Overview of IPCC 2013, Chapter 12 - Long-term Climate Change: Projections, Commitments and Irreversibility

Previously, the IPCC have been hesitant to use the term “tipping point”, possibly in reluctance to cause mass hysteria amongst the Daily Mail and similar. Even their most recent report doesn't feature tipping points strongly, though closer inspection reveals more. Their 2013 report details combined evidence from many of the best respected climate scientists to show that tipping points, or at least alternative stable states, do exist in a variety of forms, a few of which are discussed here.

The September 2013 report defines a tipping point as “a perturbed state irreversible on a given timescale if the recovery timescale from this state due to natural processes is significantly longer than the time it takes for the system to reach this perturbed state.” The Earth system has multiple and varied response timescales to climate changes. For a rapid change in forcing, much of the surface temperature response will be evident within decades. Taking that view, most aspects of the climate change resulting from CO2 emissions are irreversible due to the long residence time of CO2 in the atmosphere and the resulting warming (Solomon et al., 2009).

A number of components of Earth’s system have been proposed as possessing critical thresholds or tipping points, beyond which abrupt transitions to an alternative state result. It is important to note that abrupt changes that arise from nonlinearities within the climate system are intrinsically difficult to assess and timing of future changes difficult to predict, making mitigation difficult. This table shows some of the potential climate tipping points identified by the IPCC report.

 ARCTIC OCEAN

There is very little evidence in climate models of a tipping point from perennially ice-covered Arctic ocean to a seasonally ice-free Ocean where further ice loss in unstoppable. It is, however, very likely that Arctic sea ice will continue shrinking and thinning during the 21st century as global mean surface temperature rises. Conversely, it has been suggested by models that the surface mass balance of the Antarctic Ice Sheet may increase because increased snowfall rates outweigh melt increase. These abrupt changes in ice volume do not necessarily require the existence of a tipping point in the system. Irreversibility of ice sheet volume and extent changes can occur when a decreased elevation of the ice sheet induces a decreased surface mass balance, generally through increased melting.

ATLANTIC MERIDIONAL OVERTURNING CIRCULATION

Observations and models suggest that the present day ocean is already in a bi-stable regime, thereby allowing for multiple equilibria and a stable ‘off’ state of the Atlantic MOC (Bryden et al., 2011; Hawkins et al., 2011). It is very likely that the AMOC will weaken, but confidence in the magnitude of this is low, and crossing a tipping point similar to that of the Younger Dryas cooling is very unlikely in the next century or so.

INDIAN MONSOON

Studies with conceptual models (Zickfeld et al., 2005; Levermann et al., 2009) show that the Indian summer monsoon can operate in two stable regimes. Besides the “wet” summer monsoon, there is a stable state characterized by low precipitation over India. This suggests that any perturbation of the radiative budget that often weakens the pressure gradient could induce abrupt transitions between these two regimes.

So, there it is, there’s no denying that tipping points exist. The main point to take from this is that these changes are unpredictable in scale and in feedback response, and that most of the bigger climatic changes are unlikely to happen in the near future.

(All from WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT REPORT

CLIMATE CHANGE 2013: THE PHYSICAL SCIENCEBASIS, Chapter 12, IPCC, 2013.  See full article for internal  references.)