Question: How much higher or lower was the sea level in the early Pliocene compared to today?
About the article "Climate change", section "Sea level change".

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The sea level was high in the very early Miocene, and sandstone and calcarenite deposits are up to 30 meters above sea level in the north west and on King Island.
In the late early Miocene sea level was up to 100 meters higher than now.
There is Pliocene limestone on Flinders Island just above sea level.
The Scottsdale sub-basin is up to 225 meters thick from the late Oligocene to early Miocene. 
During the Pliocene epoch (5.3 Ma to 2.6 Ma) climate became cooler and drier, and seasonal, similar to modern climates.
The global average temperature in the mid-Pliocene (3.3 Ma–3 Ma) was 2–3 °C higher than today, global sea level 25m higher and the northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma.
The formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios and ice-rafted cobbles in the North Atlantic and North Pacific ocean beds.
Mid-latitude glaciation was probably underway before the end of the epoch.
The global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas.
During the Pliocene the earth climate system response shifted from a period of high frequency-low amplitude oscillation dominated by the 41,000 year period of Earth's obliquity to one of low frequency-high amplitude oscillation dominated by the 100,000 year period of the orbital eccentricity characteristic of the Pleistocene glacial-interglacial cycles.
The equatorial pacific ocean sea surface temperature gradient was considerably lower than it is today, mean sea surface temperature in the east were substantially warmer than today but similar in the west, this condition has been described as a permanent El Niño state or El Padre 
Global sea level change for much of the last century has generally been estimated using tide gauge measurements collated over long periods of time to give a long-term average.
More recently, altimeter measurements — in combination with accurately determined satellite orbits — have provided an improved measurement of global sea level change.
To measure sea levels prior to instrumental measurements, scientists have dated coral reefs that grow near the surface of the ocean, coastal sediments, marine terraces, ooids in limestones, and nearshore archaeological remains.
The predominant dating methods used are uranium series and radiocarbon, with cosmogenic radionuclides being sometimes used to date terraces that have experienced relative sea level fall.
In the early Pliocene, global temperatures were 1–2˚C warmer than the present temperature, yet sea level was 15–25 meters higher than today.
While the Razorbill is the only living species, the genus "Alca" had a much higher diversity in the Pliocene.
Some ornithologists also feel it is appropriate to retain the Great Auk in the genus "Alca", instead of "Pinguinus".
A number of fossil forms have been found:  "Alca "antiqua"" (Late Miocene/Early Pliocene of Lee Creek Mine, USA)  "Alca" sp.
(Late Miocene/Early Pliocene of Lee Creek Mine, USA) - possibly "A. stewarti"  "Alca stewarti" (Kattendijk Sands Early Pliocene of Belgium)  "Alca ausonia" (Yorktown Early Pliocene of Lee Creek Mine, USA - Middle Pliocene of Italy)  "Alca" sp.
(Puerto de Mazarrón Pliocene of El Alamillo, Spain) - may be "A. antiqua" or "A. ausonia" As far as is known, the genus "Alca" seems to have evolved in the western North Atlantic or the present-day Caribbean like most other Alcini.
Its ancestors would have reached these waters through the still-open Isthmus of Panama during the Miocene. 
Past changes in sea level.
The sedimentary record.
Sedimentary deposits follow cyclic patterns.
Prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level.
The rock record indicates that in earlier eras, sea level was both much lower than today and much higher than today.
Such anomalies often appear worldwide.
For instance, during the depths of the last ice age 18,000 years ago when hundreds of thousands of cubic miles of ice were stacked up on the continents as glaciers, sea level was lower, locations that today support coral reefs were left high and dry, and coastlines were miles farther outward.
During this time of very low sea level there was a dry land connection between Asia and Alaska over which humans are believed to have migrated to North America (see Bering Land Bridge).
For the past 6,000 years, the world's sea level gradually approached the current level except during marine transgressions like the Older Peron.
During the previous interglacial about 120,000 years ago, sea level was for a short time about higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas.
There are also Pleistocene coral reefs left stranded about 3 metres above today's sea level along the southwestern coastline of West Caicos Island in the West Indies.
These once-submerged reefs and nearby paleo-beach deposits indicate that sea level spent enough time at that higher level to allow reefs to grow (exactly where this extra sea water came from—Antarctica or Greenland—has not yet been determined).
Similar evidence of geologically recent sea level positions is abundant around the world.