The second millennium: The Medieval Warm Period

Further evidence for Northern Hemisphere (NH) climate variability in the past 1000 years has been shown by Mann et al (2009) (1). In Fig. 1, a compilation of proxies from ice cores, dendrochronology, corals and ocean/lake sediments have been used to show the temperature anomalies against the 1961-1990 record from 1500 years BP to present. 

Fig. 1. Proxy compilation showing temperature reconstructions over the past 1500 years (Mann et al., 2009)

The red box on Fig. 1. details the timeline of the Medieval Warm Period (MWP), also known as the Medieval Climate Anomaly (MCA) occurring from c. 950-1250 AD. This is largely only seen in the NH as SH records are sparse.

In the MWP, Mann et al. suggest that mean temperatures were significantly higher than the 1961-1990 average and possibly higher than the 1990- present period. Indeed, multi-proxy temperature reconstruction undertaken by Goosse et al (2006) (2) confirm that European summer temperatures 1000 years ago were similar to temperatures seen in the past 25 years.

Several reasons for this warm period have been presented. A paper by Graham et al. (2011) (3) developed a global physically-based model to represent temperatures since 800 AD. In that model, changes to atmospheric circulation, including a widening of the Hadley Cell as well as North Atlantic Oscillation (NAO) strengthening were presented as possible reasons for the warming of the NH climate. They suggest that this may have been caused by warming to the Indian and Western Pacific Oceans which would also affect monsoon-led circulation. As Trouet et al (2009) (4) explain, such an atmospheric change would result in a net cooling of the Eastern and Central Pacific causing La Nina like conditions. This would strengthen the NAO causing enhanced tropical westerlies, push the ITCZ further North and enhance the AMOC thus increasing heat transfer to the NH (see 'Short circuiting the thermohaline circulation' post for more information).

The cause of such atmospheric circulation changes, with enhanced La Nina and NAO, may have been caused by enhanced solar irradiance, as solar irradiance cycles are accepted in the literature, coupled with lower volcanism, which eject particles high into the atmosphere to reflect and block solar energy as suggested by Mann et al. (2009).

The relaxation of these atmospheric anomalies may have swung the NH climate into the LIA, shown by the blue box on Fig. 1. Evidence for the LIA will be presented in the next post as well as further theories to explain both events.

(1) Mann et al. (2009) doi: 10.1126/science.1177303

(2) Goosse et al. (2006) doi: 10.5194/cp-2-99-2006

(3) Graham et al. (2011)  doi: 10.1007/s00382-010-0914-

(4) Trouet et al. (2009 doi: 10.1126/science.1166349