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Studies Suggest Volcanic Activity Had Profound Long-Term Impact On Past Climate …CO2 Is No Explanation

Written by Kenneth Richard

Guest author Kenneth Richard examines the impacts of past volcanoes on climate. The findings will surely be controversial.

Volcanic activity explains long-term climate change better than CO2

By Kenneth Richard

Long-term (decadal and even centennial-scale) volcanic influence on climate has recently gained more and more attention in the scientific literature.  Previously thought to influence surface temperatures for only a few years at a time, there is now a growing body of evidence suggesting volcanic aerosols may significantly affect both short and long-term climate changes by blocking solar radiation from heating the oceans’ surface waters.

When specifying the factors contributing to decadal and centennial-scale temperature changes, solar activity and greenhouse gases are usually thought to top the list. And since 93% of the heat from global warming ends up in the oceans (IPCC, 2013), the focus necessarily should be on what mechanisms contribute most to variations in ocean heat content (OHC) and sea surface temperatures (SST).

Back in 2013, Rosenthal et al. published a paper in Science on millennial-scale ocean heat content variations (Pacific). As the graph (Fig. 4B from the paper) below illustrates, the authors document a dramatic cooling of the 0-700 m layer between the Medieval Warm Period (~1000 CE) and Little Ice Age (1600-1800 CE). While OHC has risen since the depths of the Little Ice Age, modern ocean temperatures are still significantly cooler (-0.65°C) than what they were just 1,000 years ago, or during the Medieval Warm Period.

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In a Rosenthal et al 2013 paper, he writes:

“We show that water masses linked to North Pacific and Antarctic intermediate waters were warmer by 2.1 ± 0.4°C and 1.5 ± 0.4°C, respectively, during the middle Holocene Thermal Maximum than over the past century. Both water masses were ~0.9°C warmer during the Medieval Warm period than during the Little Ice Age and ~0.65° warmer than in recent decades.”

The causal explanation for this dramatic ocean cooling during the last millennium has generally not been forthcoming from those who attribute variations in temperature/heat content predominantly to variations in carbon dioxide. After all, there was essentially no change in atmospheric CO2 concentrations (which centered around ~275 ppm) between ~1000 CE and 1600-1800 CE, implying that CO2 forcing cannot explain the long-term changes in OHC.

There are literally hundreds of scientific papers that have been published suggesting that tracts of low solar activity (i.e., Oort, Spörer, Maunder, Dalton Minimums) are well correlated with decadal- and centennial-scale cooling periods. It is also well-documented that the Medieval Maximum and Modern Grand Maximum (~1920 to ~2010) of very high solar activity are well correlated with the last two warm periods. However, the solar activity explanation is deemed quite controversial, as it is presumed that long-term variations in the Sun’s energy output are too small to have a significant impact on climate changes.  So as not to veer off track or stumble through this controversy, a more indirect explanation for global warming and cooling trends is succinctly referenced here.

In 2015, the authors of the Pages2k (2013) “global” temperature reconstruction for 0-2000 CE released their Ocean2k record of sea surface temperatures (SST) entitled “Robust global ocean cooling trend for the pre-industrial Common Era“.  Below is the definitive graph from the paper depicting this robust global ocean cooling (which conspicuously conceals the post-1900 SST record). The paper itself indicates that the 1,000-year (kyr) cooling trend for global ocean temperatures changed by a modest ~0.1°C (from about -0.45°C kyr to -0.35°C kyr) with the inclusion of the 1800 to 2000 (“anthropogenic”) SST record:

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 McGregor et al., 2015 (Ocean2K):

“Our best estimate of the SST cooling trend, scaled to temperature units using the average anomaly method (method 1), for the periods 1–2000 CE is –0.3°C/kyr to –0.4°C/kyr, and for 801–1800 CE is –0.4°C/kyr to –0.5°C/kyr”

While intriguing that the authors of the Ocean2k reconstruction effectively acknowledge that global ocean surface temperatures haven’t fallen out of the range of long-term natural variability when including the last ~200 years, even more interesting is the physical mechanism suggested for this long-term ocean cooling: a centennial-scale decrease in surface incident solar radiation (SSR) via the aerosol dust-veiling effects of “explosive volcanism.”  Here is the summary from the paper’s abstract:

“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.”

Another 2015 paper by Pausata et al. published in PNAS explains how the direct, short-lived (2 to 3 years) effects of volcanic eruptions indirectly influence major long-term decadal-scale (25-35 years here) ocean oscillations (AMOC, ENSO), which, in turn, heavily influence climate.

“Large volcanic eruptions can have major impacts on global climate, affecting both atmospheric and ocean circulation through changes in atmospheric chemical composition and optical properties. The residence time of volcanic aerosol from strong eruptions is roughly 2–3 y.

Attention has consequently focused on their short-term impacts, whereas the long-term, ocean-mediated response has not been well studied. Most studies have focused on tropical eruptions; high-latitude eruptions have drawn less attention because their impacts are thought to be merely hemispheric rather than global. No study to date has investigated the long-term effects of high-latitude eruptions.

Here, we use a climate model to show that large summer high-latitude eruptions in the Northern Hemisphere cause strong hemispheric cooling, which could induce an El Niño-like anomaly, in the equatorial Pacific during the first 8–9 mo after the start of the eruption.

The hemispherically asymmetric cooling shifts the Intertropical Convergence Zone southward, triggering a weakening of the trade winds over the western and central equatorial Pacific that favors the development of an El Niño-like anomaly. In the model used here, the specified high-latitude eruption also leads to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) in the first 25 y after the eruption, followed by a weakening lasting at least 35 y. The long-lived changes in the AMOC strength also alter the variability of the El Niño–Southern Oscillation (ENSO).”

The conclusion that explosive volcanic eruptions – or clusters of smaller eruptions spaced closely together – could have a significant long-term effect on climate is not a new one. Back during the 1960s and 1970s, when scientists were searching for an explanation for the decades-long -0.3°C global cooling trend and its possible connection to droughts and severe weather events, volcanic climate forcing was often recognized as significantly responsible (Benton, 19701; Mitchell, 19702; Budyko, 19693).

It was also usually acknowledged that the proportion of human contribution to atmospheric aerosol loading was small (~10%) relative to the predominance of volcanic aerosols (Mitchell, 1970; Cobb, 19734).  More recently, scientists have also confirmed that volcanic eruptions are the  primary source of increases in stratospheric aerosol, and that “no hint for a strong anthropogenic influence has been found” (Neely et al., 20135; Höpfner et al., 20136 ).

Further strengthening this correlation between the presence or absence of volcanic aerosols and long-term cooling or warming trends, consider this key graph taken from Oliver (1976) below (Fig. 1, page 2).

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They write:

“A period of several decades existed (~1915-1945) in which volcanic activity was unusually light and, as mentioned earlier, the temperatures were higher than the preceding [1880s to 1910s] or, in fact, the subsequent (current) [1950s-1970s] period.

… Numerous possible causes of climate change have been discussed in the literature, including both anthropogenic and natural factors. Two principal anthropogenic sources are often considered: changes in atmospheric carbon dioxide and changes in tropospheric dust. … Mitchell (1975) concluded that neither tropospheric particulates [anthropogenic pollution] nor atmospheric CO2, in concert or separately, could have accounted for the major part of the observed temperature changes of the past century.”

Notice the remarkable correlation between the warming decades (1915-1945) and the lack of volcanic eruptions during that same period, and then notice the years and decades with several large volcanic eruptions and how these periods correspond with cooling. This appears to suggest that the absence of a physical cooling mechanism – namely, clusters of volcanic eruptions – may effectively be interpreted as a warming mechanism.

This explanation could account for the ocean temperature changes since the 1880s far better than anthropogenic CO2 emissions can. After all, anthropogenic CO2 emissions were flat and low (~1 GtC [gigatons carbon] per year) during the 1915 to 1945 warming period, and they rose dramatically (up to ~5 GtC per year) between the 1940s and 1970s.  This means that as CO2 emissions increased significantly, surface temperatures cooled significantly (1940s to 1970s) – the opposite of what should have been occurring if rising CO2 emissions are largely responsible for global warming.

A more recent depiction of large-scale volcanic eruptions for the last 1,500 years comes from a new paper (Liu et al., 2016) linking global-scale precipitation patterns (monsoons) to large volcanic eruptions. Notice that the dramatic post-Medieval Warm Period centennial-scale ocean cooling described above (Rosenthal et al., 2013, Ocean2k) corresponds closely with frequent clusters of volcanic eruptions. Also, notice how volcanically quiescent the last 80 years of the 20th century have been, which has, on net, allowed more solar radiation to heat the oceans and contribute to global warming.

Liu et al., 2016 finds:

“There are 54 large explosive volcanoes during 501–2000 AD in total, and the strongest one is the Samalas volcano in 1257–1258, which is followed by three smaller eruptions in 1268, 1275 and 1284. These strong volcanoes do not allow the climate to recover, and might have triggered the Little Ice Age.”

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Other recently-published papers also document a strong correlation between volcanic eruptions and climate changes. Otterå et al. (2010)7 conclude that volcanoes have played a “particularly important part” in directly influencing sea surface temperatures and  in phasing multi-decadal variability for the last 600 years.  Ludlow et al. (2013)8 find 1,200 years of statistically significant (99.7%) associations between cooling events and explosive volcanism for the Greenland Ice Sheet.

Instead of asking what factors are contributing to ocean warming on decadal and centennial time-scales, perhaps there needs to be more of a focus on answering the question of what factors have contributed to the periods of ocean cooling during the last millennium.

Variations in CO2 concentrations or anthropogenic CO2 emissions cannot effectively explain the long-term cooling of the oceans (about -0.35 C per 1,000 years for the last 2,000 years per Ocean2k ). On the other hand, explosive volcanism and its dimming effect on surface solar radiation can much more readily explain decadal- and centennial-scale cooling and warming phases with both its presence (cooling) and absence (warming).


1. Benton, 1970

2. Mitchell, 1970

3. Budyko, 1969

4. Cobb, 1973

5. Neely et al., 2013

6. Höpfner et al., 2013

7. Otterå et al., 2010

8. Ludlow et al., 2013

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