Global Warming is observably and measurably happening, there is no ambiguity about that. Sea levels have risen by 12" along the East Coast in the past century, increasing the frequency of seawater flooding in low-lying coastal cities like Miami Beach, due to thermal expansion of the water in the oceans as the planet warms. Glaciers are melting and retreating, or disappearing completely like the one in Greenland, and long hidden cadavers, even crashed aircraft, are emerging. Polar ice is melting and retreating.
The effect of Global Warming on climate is less certain but the frequency of unprecedently severe weather events is increasing and only a fool would ignore that.
What would it take for deniers to take Global Warming and Climate Change seriously. The combination of sea level rise and a storm surge cause by an unprecedented severe storm causing widespread flooding in NYC might at least give them cause for concern.