A lot of people are completely against this idea (government intrusion on freedom, etc.) but that's the only way we've ever solved problems based on the "tragedy of the commons"
Really... government coercion the only way? No one voluntarily shares their assets for the benefit of the greater good? The facts say otherwise:
"Total giving to charitable organizations was $335.17 billion in 2013 (about 2% of GDP). This is an increase of 4.4% from 2012. Although this is the fourth straight year that giving has increased, it is still not at the pre-recession level of $349.5 billion seen in 2007."
Charitable giving increases as a person's disposable income increases -- and not linearly, either: as income grows to exceed a person's basic needs, people tend to increase the percentage of income given to charity. That's why a 10x increase in GDP would result in greater than a 10x increase in charitable giving.
So if we just resume pursuing pro-growth policies for a few more decades, private charity will be more than capable of providing the entire social safety net -- bigger and better than our current social safety net -- and government will be able to streamline itself and stop performing that function. (Which will be a quite virtuous circle that has further benefits for the economy.) There is also quite a bit of value in the fact that it will be 100% funded by voluntary contributions, and 0% by coercive confiscation.
When philanthropists perceive that the social safety net is well-funded, they will shift a portion of their giving to other charitable purposes of their choosing: for example, subsidizing clean energy projects.
Remember, pursuing pro-growth policies is the key to realizing this rosy future. That means growth we come by honestly, as opposed to short-term growth that is forced by unsustainable, house-of-cards measures -- like deficit spending, or the Fed holding interest rates artificially low.