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A recent article in the journal Science has provided a new, detailed climate record for the Medieval Climate Anomaly (MCA), also know as the Medieval Warm Period. It was the most recent pre-industrial warm period, noted in Europe and elsewhere around the globe. The researchers present a 947-year-long multi-decadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. The interesting thing is that the MCA had basically been removed from the climate record by Michael Mann's infamous “hockey stick” history graph that was adopted by the IPCC a decade ago.
The persistent positive phase reconstructed for the MCA appears to be associated with prevailing La Niña–like conditions possibly initiated by enhanced solar irradiance and/or reduced volcanic activity and amplified and prolonged by enhanced AMOC. The relaxation from this particular ocean-atmosphere state into the LIA appears to be globally contemporaneous and suggests a notable and persistent reorganization of large-scale oceanic and atmospheric circulation patterns.
In summary the researchers conclude: “[I]t may be necessary to incorporate additional mechanisms for increased ocean heat uptake when simulating the early Pliocene climate and, potentially, the response of the tropics to contemporary global warming. The enormous impacts of changes in the warm pool (such as shifts in global precipitation patterns and cloud cover), as well as tentative evidence that the tropical belt has been expanding poleward over the past few decades, make our findings especially relevant to current discussions about global warming.”
The Medieval Warm Period linked to the success of Machu Picchu, Inca Empire
Here is the abstract: The rapid expansion of the Inca from the Cuzco area of highland Peru produced the largest empire in the New World between ca. AD 1400–1532. Although this meteoric rise may in part be due to the adoption of innovative societal strategies, supported by a large labour force and standing army, we argue that this would not have been possible without increased crop productivity, which was linked to more favourable climatic conditions. A multi-proxy, high-resolution 1200-year lake sediment record was analysed at Marcacocha, 12 km north of Ollantaytambo, in the heartland of the Inca Empire. This record reveals a period of sustained aridity that began from AD 880, followed by increased warming from AD 1100 that lasted beyond the arrival of the Spanish in AD 1532. These increasingly warmer conditions allowed the Inca and their predecessors the opportunity to exploit higher altitudes from AD 1150, by constructing agricultural terraces that employed glacial-fed irrigation, in combination with deliberate agroforestry techniques. There may be some important lessons to be learnt today from these strategies for sustainable rural development in the Andes in the light of future climate uncertainty.