Volcanic activity may cause climate change by a. blocking solar energy from reaching the earth's surface. b. shifting the equatorial and temperate zones. c. increasing the earth's overall temperature. d. altering global wind and current patterns.

Volcanic activity may cause climate change by increasing the Earth's overall temperature. Volcanic eruptions produce major quantities of carbon dioxide (CO2), a gas known to contribute to the greenhouse effect. Such greenhouse gases trap heat radiated off of the surface of the earth forming a type of insulation around the planet. 

The correct answer is d