Solar radiation management from Icelandic volcanoes during the Medieval Quiet Period

“Solar Radiation Management” aims to mitigate global warming by either seeding aerosols into clouds to change their radiative properties and occurrence frequency, or injecting sulfur into the atmosphere to shield the Earth’s surface from incoming solar radiation. These approaches are inspired, among other things, by the effect of volcanic eruptions on the climate system.

Here we provide a critical reassessment of a time period commonly referred to as the “Medieval Quiet Period”. For several centuries in early Medieval times (c. 750-1050 CE) the climate system was postulated to have been relatively unperturbed by natural climate forcing, resulting in a unique period of climate stability. We present evidence that just the opposite is true. In large parts of the Northern Hemisphere and in the Arctic, atmospheric aerosol loads were persistently high during this period as a result of increased volcanic activity, especially in Iceland.

Our new insight is supported by evidence taken from an array of synchronized ice cores from Greenland with high time-resolution records of a large suite of trace elements, including volcanic volatiles such as sulfur, chlorine, fluorine, and heavy metals. We use crypto-tephra in ice cores to provenance the sources of many volcanic eruptions and sulfur isotopes (³³S, ³⁴S) to delineate if volcanic gas emissions occurred above or below the ozone layer.

We define an “Iceland Active Period”, a time period of frequent and prolonged volcanic activity, producing persistently high levels in atmospheric aerosol burdens in the Northern Hemisphere Arctic’s preindustrial atmosphere lasting for decades to centuries. The frequency and cumulative amount of emissions of climate-impacting trace substances (e.g., sulfates, halogens, ash) is unprecedented in the late Holocene. It is exceeded at times only in the Anthropocene (since about 1900) and in the early-middle Holocene (e.g. during rapid deglaciation). We demonstrate that previous reconstructions of volcanic forcing used in PMIP3 and PMIP4 strongly underestimate volcanic aerosol emissions in the early Medieval and argue that this period should not be considered a reference climate state for the Common Era.

Finally, we investigate possible aerosol-climate interactions following these eruptions using climate proxies and state-of-the-art chemistry climate models with prognostic stratospheric aerosols and chemistry.