A study of oceanic temperatures from 1-1800 AD has found a slow cooling. The findings broadly match measurements for temperatures on land, and the authors attribute the effect to increased volcanic activity. Some of the reversal since the start of the 19th century has been a consequence of decreased volcanism, but recent warming is overwhelmingly human induced, according to first author Dr. Helen McGregor.

Climatologists have used measures such as tree rings, stalactites and ice cores to track the Earth’s temperatures on land prior to the establishment of global weather stations. However, with 70% of the planet covered by water, and the oceans holding far more heat than the atmosphere, this picture has been very incomplete. Now, marine sediment cores have been collected from enough places around the world to put the oceans in the picture. These cores provide a record of the conditions in the water column above them.

McGregor of Wollongong University, Australia, and co-authors from around the world combined results from studies at 57 locations. McGregor told IFLScience that the team avoided work that focused on annual fluctuations, instead bracketing data into 200-year intervals to smooth out trends.

Beginning at 1 AD, the results, published in Nature Geoscience, show a clear cooling trend that gets stronger around 1,000 years ago. McGregor says temperatures fell about 0.5°C per millennium, but “we have more certainty in the direction than the absolute number.”

McGregor says a number of techniques were used to estimate temperatures at the time sediment was laid down, including organisms whose shells reflect the chemical conditions in which they were formed, and the presence or absence of temperature-dependent species.

The findings, particularly for the period 800-1800 AD, are “qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions,” the authors report. This includes the work of Michael Mann for which he has been vilified and harassed.

Over the course of Earth’s history, temperatures have changed for many reasons. At least from the period 800 AD onwards, the authors conclude the trend “is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism.”

McGregor explained to IFLScience that ice cores from the poles contain ash deposits from volcanoes and, “When we get ash layers from Greenland and Antarctica simultaneously we can be fairly sure they are from a tropical volcano.” McGregor adds that these have been confirmed in the cases of eruptions for which we have historical records, such as Tambora and Krakatau.

Although volcanoes can sometimes warm the planet through the release of carbon dioxide, it is more common for them to have a cooling effect when ash clouds and sulfate aerosols block out the sun. The paper argues that the increasing frequency of volcanic outbursts through the period explains the cooling observed.

McGregor told IFLScience that the relatively low numbers of large volcanic eruptions in the 20th century would have caused some rebound in ocean temperatures, “but human contributions far eclipse natural forcings” for warming in recent decades. In just 40 years, ocean temperatures have warmed more under the influence of emissions than they have cooled from a thousand years of volcanic activity.