Greenland's Ice Sheet has reached a tipping point according to lead author Michael Bevis, insofar as it looses mass rapidly when the North Atlantic Oscillation is negative (when warmer air hits Greenland) while mass loss abruptly slows when the NAO is positive. Greenland’s Melting Ice Nears a ‘Tipping Point,’ Scientists Say
Greenland’s enormous ice sheet is melting at such an accelerated rate that it may have reached a “tipping point,” and could become a major factor in sea-level rise around the world within two decades,
… most estimates of a tipping point for Greenland ice loss cite higher average temperatures than are currently occurring, more along the lines of 1.5 or two degrees Celsius above preindustrial levels.
If the warm cycles of the N.A.O. are associated with huge losses of ice, and the cool cycles only pause the melting, it suggests a threshold has been reached: As average temperatures rise further, melting will be more sustained, and the cooling cycles will have less of an effect in slowing the ice loss.
“If a relatively minor cycle can cause massive melting,” he [Michael Bevis] said, “it means you’ve reached a point of amazing sensitivity” to warmer temperatures, which could represent “the tipping point.”
… losses from the Greenland ice sheet have reached their fastest rate in at least 350 years.
A co-author of the Nature paper, Sarah B. Das,… said, “I take issue with using ‘tipping point’ to describe the accelerating mass loss Greenland is experiencing,” because “it makes it appear as if we have passed, or soon will pass, the point of no return.” [emphasis mine]
The argument for avoiding "tipping point" language seemed to be that because we might be able to control how fast the ice sheet melts in the future by reducing greenhouse gas emissions, there’s reason for hope, therefore it's not really a point of no return. I see the reluctance of Sarah B. Das to sound the alarm as a kind of soft climate denial. She does the climate science, looks at the data, but clings so tightly to "hope" that she blinks instead of speaking-truth-to-power.
Remember when IPCC scientists couldn't imagine us opting for scenario 8.5, because it was unthinkable society would be that insane? But here we are, a decade into 8.5, and last year, already at 1°C rise, we accelerated CO2 emissions. Like there's no tomorrow.
image source (unrelated) text mine