Timothy Lenton does lay folks a great service in parsing climate science on the question of tipping points…those pesky "non-linear transitions" where a small change can make a big difference.
Says Lenton, "climate policy should be more concerned with what tipping elements might be triggered by human activities in the future, and whether their tipping points can be avoided."
Lenton lists the seven most important potential tipping elements relevant to policy makers:
- Greenland ice sheet
- Amazon rainforest
- West Antarctic icesheet
- Boreal forest
- Sahara/Sahel and WAM
- Atlantic circulation
- ENSO amplitude
To derive this list, Lenton's science group defines a "tipping element" thusly:
- Human activities are interfering with the system such that decisions taken within a “political time horizon” can determine whether the critical control value is reached.
- The time to observe a qualitative change in the system plus the time to trigger it lie within an “ethical time horizon”.
- A significant number of people care about the fate of the system, either because it is integral to the overall functioning of the Earth system, and/or tipping it will impact a large number of people, and/or it has intrinsic value as a feature of the biosphere.
Lenton's science group chose 100 years for the political time horizon and 1000 years for the ethical time horizon (based on the lifetime of human cultures).
Bear in mind that "warming on the century timescale is projected to be in the range 1.1–6.4 Â°C above present (IPCC, 2007). This represents the ‘accessible neighbourhood’ of global temperatures."
Says Lenton, "we have already made a ‘commitment’ to ~0.6 Â°C of further warming even if we could stabilize greenhouse gas concentrations tomorrow (which we can’t)."
Lenton notes that "there could be tipping elements that haven’t been triggered yet, but which we are already committed to triggering."
Here's the good news:
"Our research thus far does not clearly indicate any tipping elements that fall into this category."
Sure, Lenton excludes a number of potential tipping elements "because their tipping point is estimated to be inaccessible on the century timescale (although they may become a concern for future policy makers)."
Guess that takes us more into ethical territory, no?