Part 4 of 5: Medieval Warming Period

This is the fourth installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

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!

This entry was posted in Basic Science of Global Warming. Bookmark the permalink. Both comments and trackbacks are currently closed.


  1. NL
    Posted July 6, 2007 at 1:04 am | Permalink

    I have been exploring your series (thanks to Dr. Lisa Moore posting it on daily kos), and have seen your prior articles linked to in the main text and comments. I must say that I am thouroughly impressed. Dr. Chameides. Your work is about the most accessibly written work to explain the global warming phenomenon that I have seen. Providing links to full data sources is an excellent practice as well.

    Thank you for your efforts.

  2. NL
    Posted July 6, 2007 at 1:05 am | Permalink

    … and looking forward to the final installment in this series. Are you able to mention when you are planning to publish it?

  3. Posted July 6, 2007 at 10:10 am | Permalink

    Hi NL,

    Thanks for your feedback – I appreciate it.

    The final installment in the series will be published next week. We’ve been posting them one each week.

  4. NL
    Posted July 6, 2007 at 11:27 am | Permalink

    Dr. Chameides, thanks for your reply. One point I was thinking of was that, the Live Earth concerts that are to take place tomorrow are likely to generate a strong surge of interest in people around the world looking to explore the subject. They may come over to visit your series (from search engines, recommendations, etc), and it will be very useful if the final article in the series is there when that interest peaks, which may happen over the next couple of days.

    For that reason, I would strongly recommend posting the article as soon as possible, if it is already written up, to maximize the audience and impact it would have in the context of Live Earth. If you’re unable to do so, I look forward to reading it next week.

    I also have several technical questions which I may stop by to post in the coming days.

    thanks again.

  5. Posted July 7, 2007 at 10:00 pm | Permalink

    Here’s a series you should watch. And please, by all means, feel free to provide intelligent, rational feedback adressing the FACTS in this documentary.
    Thank you.

  6. Hangman
    Posted July 8, 2007 at 2:06 pm | Permalink

    Man is just the salt on an open wound that already exists. Adding to the effect of a cause that started eons ago. The Sun is blasting out thousands more times of energy than it ever has, causing a polar shift of the Earth. The magnetic field that surrounds the Earth is changing, this is whats happening to the Honeybees, whales getting lost or beeching themselves, bird migration, etc. etc., Animals use the earths magentic field to guide themselves.
    This has happened 6 previous times in Mother earths life, every 25,837 years, 1 Solor year. This is also the reason why the other Planets in our Solor system are turbulent & changing colors. The Planets move around the sun in a sort-of Cosmic Symphony, & the end of an Epoc is near, where the Cosmic Clock or Symphony will be resetting or restarting, a new Solor Year.