Global clean energy investment has grown from about $30 billion to about $150 billion over the past four years. But the world will need to nearly quadruple that level of investment every year from now until 2030 if it hopes to avoid the worse effects of human-caused climate change.

That's the daunting task set out in a report from the World Economic Forum and New Energy Finance released Thursday at the forum's annual meeting in Davos, Switzerland. Getting there will require immediate action by governments to push private investors, the report said.

The report's $515 billion-per-year figure is aimed at reducing atmospheric greenhouse gas concentrations to 450 parts per million by 2030 – a goal that would force a 60-pecent reduction in emissions growth under a "business as usual" scenario. It's slightly higher than the $500 billion per year set out by the International Energy Agency's World Energy Outlook report last year.

"The good news is that the process of transition and the associated surge in investment have already begun," the report stated.

Last year's global investment of $142 billion – down slightly from 2007's record $148.4 billion -- still represented enough clean energy and energy efficiency projects to account for about 10 percent of the world's spending on energy, the report said.

Clean electricity generation – which excludes biofuels from the mix – added up to 42 gigawatts in 2007, nearly a quarter of the 190 gigawatts of capacity added worldwide, the report said.

The report didn't pick favorites when it comes to clean energy, saying that wind power, solar power, waste-to-energy projects, geothermal power and biofuels both from food and non-food sources would have to play a role.

 "The shift to a low-carbon energy system cannot be achieved simply through the addition of new sources of renewable energy," the report stated. "It will also be necessary to make wholesale changes in the way energy is distributed, stored and consumed."

That includes investments in more efficient generation, distribution and use of power. A report McKinsey Global Institute report estimated that $170 billion in energy efficiency investment opportunities could yield returns of 17 percent or more.

Investments in "smart grid" projects will play a part in that push, the report said (see Smart Grid Backers Push Investment for Job Growth). Making electricity distribution grids more energy efficient by building two-way communications between utilities and power users will require a startling $8.6 trillion, New Energy Finance estimates – about three-fourths of that just to repair and replace aging portions of existing distribution grids (see EPRI Plugs Smart Grid for Energy Savings).

Bringing down the cost of storing large amounts of power – now estimated at about $50 to $180 per megawatt-hour, the report said – is another key to a cleaner energy future.

And because fossil fuels, including coal, will no doubt continue to play a major role in the world's power generation for decades to come, finding cost-effective technologies to capture and store carbon emissions – now being done only at pilot scales around the world – will also be critical.

The report's authors did say that they expect the world's major countries to set prices for greenhouse-gas emissions in the coming years. But "carbon prices along, however, will not be high enough – at least for the next few decades – to prompt a large-scale roll-out of renewable energy, nor will they be sufficient to promote carbon capture and sequestration," the report said.

Valuations for clean energy businesses dropped nearly 70 percent through 2008, and debt and equity financing for large-scale clean power projects has declined dramatically since mid-2008, the report said (see Wall Street Feeling Downbeat on Solar and Tax Credit Fix for Solar in the Works).

That means that governments will have to find ways to stimulate increased investment in the midst of a global economic downturn, the report said.

The stimulus package now working its way through Congress does contain billions in incentives for clean energy, energy efficiency, smart grid and other items. But the report called for longer-term policy changes, including a big push for governments to buy renewable energy and put energy efficiency standards into place.