1. The Earth's climate has been changing for billions of years and will continue to change.

2. The biggest factors are astronomical and geological and are forever beyond human control:
-- Astronomical
2.a Variation in solar output
2.b Precession of the Earth's axis
2.c Nutation of the Earth's axis
2.d Oscillation in the eccentricity of the Earth's orbit
2.e Large impacts
-- Geological
2.f Volcanism
2.g Plate tectonics

3. Despite the above, humanity should do whatever is possible to minimize its effect upon the climate, since even small perturbations could have large repercussions due to their effect upon an unstable system. To put it another way, if your ship is being tossed around by huge waves, it is wise not to go about drilling holes in the hull.