Al Gore took full advantage of his stature among fellow environmentalists to dish out some hard home truths today – and try to prod the climate change summit towards a deal.

The former US vice-president, though among friends, was unsparing. He turned up the pressure on Barack Obama, calling on activists to press the White House and the Senate to pass a climate change law by the 30th anniversary of Earth day in late April.

"Join me in asking president Obama and the US Senate to set a deadline of 22 April for final action in the US Senate," he said. "I do not believe we can wait till next November or December."

The ultimatum to Obama was a departure for Gore who has been cautious of exerting too much pressure on the president, or causing him embarrassment.

He kept up the pace by calling for the international community to sign up to a fully fledged climate change treaty by July 2010 – and then announcing that Mexico was prepared to host a deal-making summit.

He scolded rich countries for demanding the developing world offer evidence of emissions cuts while at the same time trying to inflate the funds they were prepared to offer poor countries to deal with climate change. "This issue of transparency should also be applied to financial pledges of developed countries," he said. "How many times has same money been pledged two, three or even four times?"

But Gore also reprimanded rapidly emerging economies for balking at the idea of an international monitoring regime for emissions cuts. He advised them to be sensitive to fears that China and India could use climate change to gain competitive economic advantage.

"They should take into account the economic situation in our world today," he said. "Working people in many of our developed economies have had a bitter experience of outsourcing of jobs empowered by information technology."

He went on: "I would ask for an understanding of the difficulty that poses for elected officials who are charged with responsibility of recognising the economic interests of those men and women."

Gore was just as tough on activists who have embraced him as a hero, demanding they set aside their pride and their principles and embrace a deal – no matter how imperfect.

He said he recognised their frustration with the glacial pace of negotiations. He agreed that cap-and-trade schemes to cut carbon emissions were an imperfect solution – Gore confessed to favouring a carbon tax – but the current efforts for a deal were the best prospect of avoiding catastrophic climate change.

And there was no trace of sympathy for opponents of action on climate change. Gore began with a brief run-through of the latest science on melting of the Arctic ice cap, evidence he said "only reckless fools would ignore".

On a day that saw Arnold Schwarzenegger walking the carpet before the television cameras, and actor Daryl Hannah offering tips on green living, Gore was still the undisputed star of the meeting – at least until Obama turns up on Friday.

www.guardian.co.uk