take up the slack
Verb
Meaning
To tighten something that is slack so that it is taut.
To do work that would otherwise be left undone.
To provide extra resources that are not met by normal sources.
(mathematics) To act as a slack variable, converting an inequality into an equality.
To consume something that would otherwise go to waste.
Source: en.wiktionary.orgExamples
If Britain was to close down altogether overnight, then China would take up the slack of carbon emissions in two years. If America closed down, just the growth in China's emissions would replace America's emissions in 12 years.