Volcanic eruptions
Changes in solar output
Changes in ocean circulation
Changes in the shape of Earth's orbit.
All of them, although changes in planetry orbit happen over thousands of years, and volcanoes are more likely to cause short lived changes 2 or 3 years
Volcanic eruptions - the effect of one volcanic eruption could be profound, but would last for only a few years. Major changes in volcanic activity could cause long term climate change.
Changes in solar output - Yes
Changes in ocean circulation - Yes
Changes in the shape of Earth's orbit. -Yes
Changes in ocean circulation
Changes in the shape of Earth's orbit.
Solar output and volcanic erupts will likely result in short term changes in say a number of months at most
All of those do and are in fact linked to past events.
"Volcanic eruptions"
Kano has it partly right, changes linked to particulates can last a year or two after a major eruption causing cooling but atmospheric processes remove them fairly quickly, but volcanoes also put CO2 into the atmosphere, at current rates about 1% of the output of humans, but volcanic output in the distant past has been far larger than it is now. Events like the Siberian Traps, dwarf any volcanic activity seen during the time man has been around.
http://en.wikipedia.org/wiki/Siberian_Tr...
The Siberian Traps are linked to a major climate change and a mass extinction event.
"Changes in solar output "
Deniers try to link current changes in the solar cycle to climate changes but this is a tiny change less that 0.1% but in the longer term the Sun can and has had longer cycles when it has dipped in activity, a period denier often mention is the little ice age (LIA) but this was nothing like the fairly low activity we have seen in recent years, the LIA was a period of ~70 years with pretty much no sunspot activity, but this didn't happen in isolation there where also a string of volcanic eruptions of very large power through this period as well, culmination in the eruption of Krakatoa, probably the largest eruption of the last 1000 years.
The Sun power (or Irradience) i.e. light is measured in W/m2 this varies over the solar cycle between 1365 and 1367 a variation of just 2 a 1% variation would be 13.6 so it is in fact ~0.1%.
In the longer term the Sun is known to be warming up in the very long scale ~10% per billion years, so another period deniers try to use ~half a billion years ago when they say temps were lower with higher CO2 is not that hard to explain when the Sun was putting out 5% less energy not over 1-2 years but over millions of year.
"Changes in ocean circulation" Are know to have had effects on climate, creation of the Isthmus of Panama caused to stopping of a major current that used to flow between between the Atlantic and the Pacific, the redirection of this warm water current created the North Atlantic Current, which makes a good portion of Western Europe warmer than it should be. Also when the last two landmass's (Australia and South America) separated from Antarctic this created another current, the circumpolar current, which allowed Antarctica to cool eventually developing the massive glaciers it has today.
http://en.wikipedia.org/wiki/Antarctica#...
"Changes in the shape of Earth's orbit."
Milankovitch cycles are not so much a change in our orbit as they are a change in the tilt of the Earth to the Sun, meaning polar regions get just a little more light (energy) causing them to shrink as they do more land is exposed and CO2 (and other GH gases) are released and they add to this warming effect.
This change would have been there before the glacial cycle started ~2.5 million years ago, but it took the Antarctic glaciers growing to tip us into the glacial cycles, before this happened considerable life existed (on land) in Antarctica, but all slowly died out as the glaciers grew. In this context your last two points are linked and together, caused a long term change in the planets climate.
http://en.wikipedia.org/wiki/Milankovitc...
Which of the following may cause long-term changes in climate?
Volcanic eruptions
Changes in solar output
Changes in ocean circulation
Changes in the shape of Earth's orbit.
