Both human activities and factors unrelated to humans influence Earth’s energy balance. Large volcanic eruptions can inject sulfur dioxide gas into the upper atmosphere, where it forms tiny droplets of sulfuric acid that can reflect sunlight back to space. Short-term cooling of several years’ duration often follows in the aftermath of such eruptions. The sun’s brightness can also change, although the changes to the energy balance from the sun’s brightness that have been directly observed are very small, amounting to about 0.1%. On longer timescales, the tilt of Earth’s axis and shape of its orbit can change over periods of thousands of years. Such changes have been implicated in the periodic ice ages that have occurred during the past several million years. On even longer timescales, plate tectonics shifts the locations of the continents, altering surface characteristics and ocean circulation.
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