Al Gore has endorsed Joe Biden for the Democratic presidential nomination, telling voters that the decision "is not complicated" in the face of the glowing climate threat.

During an Earth Day town hall, the former candidate said: "If you care about the climate crisis, if you want to start solving the climate crisis, this is not rocket science."

Mr Gore served as vice president under Bill Clinton and later ran against George W Bush before a narrow loss in 2000.

He later emerged as one of the most prominent figures ringing the alarm for the climate crisis following his unsuccessful bid for the White House with the 2006 film An Inconvenient Truth.

"We need policy changes, and that means we need to change some of the policymakers, particularly the one in the White House", Mr Gore said on Wednesday. "This is the most consequential choice in a presidential election that we've ever had in American history. Donald Trump is the face of climate denial globally."

Mr Biden, now the likely nominee to face the incumbent in the November general election, also received the endorsement of former Democratic challenger Jay Inslee, the governor of Washington state who had also steered a climate-focused campaign.

But the former vice president has faced several challenges on the campaign trail from critics pushing for stronger environmental protections and the adoption of a Green New Deal like the one promoted by his chief rival Bernie Sanders.

He has set a target date of 2050 to curb carbon emissions, though critics say that's too conservative, and the candidate has not supported an outright ban on fracking.