NEW YORK (REUTERS) - Before Rex Tillerson was the ousted former US Secretary of State who said Washington "can be a very mean spirited town", he was the former CEO of Exxon Mobil and, on Wednesday (Oct 30), he took the witness stand in Manhattan state court to testify in a closely watched civil lawsuit accusing the energy giant he once ran of hiding the true cost of climate change from investors.

The case, which began last week, is the first of several climate change-related lawsuits by cities and counties across the US pending against major oil companies to go to trial.

The lawsuit, brought by New York's attorney general, alleges that Exxon did not fully account for the impact of future climate change regulations on its business, resulting in losses to investors of up to US$1.6 billion (S$2.17 billion), and that investors were misled on Tillerson's watch.

Exxon has assailed the claims as false and politically motivated.

The case has also revealed Tillerson used a secret email account under the alter ego "Wayne Tracker".

The state said Exxon deleted emails from that account, and asked the judge to rule that the missing emails be assumed damaging to Exxon's case.

Theodore Wells, a lawyer for Exxon, has defended Tillerson in court, saying that after he became CEO in 2006, the company put in place a "robust system" to manage the risk of increasing climate change, and said the alternate email account was "entirely proper."

New York and Massachusetts have both filed lawsuits accusing Exxon of misleading investors and consumers for decades about the role fossil fuels play in climate change and are seeking funds to pay for seawalls and other infrastructure to guard against rising sea levels.

Exxon and the other oil and gas companies involved in the lawsuits said in court filings that they cannot be held liable for climate change.