We observe three distinct behavioral states in field emission from single-walled carbon nanotubes between temperatures of 300 and 1800 K. At room temperature, nanotubes emit through adsorbate states correlated to the presence of water. These states are removed above 900 K. After adsorbate removal, the apparently clean nanotube state shows lower emission current and substantially reduced emission noise. Current-temperature measurements identify a deviation from metallic tunneling behavior. Nanotube field emission undergoes a second, field-stabilized transition at high temperatures which reduces the current by as much as five orders of magnitude relative to the room temperature current. This current decrease is 100% recoverable. In addition, the stable behavior of clean nanotube states breakdown at extremely high currents and temperatures. Rings form around the field emission images, similar to those observed in metals at extreme current densities. Under these extreme conditions, we also find evidence for the rearrangement of atoms on the nanotube caps.