Fog thinning out

San Francisco's iconic fog is thinning. Photo / Supplied

The coastal fog that gives San Francisco its romantic ambience is thinning out, a boon to drivers but a real threat to the giant redwoods there, researchers reported.

It in unclear if natural climate variations or human activity is to blame, but the result could be the loss of trees, they reported in the Proceedings of the National Academy of Sciences.

"Fog prevents water loss from redwoods in summer, and is really important for both the tree and the forest," biologist Todd Dawson of the University of California Berkeley said in a statement.