Recent increase in nitrate concentration of Antarctic snow.

POLAR ice cores provide a unique record of global climate change. In particular, their records of nitrate concentration can yield new insight into the atmospheric nitrogen cycle, but first it is necessary to understand the processes controlling the spatial distribution of nitrate at the ice-sheet surface, and to define any trends in its temporal distribution. Here we report trends in the nitrate time series deduced from low-accumulation sites such as Dome C and Vostok in Antarctica. These trends must be treated with caution because of the possibility of post-depositional alteration. But the increases in the concentration of the spring maximum in nitrate that we find in the South Pole record for the past few years deserve careful consideration, as they may be a result of denitrification of polar stratospheric clouds in the lower stratosphere and may hence be connected in some way with the Antarctic ozone 'hole'.