To the Point Prediction

Luckily, we are not left blindly waiting for tipping points to occur. Once we know that they exist, or have occurred in the past, we can look out for early warnings. Early warning can take several forms, as simple as the knowledge that an event could occur and that it is becoming more likely, to a forecast of its timing and modelling of future events (Lenton, 2011). Slowing down of a system before a bifurcation occurs has been noticed in present day systems, climate-model output and palaeoclimate data; it causes the intrinsic rates of change in a system to decrease, and thus the state of the system becomes more like its past, alternative state (Lenton, 2011). Similar to this are ‘small-signal amplification’ and ‘noise amplification’, where small intermittent perturbations or noise are amplified at particular frequencies depending on the type of bifurcation (Lenton, 2011). Ditlevsen and Johnsen (2010) describe the two generic characteristics of the approach to a bifurcation point as increased variance of the observed signal and the corresponding increased auto-correlation related to critical slow down. They do however, stress that the early warning of climate or structural change in any system can only be obtained if increase in both variance and auto-correlation is observed, and that conclusions drawn based solely on one of the signals and not the other are invalid (Ditlevsen and Johnsen, 2010).

http://cpa.ds.npr.org/wamc/audio/2013/11/11-25-13_harvard_forest_troubled_lakes.mp3

In this talk, (see link above) Dr. Aaron Ellison talks about ecosystems and tipping points, briefly discussing the findings of his 2013 collaborative paper – Sirota et al. 2013.

Image from North Carolina Native Plant Society,
 http://www.ncwildflower.org/index.php/plants/details/sarracenia-purpurea/

Although experimental induction of tipping points is rare due to the scale of the system in question, Sirota et al. (2013) experimentally induced a shift from aerobic to anaerobic states in a miniature aquatic ecosystem of the self-contained pools that form in leaves of the carnivorous northern pitcher plant, Sarracenia purpurea, in order to represent the shift from a clear, oligotrophic lake to a murky, eutrophic one. The plants were fed controlled amounts of dried, ground arthropod prey. In controls, the concentration of dissolved oxygen replicates exhibited regular diurnal cycles associated with daytime photosynthesis and nocturnal plant respiration. Results showed that increasing organic-matter loading led to predictable changes in O2 dynamics, with high loading consistently driving the system past a well-defined tipping point. The Sarracenia micro ecosystem therefore functions as a compliant experimental system in which to examine prediction and management of tipping points.

This, as well as other models, tests and qualitative observations, show promise for early warning of bifurcation-type climate tipping points, but there are potential limitations of ‘false alarms’ (false positives) and ‘missed alarms’ (false negatives) (Lenton, 2011) that must be considered before jumping to conclusions. There is, however, hope for a better understanding of impending tipping points and how we can mitigate, if not prevent, them.

Ditlevsen, P. D. & Johnsen, S. J. (2010). “Tipping points: Early warning and wishful thinking”. Geophysical  Research Letters, 37.

Lenton, T., M., (2011) “Early warning of climate tipping points” Nature Climate Change, 1, 201-209

Sirota, J., B. Baiser, N. J. Gotelli, and A. M. Ellison. 2013. Organic-matter loading determines regime shifts and alternative states in an aquatic ecosystem. Proceedings of the National Academy of Sciences, USA. 110: 7742-7747.

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

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.)