Scientists have the ability to take ice core samples of the polar ice caps. These ice core samples can tell scientists vast amounts of information about the Earth's climate dating back hundreds of thousands of years (to those people who think the Earth is only some seven thousand years old, sorry). Here is a description taken from a NASA Earth Observatory article of how ice core sampling works. This info. comes from the following link http://earthobservatory.nasa.gov/Features
"Throughout each year, layers of snow fall over the ice sheets in Greenland and Antarctica. Each layer of snow is different in chemistry and texture, summer snow differing from winter snow. Summer brings 24 hours of sunlight to the polar regions, and the top layer of the snow changes in texture-not melting exactly, but changing enough to be different from the snow it covers. The season turns cold and dark again, and more snow falls, forming the next layers of snow. Each layer gives scientists a treasure trove of information about the climate each year. Like marine sediment cores, an ice core provides a vertical timeline of past climates stored in ice sheets and mountain glaciers."
These ice core samples have been able to tell us information about the climate of the Earth dating back 110,000 yrs in Greenland and 750,000 yrs in Antarctica.
These core samples tell scientists the levels of greenhouse gases which were present in the atmosphere by the quantity of those gases which were trapped into the layers of ice through the years. Here is a quote from the same article of how the process works:
"As valuable as the temperature record may be, the real treasure buried in the ice is a record of the atmosphere's characteristics. When snow forms, it crystallizes around tiny particles in the atmosphere, which fall to the ground with the snow. The type and amount of trapped particles, such as dust, volcanic ash, smoke, or pollen, tell scientists about the climate and environmental conditions when the snow formed. As the snow settles on the ice, air fills the space between the ice crystals. When the snow gets packed down by subsequent layers, the space between the crystals is eventually sealed off, trapping a small sample of the atmosphere in newly formed ice. These bubbles tell scientists what gases were in the atmosphere, and based on the bubble's location in the ice core, what the climate was at the time it was sealed. Records of methane levels, for example, indicate how much of the Earth wetlands covered because the abundance of life in wetlands gives rise to anaerobic bacteria that release methane as they decompose organic material. Scientists can also use the ice cores to correlate the concentration of carbon dioxide in the atmosphere with climate change-a measurement that has emphasized the role of carbon dioxide in global warming."
These ice samples do point to times where the earth has naturally warmed and cooled, but levels of greenhouse gases are now higher than during any other period of warming for which we have record. Dating back 420,000 thousand years the highest levels of CO2 which scientists have found in the ice samples was 300 ppm (http://cdiac.ornl.gov/trends/co2/vostok.
So, what is the difference between this warming trend and previous ones. Why are CO2 levels astronomically higher than they have normally been in the past? Humans.