The climate has had periods of relative stability amidst interludes of major change. We are currently in the interglacial phase of a million-year-plus ice age. Nobody knows why there are ice ages. Nobody knows why there are interglacial phases. We DO know this:

The major determinants of climate are astronomical, geological, and biological in origin.

1 Astronomical:
1.1 Sun's variability. The Sun is a variable star. Sometimes its output increases by several percent for decades. It may be that there are larger, longer-term variations as well. These have enormous effects upon Earth's climate.
1.2 Precession of the axes combines with eccentricity of the orbit. The Earth's orbit is elliptical, not circular. The Earth is closer to the Sun during Winter of the Northern Hemisphere than during the Summer. This makes Summer longer in the Northern Hemisphere because the Earth's angular velocity is lower when it is farther from the Earth. This is constantly changing, since the axis of the Earth's rotation slowly rotates (precesses) every 25,000 years or so. So sometimes Summer is longer in the Southern Hemisphere, and will be again.
1.3 Nutation of the axes. The tilt of the Earth's axis with respect to the orbital plane is current about 23.5 degrees. This is NOT constant either, but varies from about 21 to 26 on a cycle of circa 40,000 years. This makes the seasons either more or less intense, depending upon the tilt. It also interacts with the precession (1.2) noted above.
1.4 Every so often a massive object collides with the Earth and spreads dust into the atmosphere, in addition to digging very large holes.

2. Geological:
2.1 Volcanic eruptions sporadically introduce huge quantities of dust and greenhouse gases into the upper atmosphere. There was NO Summer at all in parts of the USA for a year after Tambora erupted in Indonesia. People starved.
2.2. Plate tectonics cause the shape of the continents and oceans to change over long periods. In addition, there are important short-term changes as entire mountain ranges can rise or fall by several meters in a few seconds during earthquakes. Massive landslides can change the courses of rivers and ocean currents also as a result of Earthquakes.

3. Biological:
3.1 Plants and phytoplankton absorb CO2 and release O2 and water. When plants spread over new territory, they increase ground albedo and evaporation of water. New varieties and new species are always spreading around and replacing others, thereby changing the climate.
3.2 Animals consume plants and phytoplankton (and each other) and thereby effect the results of the previous influence.
3.3 There is an idiotic and rapidly increasing species of vertebrate that consumes more food and generates more waste than all other species of vertebrates combined. It cuts down and burns down entire forests, clears land of natural plant cover, and thereby increases erosion. I am sure that you have heard of it.