It's remarkable that "global warming" didn't make the list given its potential for $trillions in damages and vast loss of life if left unchecked. In terms of severity, there is no second place.
Global warming doesn't really affect the USA specifically and/or uniquely, so maybe that biases the answer? Also, dying of smoking is less a worry when you see a truck about to run you over. Maybe people preference immediate danger over longer term danger?
The issue of "country" is an interesting one in the other direction as well. If a state/city/county has 16% unemployment, but the country has low unemployment, some people may think it is a country-wide issue, and preference that. Ditto other issues that are state or local level issues.
In any case, I can see why almost any answer is valid, which kinda makes the whole question an odd one.
He suggests that phenomena on the scale of climate change - i.e quadrillions of gallons of water entering the ocean - are something that our brains have a hard time contemplating on a day to day basis, because we haven't evolved to think about things that happen on that scale.