Therefore some people started travelling from south to north.Such northward migration continued for several centuries and finally when these people from south India reached the banks of Himalayan Rivers, they got climatic conditions conducive to long term development of civilization on the banks of these rivers providing security of water, food and shelter for a very long time.By 1 AD, there were an estimated 170 million people knocking around the planet, and by the 19th century, a billion. Urbanization and industrialization drive growth, and humanity is ascendant.Zooming in on the past 2,000 years makes the sharp increase at the time of the Industrial Revolution even more obvious.However: tide gauges may move vertically as a result of movements of the earth's crust, such as post-glacial rebound, tectonic uplift or crustal subsidence. This greatly complicates the problem of determining global sea level change from tide gauge data (From: Tide gauge sea level). According to Fraczek: "MSL is usually described as a tidal datum that is the arithmetic mean of hourly water elevations observed over a specific 19-year cycle. For the use of GPS (Global Positioning System) for sea level measurement go to PSMSL. The height of Mean Sea Level is given relative to a certain, arbitrary chosen, reference or datum level, e.g. Though the zero point of the reference scale (such as NAP) may be so chosen as to coincide with Mean Sea Level, Mean Sea Level is not identical with the zero point. This definition averages out tidal highs and lows caused by the changing effects of the gravitational forces from the moon and sun". Mean Sea Level may rise or fall, relative to the zero point of the reference scale.
How is MSL defined: as the halfway point between mean high and mean low tide or, expressed otherwise, the mean of mean high tide and mean low tide? It must be derived from other measurements (such as hourly tide gauge measurements) or proxy data (sea level marks).
Thousands of years later, when some of these Himalayan Rivers became non-perennial or started drying up, some of these people started moving towards Central Asia and Europe.
As per ecologists this ecological cycle has been repeating itself and will get repeated after every ice age and during the beginnings of all Holocene cycles.
references are given to papers on sea level change in the Middle Ages (500-1500 AD) and the Little Ice Age (LIA; approximately 1450-1850 AD) that can be viewed online.
First, some preliminary remarks are given: - What is Mean Sea Level (MSL)?