What do scientists believe is the primary cause of earth's climate fluctuations?

Answer 1

If you mean the present climate change, then scientists agree that it is caused by man's deforestation and burning of fossil fuels.

Another View:
If we are discussing actual major temperature changes though, like the warming trend of the past 11,000 years the answer is far different and the changes far ore profound.

What is known about our long term history of climate change comes primarily from a single source. The glacial samples from Lake Vostok in Antarctica. These samples tell us, with remarkable detail some interesting things:

1. we know the planet goes through cycles. These cycles are regular and the longest is just over 100K years long. We are in the warmest section of this long cycle currently.

This warming is believed to be caused by planetary alignments of the major planets in our solar system. Our orbit around the sun is far from a perfectly circular orbit and the distance our planet is from the sun varies greatly. When all of the larger planets are scattered throughout the solar system we have a far more elliptical orbit the when the larger planets are all on a single side of the sun. This event happens once every 100K years and corresponds to these warm periods. This current cycle has seen an overall warming of about 11 degrees C from this issue. Man's contribution is an unknown portion of a partial contribution of the 0.3 degrees of warming we have seen since 1939. Even if man is the sole contributor, it is nothing compared to this warming.

Other portions of our orbit that influence our climate are listed within the Milankovich effect link below.

2. Solar variation is agreed by climate experts to have assisted in temperature variations, including the current cycle of warming. The problem here is that, according to the IPCC, (the United Nations body that is considered The Bible of climate change) that while we know it has caused some portion of the current climate change, what portion and how is still "very poorly" understood.
(see IPCC links)

While there are other issues that have caused the major climate changes of the past few thousand years, these two are in the primary category.

3. It is also likely that a wobble in the Earth's axis of rotation contributes to these ice age cycles we've seen wax and wane over the past 800,000 years (based on ice cores, so may not be globally representative in details, but it is in the big picture). The cycle has been quite regular the last 400,000 years, and seems to be getting more pronounced and longer with time. Five times in the last 400,000 years we've seen a very steep rise in temperatures to come out of an ice age, then the temperatures begin a much slower descent back into ice age conditions. The most recent cycle (before the sharp warming that started about 25,000 years ago) was most pronounced, with Earth's average temperature possibly fluctuating by as much as 15C from the peak ~125,000 years ago to the nadir ~30,000 years ago. Taking anthropomorphic global warming out of the equation, and looking purely at the ice age cycle, we appear to have reached the end of the cycle's sharp temperature increase several thousand years ago, and should be in the early stages of the gradual cool-down toward an ice age.