This is a story that's running at the moment - hugely sensationalized and made as pessimistic and gloomy as could be. in many of the papers, especially the sensationalist press, it's presented as a "Doomsday prediction". It's nothing of the sort.The number of scientists, 15,000 is correct, and that they were warning about climate change and other issues is correct, so the snesationalist press got that much right. However, they don't say the world will be destroyed.
The letter actually says there is much we are doing right, but that there is much more we have to do to see ourselves through to 2100 and beyond. Their message is that we have to continue what we are doing, and do more, in order to avoid widespread misery in the future.