We’ll continue to be vigilant about cutting ours, looking for creative ways to power our facilities, encourage even more employees to seek alternative commutes, and generally inspire Yahoos around the world to think differently about their energy use."  First, Yahoo is hiring an outside auditor to calculate its "carbon footprint" to gauge its contribution to global warming. Commuting is one of the biggest sources of greenhouse gas emissions in Silicon Valley and Yahoo already offers programs designed to get employees out of their cars - from Wi-Fi-equipped biodiesel shuttle buses to bike lockers and public transit subsidies.