Around 1.2 million years ago, a shift in global climate began that caused a change in the timing of the alternating warm and cold periods—called interglacials and glacials—that have persisted during the Pleistocene Ice Age. Prior to that time, ice age glacial periods lasted about 40,000 years but since ~700,000 years ago ice-age cycles have lasted for around 100,000 years. Orbital variations, called the Croll-Milankovitch cycles, do exert some forcing on the 100,000 year time scale, but it is relatively weak. Orbital cycles seem to many too feeble an explanation for the change in glacial-interglacial timing. Some scientists have attempted to attribute the timing shift to a drop in CO2 but a new study confirms that carbon dioxide levels were not the cause of the climate shift.
In a new report in Science, scientists present results matching neither the hypothesis of inter-hemispheric synchrony during the mid to late Holocene nor rhythmic asynchrony, downplaying the importance of global driving mechanisms. It seems that several glacier advances occurred in New Zealand during classic northern warm periods. Understanding the relationship between climate changes in the northern and southern hemispheres during the Holocene, along with their causes, remains a major problem of climate science.
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.