Rex Tillerson, the former chief executive of ExxonMobil and former secretary of state in the Trump administration, has denied in court that the oil and gas giant committed fraud by concealing from investors the true financial impact of the climate crisis.

Tillerson told the New York supreme court that Exxon had acted properly over disclosing its financial exposure to climate-related regulations while he was chief executive.

Leticia James, the New York state attorney general, is suing Exxon over claims it misled investors over the costs associated with the climate crisis.

Exxon is accused of misleading investors over the business risks caused by regulations to address the climate crisis.

The company is accused of covering up the costs it will incur from government rules designed to reduce greenhouse gas emissions.

The case represents just the second time a climate case has gone to trial in the US.

Exxon has long been a target for environmentalists, not only for being a major contributor to the climate crisis but also over revelations that it knew for decades that burning oil and gas would dangerously heat the planet.

New York’s lawsuit accuses Exxon of using a public “proxy cost” of up to $80 per ton of carbon emissions to account for regulations that would limit planet-warming emissions, while internally using figures of $40 a ton, or nothing at all, to allow it to make more environmentally damaging investments.

An Exxon spokesman has previously said that it properly applies two different metrics – a “proxy cost”, intended to reflect the impact of all climate policies on global demand, and a greenhouse gas cost which relates to actual levies placed on fossil fuel projects in a certain jurisdiction.

Under questioning, Tillerson said: “We don’t know how else to model what future policy impacts might be. But whatever policies are, ultimately they come back to either your revenues or your costs.”

The former Exxon head, who served as US secretary of state in Donald Trump’s government for just over a year before being fired in March 2018, added: “If you go back to the history of this, this was all fairly new.”

Tillerson said he did not remember the climate costs being a determining factor in any decision made during his decade-long tenure at the helm of Exxon, which ended in 2016.

James’s office has sought to cast Tillerson as a central figure in the securities fraud case, with Kevin Wallace, acting head of the attorney general’s investor protection bureau, telling the court last week that the former chief executive was “deeply involved” in working out the impact of climate science upon Exxon’s business prospects.

On Wednesday morning, Tillerson said several times that he did not recall email conversations with colleagues on the cost of carbon. During his time at Exxon, Tillerson used an email alias, calling himself “Wayne Tracker”.

Tillerson did, however, say that he supports the Paris climate agreement, which Trump announced during Tillerson’s tenure as secretary of state that the US planned to withdraw from, and that he favoured a price on carbon emissions.

Exxon has denied any wrongdoing over its pricing of carbon and accused the lawsuit of being politically motivated. The case is among the most high profile of around a dozen rather different lawsuits launched by jurisdictions across the US that are attempting to claim compensation from fossil fuel companies for the damage caused by rising sea levels, storms, heatwaves and other impacts stemming from the climate crisis.

“The attention this case received certainly places a harsh light on Exxon which must make them uncomfortable,” said Michael Gerrard, an expert in climate law at Columbia University. “They tried for years to stop this case from happening. This is all at least as much about reputation as it is about money.”

Climate activists staged protests at the start of the Exxon trial and have kept up their criticism of the company since then.

“Rex Tillerson is a climate criminal,” said Dominique Thomas, north-east regional organizer of climate group 350.org. “Yesterday was the seventh anniversary of Superstorm Sandy; a grave reminder that the lies of people like Rex Tillerson cause that kind of climate destruction.”