The more we learn, the more we discover how little we know. So it is with global climate change. We are continually learning more about the world in which we live, but we still lack the knowledge to put it all together or say what it all means.
Is the global climate changing? I would say that there are very few people, scientists and laymen alike, who would deny that it is. That doesn't necessarily mean that it is getting warmer or colder, but that it is changing. The Earth, after all, is dynamic and constantly changing, so the climate would be expected to change. There is also a great deal of give and take; when there is a drought in one region, there is often monsoon-like rain and flooding in another, as the Earth works it's magic in balancing it all.
There are a lot of proofs that the climate has not always been as it is now. Two thousand years ago, worldwide temperatures were far warmer than they are now, for example. They were warmer still, 65 million years ago, when dinosaurs were the dominant form of life. On the other hand, we know about the ice ages that gripped our planet several times for thousands of years in each instance. In fact, during the late 1700's, there was a worldwide cooling that is commonly referred to as the "little ice age". During that time, the people of England were actually skating on the ice on top of the Thames in London, and had New York existed as it is today, people could have walked from one of the boroughs to another, over the ice.
What do we know about the causes of all of this? We hear a lot about CO2 levels, but this doesn't explain much. The levels of carbon dioxide were roughly the same or a little higher during the little ice age than they are now.
Some have blamed it on volcanic eruptions. We know that one moderate volcanic eruption produces a lot of ash that can block the sunlight from reaching the ground, but on the other hand, that same eruption produces more of the greenhouse gases than man produces in a whole year. Logically, the two should balance, so this too is not much of an explanation.
Even plate tectonics can only explain so much, though it is likely that great fluctuations, one way or the other, would not occur if not for the oceans between the continents.
I believe that while there are most likely a great number of factors that are involved, including quite a few that man has yet to even consider, that the biggest one is the source of most of the heat the Earth receives: The Sun.
We've identified several cycles in the sun, though nowhere near all of them. For instance, there is an 11 year cycle of 'sun spots'. Big deal, right? Well, it is a big deal. It turns out that during the little ice age, as well as several other identified little ice ages, that the sun "skipped" quite a number of sun spot cycles. The result was that there were many years where there were practically no sun spots. I cannot say that this alone is responsible, but since the source of most of the heat of this planet comes from the sun, it certainly can't be discounted.
Maybe sometime in the future, we will begin to understand more clearly what the sun does, but it is becoming increasingly clear that the sun is far more powerful than man can even conceive, when it comes to climate. If we truly want to learn the reason for climate changes and shifts, it is my opinion that the best first step would be to try to learn more than the source of our heat, the sun.