How Volcanoes Help Resolve Climate Controversies

Aug 04, 2023

For the past two decades, global warming and climate change skeptics have cited two historical events as proof that present-day global warming is a myth and that potential climate change is a crisis generated by politicians and special interest groups as an excuse to grab power. New research on those two events, the Little Ice Age and the Medieval Warm Period, helps us understand what they were and how that knowledge can help quell current debate.

As a scientist and a Christian, I, too, am concerned about having my freedoms and the freedoms of others wrested away for illegitimate reasons. My motivation for writing Weathering Climate Change1 was to demonstrate that God has designed our planet to provide the resources humans need to manage Earth for our benefit and for the benefit of all other life-forms. I showed that we can resolve global warming and climate change while we boost the world economy and enhance its ecosystems. Such win-win solutions obviate any need for punitive taxes, draconian laws, or social engineering.

On the other hand, fear of abuse by politicians or special interest groups should never be an excuse to ignore or vilify scientific research findings. Part of obeying God's command in Genesis 1 to manage Earth's resources for the benefit of all life is to scientifically study Earth and its resources.

Medieval Climate AnomaliesThe Medieval Warm Period (MWP), also known as the Medieval Climate Anomaly, lasted from about 950 to 1250 AD.2 The Little Ice Age (LIA) endured from about 1300 to 1850 AD.3 Climatologists now acknowledge that both climate events were not global in scale but largely limited to Europe, the North Atlantic, and eastern North America.

As I showed in a previous Today's New Reason to Believe article,4 marine temperature records from offshore oceanic sites show that the global mean temperature rose by only 0.05°C during the MWP and dropped by just 0.10°C during the LIA (see figure 1). However, the most extensive and reliable continental temperature records from Europe, Iceland, and North America reveal a maximum temperature rise of about 0.4°C during the MWP and a maximum temperature drop of about 0.6°C during the LIA (see figure 2) relative to the twentieth-century average global mean temperature.

Figure 1: Global Marine Surface TemperatureAdapted from figure 2 of Osman et al., Nature 599 (2021): 241, and from figure 1 of Marcott and Shakun, Nature 599 (2021): 208.

Figure 2: European and Eastern North America TemperatureTemperature curves credit: Robert A. Rohde, Global Warming Art Project, Creative Commons Attribution; Diagram credit: Hugh Ross

The recently produced temperature records displayed in figures 1 and 2 have helped lessen the vigor and heat of debate over global warming and climate change. Previous temperature records had indicated that the MWP was 1.0°C above and the LIA 1.0°C below the twentieth-century average global mean temperature, respectively. Both events were presumed to be global in extent. Therefore, several scientists, many politicians, and laypeople concluded that the 1.0°C global mean temperature rise that occurred from 1950 to 2020 was nothing new, no more extraordinary than the MWP. The new temperature records (of a rise of .4°C during the MWP and a drop of .6°C during the LIA) eliminate this rationale for denying the reality of recent global warming. The recent temperature rise is about 60% more dramatic than during the MWP.

Climate Anomalies’ CausesThroughout the past century, scientists have debated the causes that gave rise to the MWP and the LIA. Scientists agree that human activity had little to do with either of the events. The populations, technology level, and energy use of people living in Europe and eastern North America were too low at those times to be a significant factor.

Possible causes explored by scientists included changes in the Sun's irradiance, changes in Earth's volcanic activity, and alterations in Atlantic Ocean circulation and currents. A paper published in 2012, by a team of 13 geologists led by Gifford Miller established that a lack of volcanic activity from 950–1250 AD explains the MWP whereas intense volcanic activity explains the onset of the LIA.5

Miller and his colleagues developed a computer climate model simulation that showed explosive volcanism produces abrupt summer cooling and that cold summers would be sustained by sea-ice/ocean feedbacks long after volcanic aerosols settle out of the stratosphere. They then proceeded to demonstrate that the onset of the LIA is linked to a 50-year-long episode of four large and sulfur-rich explosive volcanic eruptions. The team calculated that each of these volcanic eruptions expelled more than 60 teragrams (60 million tonnes) of sulfates into the stratosphere. They showed that the more than two-century-long persistence of cold summers is well explained by subsequent sea-ice/ocean feedbacks.

The team concluded that no large changes in solar irradiance are needed to explain either the MWP or the LIA. Though not addressed in their paper, the four volcanic eruptions would explain why both climatic events were largely regional and minimally global.

Lunar Eclipses Shed LightNow, a paper published in April 2023, offers further insight on what caused the LIA. An interdisciplinary team of 13 scientists led by Sébastien Guillet, none of whom were involved in the research endeavor led by Miller, provided independent confirmation that volcanoes or the lack thereof explain the MWP and the LIA.6 Guillet's team first cites previous research establishing that explosive volcanism is a key contributor to climate variability on annual, decadal, and centennial timescales.7 The sulfate aerosols that large explosive volcanoes eject into the stratosphere block out incident sunlight, thereby cooling Earth's surface.

The primary evidence of past volcanic events comes from ice core records and tree rings. However, these methods do not easily distinguish between tropospheric and stratospheric volcanic aerosols and dust. Furthermore, atmospheric transport of volcanic aerosols and dust can yield substantial dating uncertainties for the volcanic eruptions and their durations.

Guillet's team sought a more definitive understanding of past volcanic eruption events through an analysis of records of past total lunar eclipses. Total lunar eclipses can be seen from any location on Earth every 2.5 years.

The brightness of the Moon during a total eclipse is highly sensitive to sulfate aerosol abundance in the stratosphere. A high abundance of sulfate aerosols in the stratosphere produces a dark total lunar eclipse, whereas a clear stratosphere yields a red lunar disk, popularly known as a blood Moon.8

Guillet and his colleagues studied records of total lunar eclipses in European, Arabic, Chinese, Japanese, and Korean sources. Their database included 389 accounts of 62 total lunar eclipses spanning 1100–1300 AD. They found that European and Eastern Christian sources were meticulous about recording the brightness and color of the Moon during total eclipses but that such information was largely absent from the Asian sources. They attributed this distinction to the many references in both the Old and New Testament to the color and brightness of the Moon, specifically citing passages in Revelation.

Guillet's team found six exceptionally dark total lunar eclipses during their two-century window. These six occurred in May 1110, January 1172, December 1229, May 1258, November 1258, and November 1276. These dark total lunar eclipses correlate with five of the seven largest volcanic sulfate signals recorded in polar ice cores from 1100–1300. The correlation suggests that the strong volcanic sulfate signals dated at 939 and 993–94 in the polar ice cores also indicated the presence of sunlight-blocking stratospheric sulfate aerosols.

Thanks to this research there is now little doubt that major explosive volcanic eruptions, perhaps limited to the greater North Atlantic region, played the predominant role in the Little Ice Age cooling event. Guillet and his team conclude their paper by explaining how future analysis of expanded datasets of total lunar eclipses, ice core records, and tree rings over the past several millennia could yield substantially more detailed and reliable Earth climate models.

Management Responsibilities In the Bible's opening pages, God assigned responsibility to humanity to manage Earth's resources for the benefit of all life-forms. An accurate knowledge of Earth's past climate history and its underlying causes is essential for wise and beneficial management. It is also important for quelling hostile controversies and debates about global warming and climate change. This research demonstrates yet another way that advances in science affirm the message of the Bible.

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