If the world is counting on innovative companies to solve global warming, we may be in trouble. Venture investment in cleantech is slumping. Venture capitalists in the US poured $2 billion into 139 cleantech start-ups in the first half of 2008, according to data from PricewaterhouseCoopers. In the first half of this year, venture investments in the sector plummeted to $513 million in 89 companies.