Changes in the Earth's UV reflectivity from the surface, clouds, and aerosols

Abstract

Measurements of the Earth's 380 nm UV reflectivity combine the effects of surface reflectivity, aerosols, haze, cloud optical thickness, and the fraction of the scene covered by clouds. Changes in UV cloud and aerosol reflectivity would imply similar changes over a wide range of wavelengths, UV, visible, and near infrared (at least 0.3 to 2 μm), affecting both the transmission of radiation to the Earth's surface and the reflection back to space. Using the TOMS (Total Ozone Mapping Spectrometer) 380 nm reflectivity data, the 14-year annual mean power reflected back to space is 385.3±31 W/m², mostly by clouds, aerosols, and snow/ice. On the basis of measured long-term changes in global reflectivity, it is estimated that there is an additional 2.8±2.8 W/m² per decade reflected back to space (2 standard deviation error estimate) during the TOMS observing period of 1979–1992. Since the 380 nm surface reflectivity is low (2–8%) over most surfaces, water and land, the observed reflectivity changes are mostly caused by changes in the amount of snow/ice, cloudiness, and aerosols. Time series analysis of TOMS reflectivity over the period from 1979 to 1992 shows that there were no significant changes in annually averaged zonal-average reflectivity at latitudes within 60°S–60°N, even though there were changes at higher latitudes (e.g., 3% per decade, in reflectivity units, between 60°N and 70°N). When the effects of the 11.3-year solar cycle and ENSO (El Niño-Southern Oscillation) are removed from the data, statistically significant reflectivity changes are observed poleward of both 40°S and 40°N. The presence of a statistically significant 11.3-year periodicity in the reflectivity time series correlates with the solar cycle and suggests a possible Sun-weather relationship. There are significant regional changes in reflectivity R over land and ocean areas that affect the amount of solar radiation reaching the surface. The largest of these regions have decreases in R of 3 to 6 ± 1% per decade in central Europe, the western United States, central China, and western Russia. These decreases are offset by increases in the same latitude bands mostly over the oceans. The largest regions showing an increase in R are off the western coast of South America (near Chile and Peru), 5 to 8 ± 1%/decade and over the Weddell Sea in Antarctica of 10%/decade, but no change over the ice shelf and continent. The largest increase in R occurs over the ocean just to the north of Antarctica. This change is important because it reduces UV radiation overall (290–400 nm) and partially offsets the effect of the increased amount of UV-B radiation (290–320 nm) caused by decreasing Antarctic ozone.