It is extremely hard, in some cases impossible, to gather sufficient data about every aspect of the climate system for periods of time or events in the distant past -- and especially to do so at a temporal resolution adequate for a full and nuanced explanation. The record in the ice cores is extraordinarily rich in variety and detail, but it only goes back as far as the age of the ice sheets -- even less, as there is melting from the bottom even as there is accumulation at the top. Past that time, about a million years ago in the Antarctic, records must come from ocean sediment, rock layers, fossils, and other imaginative sources. These are harder to decipher and are much coarser in temporal resolution. The spatial extent of samples is often far from sufficient to get global information.

In contrast, today we are closely monitoring everything we can think of. We are much more able to generate quality reconstructions of the recent past for those factors we did not think of until now. We know how the sun is behaving. We know when and how hard the volcanoes are erupting. We know the atmospheric levels of ozone, CO2, CH4, NO2, etc. to a high degree of precision, and on a month to month basis, across the globe. We know where the continents are, how the oceans are flowing and the size of the ice sheets.

Consequently, our understanding of what is going on today is leaps and bounds ahead of what we possess with regard to any point in the past. It is no surprise at all that we can speak with much greater confidence about today than we can about yesterday. The real test the past can provide is new and exciting data that can't be explained by current theories -- every scientist's Holy Grail. This happens frequently in the finer details of climate theory, but the basics only become more and more certain the more and more data we recover.

So far, although we are far from that elusive Perfect Understanding (tm) for the reasons described above, there is also no known climate change in the earth's past that provides substantial contradiction of the theories that underpin anthropogenic global warming.