Al Gore in Davos. BI / Jim Edwards Here's one way to solve global warming: Spend $90 trillion (£59 trillion) over the next few years to redesign all the cities — as in all the cities on earth — so people live in more densely packed neighborhoods and don't need cars.

That is one of the more ambitious (and possibly outlandish) ideas knocking around the World Economic Forum at Davos, Switzerland, on Thursday morning. The Davos meeting is the annual conclave of the world's ruling class: presidents and prime ministers, CEOs, and religious figures (and the thousands of journalists who follow them, hoping for a sound bite or two).

The $90 trillion proposal came from former US vice president Al Gore, former president of Mexico Felipe Calderon, and their colleagues on The Global Commission on the Economy and Climate. That group hopes to persuade the world's leaders to do something about humanity's suicidal effort to heat the earth's climate.

Part of fighting climate change will mean redesigning, or building anew, towns and cities without cars, Calderon says.

"We cannot have these cities with low density, designed for the use of cars," he said. "We recommend those cities should have more density and more mass transportation." Together with a program for reforming land use, and bringing deforestation to zero, the total cost of this plan would most likely be $90 trillion in future investment, Calderon said.

Business Insider spoke briefly with Calderon after the panel to ask him to explain where this $90 trillion was going to come from and how exactly one might persuade every city on earth to go along with it.

It turns out the $90 trillion is the total of infrastructure investment that is likely to be spent anyway building and upgrading cities. Gore and Calderon are arguing that it be spent more wisely, to produce cities that don't encourage people to burn fossil fuels just to get from A to B.

The key will be to persuade the mayors — again, all the mayors on earth — that designing new cities this way will be vastly preferable to the old way, in terms of efficiency and prosperity for their residents. "The mistake we made in Mexico was to let cities develop however they want, and it's a mess," Calderon told Business Insider. It's in the mayors' "best interests" not to repeat that "mistake," Calderon said.

The main problem is that mayors are not widely aware that the cost of designing cities sustainably in the future may be cheaper than the cost of letting development run unhindered and focused on cars.

You can read more of the Gore/Calderon GCEC plan here and here.