Boston ad agency Arnold Worldwide said it is collaborating with Al Gore’s Climate Reality Project to launch Reality Drop, an initiative using online game mechanics as it seeks to build an advocacy community to take on critics of global warming. Reality Drop offers a database of “climate-change myths” along with the science to refute them. “We had to custom-design a strategy that wouldn’t just find the right people passionate about this topic, but also make it easy for them to speak up and take part in real-time discussions that work in our digital culture,” Arnold’s Pete Favat said.