All mayoral candidates say they want a greener London, so which has the best ideas for achieving this and which is the most likely to deliver? The Green Alliance think tank held a hustings at 1 Wimpole Street with Liberal Democrat Caroline Pidgeon, Conservative Zac Goldsmith, Labour’s Sadiq Khan and the Green Party’s Sian Berry on the panel. James O’Brien of LBC Radio chaired. Candidates’ hopes, dreams, bold assertions and no go areas regarding some of the main themes are compared and contrasted below.

Congestion charging

This is one of the two environment policy areas that make politicians most nervous, in spite of strong arguments that embracing it would be good for London. It isn’t only the green lobby that would like to see more congestion charging in the capital. A range of organisations, including the RAC Foundation, London First, the Royal Academy of Engineering and the Institution of Civil Engineers believe that an expanded and more advanced road-pricing system is desirable for economic as well as for air quality reasons. Even Boris Johnson’s transport strategy accepts that more congestion charging needs to be considered.

The problem is that, like any other curb on motorists, opposition to it can be fierce. That makes advocating it a risky proposition for mayoral candidates with serious chances of winning. Goldsmith has ruled it out following remarks he made to LBC last year which sounded like support for the idea. Khan has not proposed it.

By contrast, Pidgeon has argued for increasing the current charge, particularly in peak hours on polluting vehicles, and for piloting a new charging zone round Heathrow airport. And Berry is the boldest on this issue. She challenged fellow candidates to commit to “a new and smarter distance-based road charging system” in London with the money raised funding more cycling, walking and public transport. “We can’t do without traffic reduction measures in London and we can’t reduce air pollution with green vehicles alone,” she said.

Building on the green belt

This makes London politicians even more nervous than congestion charging. All four candidates say they would preserve the green belt, which takes up 22% of Greater London territory. Goldsmith has claimed that Khan can’t be trusted to do this and insisted that he can double house-building to 50,000 new homes a year by 2020 without reassigning any green belt land. That does not accord with the findings of a joint report by housing charity Shelter and planning consultants Quod, which advises many of London’s major property companies. They find that there simply isn’t enough brownfield land in London to enable a 50,000 a year target to be hit: “Any attempt to rely on brownfield land alone is doomed to fail.”

A great deal of green belt land is taken up with golf courses, pony clubs and even farming. Much of it isn’t even very green and little of it is freely accessible to the public. According to the Shelter/Quod report, persuading boroughs to redesignate some of its green belt would have to be part of any realistic plan to get 50,000 new homes built in London each year, which is the minimum target London needs to hit. Economist Paul Cheshire has calculated that a massive 1.6 million new homes could be built at average densities on London green belt space if just a fraction of its 32,500 hectares was reclassified. Yet no candidate will commit to looking seriously at doing this. Such is the harmful power of the green belt taboo.

Buses, cabs and diesel fumes

The candidates were asked to commit to phasing out all diesel black cabs and private hire vehicles (PHVs) by 2020 and to have the entire London bus fleet running on clean fuel by 2025. Doing this would mean a big step up from current policy. All new black cabs and PHVs will have to have a “zero emissions” capacity from January 2018 but diesel ones bought before that date could, as Goldsmith pointed out, still be around for years. “You can’t just push them off the road because you’ll push a lot of black cab drivers out of business,” he continued, adding that many are “right up against it anyway” because of “unfair competition with the likes of Uber.”

He said the obvious solution was to retrofit those vehicles to run on liquified petroleum gas: this would cost around £6,000 per cab, but would pay for itself within three years at most. On buses, he pledged that every one in Greater London would be “at the very least” compliant with the requirements of the forthcoming Ultra Low Emissions Zone (ULEZ) by 2025, through a combination of new buses coming on stream and retrofitting.

Pidgeon would want Transport for London (TfL) to “bulk buy huge numbers” of the latest, Chinese-made, electric cabs, which will become available next year in order to keep their prices down and would “look to help drivers in terms of subsidy.” She said there are single decker electric buses in use around the world and that London’s should be replaced with these “straight away,” along with piloting double deckers. She also stressed the importance of increasing the number of charging points for electric vehicles. Berry agreed that it was “only fair” that cab drivers should get the scrappage and retrofit grants they are seeking and said it was not acceptable that, as things stand, there would be over 2000 diesel buses still on London’s roads in 2020.

Khan warned that “we need to be realistic” about how many drivers would upgrade to new greener cabs, because taxis are expensive even at a discount price. But he wants to introduce ULEZ sooner than 2020 and look at expanding it to cover wider arterial roads. He criticised the cost of Boris Johnson’s New Routemaster and invited the audience to “imagine the race there would be” among bus manufacturers to produce an electric bus for not only London but many other cities” instead. He also favoured “clean bus corridors,” with new generations of buses introduced on to the most polluted routes.

He is not, however, scoring very well with the Clean Air in London campaign, which gives him four out of ten so far compared with eight for Berry and seven for Pidgeon. He’s doing better than Goldsmith, though. The noted environmentalist is trailing on just 3.25 out of ten.

Increasing walking and cycling

Berry said investment in making all London’s town centres better for walking should match what’s gone into Mayor Johnson’s Outer London “mini-Holland” cycling schemes and an investment levy raised on supermarkets. She stressed the importance of building support at local level for the introduction of cycling infrastructure and other street improvements to ensure they command public support.

Goldsmith too spoke about the three mini-Holland schemes. He judged the one in his parliamentary patch in Kingston, which was secured when Richmond Council was in Lib Dem hands, to be “wildly popular” with residents because “they did a good consultation job.” He contrasted this with the Enfield scheme which he said is “really, really unpopular and all the data suggests it’s been handled incredibly badly.” Why? “Because I don’t believe the ‘top-down’ approach works. You have to work with communities.” He made the same point about walking: “I am a localist and I will work with local authorities on their own schemes for walkability and making walking safer.” He said cycling injuries are “not just about infrastructure” and that action was required to remove HGVs from roads during peak travel times.

Pidgeon, who has already pledged a heavy goods and construction vehicle peak hours ban, wants “much more of London pedestrianised” and complained that “cycling and walking is buried in basically the bus directorate at TfL. The people who run it are bus men. What are they interested in? Buses.” She’d like walking and cycling given the same prominence. She wants to promote play streets and drew attention to the slow rate at which road crossings had been adapted to help visually impaired people.

Khan said he would install a “pedestrian champion” in City Hall. He invited people to think of the parts of the Thames and “green corridors” you can’t walk along “because it has become private land” and parts of London you can’t walk through because of roads. “We need to think differently about how London can work.” He praised the King’s Cross redevelopment for its public realm element. Like the others, he enthused about Mayor Johnson’s cycle superhighways and called for more quiet routes, cycle bridges and storage space.

The solar power city

Another challenge put to the candidates was to make London a “world-leading solar city” with a 10-fold increase in solar capacity, meaning something like 200,000 solar rooftops. All four accepted it and also talked more widely about energy policy. Khan, calling for a “far more ambitious” approach to solar, said he would set up a City Hall unit called “Energy for Londoners” which would look at a policy for all public buildings in London. He thinks London’s solar energy output, which he said was just 1% of the total at present, could be increased to 20% “without too much trouble.” He praised the use of solar at Blackfriars Bridge and in the Olympic Park, pointing out that TfL land could accommodate many photovoltaic cells and criticising government cuts to solar programme subsidy. He also praised the on-site combined heat and power plant at King’s Cross and the green energy centre on Islington’s Bunhill estate.

Asked about the effect of government cuts, Goldsmith said that solar, which he’d written about in The Ecologist magazine in 1996, was now an economic proposition and would be more so “if we can negotiate to get rid of the tariffs in Europe that prevent us from getting the cheapest solar energy from China.” He acknowledged that mayors can’t make that happen but said that they could negotiate with local authorities to make roof space available across the city and write “the highest possible standards” into the London Plan for new buildings, “not just for solar but all energy efficiency.”

Pidgeon said she would have “a solar task force” to audit all buildings with the GLA group of partner organisations and challenge the capital’s larger private sector organisations to do more. Help should be given through the planning process. “We need to develop a London feed-in tariff,” she said, to encourage to uptake of small scale renewable and low-carbon electricity generation and move forward the process of City Hall itself becoming a junior electricity supplier. Berry, who didn’t get much time to address the question, said she’d aim for 30% of London’s electricity to be from solar power by 2030. She said the capital had fallen behind under Johnson, and the same was true of home insulation, producing “really high levels of fuel poverty.”

And finally, Europe

One of the more striking moments of the event came when O’Brien asked each candidate to name the most dangerous policy position adopted by an opponent. Khan moved very fast to pick Goldsmith’s decision to campaign to leave the European Union. This produced a large round of applause and served as a reminder that Brussels is seen by many as both a leader and a protector on environmental issues in the UK. It’s hard to know to what extent candidates’ stances on the EU referendum will affect the mayoral vote either way, but it seems that Goldsmith’s position won’t help him with the section of the London electorate represented at 1 Wimpole Street.