Democrats will pit their climate plans against each other on Wednesday at a seven-hour CNN town hall, as world scientists warn the window is narrowing to prevent catastrophe.

With the planet already about 1C (1.8F) warmer than before industrialization, the United Nations’ climate panel says that keeping the increase to 1.5C would require “rapid, far-reaching and unprecedented changes in all aspects of society”.

Carbon dioxide emissions in the US and abroad need to fall significantly in the next decade and reach “net zero” by 2050, according to the UN. Without massive changes, rising temperatures will worsen extreme weather, water and food instability, political unrest and poverty.

Q&A What is the difference between carbon free and carbon neutral or "zero carbon"? Show Hide Carbon free means eliminating all C02 emissions from power plants, cars, industry and agriculture. Carbon neutral or "net-zero carbon" would allow some emissions to continue as long as there are offsets, such as the planting of trees that absorb heat-trapping gases.

Below we assess each candidate’s stance on the climate crisis and also the judgment of outside organizations.

Three environmental groups are evaluating the candidates based on their plans and legislative records. Greenpeace assigns a letter grade for ending fossil fuel use and supporting a Green New Deal. Data for Progress tallies which elements of a Green New Deal candidates back. And the League of Conservation Voters ranks former lawmakers based on how they have voted in Congress on environment-related laws.

However, some candidates were still releasing plans shortly before the town hall, amid pressure from the Democratic base, as well as school strikers and other protesters.

To match what science requires, the next president would need to cut heat-trapping pollution from transportation, electricity, agriculture and land use, battling with Republicans who broadly downplay the crisis and oppose restrictions for industry.

Some climate advocates worry Biden would not do enough to eliminate fossil fuels from the US economy. He stumbled over a debate question about whether coal and gas would have a place in his administration, saying “we will work it out” and promising to eliminate subsidies. But his rankings are decent and he has committed to the timeline outlined by the UN climate panel.

*Score may change based on newly released proposals.

Warren would spend $3tn over a decade to decarbonize buildings, transportation and electricity, according to a plan released on Tuesday. Her blueprint praises the extensive climate proposals of the Washington governor, Jay Inslee, who dropped out of the race last month. She had previously made pitches for green manufacturing, better land use, a green military and more clean energy research.

Q&A What are the top sources of heat-trapping emissions in the US? Show Hide Transportation produced 29% of greenhouse gas emissions, followed by electricity at 28%, industry at 22%, commercial and residential at 12% and agriculture at 9%, according to Environmental Protection Agency data from 2017.

Invoking the Green New Deal, he has proposed alongside congresswoman Alexandria Ocasio-Cortez, Sanders aims to eliminate all carbon emissions by 2050, not just reach “net-zero”. His goal would be harder because it wouldn’t count offsets to climate pollution, like forests that absorb the gas. He would also spend far more than his competitors.

Sanders wouldn’t give new licenses to nuclear plants, which currently account for more than half of carbon-free energy in the US. And he is not a supporter of technologies to capture carbon emissions – which scientists say will be necessary for the challenge ahead.

Among the candidates, he has the longest history of advocating for radical change to stop the climate crisis.

*Score may change based on newly released proposals.

Harris’ plan, released Wednesday, falls between Biden and Sanders in terms of aggressiveness and spending. She would also tax carbon pollution.

Q&A What sources of energy does the US use? Show Hide Most of the energy the US uses comes from oil made into gasoline for vehicles. For electricity production, 35% is from natural gas, 27% is from coal, 19% is from nuclear and 17% is from renewable energy (hydropower, wind, biomass and solar), according to the Energy Information Administration.

*Score may change based on newly released proposals.

Buttigieg’s plan doesn’t specify how much he would spend in total, but his campaign told Vox it could reach around $1.5tn to $2tn. He wants to create 3m clean energy jobs, and he backs a price on carbon with rebates to low- and middle-income Americans.

Yang would also tax carbon pollution and use revenues to fund alternative research, upgrade energy systems and subsidize fuel costs for low-income Americans. He says people will have to move to higher ground as seas rise.

O’Rourke stops short of calling for a carbon tax, saying he backs a “legally enforceable standard” that will “send a clear price signal” to curb carbon emissions.

Q&A Which countries contribute most to the climate crisis? Show Hide China produces the most heat-trapping pollution, followed by the US. But historically, the US has contributed more carbon dioxide to the atmosphere than any other nation. The US also has high emissions per capita, compared to other developed countries. And Americans buy products made in China, therefore supporting China's carbon footprint.

*Score may change based on newly released proposals.

Booker is largely focused on environmental justice, and calls for the White House to coordinate to protect communities from environmental threats, including lead pipes that have contaminated drinking water where he was mayor, in Newark. He would ban new fossil fuel leases on public lands and spend on massive reforestation and wetlands restoration. He supports a carbon tax with dividends returned to people.

*Score may change based on newly released proposals.

Castro wants the whole country – including local governments and private entities – to invest $10tn over a decade to create 10m jobs and transition away from fossil fuels. He wants to cut emissions in half by 2030 and neutralize them by 2045.

*Score may change based on newly released proposals.

Klobuchar says she would “take aggressive executive action to confront the climate crisis” and introduce “sweeping climate legislation in the first 100 days of her presidency”. While she sets a goal for the US to be carbon-neutral by 2050, she has said she would not ban fracking for oil and gas and she supports “cleaner coal” technologies, according to Inside Climate News. She has also said she supports a Green New Deal but thinks it is aspirational.