The process will unfold slowly but the consequences for humanity will be dire. Humanity has been reduced to small numbers in the past, from millions to thousands. This time it will be reduced from billions to thousands. Preparations have been underway for years along with vast shifting of resources to save some portions of humanity, this is going on around the planet. No they're not underground alien bases, they're human/earth bases and they may support limited populations for thousands of years. When most of the people are gone the earth will recover but the recovery process will require another ice age to lock up the methane and other gasses back into the ocean depths and permafrost and ice packs.