Hot off the press...

A quick post to highlight a new report published by the Arctic Council highlight resilience and potential tipping points in the Arctic. This report is particularly relevant to this blog and the past few posts on tipping points in and around the Arctic region. 

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The authors identified 19 explicit tipping points which consists of both climatic and non-climatic (socio-ecological) elements with local to global drivers and impacts. The tipping points identified fully embraced the very definition of tipping points I intended to define in the first post. Interestingly, this report also uses an ecosystem services approach when classifying and analyzing various impacts of crossing certain tipping points. Popularized in the Millennium Ecosystem Assessment 2005, the ecosystem services framework aims to link anthropogenic change to biophysical and economic values of ecosystem functions. [Self promotion: I have been blogging about ecosystem services in another blog for my other university module (check it out here!).]

Source

This report efficiently sums up the wide variety (climatic and non-climatic, local or global) of impending tipping points with a multitude of different drivers (socio-ecological, climate change) with a wide range of impacts (local to global, ecosystem services).  An example of such impacts would be a loss of Arctic summer sea ice after surpassing a critical threshold and a subsequent loss of cultural ecosystem services of subsistence hunting and transportation of indigenous Alaskans. Although I focused on climatic tipping points until now, I will also be blogging about ecological and societal tipping points in future posts. 

Detecting Critical Thresholds

I have yet to touch upon how scientists identify or detect tipping points and how they determine where a critical threshold is located in time. In this post, I will outline the major characteristics of a system approaching critical thresholds and how scientists determine them.

General properties

Three general properties characterizes the point at which a critical threshold is surpassed:

1. Rate of change increase sharply then what prevailed over previous stable periods
2. System state exceeds range of historical variations 
3. Rate of change increased at a pace which exceeds the abilities of nations to respond

The timing, type of transition and magnitude of change depends on the nature of interactions and heterogeneity within the system. Recent increases in attempts to predict the type and temporal onset of tipping points can mainly be characterized in two ways (Thompson and Seiber 2010):

1. Models

Climate models have been widely used to predict the presence, timing and magnitude/extent of changes in identified tipping elements. However, the nature of climate models means that the magnitude of threshold effects and when feedback induced thresholds are reached are highly uncertain and varies among model specifications and parameterization (Maslin and Austin 2012). Models also have varying interpretations of mechanisms behind the earth system and varying sophistication in processes represented. Climate models are often used to predict future changes (eg. Arctic summer ice loss seen in Holland et.al. 2006) and validated by its ability to adequately simulate past abrupt changes (eg. 'dangerous climate change' prediction model in Hansen et.al. 2006).

2. Time Series - statistical analysis from observational climatological or ecological time series data to identify statistical characteristics that precedes tipping points/bifurcation

A prime example of this is early warning systems for natural hazards risk management. As Earth have underwent a long history of abrupt climatic changes, scientists often make use of natural archives (eg. ice cores; diatoms; pollen etc.) to infer past climatic change and fluctuations (Thomas 2016). Paleo records of Earth surpassing ancient critical thresholds provides scientists with an observational records from which early warning signals can be identified. Scientists using time series statistical analysis to identify early warning signals showed universal signals across multiple ancient abrupt climate shifts (icehouse -> greenhouse; Younger Dryas; N.Africa climatic shift) (Davos et.al. 2008).

Early warning signals

Tipping points could affect decision making if adequate knowledge on timing, occurrence, and impacts were available. There is therefore a whole field of research aimed at finding preceding signals and characteristics of tipping points from historical abrupt transitions. Early warning identification can take the path of qualitative assessment or quantitative prediction of timing of impending thresholds (Lenton 2011). Listed below are early warning signals identified in past abrupt tipping points (Scheffer et.al. (2012). These are true for a range of complex systems, ranging from climatic systems to financial and social systems.

1. Critical slowing down - Decreased rates of change as system approaches critical thresholds; increasingly sluggish system response; increase in amplitude and reduction in fluctuations

2.  Skewness and kurtosis - Asymmetric fluctuations; presence of extreme values measured through high skewness and kurtosis as system approaches bifurcation

3.  Increased autocorrelation - Decreased rates of change lead to state of system being more and more like its past state prior to critical threshold, thus increase in correlation

4. Spatial patterns - often ecological; signals derived from spatial/temporal persistence and presence of species 

Thank you for reading! In the next post, I will continue this discussion by looking at identified tipping points in Earth's history.

Tipping Point II - Circulation Change

This week, I will be outlining research and modelling exercises on the potential shutdown of the Atlantic Meridional Overturning Circulation (AMOC), also known as the Thermohaline circulation. 

Thermohaline Circulation (THC)

The thermohaline circulation, popularly called the global ocean conveyer belt, is an integral feature of the present day climate system. It sustains the current climate and is a major contributor to the global heat budget. As illustrated in Fig 1,  global oceanic circulation is driven by density gradients related to the formation of deep water. The density of seawater is governed by temperature and salinity. The THC process is briefly outlined below (Broecker 1997):

1. Warmer, saltier water brought into NE Atlantic, warms the European continent
2. Warm water cools and mixes with cold Arctic water, becomes dense and sinks, forming North Atlantic Deep Water 
3. Further sinking of dense water occurs near Antarctica with cool water sinking from the effect of the circumpolar currents, forming Antarctic Bottom Water 
4. Cold dense water returns to the surface throughout the world's oceans and forms a closed loop of exchanges between warm, surface water and cool, dense deep water

Modes of THC

Driven by density differences, the THC is particularly sensitive to the freshwater budget which will disrupt overturning of deepwater by reducing salinity. Looking in past abrupt changes to the THC, scientists have identified three possible modes (stable states) with fundamentally different climates (Rahmstorf 2000):

1. Warm - interglacial (current) mode where deep water forms in Nordic Seas 
2. Cold - glacial mode where deep water forms south near Greenland, Iceland and Scotland
3. Off - shutdown of THC, no formation of deep water in North Atlantic

Transitions between modes would cause abrupt climate changes. Previous transitions between modes have occurred in the form of Dansgaard-Oeschger cycles and Henrich events where large scale breakdown of N Atlantic icebergs dramatically increases freshwater input (Alley 2000). 

Tipping Point

As the thermohaline circulation is driven by density differences which are particularly sensitive to temperature and salinity, both sufficient heat from continued increase in GHG concentrations and alteration of the freshwater budget from melting ice can lead to fundamental re-organization of ocean circulation and transition to a alternative state (Clark 2002). Classified as being 'low probability with high impacts' by the IPCC, a critical threshold may be observed with hysteresis characteristics from non-linear behaviour. Modelling studies using coupled atmosphere-ocean General Circulation Models have suggested that the THC is particularly sensitive to freshwater infiltration on the order of 0.1 Sv with a transition across critical threshold within range between 0-0.15 Sv (Clark 2002; Rahmstorf 2000).

Modelling responses of the THC to rising CO2 concentrations with warming and melting ice effects, Wood et.al. 1999 proposed that a shutdown of convection in the Labrador Sea would result in a 20-25% reduction in deep water formation by the time CO2 quadruples pre-industrial levels. Salinity in the Nordic seas declined since the 1960s, suggesting a possibility of critical threshold of freshening in this century (Curry and Mauritzen 2005). A 20th century slowing down of the AMOC was witnessed by a region of cooling in Northern Atlantic after 1970 due to increased freshwater input with further uncertainty from melting of the Greenland ice sheet (Rahmstorf et.al. 2015). 

Impacts

The impacts of a THC tipping point has been widely studied with models and experiments. A shutdown of AMOC would lead to cooling effects which may outweigh and reverse current CO2 induced temperature trends whereas a AMOC weakening are dominated by increased CO2 domination (Yin et.al. 2006). Using the Met Office HadCM3 GCM in a modelling experiment, an artificial collapse of the AMOC in 2049 would cause reduction in precipitation in Western Europe and a regional cooling of the Northern Hemisphere by -1.7 degrees with up to -9 degrees cooling locally. Global primary production from vegetation will also decrease by 5% due to temperature and moisture changes. Drying trends will also be noticed in Central America and SE Asia with impacts extending globally within 30 years (Vellinga and Wood 2003; Vellinga and Wood 2008). 

Causing the shutdown of the THC would no doubt constitute as 'dangerous level of interference' (Hansen et.al. 2006), possibly characterising the Anthropocene epoch. The uncertainty in coupled GCMs on the full range of feedback responses and the 'low probability' claim of the IPCC is certainty not an excuse for inaction and a shutdown in the current century should not be ruled out. This also highlights the need for better oceanic monitoring equipment and research to ensure development of early warning systems and proper anticipation of global impacts.

Thank you for reading this post! In the next post, we will look at the possibilities of detecting critical thresholds.

Tipping Point I - Melting Ice

Welcome back! This has definitely been an eventful week! It started off well with the release of Leonardo Dicaprio's Before the Flood documentary but ended disastrously with the electoral victory of a climate sceptic as President of the United States. I hope that those in power would recognize the consensus of anthropogenic climate change and the increasing likely reality of irreversibility and impending tipping points. 

In one of the first comprehensive study of tipping elements in the climate system, Lenton et.al (2008) identified 8 major scientifically probable tipping elements regional to global implications. Three major classes of tipping elements were identified as shown below:

1. Melting Ice

2. Circulation Change (Atmospheric and Oceanic)

3. Biome/Ecological Loss

It should be noted that by separating tipping elements in distinctive classes does not necessarily mean that tipping elements operate independent of each other but are instead interconnected to each other and other areas of concerns. I will first be focusing on the Arctic as it has the greatest number of identified potential tipping elements across the three classes. In this post, I will focus on melting ice and in particular the current status of Arctic Sea Ice. 

Arctic Ice Sheet

The Arctic ice sheet is highly sensitive to climatic changes. Considerable thinning, record minimum in multi-year ice extent and a declining in summer (September) sea ice extent have all been observed in the past century (Holland et.al. 2006). Six of the lowest summer ice extent in satellite history had been observed in the period between 2007 and 2012 (Livina and Lenton 2012). Observations and model simulations have shown, with relatively high certainty, the presence of a critical threshold by which summer ice would permanently disappear. 

Source

Warming in Arctic temperature affects the entire suite of ice-ocean system and may have the potential to cause rapid changes in the earth system. The dominance of positive feedbacks in the Arctic is instrumental in amplifying warming and accelerates ice retreat and thinning, resulting in possible ice-free summer conditions. This has been termed 'Arctic amplification' and has largely been used in paleoclimatology. Outlined below are the three major positive feedback loops amplifying rises in global mean sea surface temperature and subsequently drives rapid decline in thickness and extent of sea ice (Miller et.al. 2010): 

1. Ice-Albedo Feedback - Fresh snow and sea ice has the highest albedos (reflectivity of solar radiation) of any land surface on Earth. 

Increases in temperature from anthropogenic warming --> reduction in seasonal/areal extent of sea ice --> increase exposure of open ocean--> reduction in albedo -->  stronger absorption of solar radiation --> further rise in sea surface temperature

2. Vegetation Feedback - Tundra and vegetated lands are abundant in Arctic inlands and can contribute to warming through positive feedbacks

Seasonal reduction in extent and duration of snow cover --> increased vegetation response with advancement of dark shrubs --> reduction in albedo -> stronger absorption of solar radiation --> further rise in sea surface temperature

3. Permafrost Feedback - Large stocks of methane hydrates are present in continental shelves of the Arctic. It is also a carbon sink in recent decades (McGuire et.al. 2009)

Warming melts ground ice --> extensive permafrost thaw --> release of frozen carbon and methane into the atmosphere --> accelerate rate of climate change 

It is therefore clear that these positive feedbacks may induce non-linear behaviour which, at a critical threshold, may tip the entire system into a qualitatively different state. Non-linear shrinking and thinning of Arctic sea ice have been observed since 1988, leading to some suggesting that a critical threshold has already been passed. Lindsay and Zhang (2005) suggested the dominance of internal system response over response to external forcing since 1989 beyond which positive feedback loops were increasingly capable of triggering initiation of continual rapid thinning and shrinkage even when external forcing remained relatively unchanged. Other studies have employed an alternative definition of tipping point in terms of summer sea ice extent but are in agreement about the role played by ice-albedo feedbacks and open water formation.  Using 7 ensemble model simulations from the IPCC, there was general agreement that reduction in summer sea ice is a universal feature in the 21st century with a 60% decrease in sea ice in a decade and summer ice-free conditions by 2040 (Fig.1) (Holland et.al. 2006). Furthermore, some even suggests impending year round ice-free conditions when polar temperature rises above -5 degrees and positive feedbacks disturbs linear relationships between sea ice and climate (Winton 2006). 

Fig.1 Critical threshold at around 2040 with ice-free conditions in
the summer in 7 runs of ensemble models in the IPCC AR4 report

However, some have suggested that ice extent recovers and a tipping point is unlikely to exist in the foreseeable future. It was suggested that positive ice-albedo feedbacks are not permanent and anomalous summer ice loss is reversible by large-scale recovery mechanisms. Anomalous summer ice loss due to positive ice-albedo feedbacks are reversed when anomalously warm atmosphere causes increased heat loss and decreased heat gain adaptations at the top of the atmosphere (Tietsche et.al. 2011). It is therefore clear that there are considerable uncertainties in whether or not a certain critical threshold exists and whether it has already been passed or not. To sum up, Duarte et.al. (2012) raises an important point that descending into a semantic argument of what constitutes a tipping point in Arctic sea ice loss and whether or not a threshold has been passed detracts from the urgency of the situation and the need to avoid the increasing reality of dangerous climate change in the Arctic. 

Political Tipping Point

The US presidential election across the pond is only one week away! Climate change had never featured this much in a presidential election before. From asserting that climate change is the biggest threat to national security by Bernie Sanders to outright rejection of climate science by Donald Trump, it is fair to say that the next decade of global environmental cooperation depends on this historic election.

I came across an open letter sent by >300 scientists and 30 Nobel laureates warning of the serious risks of climate change and the consequences of opting out of international climate cooperation. The letter highlights the consensus in the presence of climate tipping points and the risks of inaction. 

"We know that the climate system has tipping points. Our proximity to these tipping points is uncertain. We know, however, that rapid warming of the planet increases the risk of crossing climatic points of no return"

Just like the climate system, 'the political system also has tipping points'. Handing over power to a president who believes climate change is an invented hoax and a vice president who have received large amounts of his campaign money from donors in the fossil fuel industry would represent a political tipping point where global environmental cooperation are undermined and downplays the urgency of impending climate crisis. 

Are we doomed?

I recently stumbled across this video taken from 'Disruption', a climate change documentary premiered in 2014. It is a very nice introduction to three major and arguably most urgent climatic tipping points which are scientifically probable and will have regional to global impacts. It is also a nice video to introduce you all to the next part of my blog where I will delve into the scientific basis of the various tipping points. 

Enjoy the video! My next post will be on the first climate tipping point - potential tipping behaviour of the Arctic sea ice.