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The Deep Ocean May Be Colder Today Than Any Time in the Last 4.5 million Years

A serene underwater scene depicting glowing jellyfish swimming in dark blue water, illuminated by shafts of light filtering down from the surface.

From No Trick Zone

By Kenneth Richard

A new study finds Earth’s bottom water temperatures (BWTs) have cooled by 2-3°C over the last 4.5 million years through to the pre-industrial era (1750).

Since 1750, however, global BWTs have not risen in a detectable way, nor have they exceeded the warmth achieved during the Medieval Warm Period (Gebbie and Huybers, 2019). The Pacific Ocean as a whole has continued to cool in the last centuries.

Graph depicting changes in bottom water temperature (BWT) over the past 4.5 million years, showing cooling trends and reconstruction data points, with highlighted Pacific and Atlantic ocean temperature changes.
Image Source: Clark et al., 2025 and Gebbie and Huybers, 2019

Regionally, today’s North Atlantic’s bottom water temperatures hover around 4°C – just as they did throughout the Late Holocene. The BWTs averaged ~5°C, or “slightly warmer than present-day,” during the last glacial, with anomalies reaching 10°C both 13 and 16 thousand years ago and about 7°C both 15 and 19 thousand years ago (Yasuhara et al., 2019).

Scientific article discussing bottom-water temperature variability in the North Atlantic over the past 20,000 years, highlighting key findings and data.
Image Source: Yasuhara et al., 2019

As recently as 10,000 years ago, the Arctic Ocean’s bottom water temperatures were 6-10°C warmer than they are today (Beierlein et al., 2015).

Graph showing seasonal water temperature cycles in the Arctic Dicksonfjord during the Holocene Climate Optimum and modern BWT comparisons, detailing maximum, average, and minimum temperatures.
Image Source: Beierlein et al., 2015

These global and regional BWT reconstructions do not support the narrative that modern ocean temperatures are unprecedentedly warm due to human activity.

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