A monthly and latitudinally varying volcanic forcing dataset in simulations of 20th century climate

Caspar M Ammann,Warren M Washington,Charles S Zender,Gerald A Meehl

doi:10.1029/2003gl016875

Abstract

A new monthly volcanic forcing dataset is included in a coupled GCM for a more physically consistent treatment of the stratospheric sulfate aerosol history from explosive volcanism. The volcanic forcing is different from previous versions in that there is an individual evolution of the aerosol for each event. Thus the seasonal and latitudinal dependence of the volcanic aerosol can affect global climate in a more realistic way prior to the satellite period, compared to earlier volcanic forcing datasets. Negative radiative forcing from volcanic activity is greatest in the early 20th century prior to 1915 and in the late 20th century after 1960. The combination of volcanic and solar forcing contributes to an early‐20th century warming, followed by relative cooling in late 20th century. Consequently, the addition of natural forcing factors to the anthropogenic GHG forcing in late 20th century is required to simulate the observed late 20th century warming.

Highlights

[2] Global coupled climate model simulations have shown that natural forcings from either solar or volcanic sources are the main contributors to the observed global warming in the first half of the 20th century, and that radiative forcing from anthropogenic sources was primarily responsible for the observed late 20th century warming [Stott et al, 2000; Mitchell et al, 2001; Meehl et al, 2003]
Because of the still limited amount of optical perturbation data, we chose to base our simulated volcanic dataset on total sulfate loading that is in close agreement with Stothers [1996, 2001a] but larger than Sato et al [1993]
The timing of the volcanic eruption in the seasonal cycle can affect the transport of aerosols into either hemisphere, and affect the nature of the volcanic forcing on the climate

Summary

Introduction

[2] Global coupled climate model simulations have shown that natural forcings from either solar or volcanic sources (or both) are the main contributors to the observed global warming in the first half of the 20th century, and that radiative forcing from anthropogenic sources was primarily responsible for the observed late 20th century warming [Stott et al, 2000; Mitchell et al, 2001; Meehl et al, 2003]. [5] Here we describe a new volcanic forcing dataset that is based on the total amount of sulfate released by each eruption and that contains in a consistent way the meridional spread and decay of volcanic aerosol at monthly resolution taking into account the seasonally changing stratospheric transport. While this method promises an improvement of the stratospheric aerosol history prior to satellite measurements, it clearly simplifies the volcanic cloud evolutions for the most recent events. AMMANN ET AL.: VOLCANIC FORCING DATASET OF 20TH CENTURY tures on the inter-annual timescale with the inclusion of the new volcanic forcing dataset

Configuration and Setup

Results

Conclusions