This paper has its origin in speaking notes for my presentation to the Berlin meeting, together with additional material provided in a leaflet available there.

It provides the first articulation of my concern that a significant metric for precipitating catastrophic abrupt climatic change, passing a “tipping point”, may be neither the increase of average surface temperature (“2 degrees Celsius”) nor the rate of change of surface temperature (“0.2 degrees Celsius per decade”), but the integral of greenhouse gas excess over the pre-industrial, as a measure of the quantity of heat injected into the earth system.

Some implications for mitigation policy are noted – a shift of emphasis from emissions reductions to carbon removals – and some suggestions for future research directions and for the work of the IPCC are advanced.