Taken from: http://www.rgs.org/OurWork/Schools/Teaching+resources/Key+St...
There are other sources, but this is one example that makes it clear we have been in a warming phase the last few thousand years.
For example: https://en.wikipedia.org/wiki/Temperature_record#/media/File...
We would naturally now have started cooling again were it not for the additional greenhouse gases in the atmosphere. As I've already explained.
> "Since the industrial revolution in the 18th century, the concentration of carbon dioxide in the atmosphere (an important greenhouse gas) has risen significantly above the level it would be naturally. It now stands at over 400 parts per million (ppm) in the atmosphere. Evidence from ice cores (see Lesson four) tells us that the normal level for CO2 in the atmosphere during ‘interglacial’ times (such as the Holocene in which we now live) is 270 to 290 ppm, and that at no time over the last 800,000 (the time covered by ice cores) has the CO2 level been as high as it is now. (CO2 is thought to be at its highest level for three to five million years.)"
http://www.rgs.org/OurWork/Schools/Teaching+resources/Key+St...
If one had a time machine and went back 1000 years, took temperatures and compared them to those from previous thousand years, one would not be surprised to see that on average they were higher.
Even if you were right and we were in a "warming" phase, the velocity of the change should be on a geological scale; that is, very very very tiny. We are seeing rates of warming orders of magnitudes higher, which cannot possibly be anything but man made.
4000 years ago there were still mammoths and far greater glaciation than now, so clearly climate has been warming over the last few thousand years.