Global trends in visibility: implications for dust sources

[摘要] There is a large uncertainty in the relative roles of human land use,climate change and carbon dioxide fertilization in changing desert dustsource strength over the past 100 years, and the overall sign of humanimpacts on dust is not known. We used visibility data from meteorologicalstations in dusty regions to assess the anthropogenic impact on long termtrends in desert dust emissions. We did this by looking at time series of visibilityderived variables and their correlations with precipitation, drought,winds, land use and grazing.Visibility data are available at thousandsof stations globally from 1900 to the present, but we focused on 357stations with more than 30 years of data in regions where mineral aerosolsplay a dominant role in visibility observations. We evaluated the 1974 to2003 time period because most of these stations have reliable records onlyduring this time. We first evaluated the visibility data against AERONETaerosol optical depth data, and found that only in dusty regions are the twomoderately correlated. Correlation coefficients between visibility-derivedvariables and AERONET optical depths indicate a moderate correlation (0.47), consistent with capturing about 20% of the variability in opticaldepths. Two visibility-derived variables appear to compare the best withAERONET observations: the fraction of observations with visibility less than5 km (VIS5) and the surface extinction (EXT). Regional trends show that inmany dusty places, VIS5 and EXT are statistically significantly correlatedwith the Palmer drought severity index (based on precipitation andtemperature) or surface wind speeds, consistent with dust temporalvariability being largely driven by meteorology. This is especially true forNorth African and Chinese dust sources, but less true in the Middle East,Australia or South America, where there are not consistent patterns in thecorrelations. Climate indices such as El Nino or the North AtlanticOscillation are not correlated with visibility-derived variables in thisanalysis. There are few stations where visibility measures are correlatedwith cultivation or grazing estimates on a temporal basis, although this maybe a function of the very coarse temporal resolution of the land usedatasets. On the other hand, spatial analysis of the visibility datasuggests that natural topographic lows are not correlated with VIS5 or EXT,but land use is correlated at a moderate level. This analysis is consistentwith land use being important in some regions, but meteorology drivinginterannual variability during 1974–2003.

[学科分类] 大气科学