The world’s biggest corporate law firms have been making millions of dollars representing fossil fuel companies but, as the climate crisis intensifies, this work is coming under increasing scrutiny.

Over the last five years, the 100 top ranked law firms in the US facilitated $1.36tn of fossil fuel transactions, represented fossil fuel clients in 358 legal cases and received $35m in compensation for their work to assist fossil fuel industry lobbying, according to a “climate scorecard” published in August.

The scale of law firms’ work for the fossil fuel industry is huge, said Tim Herschel-Burns, a third year student at Yale Law School and co-founder of Law Students for Climate Accountability, which developed the scorecard. “As we started digging we realised how holistic this is. Everything fossil fuel companies want to do, they need lawyers to accomplish.”

Fossil fuel companies rely heavily on armies of lawyers to advise on projects, lobby, negotiate contracts, secure permits and navigate an increasing number of climate lawsuits. Law firms’ fossil fuel industry work has increased compared with the previous year’s scorecard, even as climate warnings become more dire and the International Energy Agency has warned new fossil fuel development is incompatible with the target of net zero emissions by 2050.

The climate scorecard awarded firms grades based on their involvement in lawsuits “exacerbating climate change”, their support for fossil fuel transactions and fees received for lobbying on behalf of the fossil fuel industry.

Paul Weiss, a top 10 US firm according to Vault Law’s rankings, was one of 37 to receive the lowest F grade. The firm, which has its own sustainability practice, has acted for fossil fuel companies in 30 cases over the last five years, according to the scorecard. Among the most high-profile was the firm’s work representing ExxonMobil in a landmark trial where the company was accused of having misled investors about the risks of climate change to its business. The court ruled in favour of Exxon in 2019. Paul Weiss did not respond to a request for comment.

Only 12 law firms were rated an A or B in the scorecard, which meant they did not conduct work for fossil fuel clients. Three firms – Cooley; Wilson Sonsini Goodrich & Rosati; and Schulte Roth & Zabel – have actively addressed the climate crisis through renewable energy transactions, lobbying or pro-climate litigation, according to the scorecard report.

“We definitely agree that the law can be this force for good,” said Herschel-Burns. “But one thing that we found really striking is that overwhelmingly the top law firms are [representing] the wrong side of it.”

There’s a tangible human cost, said Alyssa Johl, legal director for the Center for Climate Integrity. “Elite law firms are representing the oil and gas companies and providing them with a deep bench of high-priced lawyers,” she said. “For the communities across the country that are seeking justice, the end result is that their cases have been delayed and bogged down by procedural hurdles put forward by some of the biggest law firms in the country.”

Law Students for Climate Accountability is calling on law firms to pledge to stop taking on new fossil fuel industry work, phase out their current work by 2025 and ramp up their work for the renewable energy industry and in support of litigation to tackle the climate crisis.

It’s a potentially controversial stance given the principle that everyone should have access to legal representation. But Herschel-Burns said this principle is often used in “really sloppy ways which end up justifying law firms being able to represent whoever pays the most”.

Some firms have acknowledged a choice. Speaking at a conference last year about the link between law and climate change, the global senior partner at Clifford Chance Jeroen Ouwehand said firms “can choose what we support, and what we don’t support. We do not have to be neutral professional service providers”.

Law firms are starting to ramp up their own climate action, even as they continue their fossil fuel work. A number of firms that scored F grades on the climate scorecard, including Shearman & Sterling and Hogan Lovells, have signed up to the Net Zero Lawyers Alliance, which launched in July. Members pledge to achieve net zero emissions by 2050 and to work with clients to embed climate goals and to help drive “systemic change”. Neither firm responded to the Guardian’s request for comment.

Another alliance member, DLA Piper, has set its own science-based target to halve all emissions by 2030, including indirect emissions from the firm’s supply chain. The firm was recently appointed official legal services provider for the forthcoming Cop26 climate talks. But it scored only a D on the climate scorecard and has represented clients including Shell, ExxonMobil and BP. The firm declined to comment.

Thom Wetzer, law professor and director of the Oxford Sustainable Law Programme, one of the organisations supporting the alliance, said the firms may be serious about taking steps in the right direction but they do not necessarily have the best practices yet. “We need to move the whole sector, and for many firms involved this is the start of a journey.”

Some law firms that ranked low on the climate scorecard were keen to promote their green energy work. Allen & Overy, which according to the scorecard worked on fossil fuel transactions worth $125bn over the last five years, said in a statement that it does “more renewables work than any other law firm in the world by most key measures”. Clifford Chance, which the scorecard calculated worked on fossil fuel transactions worth $123bn, said it is “perennially at or near the top” for advising on renewables financing.

But neither firm, both of which scored an F, responded to questions about how they reconcile this work with their representation of fossil fuel firms.

Lawyers have a responsibility to reflect on their own role and to ask whether their clients’ values align with their own, Wetzer said. “Firms that engage constructively with the net zero transition will be rewarded; clients will value their judgment and expertise, top talent will be more easily attracted and retained, and these firms will strengthen their social license to operate.”

Law firms’ best resource is their employees, said Sam Sankar, of the nonprofit environmental law organization Earthjustice, which makes the climate scorecard a powerful tool. “In the future nobody is going to think twice about making career decisions with an eye to whether it aligns with their climate ethics.”

There’s now a conversation about legal ethics and climate which is well overdue, Sankar said. “The [fossil fuel] industry is paying law firms tons of money in an effort to dodge responsibility and block regulatory reforms that could help avert this crisis.”