Several careers involve working outdoors. I have made a small list, but keep in mind it is not all-inclusive. As far as the outdoor rescue work goes, yes it is almost all volunteer. Average is 98% volunteer. The remaining are made up from employers like the National Park Service (US) and Border Patrol (US) even some Fire and EMS departments have a “specialized rescue” organization, such as Los Angeles Fire Department’s Urban SAR.

Forest Service (US)
National Park Service (US)
Ministry of Lands, Air & Water (Canada)
Coast Guard (US and Canada)
Wildlife and Fishery
Archaeology
Astronomy

These are very broad and each has its own sub-category. As Chris K said, the main idea is to get outdoors.