thumb|Aerial view of Portsmouth and Portsmouth Harbour|alt=An aerial view of western side of Portsmouth (including Gunwharf Quays, the dockyard and the Spinnaker tower), the harbour itself, and the town of Gosport thumb|England population density and low elevation coastal zones. Portsmouth is particularly vulnerable to sea level rise. Portsmouth is by road from central London, west of Brighton, and east of Southampton. It is located primarily on Portsea Island and is the United Kingdom's only island city, although the city has expanded to the mainland. Gosport is a town and borough to the west. Portsea Island is separated from the mainland by Portsbridge Creek, which is crossed by three road bridges (the M275 motorway, the A3 road, and the A2030 road), a railway bridge, and two footbridges. Portsea Island, part of the Hampshire Basin,Melville, R.V. & Freshney E.C. (4th Ed 1982), The Hampshire Basin and adjoining areas, British Regional Geology series, Institute of Geological Sciences, London: HMSO is low-lying; most of the island is less than above sea level. The island's highest natural elevation is the Kingston Cross road junction, at above ordinary spring tide.
thumb|Aerial view of Portsmouth and Portsmouth Harbour|alt=An aerial view of western side of Portsmouth (including Gunwharf Quays, the dockyard and the Spinnaker tower), the harbour itself, and the town of Gosport thumb|England population density and l:ow elevation coastal zones. Portsmouth is particularly vulnerable to sea level rise. Portsmouth is by road from central London, west of Brighton, and east of Southampton. It is located primarily on Portsea Island and is the United Kingdom's only island city, although the city has expanded to the mainland. Gosport is a town and borough to the west. Portsea Island is separated from the mainland by Portsbridge Creek, which is crossed by three road bridges (the M275 motorway, the A3 road, and the A2030 road), a railway bridge, and two footbridges. Portsea Island, part of the Hampshire Basin,Melville, R.V. & Freshney E.C. (4th Ed 1982), The Hampshire Basin and adjoining areas, British Regional Geology series, Institute of Geological Sciences, London HMSO is low-lying; most of the island is less than above sea level. The island's highest natural elevation is the Kingston Cross road junction, at above ordinary spring tide.