This week I found out what it feels like to go beyond your wildest dreams. A case I fronted won at the supreme court, with potentially huge positive impacts for the climate. For almost five years, I had been mounting a legal challenge to fossil-fuel production at Horse Hill in the Surrey countryside. A group of residents, activists and lawyers had been pursuing a routine legal review of a council planning decision that had given an oil company the green light to drill four new oil wells and produce oil for 20 years.

The supreme court ruling means it will now be much harder for new fossil-fuel projects to go ahead as their full climate impact will need to be factored in from the start. Our challenge centred on the fact that the oil produced by the Horse Hill site would inevitably be burned, throwing carbon into the atmosphere and heating the planet. We expected it to be a routine legal procedure lasting six months. But as the case came together, its wider significance for the climate and the fossil-fuel industry at large became clearer, and months turned into years as it worked its way through the courts.

Emissions from burning the fuel had been explicitly excluded from the environmental statement prepared as part of the environmental impact assessment process for Horse Hill. I – and others – thought it was wrong that the council had ignored these “downstream” greenhouse gas emissions when they gave permission to pump the oil out of the ground.

The decision will have a knock-on effect for a number of already approved projects, which are subject to separate legal challenges including similar grounds to the Horse Hill one, such as the planned new coalmine in Whitehaven, Cumbria, and the giant Rosebank and Jackdaw oil and gas fields in the North Sea. All these projects omitted to assess the impacts of use of the fuels in their environmental statements. So the Horse Hill ruling strengthens the cases against them.

Horse Hill was set to become a flagship project for the oil and gas industry across the Weald region; it hoped that if this site proved successful, others would follow, and the planning permission granted in 2019 raised the game. Twenty years of production rights was a big step up from the short-term exploratory permissions that had been seen before. And the planning decision was made just months after Surrey county council had declared a climate emergency.

I had been opposing oil developments at this site for several years already. Along with local people and activists, we used everything in our toolkits to oppose the drilling, from letter-writing and lobbying to protest camps and direct actions, monthly multifaith prayer gatherings at the gates and more besides. Horse Hill wasn’t the only new onshore fossil-fuel development in the region and, through networking, local groups and individuals came together in 2016 to share skills and knowledge. We called ourselves the Weald Action Group.

For the next three years we had a series of hearings, in the high court and the court of appeal. Each time we lost. But a split judgment in the court of appeal, where one judge agreed with us and two didn’t, and the fact that the court’s grounds for rejecting our case differed from those of the high court, meant we were allowed to appeal to the UK’s highest court. That hearing took place on Midsummer’s Day in 2023.

This week, almost exactly a year later, the supreme court ruled that the Horse Hill planning permission was unlawful because the environmental impact assessment for the project failed to assess the effect on the climate of the oil to be produced there. Exactly what we had argued all along.

It is a huge win; a win for the community around Horse Hill, who no longer face the prospect of a large industrial development on their doorstep. But its impacts are wider than that. The court has made clear that the impacts of using the fuel produced must be assessed before a fossil-fuel development is given the go-ahead.

That may sound obvious, but these emissions are increasingly being left out of environmental impact assessments when planning applications are made for fossil-fuel projects – including those at Whitehaven, Rosebank and Jackdaw – despite the huge impact they would have on the escalating climate crisis.

We often hear about tipping points in climate – such as Amazon deforestation and melting permafrost, where small changes accelerate global heating in an unpredictable way. I believe that the supreme court’s ruling is a tipping point in the right direction. A small step that will actually make the future better, not worse.

The days of fossil-fuel applications being waved through without full consideration of their climate impacts are over. I am proud and elated that the Weald Action Group has made this happen.