In 1623 the Dutch navigator Jan Carstensz saw a snow covered mountain range in inner New Guinea from his ship near the south coast of the island, becoming the first European to see this amazing sight. After colonization by the Netherlands in 1899, a surveying expedition saw the glaciated mountains from the lowlands and called them "Carstensz Toppen" (Carstensz Mountains). In 1912 the Wollaston expedition reached the foot of the glaciers in the Carstensz area from the south, but they couldn't climb to the inner glacier valley (Meren Valley).
In 1936 the explorers Anton Colijn, Jean-Jaques Dozy and Frits Wissel were the first to entered the inner glaciated areas of the Carstensz Mountains, and they made first ascent of East Castensz Top and the highest peak on the Northwall Firn Glacier, the large ice cap of Ngga Pulu. Trigonometric measurements showed that Ngga Pulu was at this time (and had been for many centuries before) the highest mountain of New Guinea and also the highest summit of the Australia/Oceania continent. The altitude of Ngga Pulu was in 1936 about 4907m/16100’, and it was the highest and most prominent peak between Himalaya and the Andes. But the expedition also documented a strong recent retreat of all glaciers in the area, foreshadowing changes to come.
In 1900 all the highest mountains in Carstensz area, as well as Puncak Mandala (Juliana Peak), Puncak Trikora (Wilhelmina Peak) and Ngga Pilimsit (Idenburg) were covered by glaciers. Also, all prominence key cols in the inner Castensz area were entirely ice-covered, so Carstensz Pyramid at this time was a just a sub-peak of Ngga Pulu with around 200m/656’ of prominence. But due to glacial melting, Ngga Pulu lost a lot of altitude in the mid-20th century.
In 1963 the international expedition of Heinrich Harrer made the first ascent of the rocky summit of Carstensz Pyramid (Puncak Jaya), which was now significantly higher than Ngga Pulu and the new high point of New Guinea at 4884m/16,024’. The expedition also documented, once again, a strong retreat of all the glaciers in the area--more than 400 meters since 1936 for the main Meren Valley Glacier alone.
In the second half of the 20th century the glacier melting process was documented worldwide, and in New Guinea the glaciers on the highest peaks lost more and more ice volume. The Australian scientific expeditions of 1971-73 measured Ngga Pulu at 4862m/15951’, and the ice melting in the key col resulted in a prominence of around 300m/1000’. By the year 2000 all New Guinea glaciers outside the Carstensz area had disappeared. Inside the Castensz area, a former sub-peak, the now rocky summit of Sumantri, is now some meters higher than the still ice covered Ngga Pulu. So now Ngga Pulu has less than 100m/328’ of prominence as a sub-peak of Sumantri (4870m /15,978’), which itself has a prominence around 350 m/1149’.
The glacial melting has produced significant altitude changes for prominence key cols in the inner Carstensz area and it is also quite probable that other former big ice-capped peaks in New Guinea lost various meters in altitude, such as Puntak Mandala, East Carstensz Top, and Ngga Pilimsit. Scientists are monitoring this warming and estimate that by about 2020-2030 all New Guinea glaciers may disappear.
(Thanks to Chris Myagar for providing this information.)