The five Conservative leadership contenders have been challenged on climate change by a Scottish teenager.
During the debate broadcast on BBC One, climate striker Erin, 15, asked the candidates to commit to net zero carbon emissions by 2025.
Last week the UK government said it would cut emissions to almost zero by 2050.
None of the leadership hopefuls agreed to bring forward the deadline, and Erin said she was not impressed.
Boris Johnson, Jeremy Hunt, Michael Gove, Sajid Javid and Rory Stewart took questions from members of the public during the televised debate.
They are in the race to be the next leader of the Conservative party after Dominic Raab was earlier eliminated in the second round of voting.
Erin, who joined the broadcast from Glasgow, said she was asking her question on behalf of UK youth strikers, who walked out of school to raise awareness of climate change.
She said: "Can you promise that the environment will be your top priority if elected? Will you commit to net zero carbon emissions by 2025?"
Emily Maitlis, who chaired the debate, asked for a show of hands from those who would agree to the 2025 deadline - but there were none.
Rory Stewart said he was "deeply proud" of the new 2050 target and it was the most ambitious set by an advanced industrial economy.
He pointed out that, as international development secretary, he has said he wants to double the amount spent on climate and the environment.
And he said the UK has "fantastic resources" in the North Sea for carbon capture and storage, and for offshore wind.
"I would like to make us the leader in the world in driving green and environmental change," he added.
Michael Gove, who has met leading activist Greta Thunberg, praised climate strikers for raising awareness of environmental issues - but added "even though I think folk should be in school".
As environment secretary, he said: "I've introduced a 25-year environment plan to ensure that we help increase the amount of energy that is generated by renewables but also that we do everything from restoring the habitats - the trees and forests which act as carbon sinks - to making sure that we rid our air of the pollution that is choking the lungs of young people, and also rid our seas of the plastic which is a scourge."
He told Erin: "As environment secretary, I have one of the best jobs in the world because I can do everything I can to make sure that your generation get a cleaner and greener planet."
Meanwhile Boris Johnson was challenged on plans to expand Heathrow airport, of which he has been a vocal opponent in the past.
He said he continues to have great reservations about building a third runway for reasons of air quality and noise pollution.
And on the broader issue of the environment, he pointed to his record as London mayor.
He said that the city's population increased by 200,000 over about eight years, but CO2 emissions were cut thanks to the use of technology.
Foreign Secretary Jeremy Hunt welcomed Erin's challenge to be ambitious with the government's environmental goals.
"The question is not whether we're doing something, it's whether we're doing enough," he said.
"It's a fundamental matter of trust between teenagers and all of us here who are in our 40s and 50s, as to whether young people can actually trust the next generation."
Home Secretary Sajid Javid admitted that he gets similar questions from his 16-year-old daughter.
"We have done quite a few things over the last few years... but nowhere near enough," he added.
"We have to be a lot more ambitious especially in terms of renewable energy, in terms of helping people to buy electric cars, also to charge electric cars, so we can be ambitious, and at the same time create a lot more jobs and be a world leader."
However, Erin said none of the contenders "really impressed" her.
"Climate change isn't an issue of tomorrow, it's an issue of today and we need to create drastic, critical action and I don't think any of you are willing to offer that," she said.
The Conservative leadership race will continue on Wednesday, with a third ballot of party of MPs.