Statement here and at the California Governor's office. IIRC, it originally just had scientist signers but is now open to the rest of us. While it leads with climate change, it includes biodiversity, loss of natural lands, pollution, and (brave for a politician) overpopulation.
I could see some quibbling with the one-page summary describing quality of life to suffer "substantial degradation by 2050" if problems aren't fixed, but not enough to avoid signing. For certain aspects of quality of life it's definitely true (four of those five challenges, mixed result for pollution) and the quality of life will be much worse than it would be if we don't address the challenges. A large number of people will die as a result of the failure to solve those challenges. Whether economic outcomes will be worse than present seems hard to justify, but that's not the only way to determine quality of life. The important thing is to do something about these challenges, and taking 30 seconds to endorse it seems reasonable.