Response of California temperature to regional anthropogenic aerosol changes Page: 4 of 9
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92 Further support for the role of aerosols in the post 1990 warming in California is
93 suggested by seasonal temperature and aerosol trends. In Fig. 2b and Fig. 2c we show
94 summer and winter BC concentrations and summer and winter temperature taken from
95 Fig 1 in [Duffy, Bonfils, and Lobell, 2007]. In Figures 2b and 2c BC concentrations are
96 shown on inverted concentration scale so a reduction in aerosol concentration (trending
97 upward) would be suggestive of warming. The data in Fig. 2b and 2c show that the BC
98 decrease is larger in winter than in summer. This is consistent with observed warming
99 being higher in winter than in summer. The agreement between the observed temperature
100 change and the change in BC concentration for summer for the entire 1970 - 2000 period
101 and for winter from about 1980 to 2000 is remarkably good. The reason for the large
102 scatter in the pre-1980 winter data remains unknown at present.
103 That attenuation of sunlight by anthropogenic aerosols has an effect on the surface
104 temperature in California is suggested by the relationship between aerosol optical
105 thickness (AOT) and aerosol concentrations in Fresno, California, the only site in one of
106 air basins discussed here where AOT is measured since 2000. AOT (at 870nm) data are
107 from NASA's AERONET network [http://aeronet.gsfc.nasa.gov/index.html]). Hourly
108 AOT plotted against corresponding Bsc averages for May 2002 through October 2002
109 period are presented in Fig. 3. The close correspondence among these short- term values
110 suggests that AOT and Bsc changes occur synchronously.
111 In a separate study (unpublished) we examined the relationships between
112 seasonal, daily, and hourly diurnal variations in ground level aerosol scattering
113 coefficient Bsc, and aerosol optical thickness at 870 nm in Fresno. We found that from
114 May through November 2002 the hour by hour diurnal variations of simultaneously
115 measured Bsc and AOT were well correlated when Bsc was above ~70 to 80 Mm1.
116 These results demonstrate that short-term changes in aerosol concentrations (BC or Bsc)
117 occur synchronously with AOT values and consequently influence the changes in the
118 surface solar flux, which in turn influences the surface temperature.
120 3. Conclusions
121 We have shown that: 1) annual and seasonal ambient anthropogenic aerosol
122 concentrations in three major air basins in California show a distinct decrease after about
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Kirchstetter, Thomas; Novakov, T.; Kirchstetter, T.W.; Menon, S. & Aguiar, J. Response of California temperature to regional anthropogenic aerosol changes, article, May 12, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc894990/m1/4/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.