In four different ways, he says, from rainfall patterns and sea levels rising to food production and ocean acidification. First, "as the planet warms up, we're going to redistribute rainfall, which is going to affect our water resources and parts of North America may get a lot drier."
Second, "as the polar ice melts, sea levels are going to rise." The world's major cities, and a lot of people, are right on the coasts and rising sea levels are going to impact them.
Third, thinking about food, the "distributions where we can grow food are going to change as the planet warms up." So the range over which you can grow corn and other crops will change.
Fourth, says Tom Wagner, "the oceans are going to get more acidic as more CO2 dissolves in them." There are untold ramifications from that, including the possibility of radically altering the food web in the ocean, "which can affect everything from the composition of the atmosphere to the ability of the oceans to provide food for us."