What human activity is thought to contribute to climate change?

There are plenty more than I can think of at this point, which include factories releasing pollution, greenhouse gases, and also burning fossil fuels, deforestation (plants help convert carbon dioxide into oxygen and without then there will be more carbon dioxide released into the atmosphere), and farming livestock. Fueling cars and using fossil fuels for electricity/power plants also contributes to climate change.

doctrine that the us had the right to expand throughout north america.

separation of church and state? not sure what you mean.