The part of the sentence "has been a decrease" is correct and usable in written English.
You can use it to express that there has been a decline or decrease in something. For example: "There has been a decrease in the number of people visiting the cinema this year."
Part of it has been a decrease in the aggressiveness of the defensive play-calling.
There has been a decrease in overall crime in all but two of the city's 76 police precincts.
Although there has been a decrease in the cultivation of opium poppies in the area, there is a thriving illegal drugs economy.
There has been a decrease of attacks involving a particularly lethal type of roadside bomb that American intelligence says is supplied by the Iranian Islamic Revolutionary Guard Corps.
"There has been a decrease in participation in formal extracurricular activities, like student government," he said, "and an increase in self-organized activities, like a cappella singing groups".
"It is disappointing to hear, however, that there has been a decrease in the take-up of modern foreign languages at GCSE".
"There has been a decrease in the level of violence -- it hasn't gone to zero, but there's been a decrease -- so these people feel more comfortable coming out".
In the last decades, there has been a decrease in the number of children walking to school.
The result has been a decrease in the number of Neets, although the problem remains because young people can still leave school after the age of 16, while the legislation lacks teeth.
"We agree with the findings of this analysis [by the House of Commons library] that between 2015-16 and 2016-17 thase has been a decrease in this element of police funding in real terms," said Dilnot.
In the case of any such twelve-month period in which there has been a decrease in the total reservable liabilities of all depository institutions, no adjustment shall be made.
