Monsoons and their tipping points

The future evolution of monsoon rainfall under increasing levels of atmospheric CO2 and aerosol pollution is highly uncertain, and although it is generally accepted that greenhouse gases tend to increase monsoon strength, distribution of aerosols may have other effects, leaving the overall effect uncertain (Leverman et al. 2009). Summer monsoons are known to vary at orbital and longer timescales as a result of external forcing, but of more interest are the abrupt changes that have occurred without any known forcing over shorter millennial and suborbital scales (Gupta et al. 2003). Millennial-scale abrupt monsoon events within the current interglacial have been linked to North Atlantic climatic changes, according to the hypothesis that increased winter snowfall weakens the monsoon the following summer (Meehl, 1994).

Figure 1 - results of study of G. bulloides and haematite to
show periodically weakened monsoon.

Tipping points have been crossed in monsoon systems before, switching between two stable states. During the Holocene and last glacial period, rainfall in India and China has undergone strong and abrupt changes associated with these states (Leverman et al. 2010). Zickfeld et al. (2005) developed a simple model which identified two stable states in the Indian summer monsoon, potentially allowing for rapid transition between radically different monsoon circulations. We can examine these state shifts by looking at magnetic susceptibility in sediments, oxygen isotopes, dust flux in marine cores, pollen and many others (Feng et al, 2006). Gupta et al (2006) observed seven intervals of weak summer monsoon during the Holocene  (highlighted in the grey bars in Fig. 1) that can be correlated within age uncertainties to millennial-scale Dansgaard-Oescher events in the North Atlantic. These were identified using haematitie and planktonic foraminifera Globigerina bulloides, a unique proxy for monsoon circulation in the tropics that shows linear correlation with the surface cooling due to upwelling (Gupta et al. 2006). These weak phases correlate with North Atlantic cool peroids, and monsoon maxima with warm periods in the North Atlantic. Gupta et al (2006) suggest that the monsoon could be sensitive to relatively small changes in forcing of 0.25% change in solar output, or a 2°C change in sea surface temperature.

Leverman et al (2010) suggest that North Atlantic climatic events had a very small effect, but internal feedback of the monsoon amplified changes. The main feedback is the release of latent heat from precipitation over land, which increases to the temperature difference between land and ocean. This pushes stronger winds from ocean to land, thus increasing advection of moisture over land, enhancing precipitation and associated release of latent heat. If monsoon winds get even slightly weaker, this feedback is reduced significantly. The abrupt transition emerges through an additional stabilizing effect of the direct heat advection which is cooling the atmospheric column and is also reduced for reduced monsoon winds. This continues, along with other feedbacks, until a threshold is reached at which condensation/precipitation cannot provide the latent heat needed to maintain a circulation (Leverman et al, 2010).

Monsoons are depended on by millions of people in some of the world’s most densely populated regions for water for agriculture and fisheries, recharge of aquifers and maintenance of ecosystems amongst others. It is therefore in our best interest to mitigate or at least understand the impact that natural variability and global climate change may have on reaching a critical threshold in monsoon systems and turning of the cycle for good.

Gupta, A.K., D. M. Anderson, J.T. Overpeck (2003) “Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean”. Nature 421, 354–356.

Levermann, A., J. Schewe, V. Petoukhov, and H. Held (2009) “Basic mechanism for abrupt monsoon transitions” PNAS, vol. 106, 49, 20572–20577

Meehl, G. A. (1994) “Influence of the land surface on the Asian summer monsoon, external conditions versus internal feedbacks”. Journal of Climate. 7, 1033–1049

Zickfeld K, B. Knopf, V. Petoukhov and H. J. Schellnhuber (2005) “Is the Indian summer monsoon stable against global change?” Geophysical Research Letters 32:L15707.

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.)