This is the fourth installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.
In my previous article in this series, I talked about climate change patterns over geologic time. I showed how today's spike in temperature and carbon dioxide (CO2) concentration breaks patterns and records going back 600,000 years.
This departure can be seen even more clearly when you look at just the last 1000 years (see graph in previous post). So what is this you hear about a Medieval Warming Period?
The Medieval Warming Period refers to a relatively warm period lasting from about the 10th to the 14th century. Some argue that since the Medieval Warming Period was a natural event, the current warming also must be due to natural processes.
This argument is wrong on two counts – one factual and one logical. I'll start with the logical error.
Let's say that the premise is true – that there was a global warming period in Medieval times. (It's not true, but for the sake of argument, let's say it is.) It does not then follow that the current warming period has the same cause! If you and I both have purple shoes, it does not then follow that we got them in the same place. This logical fallacy, commonly used by climate change skeptics, is called Ignoratio Elenchi – Latin for "irrelevant conclusion".
So the logic makes no sense to begin with, but in this case even the premise is wrong! There may have been a warm period during Medieval times, but current studies show that (a) temperatures were not as warm as today, and (b) it wasn't global.
There's been a lot of argument and misinformation on this topic. For a great summary of the history, see "Myth vs. Fact Regarding the "Hockey Stick"" on the RealClimate Web site.
The graph below is from a sidebar in the IPCC assessment called "Hemispheric Temperatures in the 'Medieval Warm Period' [PDF - 7.9MB]" (page 468). As you can see, when you look at temperature estimates from around the world – not just Western Europe – there's no clear spike in temperature until our own time.
If you don't trust the temperature proxies, here's proof that is perhaps even more telling. Andean glaciers that have been intact for more than 5,000 years are now rapidly melting (see original study, news coverage). If the Medieval Warm Period was truly global, these glaciers would not have survived.
So there's one more climate change myth debunked. Hopefully this article will help you to explain the "Medieval Warming Period" to others. Any comments? Questions? Post them here!