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.

Tipping the biosphere

My previous posts have described how critical transitions lead to state shifts, causing abrupt changes and unanticipated effects. Although humans appear to dominate Earth, we have a huge dependence on the biosphere and ecosystem functioning for resource capture, primary production, and decomposition and recycling of nutrients, as well as potentially ecosystem stability (Cardinale et al, 2012). If the relationships mentioned by Cardinale et al (2012) transpose to a planetary scale, the implication is that global biodiversity and species richness positively correlate with the resilience and functioning of the biosphere (Lenton et al. 2013). For this reason, there has been an almost compulsory growth in interest in forecasting biological responses on all temporal and spatial scales (Barnosky et al, 2012).

But how do these changes occur?

Barnosky et al. (2012) describe biological states as neither steady nor in equilibrium, and say critical thresholds may be crossed by a ‘threshold’ effect in incremental values or a ‘sledgehammer’ effect from a large event, such as forest clearance. Localized ecological systems are known to shift abruptly and irreversibly across critical thresholds to new mean conditions outside the range of fluctuation of the previous state (Barnosky et al, 2012). Tipping points in the terrestrial biosphere can also cross continents if vegetation and atmosphere are tightly coupled, (Lenton et al. 2013), potentially becoming global if there are interrelated drivers acting on a global biological or ecological threshold, causing all locations to ‘tip’ simultaneously (Brook et al. 2013). Brook et al (2013) think this is unlikely given the heterogeneity of climate change and ecosystems. Jefferies et al. (2006) show that intercontinental biotic connectivity and coupled regime shifts have been demonstrated by intensive agriculture in western USA, causing dramatic losses of Arctic ecosystem structure and biogeochemical cycling due to increased populations of migrating snow geese, promoted by agricultural crop as increased food source. Similarly, coral reef ecosystems appear to have disappeared globally and suddenly at the Triassic–Jurassic transition, driven by global increase in CO2 causing increased ocean acidity and temperature (Brook et al. 2013).

What are the consequences?

Several extinction events have been linked to oceanic anoxic events, crossing the tipping point in which the onset of anoxia on shelf seas triggered is phosphorus recycling from sediments, fuelling a spread of anoxia, and Lenton et al. (2013) state that the effects on biodiversity were a consequence rather than an intrinsic part of the tipping mechanism. As well as this may be, feedback loops often mean that a biological forcing applied on one scale can cause a critical transition to occur on another scale, for example, anthropogenic selection for younger maturation of individual cod as a result of heavy fishing pressure; and cascades of ecological changes triggered by the removal of top predators (Barnosky et al. 2012). Lenton et al (2013) suggest that species richness is a poor and misleading indicator of Earth-system function, with minimal basis in ecological theory for identifying a number of unique species required to maintain the general health of the biosphere. They also point out the distinction between tipping points in climate or biogeochemical dynamics and subsequent ecological responses to them (Lenton et al. 2013).

To summarize, the terrestrial biosphere, in isolation, is not the right place to be looking for a planetary-scale tipping point; the complex coupled dynamics of the Earth system as a whole need to be assessed (Lenton et al, 2013). Many of the feedbacks, and their consequences for other systems and scales, in the face of changing global climate are as yet unknown. However, planetary scale critical transitions have occurred previously in the biosphere, and evidence suggests that humans are now forcing another such transition, potentially transforming Earth into an irreversible state unknown in human history (Barnosky et al. 2012). As Hobbs et al. (2006) suggest, ‘we should perhaps move away from the one-dimensional dichotomy between natural and human dominated to a more effective depiction of how human beings interact with nature’.

Barnosky et al. (2012) “Approaching a state shift in Earth’s Biosphere”, Nature, 486, 52-58
Brook, B.W. et al. (2013) "Does the terrestrial biosphere have planetary tipping points?" Trends in Ecology & Evolution, 28, 396–401.

Cardinale, B.J. et al. (2012) "Biodiversity loss and its impact on humanity". Nature 486, 59–67
Hobbs, R.J. et al. (2006) "Novel ecosystems: theoretical and management aspects of the new ecological world order". Global Ecology and Biogeography. 15, 1–7

Jefferies, R.L. et al. (2006) "A biotic agent promotes large-scale catastrophic change in the coastal marshes of Hudson Bay". Journal of Ecology. 94, 234–242

Lenton, T., M., and H. T. P. Williams (2013) “On the origin of planetary-scale tipping points, Trends in Ecology & Evolution, 28, 7, 380-382