Editing IPCC:AR6/SROCC/Chapter-6 (section)

== Box 6.1 Multiple Hazards, Compound Risk and Cascading Impacts ==

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The following case studies illustrate that anthropogenic climate change including ocean changes is increasingly having a discernible influence on elements of the climate system by exacerbating extreme events and causing multiple hazards, often with compound or sequential characteristics. In turn these elements are interacting with vulnerability and exposure to trigger compound events and cascading impacts.

'''''Case Study 1: Tasmania’s Summer of 2015''''' '''–''' '''''2016'''''

'''Tasmania in southeast Australia experienced multiple extreme climate events in 2015–2016, driven by the combined effects of natural modes of climate variability and anthropogenic climate change, with impacts on the energy sector, fisheries and emergency services.''' The driest warm season on record (October to April), together with the warmest summer on record, brought agricultural and hydrological droughts to Tasmania and preconditioned the sensitive highland environment for major fires during the summer. Thousands of lightning strikes during the first two months of the year led to more than 165 separate vegetation fires, which burned more than 120,000 hectares including highland zones and the World Heritage Area and incurred costs to the state of more than 50 million AUD (Press, 2016).

In late January an intense cutoff low-pressure system brought heavy rainfall and floods, so that emergency services were simultaneously dealing with highland fires and floods in the east and north. The floods were followed by an extended wet period for Tasmania, with the wettest wet season (April to November) on record in 2016. Meanwhile, an intense marine heatwave (MHW) off the east coast persisted for 251 days from spring 2015 through to autumn 2016 (Oliver et al., 2017).

The driest October on record was influenced by both the El Niño and anthropogenic forcing (Karoly et al., 2016). Warmer sea surface temperatures (SSTs) due to anthropogenic warming may have increased the intensity of rainfall during the floods in January (e.g., Pepler et al., 2016a). The intensity and the duration of the MHW was unprecedented and both aspects had a clear human signature (Oliver et al., 2017).

Tasmania primarily relies on hydro-electric power generation and the trading of power over an undersea cable to mainland Australia, ‘Basslink’, for its energy needs. Lake levels in hydro-electric dams were at relatively low levels in early spring 2015, and the extended dry period led to further reductions and significantly reduced capacity to generate power (Hydro Tasmania, 2016). An unanticipated failure of the Basslink cable subsequently necessitated the use of emergency diesel generators (Hydro Tasmania, 2016).

The compound events caused many impacts on natural systems, agriculture, infrastructure and communities. Additional emergency services from outside the state were needed to deal with the fires. The MHW caused disease outbreaks in farmed shellfish, mortality in wild shellfish and species found further south than previously recorded. The energy sector experienced a severe cascade of impacts due to climate stressors and system inter-dependencies. The combination of drought, fires, floods and MHW reduced output from the agriculture, forestry, fishing and energy sectors and reduced the State of Tasmania gross state product (GSP) to 1.3%, well below the anticipated growth of 2.5%. To address the energy shortages, Tasmania’s four largest industrial energy users, responsible for 60% of Tasmania’s electricity usage, agreed to a series of voluntary load reductions of up to 100 MW on a sustained basis, contributing to a 1.7% reduction in the output of the manufacturing sector (Eslake, 2016). The total cost of the fires and floods was assessed at 300 million USD. In response funding has been increased to government agencies responsible for managing floods and bushfires, and multiple independent reviews have recommended major policy reforms that are now under consideration (Blake, 2017; Tasmanian Climate Change Office, 2017) .

This case illustrates the concepts presented in Figure 6.1. Anthropogenic climate change ''likely'' contributed to the severity of multiple hazards; including coincident and sequential events (droughts and bushfires, followed by extreme rainfall and floods). Compound risks, including risks for the safety of residents affected by floods and fires, the natural environment affected by MHWs and fires and the economy in the food and energy sectors arose from these climate events with cascading impacts on the industrial sector more broadly as it responded to the shortfall in energy supply.

Extremes experienced in 2015–2016 in Tasmania are projected to become more frequent or more intense due to climate change, including dry springs and summers (Bureau of Meteorology and Australian CSIRO, 2007), intense lows bringing extreme rains and floods in summer (Grose et al., 2012), and MHWs on the east coast associated with convergence of heat linked to the East Australia Current (Oliver et al., 2017) indicating that climate change by increasing the frequency or intensity of multiple climate events will ''likely'' increase compound risk and cascading impacts ( ''high confidence'' ).

'''''Case Study 2: The Coral Triangle'''''

'''The Coral Triangle is under the combined threats of mean warming, ocean acidification, temperature and sea level variability (often associated with both El Niño and La Niña), coastal development and overfishing, leading to reduced ecosystem services and loss of biodiversity.''' The Coral Triangle covers 4 million square miles of ocean and coastal waters in Southeast Asia and the Pacific, in the area surrounding Indonesia, Malaysia, Papua New Guinea, the Philippines, Timor Leste and the Solomon Islands. It is the centre of the highest coastal marine biodiversity in the world due to its geological setting, physical environment, and an array of ecological and evolutionary processes which makes it a conservation priority. Together with mangroves and seagrass beds, the 605 species of corals including 15 regional endemics (Veron et al., 2011) provide ecosystem services to over 100 million people from diverse and rich cultures, in particular for food, building materials and coastal protection.

The riches of the ecosystems in the Coral Triangle led to expanding human activities, such as coastal development to accommodate a booming tourism sector and overfishing. There is agreement that these activities, including coastal deforestation, coastal reclamation, destructive fishing methods and over-exploitation of marine life generate important pressures on the ecosystem (Pomeroy et al., 2015; Ferrigno et al., 2016; Huang and Coelho, 2017). As a result, the coastal ecosystems of the Coral Triangle have already lost 40% of their coral reefs and mangroves over the past 40 years (Hoegh-Guldberg et al., 2009).

Risks from compound events include increase in sea surface temperature (SST), SLR and increased human activities. The increasing trend in SSTs was estimated to be 0.1oC per decade between 1960–2007 (Kleypas et al. 2015) but increased to 0.2oC per decade from 1985–2006 (Penaflor et al. 2009), an estimation comparable with that in the South China Sea (Zuo et al. 2015). However, waters in the northern and eastern parts are warming faster than the rest of the region, and this variability is increased by local parameters linked to the complex bathymetry and oceanography of the region (Kleypas et al. 2015). Areas in the eastern part have experienced more thermal stress events, and these appear to be more likely during La Niña events, which generate heat pulses in the region, leading to bleaching events, some of them already triggered by El Niño Southern Oscillation (ENSO) events. In the Coral Triangle, El Niño events have a relative cooling effect, while La Niña events are accompanied by warming (Penaflor et al. 2009). The 1997–1998 El Niño was followed by a strong La Niña so that degree heating weeks (DHW) values in many parts of the region were greater than four, which caused widespread coral bleaching (DHW values greater than zero indicate there is thermal stress, while DHW values of 4 and greater indicate the existence of sufficient thermal stress to produce significant levels of coral bleaching; Kayanne, 2017). However, in Indonesia, the 2015–2016 El Niño event had impacted shallow water reefs well before high SSTs could trigger any coral bleaching (Ampou et al. 2017). Sea level in Indonesia had been at its lowest in the past 12 years following this El Niño event and this had affected corals living in shallow waters. Substantial mortality was likely caused by higher daily aerial exposure during low tides and warmer SST associated with shallow waters. Another climate change-associated impact in the Coral Triangle is ocean acidification. Although less exposed than other reefs at higher latitudes (van Hooidonk et al. 2013), changes in pH are expected to affect coral calcification (DeCarlo et al. 2017), with an impact on coral reef fisheries (Speers et al. 2016).

At present, different approaches are used to manage the different risks to coral ecosystems in the Coral Triangle such as fisheries management (White et al., 2014) and different conservation initiatives (Beger et al., 2015), including coral larval replenishment (dela Cruz and Harrison, 2017) and the establishment of a region-wide marine protected area system (e.g., Christie et al., 2016). There is ''high confidence'' that reefs with high species diversity are more resilient to stress, including bleaching (e.g., Ferrigno et al., 2016; Mellin et al., 2016; Mori, 2016). Sustainable Management of coastal resources, such as marine protected areas is thus a commonly used management approach (White et al., 2014; Christie et al., 2016), supported in some cases by ecosystem modelling projections (Weijerman et al., 2015; Weijerman et al., 2016). Evaluations of these management approaches led to the development of guiding frameworks and supporting tools for coastal area managers (Anthony et al., 2015); however biological and ecological factors are still expected to limit the adaptive capacity of these ecosystems to changes (Mora et al., 2016).

'''''Case Study 3: Severe Atlantic Hurricanes of 2017'''''

The above-average hurricane activity of the 2017 season led to the sequential occurrence of Hurricanes Harvey, Irma and Maria on the Caribbean and southern US coasts (Klotzbach and Bell, 2017), collectively causing 265 billion USD damage and making 2017 the costliest hurricane season on record (Blake et al., 2011; Blake and Zelinsky, 2018).

The role of climate change in contributing to the severity of these recent hurricanes has been much discussed in the public and media. It has not been possible to identify robust long – term trends in either hurricane frequency or strength given the large natural variability, which makes trend detection challenging especially given the opposing influences of greenhouse gases (GHGs) and aerosols on past changes. However, observational data shows a warming of the surface waters of the Gulf of Mexico, and indeed most of the world’s oceans, over the past century as human activities have had an increasing impact on our climate (Sobel et al., 2016).

Hurricane Harvey brought unprecedented rainfall to Texas and produced a storm surge that exceeded 2 m in some regions (Shuckburgh et al., 2017). Climate change increased the rainfall intensity associated with Harvey by at least 8% (8–19%; Risser and Wehner, 2017; van Oldenborgh et al., 2017) ( ''high confidence'' ). Emanuel (2017) estimated that the annual probability of 500 mm of area-averaged rainfall had increased from 1% in the period 1981–2000 to 6% in 2017. Furthermore, if society were to follow RCP8.5, the probability would increase to 18% over the period 2081–2100.

The event attribution method of Emanuel (2017) indicates that for TC Irma, which impacted the Caribbean islands of Barbuda and Cuba, the annual probability of encountering Irma’s peak wind of 160 knots within 300 km of Barbuda increased from 0.13% in the period 1981–2000 to 0.43% by 2017, and will further increase to 1.3% by 2081–2100 assuming RCP8.5. TC Maria followed Irma, and made landfalls on the island of Dominica, Puerto Rico, and Turks and Caicos Islands. The annual probability of encountering Maria’s peak wind of 150 knots within 150 km of 17°N, 64°W increased from 0.5% during 1981–2000 to 1.7% in 2017, and will increase to 5% by 2081–2100 assuming RCP8.5.

At least 68 people died from the direct effects of Harvey in Houston (Blake and Zelinsky, 2018). The Houston metropolitan area was devastated with the release of about 4.6 million pounds of contaminants from petrochemical plants and refineries. Irma caused 44 direct deaths (Cangialosi et al., 2018) and wiped out housing, schools, fisheries and livestock in Barbuda, Antigua, St. Martin and the British Virgin Islands (ACAPS et al., 2017). Maria caused 31 direct deaths in Dominica, two in Guadeloupe and around 65 in Puerto Rico (Pasch et al., 2018), and completely vacated Barbuda. Maria destroyed almost all power lines, buildings and 80% of crops in Puerto Rico (Rexach et al., 2017; Rosselló, 2017), and damaged pharmaceutical industries that provided 33% of Puerto Rico’s gross domestic product (GDP) causing shortages of some medical supplies in the USA (Sacks et al., 2018). The effects of Maria are expected to increase the poverty rate by 14% because of unemployment in tourism and agriculture sectors for more than a year in Dominica (The Government of the Commonwealth of Dominica, 2017), and resulted in outmigration to neighboring countries or the USA (ACAPS et al., 2017; Rosselló, 2017). These economic and social consequences are indicative of the cascading impact of the 2017 hurricanes. The post-disaster reconstruction plan is to renovate telecommunications, develop climate resilient building plans and emergency coordination (Rosselló, 2017; The Government of the Commonwealth of Dominica, 2017).

Collectively, these case studies indicate that climate change has played a role in multiple coincident or sequential extreme events that have led to cascading impacts ( ''high confidence'' ). Climate change is projected to increase the frequency or intensity of multiple climate events in the future and this will ''likely'' increase risks of compound event and cascading impacts ( ''high confidence'' ).

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