Two numbers may frame the future of the world’s climate: 51 billion and zero. Microsoft co-founder and philanthropist Bill Gates stressed both figures during an online Harvard Science Book Talk. Fifty-one billion tons of greenhouse gases are released into the atmosphere each year. Zero is the number the planet needs to reach by the year 2050 to avoid worldwide climate catastrophe, Gates told viewers on Monday night. “There are points at which when the corals die off, they never come back,” Gates said. “This is acidifying the ocean, and all the aqua ecosystems die off as that acid level goes up. As forests dry out, they are subject to both fires and infestations that kill all the trees, so you get a lot less trees. As the sea level goes up, the beaches go away.”