Have there been steep climate changes in the recent past? Yes, especially in Europe in the 1500s/1600s, there was a mini-ice age.
Do I believe the last 100 years has drastically changed our planet? How could it not? It makes sense that 100 years of cars, land fills, chemical waste and other negative human impact is changing the planet in ways that it may not recover from quickly. That just seems like common sense.
Stronger hurricanes, more frequent earthquakes, stronger tornados, tsunamis, hotter summers, colder winters, El Nino, La Nina, worldwide droughts - these all point to big changes in the future. Though I have not done extensive research into it either.
I try to decrease my environmental footprint in several ways, and even if global warming isn't a major threat today, it makes sense that if we don't change our ways it will be sooner than later.
Newton said it best with his 3rd law: For every action, there is an equal (in size) and opposite (in direction) reaction force.