For the first time since record keeping began, Earth has surpassed a critical temperature threshold that scientists have long warned could unleash the worst effects of climate change. On Friday the planet soared 2.07 degrees Celsius above pre-industrial levels, or the 1850 to 1900 average, according to Europe’s Copernicus Climate Change Service. Not all hope is lost, however. The Fifth National Climate Change Assessment, released last week by the White House, underscored that every fraction of a degree of warming added or averted will make a difference.