We are witnesses to history as we watch countries come together this Earth Day to formally sign on to the Paris climate agreement. In December, we cheered as 195 countries agreed to keep global warming beneath a 2C threshold, and aim for just 1.5C from pre-industrial levels. While that agreement was hard fought and much celebrated, it wasn't the end of our work. It was just the start for governments, businesses and organizations. And now we've come to the next step: signing the agreement.