Runaway tipping points of no return

By far the most common examples of tipping points though are in relation to ecosystems. The extremely complex web of interdependencies that keep ecosystems dynamic and healthy give rise to plenty of potential thresholds and it is extremely difficult to predict consequences of external changes. The myriad influences on the health of ecosystems (habitat loss, logging, urbanization, species introduction etc. as well as climate change) means that it is most likely here that the tipping point concept will be most applicable. Examples such as a rise in minimum winter temperatures that allow a new insect species to gain a foothold in a new ecosystem (pine bark beetles in Alaska), or warming that leads to movement upward in altitude of ecosystem zones that end up reducing the area of existing alpine biomes. As the planet warms, it is easy to imagine an increasing number of ‘tipping points’ being passed, each related to some different sub-system of the climate or biosphere.

Points of no return

Are ‘tipping points’ the same as the ‘points of no return’ oft used in the media? For a species that becomes extinct as a result of crossing a threshold, the answer is obviously yes. But in the physical climate system, are there genii that can’t be put back in the bottle? This is really a question of time scale. Changes to aerosol concentrations can be reversed in a few weeks after an emission change. CO2 levels however are much slower to change and are already very unlikely to revert to pre-industrial values in any scenario over the next few hundred years. In this minimal sense the climate is already past the point of no return compared to pre-industrial climate.

The ‘known’ physical tipping points described above have natural timescales that determine whether ‘returns’ are possible. The Arctic sea ice, for instance, has timescales of around 5 years to a decade, and so a collapse of summer ice cover could conceivably be reversed in a ‘cooling world’ after only a decade or so (interactions with the Arctic ocean stratification may make that take a little longer though). Model simulations of the thermohaline circulation indicate that for small perturbations, recovery can occur in a few decades. For larger perturbations (i.e. complete collapses) intermediate-complexity models suggest that in some regimes these changes can be quasi-permanent, although this behaviour has not yet been fully explored in current state-of-the-art GCMs. The clues from the paleo-record indicate that there is likely a bi-modal spectrum of overturning states in glacial climates, but there is no evidence of such multiple steady states in the Holocene. Thus there is no strong reason to think either of these ‘tipping points’ are really irreversible – though that is not to imply that the process of loss and recovery wouldn’t have significant impacts.

The big ‘point of no return’ though is usually associated with the melting of the ice sheets – in particular, Greenland and the West Antarctic Ice Sheet (WAIS). Currently the ice sheets exist in part because they already exist i.e. the reason it snows on Greenland is in some large part because there is a large ice sheet there. Should the ice sheet start to melt in a serious way (i.e. much more significantly than current indications suggest), then lowering of the elevation of the ice sheet will induce more melting simply because of the effect of the lapse rate (air being warmer closer to sea level due to pressure effects). Thus if Greenland disappeared, it is unlikely that it would grow back even under current climate, let alone in a warmer world. So loss of either of these ice sheets would indeed be an effect with ‘no return’, at least on any reasonable human timescale.

10 years?

Page 2 of 3 | Previous page | Next page