a new study shows that this spike in temperatures is unprecedented going back over one hundred centuries. They looked at global temperature anomalies—deviations from an average or standard temperature—for 73 sites distributed across the planet, using fossils in sediments as a proxy for temperature. The chemical and isotopic composition of the fossils yields a fairly accurate measure of the environment temperature at the time the animal or plant making up the fossil lived.
What they found is simply stunning: The rate at which the globe is warming right now is far, far faster than it ever has going back as far as they could measure, up to 11,300 years ago. In fact, over the past 5000 years, the Earth actually cooled by about 1.3°F…until the last 100 years, when our temperature spiked upwards by about the same amount.
Mind you, this is the rate of warming, how quickly the global temperature is increasing. But they also showed the actual temperature of the planet is warmer now than it has been for 70-80 percent of the past over that time period. There have been times when the Earth was warmer, but the important point isn’t the actual temperature, but what it’s doing.
And what it’s doing now is skyrocketing.
source
http://www.slate.com/blogs/bad_astronomy/2013/03/13/global_warming_new_study_shows_warming_is_faster_than_it_has_been_in_11.html