What are some things that can change weather over long periods of time?

Question

Man made or natrual

Answer 1

I think you probably mean climate.
Some of the natural causes of climate change are continental drift, volcanoes, ocean currents, the earth's tilt,comets, meteorites, variations in the Earth's orbital characteristics, and variations in solar output.
As for man made causes, the most obvious is the burning of fossil fuels which produces the so-called greenhouse gases.

Answer 2

Here is something that may surprise you. If we were to take away the effects of global warming due to an anthropogenic increase in greenhouse emissions, there would remain according to James Lovelock, author and progenitor of Gaia, a far worse fate awaiting the earth. Permit me to quote from his recent book "The Revenge of Gaia"; "In the very long term, solar warming is a far greater problem for life than our present-day battle with man-made global heating. In about one billion years, and long before the sun's life ends (forecast to be about 5 billion years from now), the heat received by the Earth will be more than two kilowatts per square metre, which is more than the Gaia we know can stand; she will die from overheating." What this means is that the Sun is about 25% hotter than when life began on earth. This is not to suggest we throw up our hands and do nothing about our current problems in managing our bio-sphere but it does put it in a different perspective and if true certainly throws out any concern again about global cooling. While this problem is in God's hands, Global warming is in ours.