There has been a progressive decrease in population every year in this city.

这个城市的人口在逐年递减。

《新英汉大辞典》

There has been a steady decrease in population in this city.

这个城市的人口在不断减少。

youdao

There has been a steady decrease in population in this city.

这个城市的人口在持续下降。

youdao

The result of these changes was that the natural decrease in population plummeted by 56% from 129, 500 in the analogous period of 2011 to just 56, 900 in 2012.

FORBES: Russia's Demographics Continue to Rapidly Improve

An actual decrease in emissions from cars as population grows faster than ever is kind of a landmark event.

FORBES: Cars Now Getting Better Climate Grades

Russia will probably achieve natural population growth in 2012, but, before the recent uptick in births and decrease in deaths, the country experienced a substantial natural population increase as recently as 1991.

FORBES: Walter Russell Mead's Very Strange View of German Ascendancy and Russian Decline