According to scientific evaluation, compiled by more than 200 scientists, climate change has put the world at risk of crossing multiple planetary "tipping points". Exceeding these points could lead to irreversible effects on natural systems , which are critical to human livelihoods, write the authors, who add that it is time to face these dangers head-on—and accelerate efforts to prevent them . Some scientists worry about overemphasizing tipping points because it is difficult to define risks and assess their likelihood. But few scientists doubt that the risks are real or that they increase when global temperatures are escalating . "These tipping points represent threats of a magnitude that humanity has never faced before," says Tim Lenton, a climate scientist at the University of Exeter in the UK who led the report with support from the Bezos Earth Fund, a philanthropic organization based in Washington DC and founded Amazon founder Jeff Bezos. (Jeff Tollefson, nature.com).