Emissions that are either physically extremely difficult to eliminate within a certain time-frame (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution, or because avoidance would require a technology that relies on a scarce resource) or which would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).
source: J Wilcox, B Kolosz, & J Freeman (2021) CDR Primer; Concepts