Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The variations in atmospheric warming and cooling result in changes in tropospheric and stratospheric circulation. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events. Sulfur aerosols, especially stratospheric sulfur aerosols have a significant effect on climate. Thus human activity could affect the stratospheric ozone layer. The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. The EPA also researched the implications of stratospheric ozone depletion. Gaseous hydroperoxyl is involved in reaction cycles that destroy stratospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. The treaty is structured around several groups of halogenated hydrocarbons that deplete stratospheric ozone. Proposed methods include increasing the planetary albedo, for example using stratospheric sulfate aerosols. It has attracted much attention for its role in the depletion of stratospheric ozone. Certain efficacy issues are specific to stratospheric aerosols. Direct attribution of the stratospheric aerosols to pyroCbs only occurred in the last decade. However, some existing legal instruments would be relevant to stratospheric sulfate aerosols specifically.