So we have seen that Earth has had dramatic climate changes in the past caused by both variation in the Earth's orbit and varying levels of greenhouse gases. Al Gore's attempts to link current climate change with past changes do not quite work. What is happening currently has no analogue; for the first time CO2 levels are rising on their own, ahead of other normally synchronous changes.
We are currently in the Holocene, which began around 11,000 years ago, marking the end of the last ice age. Glaciers covered many of the continents during the last glacial maximum, but beginning around 20,000 years ago Earth has warmed. The glaciers have retreated and left only arctic and Antarctic ice as well as mountain glaciers. The retreat of the glacier covering North America left behind the Great Lakes as well as the many kettle ponds here in New England. Cape Cod and Long Island are remnants of the debris left behind by the glacier's movement.
In other words, melting glaciers are nothing new. However, beginning with the industrial revolution humans have begun to add carbon dioxide to the atmosphere in appreciable amounts. From around 280 ppm (parts per million; within 1 million air molecules 280 of them would be CO2) before 1900 CO2 levels have risen to around 390 ppm. This is still an extremely small part of the atmosphere, but CO2 is a greenhouse gas that absorbs heat radiated from the Earth (originally from the sun). Venus, for example, has an atmosphere almost entirely made of CO2 and has surface temperatures around 400 degrees Celsius. While the Earth has many sinks for excess CO2 (only around half of total human emissions actually stay in the atmosphere), humans are adding enough to slowly but surely increase global levels.
Over the 20th century global temperatures have risen around 3/4 a degree Celsius or a little over one degree Fahrenheit. This may not sound like much, but remember, a lot of heat is needed to warm the entire planet by even 1 degree. Is all of this warming due to humans? Probably not; remember, Earth has been warming for the last 20,000 years (with a few notable exceptions), and sea levels have been rising steadily at about 2 mm per year over the same period. Most likely humans are accelerating the warming of the current interglacial period. And here we come back to the main point: this is essentially new territory. We simply do not know exactly what will happen as we add more and more CO2 to the atmosphere.
The Earth has various feedback mechanisms that have acted to stabilize climate throughout Earth's history. These seem to have prevented both an irreversible global icehouse as well as an irreversible runaway greenhouse effect. Can the Earth deal with the amount of CO2 we are adding? There are studies showing that even if we stopped adding CO2 today, climate change might continue for centuries. While modeling future climates is extremely difficult, there seems to be a general consensus that, in the words of the IPCC, "very likely" humans are contributing to the current warming and that warming will continue as long as we keep adding greenhouse gases.
Next: science vs policy.