Was there a significant Medieval Warm Period?

The answer is "yes".


One way we know this is that in 1995, the IPCC itself admitted that the Medieval Warm Period existed (above), before someone evidently decided that it needed to be purged:

I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, "We have to get rid of the Medieval Warm Period."

A massive amount of supporting evidence is found at the CO2 Science web site.

From their Medieval Warm Period Project overview:

What is it?
Our Medieval Warm Period Project is an ongoing effort to document the magnitude and spatial and temporal extent of a significant period of warmth that occurred approximately one thousand years ago. Its goal is to ultimately provide sufficient real-world evidence to convince most rational people that the Medieval Warm Period was: (1) global in extent, (2) at least as warm as, but likely even warmer than, the Current Warm Period, and (3) of a duration significantly longer than that of the Current Warm Period to date.

Why is it?
The project's reason for being begins with the claim of many scientists - and essentially all of the world's radical environmentalists - that earth's near-surface air temperature over the last two decades of the 20th century (and continuing to the present) was higher than it has been during any similar period of the past millennium or more. This claim is of utmost importance to these climate alarmists; for it allows them to further claim there is something unnatural about recent and possibly ongoing warming, which allows them to claim that the warming has its origins in anthropogenic CO2 emissions, which allows them to claim that if humanity will abandon the burning of fossil fuels, we can slow and ultimately stop the warming of the modern era and thereby save the planet's fragile ecosystems from being destroyed by catastrophic climate changes that they claim will otherwise drive a goodly percentage of earth's plants and animals to extinction. Believing all of these claims to be false, we felt that disproving the first of them was the best way of refuting all the rest, or at least making them moot. The course of action we therefore take in this endeavor is to demonstrate that approximately one thousand years ago, when the atmosphere's CO2 concentration was approximately 25% lower than it is currently, earth's near-surface air temperature was equally as warm as, or even warmer than, it is today, demonstrating that today's temperatures are not unnatural and need not be due to the historical rise in the air's CO2 content. Indeed, these and other data covering a much longer timespan suggest that a more logical cause of our present warmth is the recurrence of whatever cyclical phenomenon produced the higher temperatures of the Medieval Warm Period.

From their Study Description and Results page, you can learn some details about related scientific papers. One example is here.