Final answer:
The globally averaged glacier mass balance has been highly negative since around 1850 AD, which marks the end of the Little Ice Age. Glacier retreat and decreasing ice sheet mass have been observed due to climate change and increased mean annual temperatures, leading to considerable impacts on local ecosystems and global sea levels.
Step-by-step explanation:
The student asked about the period since which the globally averaged glacier mass balance has been highly negative. Globally, the glacier mass balance started to decline post the Little Ice Age, which reached its maximum size around 1850 AD. Due to climate change and increasing temperatures post the industrial revolution, glaciers have been retreating. Early observations of glacier retreat were noted in Glacier National Park (GNP) in 1914, indicating the glaciers have been melting and retreating for over a century. By the end of the Little Ice Age around 1850, over 150 glaciers were present in the park. However, by 2015, only 26 active named glaciers remained.
Presently, due to global warming, the mass of the ice sheets in Greenland and the Antarctic is also decreasing, contributing to sea level rise. In addition, the Arctic sea ice is also decreasing in size and thickness. The mean annual temperature in Glacier National Park has increased by 1.33 °C since 1900, illustrating a marked warming that corresponds with the global trend of glacier retreat.