Volcanic forcing of climate over the past 1500 years: An improved ice core‐based index for climate models

Abstract

This dissertation has investigated one of the most important natural causes of climate change, volcanic eruptions, by developing an ice core-based volcanic forcing index, using 54 ice core records from both the Arctic and Antarctica. The extensive collection of ice core data reduces errors inherent in reconstructions based on a single or small number of cores. This enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects. We extracted volcanic deposition signals from each ice core record by applying a high-pass loess filter to the time series and examining peaks that exceed twice the 31-yr running median absolute deviation. We then studied the spatial pattern of volcanic sulfate deposition on Greenland and Antarctica, and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing index that is a function of month from 501 to 2000 CE, latitude in 10° bands, and height from 9 to 30 km at 0.5 km resolution. This index is the longest and iii most advanced volcanic forcing index of the type. It eliminates or minimizes many aspects of problems previous reconstruction had with the ice core records. The estimated uncertainty is a significant reduction from the factor of two uncertainty reported in previously constructed volcanic forcing indices.We forced an energy balance climate model with this new volcanic forcing index, together with solar and anthropogenic forcing, to simulate the large scale temperature response. The results agree well with instrumental observations for the past 150 years and the proxy records for the last millennium. Through better characterization of the natural causes of climate change, this new index will lead to improved prediction of anthropogenic impacts on climate.Using 33 ice core records we investigated the 15 th century Kuwae eruption. We found it was indeed a single-phase eruption occurred during late 1452 to early 1453 CE and it emitted about 140 Tg of sulfate aerosols into the stratosphere with 2(SH):1(NH) hemispheric partitioning. This finding provides an important reference to evaluate and improve the dating of ice core records.iv My Contribution to the WorkThis dissertation work utilized ice core records from both Greenland and Antarctica to reconstruct a monthly and spatially dependent volcanic forcing index that is applicable to general circulation models. Dr. Alan Robock and Dr. Melissa Free developed the first ice-core-based volcanic forcing index (IVI) using multiple ice core records [Robock and Free, 1995]. With the recovery of large number of new ice cores, Dr.Robock and Dr. Caspar Ammann proposed to develop a new index utilizing all of the available ice cores. My primary contribution to the work was to collect and process the ice core records, identify the steps to reconstruct the index, search and decide the methodology involved in each steps through numerous communications with Dr. Robock and other scientists, write programs to reconstruct the index and repo...

The Holocene temperature conundrum

“…1). This follows because the volcanic activity is already very weak at MWP, with few events and each with aerosol loading of less than ∼25 teragram (Tg) (31). Current models are able to produce a cooling of ∼0.25°C from the MWP to the LIA, with large volcanic events of aerosols of 100-250 Tg.…”

Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium

“…The use of more recent forcing reconstructions (e.g., Krivova et al 2007;Gao et al 2008;Pongratz et al 2009) or simulations and models in the Coupled Model Intercomparisons Project phase 5/Paleoclimate Modeling Intercomparison Project Phase 3 (CMIP5/PMIP3) (Taylor et al 2012) might improve the model-data comparison but would not be expected to have an impact on the validation of the reconstruction technique performed here.…”

Using data to attribute episodes of warming and cooling in instrumental records

“…3). There were no major known volcanoes from 1680 to 1707 [although there were some unknown ones (38)], and it started to warm. Although commonly attributed to the Sun (1), the rapid warming of ∼1°C at the end of Maunder Minimum is 10 times greater than our understanding of the solar radiation change (39) can explain but is within the range of a speculative theory (40) if we remove 0.4°C as due to the AMO.…”