For at least 30 years- https://4.bp.blogspot.com/-_U0c8gHecnY/W1JhweA4VFI/AAAAAAADDrw/v3SwEG0bQCAJ5fwY4swb3Xa6aqZXjboIACEwYBhgL/s400/DEM%2B%25289%2529.jpg