In very broad terms, there are three factors that affect climate: the total energy we receive from the sun, the amount reflected versus the amount absorbed by the earth, and the amount reflected back (and therefore remaining in the climate) by the atmosphere. Each varies over time.
The energy put out by the sun varies some. But the bigger deal as I understand it are slight variations is the earth's axis, the shape of the earth's rotation around the sun, and which direction the earth points. All three affect how directly the sun's energy strikes the earth. When the three point in the same direction (hotter or colder), the result is significant. Since their cycles vary, they don't usually point in the same direction. This accounts in part for climate change throughout earth history.
The amount of energy absorbed by the earth or else reflected back up depends in part on the location of the continents. If most of the land is at the poles, it gets covered with snow and ice which reflects back a lot of energy. If the land concentrates around the equator, there is less snow and ice and so more energy is absorbed. At present, something like 80% of land is north of the equator. This changes at a very slow pace. (Another factor, and maybe a bigger one, associated with the location of land are ocean currents. The ocean redistributes heat around the globe. When continents open up or else close, ocean currents shift dramatically.)
The factor we mostly talk about in climate conversations is the last--the heat re-reflected back to earth and so retained in the climate system as opposed to being released into space. Lots of things affect how much heat is retained. Volcanic activity is a major factor as is the respiration of plant life. Mass extinctions tend to happen when a meteor strikes earth, triggering volcanic activity that dramatically changes the climate virtually overnight. Otherwise, the climate changes slowly, but constantly.
We can't control any of that, as climate change skeptics routinely and rightly point out. But the amount of CO2 (and other green house gasses) that we have pumped into the atmosphere measurably increases the ability of the atmosphere to retain the sun's energy. We are only one small factor in the determining the composition of the atmosphere. But we are a factor.
The metaphor that I find most helpful is thinking of our emissions as a slow volcano. The "super-volcanoes" spew more than we do in a year. But we have been spewing consistently, and increasingly, for 250 years. As a result, long-term cooling trends have been reversed and the climate is warming at a rapid (in geological terms) rate.
Another metaphor is to think of the earth as having a fever. Like us, the temperature of the earth varies relatively slightly over time. But even slight variations make you feel bad. Relatively modest variations kill us. The same is true for eco-systems.
The result is what one would expect. Species that can't adapt quickly enough go extinct, so we now are in the midst of one of the handful of great extinction events in earth's history. The changes also affect storms (bigger and more destructive) and patterns of precipitation, leading some once fertile areas to experience long-term droughts. Already environmental stresses have become a global security issue recognized by the Pentagon and a significant challenge to insurance companies.
What to do about this is a matter of politics. But at this point, several things are clear to virtually all informed observers, I believe. The climate is changing. There is much that we cannot control, but we are contributing to the changes. Climate change is creating problems. And the problems will only get worse.
The real question, as I see it, is, how serious is the problem (my answer is, very) and therefore how aggressively do we need to act? Given that our actions today will continue to have an impact for decades and maybe centuries, our descendants have a stake in our answer.
Good things are happening, but I don't think they are happening fast enough. So my interest is supporting policies that will increase the pace of our response as quickly but also as painlessly as possible. The longer we wait, the more painful the necessary response will be.