The evidence is pretty clear that the globe is cooling on most time scales, and right now we are at one of the coolest points in the history of the earth. Looking first at the last 65 million years, since T-Rex was running around:
Global Temperature estimates over the last 65 million years based on oxygen isotope thermometry of deep-ocean sediment cores from many parts of the world. Data from Zachos et al (2001). Graph by Robert A. Rohde.
Looking more recently at the last 5 million years, we see a consistent cooling trend with rapid cycling ice ages for the last 3 million years or so. Thirty-three ice ages cycled approximately every 41,000 years from 2.6 to 1.25 million years ago, and more recently thirteen ice ages have occurred on a 100,000-year cycle for the last 1.25 million years.
5 million years of global temperatures from work done by Lisiecki and Raymo in 2005. Graph by Robert A. Rohde.
The interglacial warm periods have lasted around 10,000 years, and guess what? Our most recent interglacial (that we are currently in) is about 10,000 years old.
So what has happened in the last 10,000 years? Well, one of the best ice cores we have from the last 10,000 years is from Greenland where from 1988 to 1993 scientists drilled 3 km through the ice sheet all the way to the bedrock yielding a high-resolution record of the last 200,000 years of history including two glacials and two interglacials. Here’s what GISP2 tells us about the last 10,000 years.
During the vast majority of the last 10,000 years (i.e. the age of human agriculture) it was warmer at GISP2 than it is now! Furthermore, during the last 4,000 or 5,000 years the general temperature trend is…you guessed it…down. While the single ice core doesn’t tell us the global temperature, we do have hundreds of studies confirming the Medieval Warming shown very clearly here. At GISP2, the Roman Warming was another 1ºC warmer than the Medieval Warming, and the Minoan and two earlier warmings were another .7ºC warmer than that.
To quote Don J. Easterbrook from his post on WUWT,
Temperature changes recorded in the GISP2 ice core from the Greenland Ice Sheet show that the magnitude of global warming experienced during the past century is insignificant compared to the magnitude of the profound natural climate reversals over the past 25,000 years, which preceded any significant rise of atmospheric CO2. If so many much more intense periods of warming occurred naturally in the past without increase in CO2, why should the mere coincidence of a small period of low magnitude warming this century be blamed on CO2?
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