Mean sea level (MSL) is a datum representing the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an important role in marine navigation as a chart datum and aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels. Detection of long-term variations of mean sea level requires careful measurement but can provide information on climate change. Sea level responds to a variety of influences and has varied greatly over geologic time. Currently, sea level is rising, which scientists cite as evidence supporting the view that the climate system is warming.