In 1965 Hubert Lamb notice there was evidence from around the world of a warming from 1000 to 1200 AD calling it the ‘Medieval Warm Epoch’ or ‘Little Optimum’ (AR4 Box 6.4 Page 468)

More is known about the medieval warming period today which now covers the time period from 800 to 1200 years ago and is a primary focus of the IPCC in both AR4 and AR5. The main reason for this focus is there are existing proxies covering this period providing valuable data.

Proxies are from things like tree rings, coral reefs, ice cores, pollen, and more. However, the IPCC goes on to explain proxies are not as accurate as instrumental measurements for many reasons.

For one, the IPCC tells us as times goes on the data is sparser or lacking after a 1000 or so years. Second, there are questions on if the link is related to temperature or something else. For example, were past tree ring sizes influenced by changes in temperature or moisture, ph., or maybe something else? (AR5 Section 10.7 Page 917)

Here, the IPCC tells us “forcing and temperature reconstructions are subject to large uncertainties”. They handle such uncertainties with the use of several proxies covering the past 1000 years. (AR4 Section 9.6.3.1 Page 724)

During this period the IPCC notes a study covering the NH (northern hemisphere) showing the medieval was warmer on the 2-kyr time scale. However, they cannot prove whether it was as warm as today, or how long the warming lasted in different regions. (AR4 Box 6.4 Page 468)

The IPCC explains the medieval warming was probably .1°C to .2°C lower than the temperatures seen the last 50 years, but they also admit there is uncertainty as record updates are needed because many are over 20 years old. They express a need for wider geographic coverage as there are very few records from oceans, tropics, and SH (Southern Hemisphere). (AR4 Box 6.4 Page 468)

The IPCC states proxies show today’s global average temperature is “at least as warm as any other century since at least 1400 AD”. They admit data is lacking to “allow the reliable estimation” of earth’s average temperature past 1400 AD. (AR5 Table 1.1 Page 124)

The IPCC does not address much of the data available from past civilizations during the medieval warming period. For example, the IPCC states looking at past climate changes effect on bio systems or past civilizations are not within their scope or to be covered. (AR5 Section 5.1 Page 388)

However, the IPCC does admit there is evidence from past civilization of a warmer period for a few centuries a 1000 years ago. For example, they describe how Norse settlers were able to settle Greenland because of less sea ice in the 10^th century. (AR4 Box 6.4 Page 468)

We have a lot to learn from our ancestors as this provides us some real data points. For example, we know the Anasazi Indians thrived in America’s southwest desert in a wetter warmer environment for 400 years during the medieval period. Many others civilizations from the Mayans to the Vikings who also thrived during the medieval period.

However, despite evidence from past several cultures thriving in wetter warmer environment for hundreds of years the IPCC does little to address these while claiming many studies show today is warmer or just as warm as 1000 years ago.

The length of time is also not addressed by the IPCC despite the Medieval warming lasting for 400 years or so compared to today’s warming of maybe a few decades.

As a side note, I have yet to find one past great civilization that thrive when temperatures went down. I found only the opposite as past civilizations seem to do very well during periods when temperatures in the Northern Hemisphere were rising. As far as I can tell if our civilization does fail because of rising temperatures it would be the first to do so since the ice age, but I am still looking.

The IPCC for the most part ignores temperature data during the 3000 to 7000-year ago timeframe as well despite GISP showing the temperature on the century scale was much higher than today. I find the IPCC does not try to explain what caused past warming as they seem more concerned with trying to portray today’s temperature higher than it was during the Medieval.