A key observable metric and important effect of global climate change is the extent of the Arctic ice sheet, which varies seasonally but also has a significant downward secular trend, presumably tied to global temperature increase.
The total extent of the sheet as a function of time is tracked in detail via a combination of satellite data and can be seen as an image as a function of time. Additional graphs and data can be found on the Arctic Data archive System website.
Arctic sea ice shrank to its smallest recorded extent in September of 2012. Thankfully, ice sheet coverage has not yet shrunk to an area smaller than that of 2012. See here and here for the previous Metaculus questions where we asked about the possibility for 2016 and 2017 to reach record low.
Will the extent of the Arctic ice sheet reach its lowest yet recorded value in 2019?
The resolution will be positive if the 2019 curve dips below the lowest point on the 2012 curve as found on the Arctic Data archive System website.