Not subject to urban heat island effects, the trend in global sea ice extent is a primary indicator of climate change, or the lack thereof. (climatechangefacts.info)
Until the primary sea ice instrument failed in the spring of 2016, there was no loss of global sea ice extent. (climatechangefacts.info)
I have to add that another graph has been making the rounds, showing the total global sea ice extent. (slate.com)