Earth and Sky: Our planet may have reached what scientists call a "climate tipping point." Earth & Sky spoke with James Hansen, director of the NASA Goddard institute for Space Studies in New York. He described what he calls a "climate tipping point," when little if any additional forcing – from the burning of fossil fuels – will be needed to cause large, relatively rapid climate change and impacts. Is our planet there yet? James Hansen: We’re a lot closer than we thought we were. If we …