North African desert dust: Human impacts and climate effects.

摘要

Desert dust may be playing an active role in the climate system through its radiative forcing, and changes in the land surfaces due to climate change and human impacts may be modulating the dust source. However, these effects are not well understood. Two sets of studies investigate the interactions between North African desert dust and the climate.; The first study addresses the question to what degree vegetation change and cultivation are contributing to the total dust source in North Africa. The dust distributions simulated with an off-line atmospheric transport model are converted to Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Indices (AAIs) using radiative transfer calculations, and compared with the actual satellite observations. The AAI gradients between Sahara (north) and Sahel (south) suggest that the optimal fraction of dust sources due to vegetation change and cultivation are 20-25%, while spatial and temporal correlations suggest 0-15% with the upper bound of 25-40%. However, sensitivity studies show that the uncertainties associated with meteorology and source parameterization used in the model may undermine the findings derived from the simulations. Such uncertainties need to be reduced in order to better constrain the roles of different types of dust sources using AAI simulation.; The second study investigates the role of direct radiative forcing of dust in the Sahelian drought observed in the last three decades of 20 th century using simulations with an atmospheric general circulation model. Simulations forced by observed sea surface temperatures (SSTs) produce decrease of Sahel rainfall and increase of North African dust from 1950s to 1980s, consistent with the observations, but underestimate the magnitudes of observed changes. The effects of simulated changes in Atlantic and Indian Ocean SSTs, dust, vegetation, and greenhouse gas warming explain about 70% of the precipitation changes in Sahel between wet and dry periods. Dust accounts for about 20% and two thirds of this comes through cooling the troposphere over North Africa. The dust impact on Atlantic SST is of a secondary importance. Each of greenhouse gas warming of Indian Ocean and vegetation loss in Sahel accounts for about 25%. Processes excluded in the simulations such as land use impacts on dust, dynamic vegetation feedbacks, and indirect radiative effects, or errors in the model could be responsible for the remaining differences between observed and simulated precipitations.