Does a severe sunburn cause skin aging, or is it caused by long-term sun exposure?

Both!: Acute sunburn causes more skin damage, and may increase risk of melanoma somewhat more than long-tern sun exposure, which still causes wrinkles, spots, and skin cancers as well. Radiation damage is cumulative over a lifetime, so both are not good! sort of like many toxic things--a little over a long time will kill you just as dead as a lot all at once!

Both: Sun exposure causes the skin to have to repair itself. Constant damage to cell's occurs by daily uv exposure. Severe sunburns greatly increase this damage. In short, both severe short term exposure and longterm exposures cause damage to the skin which is reflected in premature wrinkling, discoloration and early skin cancers.