Question

Which of the following may cause long-term changes in climate?
volcanic eruptions
changes in the shape of Earth's orbit
changes in ocean circulation
changes in solar output

Answer 1

Volcanoes have played a noticeable role in climate. Volcanic particles that reach the upper atmosphere can reflect enough sunlight back to space to cool the surface of the planet by a few tenths of a degree for several years. These particles are an example of cooling aerosols. Volcanic particles from a single eruption do not produce long-term change because they remain in the atmosphere for a much shorter time than GHGs.
http://www.epa.gov/climatechange/science/causes.html

Answer 2

change in ocean circulation i guess

Answer 3

all of the above

Answer 4

proximity to sun affects amount of solar radiation earth absorbs, so yes
oceans currents definately affect climate, though usually locally rather than globally, an change in one of the major currents would redirect the warm water to a new destination, affecting the weather and climate at both the new and the old destination
and volcanoes extremely affect the amount of dust particles in the atmosphere at any one time, as well as contribute greatly to the overall carbon dioxide percentage of the atmosphere, both of which have a massive effect on climate globally