions challenge.” Taxing carbon features prominently in his plan, a mechanism he (and virtually every other presidential candidate) sees as a way to “quickly and cost-effectively lower carbon emissions.”

What’s behind the recent policy push?

For decades, climate change failed to make headlines or top Americans’ list of concerns, despite scientific warnings. A Science Advisory Committee convened in 1965 warned President Lyndon Johnson that climate changes caused by burning fossil fuels could be harmful. A 1989 report by the California Energy Commission advised local lawmakers that fossil fuel emissions would probably bring more floods, fires and disease.

But that may be changing. A recent Pew Research Poll found that two-thirds of Americans say too little is being done to address climate change. Of course, Republicans and Democrats are sharply divided on the issue. More than 60 percent of Republicans and just 20 percent of Democrats think offshore drilling and coal mining should continue. But members of both parties overwhelmingly agree that the United States should invest in renewable energy technologies such as solar panels and wind turbine farms.

The Democratic candidates are responding to their base’s strong concern about the climate: More than 82 percent of Democrats or Democratic-leaning independents say climate change is a top priority.

Would Democratic proposals work?

The Democrats’ plans largely rely on carbon taxes, “green technologies” or both to address climate change.

So what do these plans involve? Under a carbon tax, consumers and businesses are charged a fee for using fossil fuels such as oil and natural gas. This pushes them to reduce energy consumption, increase energy efficiency and switch to cleaner alternatives. One group of prominent economists argues that carbon taxes send a “powerful price signal that harnesses the invisible hand of the marketplace to steer economic actors towards a low-carbon future.”

Canada, Sweden and Britain already have carbon taxes. But carbon taxes disproportionately affect low-income households, which spend a larger-than-average percentage of their earnings on energy bills. As a result, many Democrats are wary, citing their concern for the poor.

What’s more, carbon taxes would have to be significant to curb climate change. An independent watchdog concluded that Canada’s carbon tax was inadequate to reach the country’s climate goals — and would have to increase significantly to make a dent in emissions. In the United States, researchers estimate that a dime-per-gallon increase in the gasoline tax — which currently averages 52.4 cents per gallon — would reduce vehicle carbon emissions by just 1.5 percent. But raising fuel taxes too quickly can cause unrest. France experienced that last year after President Emmanuel Macron proposed boosting the gas tax to curb emissions; “yellow vest” protests erupted across the country, forcing him to back down.

Green technologies pose their own challenges. Batteries and wind turbines rely on raw materials that will be difficult to source. For instance, cobalt comes from Congo, where mining practices have raised questions about human rights abuses and child labor. A 2016 Amnesty International report found that children as young as 14 were spending up to 12 hours a day in some mines there searching for cobalt. Electric vehicles remain costlier than traditional gas guzzlers, and the Trump administration has resisted efforts to subsidize purchases.

Switching to mostly renewable energy would also require dramatic changes in infrastructure — from modernizing electric power grids to improving power storage. Upgrades are estimated to cost billions or trillions of dollars. Finally, the United Nations has warned that using green technology could be less beneficial (and in some instances more harmful) than expected.

Democratic presidential hopefuls have — thus far — avoided these issues, opting instead to emphasize the policies’ broad strokes. Making the promises might be easy, but delivering on them might be much less so.

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Ashley Nunes is a senior research associate at Harvard Law School where he studies how innovation affects economic growth.