subject

Lifeline infrastructures are vital systems that support a nation’s economy and quality of life. Modern economies rely on the ability to move goods, people, and information safely and reliably. Adding to their importance is that many of the lifeline systems serve vital roles in disaster recovery. Consequently, it is of the utmost importance to government, business, and the public at large that the flow of services provided by a nation’s infrastructure continues unimpeded in the face of a broad range of natural and technological hazards. The linkage between systems and services is critical to any discussion of infrastructure. Although it is the performance of the hardware (i.e., the highways, pipes, and transmission lines) that is of immediate concern following an earthquake, it is actually the services that these systems provide that are the real loss to the public. Therefore, a high priority in protecting these systems from hazards is ensuring the continuity (or at least the rapid restoration) of service.

Preventing Earthquake Disasters: The Grand Challenge in Earthquake Engineering: A Research Agenda for the Network for Earthquake Engineering Simulation (NEES) ( 2004 )

What is the subject of the sentence 'Adding to their importance is that many of the lifeline systems serve vital roles in disaster recovery'?