SARA STROUD — Carsharing is on the rise, but it must be more scalable to have a real impact on easing traffic congestion and cutting carbon emissions, according to cleantech investor Sunil Paul. That’s the idea behind Spride Share, a San Francisco-based carsharing startup that came out of stealth in late April and is backed by Paul’s early-stage venture fund Spring Ventures, which has funded cleantech startups such as Nanosolar and algal fuel company Solazyme.