Read an interesting report on the Berkeley Earth Surface Temperature project.
Partly funded by climate sceptics, such as the Heartland Institute.
Prof Richard Muller, a physicist and climate change sceptic who founded the Berkeley Earth Surface Temperature (Best) project, said he was surprised by the findings. “We were not expecting this, but as scientists, it is our duty to let the evidence change our minds.”
He added that he now considers himself a “converted sceptic” and his views had undergone a “total turnaround” in a short space of time.
“Our results show that the average temperature of the Earth’s land has risen by 2.5F over the past 250 years, including an increase of 1.5 degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases,”
— http://www.guardian.co.uk/science/2012/jul/29/climate-change-sceptics-change-mind
To my mind the data sets are too incomplete to be able to use past evidence to predict future trends.
I suspect that will always be the case though. No matter how much past data you have, predicting the future is always a “best guess”.
FWIW the Berkeley project used “a collection of 14.4m land temperature observations from 44,455 sites across the world dating back to 1753”