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Volcanic eruptions: Volcanoes emit carbon dioxide, but they also emit aerosols, such as ash or dust, and sulfur dioxide. Aerosols are liquids and solids that float in the air. They may also include soot, dust, salt crystals, bacteria and viruses. Aerosols scatter incoming solar radiation, causing a slight cooling effect. Volcanic aerosols block a certain percentage of sunlight and cause cooling that lasts for 1-2 years.
Orbital change: the earth's orbit is an ellipse, so the distance between the earth and the sun changes periodically, so does the temperature on the earth