(1) Dry ice is easy to sublimate and absorb heat, and can be used for artificial rainfall.

(2) Carbon dioxide can react with water to generate carbonic acid, which is acidic and can turn litmus test solution red. The text is: carbon dioxide+water → carbonic acid;

(3)① Developing solar energy, hydro energy, wind energy, geothermal energy and other energy sources can reduce carbon dioxide emissions and help to slow down the greenhouse effect; (2) Reducing the use of fossil fuels can reduce carbon dioxide emissions and help to slow down the greenhouse effect; (3) Planting trees and grass in large quantities and prohibiting deforestation can increase the absorption of carbon dioxide and help slow down the greenhouse effect, so choose: (1) ② ③.

So the answer is: (1) absorption;

(2) The purple litmus test solution turns red, and carbon dioxide+water → carbonic acid;

(3)①②③.