SAN FRANCISCO — Google is crossing a milestone in its quest to reduce pollution caused by its digital services that devour massive amounts of electricity. The company says it believes that beginning next year, it will have amassed enough renewable energy to meet all of its electricity needs throughout the world. That’s significant, given Google’s ravenous appetite for electricity to power its offices and the huge data centers that process requests on its dominant search engine, store Gmail, YouTube video clips and photos for more than a billion people. Google says its 13 data centers and offices consume about 5.7 terawatt hours of electricity annually — nearly the same amount as San Francisco, where more than 800,000 people live and tens of thousands of others come to work and visit. The accomplishment announced Tuesday doesn’t mean Google will be able to power its operations solely on wind and solar power.