Unknown Eruption Source Parameters Cause Large Uncertainty in Historical Volcanic Radiative Forcing Reconstructions

Reconstructions of volcanic aerosol radiative forcing are required to understand past climate variability. Currently, reconstructions of pre-20th century volcanic forcing are derived from sulfate concentrations measured in polar ice cores, mainly using a relationship between the average ice-sheet sulfate deposition and stratospheric sulfate aerosol burden based on a single explosive eruption-the 1991 eruption of Mt. Pinatubo. Here we estimate volcanic radiative forcings and associated uncertainty ranges from ice-core sulfate records of eight of the largest bipolar deposition signals in the last 2,500 years using statistical emulation of a perturbed parameter ensemble of aerosol-climate model simulations of explosive eruptions. Extensive sampling of different combinations of eruption source parameters using the emulators reveals that a very wide range of eruptions in different seasons with different sulfur dioxide emissions, eruption latitudes, and emission altitudes produce ice-sheet sulfate deposition consistent with ice-core records. Consequently, we find a large range in the volcanic forcing that can be directly attributed to the unknown eruption source parameters. We estimate that the uncertainty in volcanic forcing caused by many plausible eruption realizations leads to uncertainties in the global mean surface cooling of around 1 degrees C for the largest unidentified historical eruptions. Our emulators are available online () where eruption realizations for given ice-sheet sulfate depositions can be explored. Plain Language Summary Explosive volcanic eruptions inject sulfur into the atmosphere, forming sulfate particles that reflect sunlight and cause surface cooling. To understand past climate variability it is important to quantify the climate effects of historical eruptions. However, there are no direct observations of the sulfur emissions and cooling from eruptions that occurred hundreds of years ago, and volcanic climate effects must be inferred from indirect measurements. Volcanic sulfate is deposited onto Earth's surface and can be found in layers of polar ice. Ice-core sulfate records are used to identify the occurrence of past eruptions, and the size of the sulfate signals is used to estimate the amount of sulfate that was present in the atmosphere and consequently the climatic effect of the eruption. We explored the uncertainty in this calculation by using climate model simulations and novel statistics to infer the properties of eruptions that could lead to sulfate deposition that matches that from ice-core records. We found there are many eruption scenarios that could lead to the same polar sulfate deposition but have different resulting magnitudes of surface cooling. Consequently, there is a large uncertainty in the climatic effect of historical eruptions, which affects our understanding of past climate variability. Key Points We use an aerosol-climate model and statistical emulation to find eruption source parameters from eight bipolar sulfate deposition signals Plausible eruptions have a wide range in sulfur dioxide emissions and latitude, and consequently, a large range in radiative forcing Resulting uncertainties in global mean surface cooling for the largest unidentified historical eruptions are on the order of 1 degrees C