Professor Jeffrey Sachs says it is time to ''get real'' on climate change. The Columbia University economist arrived in Canberra yesterday with a tough message. He says America will have a recession, advocates nuclear power and calls the focus on emissions trading ''putting the cart before the horse''.

Professor Sachs launched a book at the Australia National University, China Update 2008: Confronting Global Challenges conference last night.

He said he thought America was in a recession and that it would be a moderate not light downturn.

''There are so many imbalances in the US economy at the moment. The financial markets ... falling house prices, the vulnerability because of the heavy US borrowing abroad, high energy prices, big fiscal debt,'' he said.

''It all adds up in my view to a weak situation that will persist.''

The United States would not lead the fight against climate change after ''essentially eight wasted years'' under the Bush presidency.

He said the new president would grapple with the legacy of Iraq, the US economic crisis, food crisis and the climate change challenge.

World cooperation was preferable to leadership.

''We better broker our differences which are smaller than our common interests in this and find a way through this.''

He said China would have to be part of the solution and the idea that rich countries would lead and China and other developing countries would take on responsibilities later was a ''non-starter''.

''The world is moving too fast. The climate is changing too fast. China is growing too fast.

''It's too big an emitter to say, 'Oh this isn't something for you right now'.''

He said it seemed serious negotiations were in a holding pattern until the Olympics finished and there was a new US president.

Professor Sachs said climate change had played a part in increasing the cost of food but an erroneous biofuels policy had added to the problem.

''It is a misguided policy that ought to be reversed because of the impacts it's having on food prices combined with the known fact that the advantages of corn-based ethanol or maze-based ethanol for carbon emission are very small.''

Australia had to decide where its power would come from in the future before it discussed carbon pricing. He said a more direct discussion about the kind of technologies used would help us to understand what kind of carbon pricing system we were going to need.

''Putting a price on carbon is not going solve the question of nuclear power,'' he said.

He said he supported the reintroduction of nuclear power. ''It's hard to see how we are going to get to enough energy with low carbon emissions without nuclear playing a significant role in the world.''

Professor Sachs is author of Common Wealth: Economics for a Crowded Planet and has been referred to as the people's economist.

He will give the keynote address, ''Confronting a global challenge'', at the conference today.