Sir Jonathon Porritt doesn't mince his words. The long-time environmental campaigner isn't afraid to dish out some harsh criticism on the government's track record of sustainability and eco-friendly policies.
"I'm absolutely certain that history will judge this particular Conservative Government to be one of the worst on environmental issues," he says, frankly. "Its calamitous energy policy - turning its back on energy efficiency and renewables in favour of fracking and nuclear; its refusal to address chronic air pollution issues in most of our big cities; its damaging reforms to the planning system, making it so much easier for developers to push through their proposals whilst putting the green belt more and more at risk - I could go on.
"It's a long list!"
The former Green Party co-chair attributes this government's shaky record to there simply being not enough Conservative MPs who "really care" about the environment.
"Many are in a state of deep ideological hostility towards anything that is seen to challenge the dominance of conventional growthist thinking," he muses.
And when it comes to Boris Johnson, Porritt says he "failed utterly" in tackling housing and air quality during his tenure as Mayor of London.
The recently elected Sadiq Khan has pledged to tackle the latter issue, setting a target for London to run on clean energy by 2050.
But, Porritt warns, no vision for the capital city will be worth the time it took to dream it up if it ignores the mandate from the Paris Agreement, which the UK signed up to last year.
"The basic message from Paris is incredibly simple: that every nation and every city, rich world or poor world, will need to move towards and ultra-low carbon economy as fast as possible.
"And those decisions have to start right now, given the length of time it takes for major planning and infrastructure decisions in big cities to work their way through the system."
And it's important for cities such as London to take the lead, adds Porritt, who was chair of the government's independent advisory Sustainable Development Commission for nine years, which was abolished in 2010 by the Lib-Con coalition.
"Take transport, for starters. By 2030, working together, governments and City Mayors will have to have eliminated new sales of all petrol and diesel vehicles.
"Almost all vehicles will need to be either all-electric or using hydrogen-powered fuel cells. Absolute priority will need to be given to cycling and pedestrian infrastructure. Between now and then, London’s sub-optimal congestion charge will be scrapped and replaced by a radical - ie, genuinely behaviour-changing - greenhouse gas protocol."
If Khan does succeed in introducing the ultra-low emission zone he's promised, the air Londoners will be breathing will be unpolluted, for the first time in a very long time.
"Thousands of deaths and hundreds of thousands of hospital admissions will have been avoided," Porritt continues. "Streets will be safer, kids will play outside across large parts of the city zoned primarily for people, not for cars. Suburbs will be ‘regreened’, with more space set aside for trees, parks, city farms and mini-market gardens."
The UK certainly has its work cut out for itself, although our international reputation for priorities is "still reasonably solid", according to Porritt.
"The fact that we’re one of the world leaders in terms of the percentage of GDP that we commit to international aid (0.7%) is enormously important – and some of this money goes to address important international environmental challenges."
However the green campaigner does feel the nation has slipped when it comes to being leaders on climate change.
"We were once.. but that's not the case any longer.
"Although the Climate Change Act [legally binding targets introduced in 2008 to limit the UK's greenhouse emissions] remains a uniquely significant legislative measure, it’s already clear that we’re not going to meet either the targets laid down in that Act, or the targets that we’re committed to under the EU’s Renewable Energy Directive.
"Far too many MPs in the Conservative Party have followed the line of George Osborne, Chancellor of the Exchequer, in arguing that addressing climate change and building up a green economy is somehow inconsistent with the business of achieving increased GDP. The level of ignorance on these things beggars belief."
But it's not just Tory politicians who Porritt takes to task. When I ask how the government can be held to account on environmental issues, he tells me: "Good government backbenchers - of whom there are now very few in the Conservative Party; a focussed and smart Opposition - unfortunately, most Labour MPs still don’t prioritise environmental issues, and for Jeremy Corbyn himself it’s clearly not a big thing; and citizens always ready to mobilise to help stop bad things happening and help good things to happen instead."
Porritt is keen to highlight this kind of activism is dependent on environmental NGOs, "for which the UK has been rightly renowned for over the past few decades". But, he adds, they're "a lot less effective" than they used to be.
"Big organisations like the WWF and RSPB are all but invisible in terms of UK policy, let alone any protest; Friends of the Earth only just rediscovered its campaigning mojo after at least five years in the wilderness.
"Which means that more and more is now dependent on Greenpeace and the social media initiatives like Avaaz and 38 Degrees."
Although Porritt withdrew from party politics in 1996 when he founded sustainable development charity Forum for the Future, he remains a public advocate for the Green Party - and says the party has "two big challenges" following Natalie Bennett's decision to step down.
First: the "unyielding reality" that is the UK's first past the post electoral system.
"It’s incredibly difficult, if not possible, to get any major electoral breakthrough in a General Election," he admits. "The Green Party only has one MP, Caroline Lucas in Brighton – and it will be a huge challenge to increase that number in the 2020 Election.
"The success of other Green Parties in Europe is closely correlated with the ‘proportionality’ of their electoral systems – just look at the recent parliamentary elections in Scotland, where we saw the election of six Members of the Scottish Parliament."
The second? The way the Green Party positions itself on the political spectrum.
"Because of the Party’s uncompromising commitment to a fairer economy and to the redistribution of wealth, most individual Greens locate themselves on the left of that spectrum," Porritt explains.
"But Natalie Bennett and the Party in general were very strongly criticised by some during the General Election for failing to give sufficient weight to environmental issues – and to accelerating climate change in particular.
"I don’t think that was actually fair, but this is never going to be an easy balancing act."
When Bennett announced she would not be standing for re-election, she criticised the "political culture and the media circus".
"As the party began to be taken more seriously than ever the level of media scrutiny and, at times hostility, has been gruelling," she wrote in a HuffPost UK blog. "I’m not a smooth, spin-trained, lifetime politician."
Porritt has similar feelings regarding the way in which members of the media treat politicians.
"I think Natalie has been a good party leader - she had to do a lot of heavy lifting on internal issues, and she was particularly good at motivating and inspiring local Green Party groups.
"But she didn’t much like the media stuff, and had a couple of ‘bad moments’, at which point the media went into attack-dog mode – which is miserable for anyone. Regardless of their party.
"And a big part of the problem does indeed lie with our media," he continues. "They’re often not very well-informed, and some are positively hostile, reflecting the ideological positioning of mainstream right-wing media in the UK."
Porritt describes informed and objective environmental journalists in particular as "a vanishing breed here in the UK", and reiterates their role in holding politicians to account when individuals can't.
"People do care a lot," he says. "and one of their biggest concerns is what they can actually do to address the things that matter to them most.
"Inevitably, however, there are some serious inconsistencies in the way in which people articulate these concerns.
"They often say that they want companies to be doing more about the environment, but when they have a chance to demonstrate that in practice - at the point of purchase - they seem to be rather more reluctant. And though people often talk about their readiness to often vote for green politicians, outside the Green Party, come an election, they often vote for much more traditional reasons."
But when it comes to climate change, it's perhaps time both politicians and the public woke up to the realities.
"By far the biggest issue that we face here in the UK is just how much we’re going to have to do to meet the challenge of accelerating climate change," Porritt says.
"In Paris, world leaders got their heads around the threat of runaway climate change for the very first time."
Every country needs to do "everything in its power" to limit the average temperature increase by the end of the century to no more than 2C, and then to aim for an even lower threshold of 1.5C, in order to halt climate change.
"You may think the difference between 2C and 1.5C doesn’t sound like much," Porritt continues. "But the difference is massive.
"Forget all that political stuff about targets and deadlines and so on, and just think carbon budgets, which is the metric climate scientists prefer.
"Scientists have calculated we can put no more than 650bn tonnes of carbon into the atmosphere if we want to stay below that 1.5C threshold.
"We’ve already put 550bn tonnes into the atmosphere, leaving a residual ‘budget’ of just 100bn tonnes.
"At the moment, we emit about 10 billion tonnes per annum. Which means, putting it as starkly as people need to hear it, that our remaining budget will be all used up in just 10 years’ time.
"I doubt a single world leader understood the implication of that in Paris," Porritt adds. "But they will by 2020!"