Climate change deniers will deny anything and everything that vaguely looks like it might support anthropogenic global warming. They think the Medieval Warm Period wasn’t regional but global and that temperatures rose a lot all around the world and all at the same time. Because the MWP wasn’t due to our emissions, they argue that the current warming isn’t necessarily due to us either. Part of my response to a recent comment on the Quora site follows …

“The MWP was regional rather than global. And the warming didn’t happen at the same time but was spread out over many hundreds of years in different places at different times; some places cooled. Those hand-drawn denier graphs showing a large rounded lump for the MWP around 950–1250 CE are nonsense. Denier Anthony Watts thought he was showing how the MWP was global but he actually showed in detail and perhaps unwittingly how it was regional. See More evidence that the Medieval Warming Period was global, not regional.

The map he pins there (see below) shows more warming (red markers) than cooling (blue markers) in the high latitudes. But there are many green (wet) and yellow (dry) markers across the middle and lower latitudes where wet or dry dominated the climate rather than warm or cool. The map gives the impression that red (warm) dominates overall, but the particular map projection has the higher latitudes taking up far more of the globe than in reality.

If you click on “this map” in the fourth line of text, you’re taken here: Medieval Warm Period – Google My Maps. When you click on the markers or on the list of 1272 MWP studies that were used to put the map together, you’ll see that the studies found warming at very different times across the globe (and cooling, wet and dry at different times too), with sometimes part or all of the period extending outside the usual timeframe given for the MWP of around 950–1250.

On another page, he posts the following map relating to various studies on the MWP. It reveals warming to very different extents and at very different times across the globe. In some areas, there were several peaks. And some peaks lasted several centuries while others only a few years. Canada’s Boothia Peninsula’s MWP peak was around 600 CE. An area at the top of Norway peaked around 1480. And other areas recorded peaks at various times in between. …

[Michael] Mann’s hockey stick and the numerous replications of it reflect these ups and downs. There is warming (and cooling) on every continent but not all at the same time. It’s a similar story with other warm periods and also the Little Ice Age. In contrast, the current warming is global, or about 98% of the globe, and it’s all happening at the same time.”

Deniers also accuse scientists of trying to erase the MWP, usually that well known climate scientist Michael Mann tried to do this. An earlier part of my same response follows …

“No one erased or tried to erase the Medieval Warm Period. Michael Mann’s study started at 1000 CE, well after the start of the MWP. His graph therefore started high and fell through the end of the MWP, through the Little Ice Age, and then rose sharply in the current era, thus its hockey stick shape. More than two dozen studies have replicated his study. Here are some of them. Note that the blade is much longer in later studies due to the current ongoing rapid warming.”