The largest contributors to greenhouse gases in the US are power plants and cars. All coal & gas power plants should be replaced by non-carbon power like nuclear, solar, and wind. Building a national high speed rail network and expanding public transport everywhere will also cut emissions. I'd immediately build a 30 minute high speed rail line with 5 stops: Hagerstown -> Frederick -> Gaithersburg -> Bethesda -> DC. Those are all very expensive projects, but can and should be done, and will create jobs.
Let's bring back winters