estuvieron descendiendo
-they/you were falling
Preterite progressive ellos/ellas/ustedes conjugation of descender.

descender

Desde 1940 hasta fines de los 60, las temperaturas estuvieron descendiendo a pesar del rápido aumento de las emisiones del CO2 antropogénico.
From 1940 through the late 1960s temperatures were falling in spite of the fast rise of anthropogenic CO2 -emissions.
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to season