Satellite nadir-viewing geometry affects the magnitude and detectability of long-term trends in stratospheric ozone

Abstract. The continued monitoring of the ozone layer and its long-term evolution leans on comparative studies of merged satellite records. Comparing such records presents unique challenges due to differences in sampling, coverage, and retrieval algorithms between observing platforms, all of which complicate the detection of trends. Here we examine the effects of broad nadir averaging kernels on vertically resolved ozone trends, using one record as an example. We find errors as large as 1 % per decade and displacements in trend profile features by as much as 6 km in altitude due to the vertical redistribution of information by averaging kernels. Furthermore, we show that averaging kernels tend to increase (by 10 %–80 %, depending on the location) the length of the record needed to determine whether trend estimates are distinguishable from natural variability with good statistical confidence. We conclude that trend uncertainties may be underestimated, in part because averaging kernels misrepresent decadal to multidecadal internal variability, and in part because the removal of known modes of variability from the observed record can yield residual errors. The study provides a framework to reconcile differences between observing platforms and highlights the need for caution when using records from instruments with broad averaging kernels to quantify trends and their uncertainties.