According to the author,
"If the new reconstruction of Southern Hemisphere temperature is accurate, then estimates of climate sensitivity — the response of global temperature change to a given amount of external radiative forcing [mostly from CO2] — may be lower than those calculated based solely on Northern Hemisphere reconstructions10. Indeed, instrumental temperature data suggest that warming in the Northern Hemisphere has been greater than that observed in the Southern Hemisphere over the past two decades (Fig. 1c) — a feature reproduced in the current suite of climate models11. Therefore, this hemispheric asymmetry may be a fundamental feature of the climate system's response to a change in radiative forcing12, whereby the ocean-dominated Southern Hemisphere acts as a buffer of sorts to global temperature change on decadal to centennial timescales. On the other hand, Neukom et al. propose that divergent hemispheric temperatures arise from strong natural climate variability in the Southern Hemisphere, and have been a constant feature of the past millennium."
"Given the new information now available from the Southern Hemisphere, climate scientists must consider a larger role for natural climate variability in contributing to global temperature changes over the past millennium. While the new reconstruction brings strong additional support to the phrase 'anthropogenic global warming', it also highlights the limits of our current ability to understand, and predict, global temperature variations from decade to decade."The new Southern Hemisphere temperature reconstruction of Neukom et al referred to in this commentary shows the globe is only about 0.25C warmer than during the Medieval Warm Period a millennium ago, and that's only by ex-post elimination of temperature proxies that didn't have a hockey stick shape, as has been noted by Steve McIntyre and others.