via Eurekalert today…
The study emphasises that this trend came to an end with the beginning of the Industrial Revolution and the resulting global warming caused by human activity.
From the UNIVERSITAT AUTONOMA DE BARCELONA
The Industrial Revolution put an end to 1,800 years of ocean cooling
The high frequency and magnitude of volcanic eruptions could have been the cause of the progressive cooling of ocean surfaces over a period of 1,800 years. This is made apparent in an international study published recently in the journal Nature Geoscience, involving researcher P. Graham Mortyn of the Institute for Environmental Science and Technology (ICTA-UAB) and the UAB Department of Geography.
The study emphasises that this trend came to an end with the beginning of the Industrial Revolution and the resulting global warming caused by human activity. It further shows that the lowest temperatures in the first 1,800 years of the Common Era were recorded between the 16th and the 18th centuries, a period known as the “Little Ice Age”.
Earlier research had already shown that volcanic explosions cause the atmosphere to cool. The present study demonstrates that the oceans can absorb and capture more heat than the atmosphere over longer periods of time, thus attenuating global temperature changes in the short term. These alterations in temperature can be prolonged when the volcanic eruptions are concentrated into a short space of time.
These findings bring a new perspective to the study of regional and global variations in ocean-surface temperature over the centuries, before the appearance of anthropogenic (human activity-induced) climate change.
The researchers combined, for the first time, 57 previous works on sea-surface temperature changes, which are calculated from fossil remains extracted from oceans all over the world, from the Poles to the Tropics. The results were compared with data from terrestrial indicators, such as tree rings or ice cores. These revealed a similar cooling trend.
To investigate the causes of this cooling – more robust between the years 800 and 1800 – they used climate models and they examined how the ocean surface reacted to factors like solar activity, changes in the Earth’s orbital patterns, land use, greenhouse gases and volcanic activity. The last of these proved to be of importance. In order to analyse long-term trends, the team grouped the data into periods of 200 years.
P. Graham Mortyn is a member of the Ocean2k group within PAGES (Past Global Changes), which has over 75 members connected to a network of close on 600 highly-experienced scientists. Mortyn lays emphasis on how this group worked with data from previous publications available to the public and archived databases that meet extremely strict criteria, and how they made regular use of Skype conferencing, shared Google documents and other innovative technological resources.
The researchers point out that understanding how these factors caused changes in ocean temperature in the past could give us an insight into future changes in climate.
As is typical with shonky alarmist press releases in climate science these days, they don’t bother to name the study or present a DOI. So I looked it up. Laughably, there’s that favorite climate buzzword again, “robust”. It turns out this was published on August 17th, and P. Graham Mortyn is the 7th from the top named author. Given that and the lateness of the press release, this looks like a ploy for attention. And the press release reads quite a bit more alarmist than the abstract, which does not mention the industrial revolution.
Robust global ocean cooling trend for the pre-industrial Common Era
Helen V. McGregor, Michael N. Evans, Hugues Goosse, Guillaume Leduc, Belen Martrat, Jason A. Addison, P. Graham Mortyn, Delia W. Oppo, Marit-Solveig Seidenkrantz, Marie-Alexandrine Sicre, Steven J. Phipps, Kandasamy Selvaraj, Kaustubh Thirumalai, Helena L. Filipsson & Vasile Ersek
Nature Geoscience 8, 671–677 (2015) doi:10.1038/ngeo2510 Received 24 October 2014 Accepted 17 July 2015 Published online 17 August 2015
The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the past 2,000 years — a key interval for understanding the present and future climate response to these forcings — global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface temperatures for the Common Era (CE) derived from 57 individual marine reconstructions that meet strict quality control criteria. We observe a cooling trend from 1 to 1800 CE that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 CE, the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with a sea surface temperature composite derived from an ensemble of climate model simulations using best estimates of past external radiative forcings. Climate simulations using single and cumulative forcings suggest that the ocean surface cooling trend from 801 to 1800 CE is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism. Our results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global scale cooling trend via a decline in mixed-layer oceanic heat content.
UPDATE: Steve McIntyre looks at the oceans2K data, and surmises it’s a bit hyped up. (h/t CTM)
The long-awaited (and long overdue) PAGES2K synthesis of 57 high-resolution ocean sediment series (OCEAN2K) was published a couple of weeks ago (see here here). Co-author Michael Evans’ announcement made the results sound like the latest and perhaps most dramatic Hockey Stick yet:
Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years,” said Michael Evans, second author of the study and an associate professor in the University of Maryland’s Department of Geology and Earth System Science Interdisciplinary Center (ESSIC). “This study truly highlights the profound effects we are having on our climate today.”
A couple of news outlets announced its release with headlines like “1,800 years of global ocean cooling halted by global warming”, but the the event passed unnoticed at realclimate and the newest “Hockey Stick” was somehow omitted from David Appell’s list of bladed objects.
The OCEAN2K Reconstruction
One of the reasons for the strange lack of interest in this newest proxy “Hockey Stick” was that the proxy data didn’t actually show “the climate was warming about 20 times faster than it cooled during the past 1,800 years”. The OCEAN2K reconstruction (see Figure 1 below) had a shape that even David Appell would be hard-pressed to describe as a “Hockey Stick”. It showed a small decrease over the past two millennia with the most recent value having a tiny uptick from its predecessor, but, whatever image one might choose to describe its shape, “Hockey Stick” is not one of them.
FAQ Figure 1: Results of the global sea surface temperature compilation from Ocean2k: A cooling over the past two millenium was reversed only in the most recent two centuries. Fifty-seven previously published and publicly available marine sea surface temperature reconstructions were combined and compiled into 200-year brackets, represented by the boxes. The thin horizontal lines dividing each box are the median of the values in that box. The thick blue line is the median of these values weighted for differences in the region of the global ocean in which they were found. (More in Figure 2a in the paper and Supplementary Table S13. ) Link
Read it all here: http://climateaudit.org/2015/09/04/the-ocean2k-hockey-stick/#more-21349