A new study just released throws all of that into question, however, by producing compelling evidence that the MWP was actually a world-wide event. The Daily Mail has the story. Here's an excerpt:
A team of scientists led by geochemist Zunli Lu from Syracuse University in New York state, has found that contrary to the ‘consensus’, the ‘Medieval Warm Period’ approximately 500 to 1,000 years ago wasn’t just confined to Europe. In fact, it extended all the way down to Antarctica – which means that the Earth has already experienced global warming without the aid of human CO2 emissions.The article goes on to describe how Lu and his team used ikaite to ascertain the temperatures which prevailed in Antarctica during the MWP. You can find the technical details at the link. The take home message, though, is that a severe period of global warming lasted for several centuries and then naturally diminished, and none of it was the result of human activity.
At present the Intergovernmental Panel on Climate Change (IPCC) argues that the Medieval Warm Period was confined to Europe – and that therefore the warming we’re experiencing now is a man-made phenomenon. However, Professor Lu has shown that this isn’t true – and the evidence lies with a rare mineral called ikaite, which forms in cold waters.
Perhaps the current upward trend in global temperatures is indeed caused by increased greenhouse gasses generated by human activity, but there appears to be historical precedent for temperature surges that are unrelated to industrialization and CO2 emissions so it would seem very premature to assume that something similar is not happening today.