At current emission levels of 2 ppm/year CO2 above the present combined CO2 + methane level of 450 ppm (CO2-e), this threshold will be crossed in a few decades. Feedbacks from the carbon cycle and from ice melt/water interaction dynamics may accelerate this process.

The lag effects of atmospheric carbon rise, including the melting of Greenland and the Antarctic ice sheets and sea level rise, will take unspecified periods, shifting the climate system into a transitional state, which the climate system may have already entered. The transition is more than likely to be associated with abrupt tipping points, such as have been recorded during the recent history of Earth (at 14 - 11 thousand years-ago and at 8200 years-ago).

The unique nature of the current “experiment” Homo sapiens is conducting with the atmosphere, in terms of the rate of CO2 forcing (two orders of magnitude faster than during recorded past climate changes), precludes precise timing of future events.

As stated by John Holdern, Obama’s science advisor: When driving in the fog, knowing there is a cliff ahead, start pulling the breaks asap …

There is a chance that, should humanity underake every possible step in an attempt to stem dangerous climate change, the process could be slowed down, possibly even reversed (?)
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Such steps need to include:

(1) Rapid deep reductions in emissions – 80 percent by 2020 relative to 1900.

(2) Fast track development of CO2 draw-down technology, i.e. CO2 sequestering vegetation, sodium capture (technologically not more complex than space projects and financially not more expensive than military expenditure).

(3) Rapid transition to alternative clean energy and transport systems.

Unfortunately the alternative to such efforts does not bear contemplation.