EAPS

Title: The Little Ice Age and 20th Century Deep Pacific Cooling

Abstract: The deep ocean adjusts to changes in surface temperature over centuries and millennia. Forcing an ocean circulation model using surface temperature reconstructions over the last 2,000 years results in a deep Atlantic that warms and a deep Pacific that cools over the 20th Century. Modern deep-Pacific cooling is a consequence of delayed adjustment to the transition from the Medieval Warm Period to Little Ice Age. A similar pattern of deep-Atlantic warming and deep-Pacific cooling is identified in temperature changes since the 1870s HMS Challenger expedition. Constraining the ocean circulation model with these observed temperature trends indicates that, below 2000 meters depth, the ocean was a net source of heat into the early 20th century on account of vestigial heat from the Medieval Warm Period. Ocean heat uptake calculations that assume the ocean was in equilibrium in 1750 C.E. miss the lingering effects of the Medieval Warm Period and Little Ice Age, which leads to an overestimate of total ocean heat uptake over the 1750-2015 C.E. interval.

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