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Towards improved understanding of cascading and interconnected risks from concurrent weather extremes: Analysis of historical heat and drought extreme events [1]

['Laura Niggli', 'Department Of Geography', 'University Of Zurich', 'Zurich', 'Christian Huggel', 'Veruska Muccione', 'Raphael Neukom', 'Department Of Geosciences', 'University Of Fribourg', 'Fribourg']

Date: 2022-08 

In this section we analyze the most recurring impacts and effects on critical systems and interlinked sectors to understand how these impacts propagate within the system and which sectors and subsystems are most vulnerable to compound heat and drought extremes.

Culture and society experienced manyfold consequences arising from heat and drought extreme events. In several European countries tourism and recreation in water bodies were limited due to low water levels, and access to recreation areas like forests was restricted during high fire risk [ 82 ]. Hiking and climbing routes were closed due to an increased level of ice and rock falls in the Alps [ 42 ]. And in 2019/2020 due to the fires in Australia, some Australian Open tennis matches were delayed or abandoned and other sports events were cancelled [ 72 ].

Extreme heat and drought events caused losses and damage in the natural environment especially affecting forests and acquatic ecosystems, but also birds and insects [ 17 , 59 ]. Hot temperature records in many water bodies in 2003, 2015 and 2018 threatened fish and water ecosystem plants in Europe [ 41 , 59 , 67 , 82 ]. At the same time many European countries reported significant increases in woodborer infected wood [ 41 , 59 , 67 ] and high tree mortality [ 89 ]. The natural environment of Australia suffered most from wildfires. Altough in many ecosystems of Australia and other dry regions, forest fires are part of the natural ecological cycle [ 110 ], climate change has likely increased the fire risk and contributed to the recent large Australian bushfires [ 52 ], which also affected ecosystems not adapted to natural fires, such has rainforests. Thousands to millions of hectares of bushland and forests burnt down in wildfires in 2009, 2012/2013 and 2019/2020 leading to the direct death of more than 1 billion animals [ 72 , 108 ]. Fires in Europe burnt particularly large and critical areas in Portugal, Spain, France and Greece [ 41 , 42 , 91 ].

In Australia in 2009 financial losses arose mainly as a consequence of wildfire destruction, power outages, transport service disruptions and response costs [ 94 ]. The wildfires of 2019/2020 in Australia caused an economic loss of around US$ 80.6 billion (i.e., 5.71% GDP) until January 2020 [ 109 ]. In Portugal the wildfires of 2003 caused losses of more than € 1.3 billion (i.e., 0.68% GDP) [ 41 ]. And in the fires around Athens in 2018, 305 vehicles were burnt and lost [ 90 ]. Apart from these exceptional fires, in Europe, losses were mostly documented for the agricultural and forestry sectors and for mobility and transport disruptions as well as power restrictions [ 82 ]. In 2003 damage in agriculture and forestry exceeded € 17.4 billion (i.e., 0.17% GDP) in Europe [ 41 , 42 ]. Crop and fodder imports in order to compensate for harvest losses, caused additional costs [ 67 ]. In 2010, Russia’s agricultural sector suffered losses of US$ 1.7 billion (i.e., 0.18% GPD) which pushed a steep increase in grain prices and widespread market speculation [ 80 ].

Buildings and infrastructure were most often damaged by wildfires. In Australia, close to 200 properties and 21 businesses were destroyed in the summer 2012/2013 [ 108 ] and about 5900 buildings were destroyed and another 1021 homes and thousands of facilities and outbuildings damaged in the summer 2019/2020 [ 52 , 72 ]. In Greece, the 2018 fires close to Athens led to the total or partial destruction of approximately 3000 buildings [ 91 ]. In the UK, the extreme heat in 2018 led to the melting of a roof of a building [ 17 ]. And in the same year the Netherlands experienced stability issues in their dike systems due to a lack of freshwater [ 88 ].

The sector of transport and mobility was affected by both, heat and drought. Extreme heat led to the buckling of rail tracks in the public train transport system and it caused damage in road transport through bitumen sticking to tyres, both observed in Australia as well as in Europe [ 17 , 45 , 67 , 107 ]. Drought on the other hand mainly affected waterway transport. In Europe this was strongly experienced in waterway cargo transportation that had to be reduced and up to suspended in several occasions in the past two decades [ 59 , 67 , 82 , 88 ]. Lastly, wildfires led to road closure and transportation blockages [ 52 ]. Said transport and mobility interruptions caused far-reaching delays in Europe [ 107 ] and in occasions led to the isolation of communities that were then only accessible by water or air in Australia [ 52 ].

The energy system during heat and drought extreme events is influenced by an altered energy supply and demand. Increased air-conditioning beside others led to a substantial rise in electricity demand in Melbourne in 2009 as well as for example in France in 2003 [ 1 , 45 ]. On the other hand power production was lower in concurrent heat and drought situations, due to low water levels in hydropower plants and a lack of cooling water in nuclear power plants [ 59 , 67 , 82 , 105 , 106 ]. The electricity sector is generally found vulnerable to heat, due to a loss of efficiency, difficulty of machine cooling, expansion of power lines and their sagging below minimum height beside others [ 45 ]. Shutdowns in Europe as well as in Australia led to restrictions, shortages and regional power outages [ 42 , 45 , 82 , 90 , 99 ].

Agricultural losses were experienced in most of the analyzed events in the last two decades. Large losses in crop yield were observed in large parts of Europe in 2003 [ 42 ] as well as in 2015 [ 99 ] and in 2018 [ 88 ]. The concurrent heat and drought events affected grain crops and other arable crops like potatoes and sugar beet, as well as pasture harvests for animal fodder [ 17 , 59 , 89 , 100 , 101 ]. In 2003, the fodder deficit in Europe varied from 30% to 60% in Germany, Austria, Spain and France [ 41 ]. In 2010, the affected regions of Russia experienced crop losses of 1/3 of the cultivated area [ 80 ]. Australia registered large losses in their wine grape harvest [ 102 ], and the irrigated cotton and rice production dropped to 20% and to virtually 0% in the catchment of the country’s largest river system in 2009 [ 103 ]. While drought often causes yield losses in agriculture, livestock are vulnerable to heat stress affecting their production levels as well as their health [ 104 ]. In 2003 a decrease in milk production, as well as an increase in the mortality in the livestock and poultry stocks were recorded in several European countries [ 41 , 42 , 59 ].

Water availability in Europe was temporally low in all three years 2003, 2015 and 2018, but drinking water supply shortfalls have so far been locally bounded [ 41 , 59 ]. The millenium drought 2000–2009 in Australia led to a substantial decrease in water resources and caused a sense of water crises latest by 2009 [ 97 ]. Water provision issues exacerbated when water treatment plants were affected by evaporation, algae, power outages and the breakdown of pumps during the extreme heatwave of 2009 [ 98 ]. The extreme drought in Cape Town between 2015 and 2018 caused severe water scarcity and anxiety due to the possibility of a city-wide water crisis [ 86 ].

Excess mortality increased during all of the mentioned extreme heat and drought events, except for the case of Cape Town, for which no excess deaths were reported specifically in relation to the extreme drought years. For Europe most of the excess mortality was caused by extreme heat. In 2003 Europe counted more than 80’000 excess deaths [ 93 ]. Garcia-Herrera et al. (2010) argue that increased mortality in Europe in summer 2003 was caused by a combination of extreme temperatures and poor air quality. During the heat wave in Moscow in 2010 more than one fifth of the 10’000 excess deaths can be attributed to interactions between high temperature and air pollution from wildfires [ 79 ]. In 2009, around 500 excess deaths were recorded in southeastern Australia [ 94 ]. However, we did not find complete and publicly available excess mortality data for most years in Australia and [ 95 ] suggest substantial under-reporting of heat-related mortality for Australia. The fatalities associated with wildfires in Australia are substantially higher than in Europe with dozens of deaths in 2009 and 2019/2020 directly due to the fires and hundreds of deaths as well as thousands of hospitalizations due to hazardous air quality resulting from the wildfires [ 45 , 52 , 96 ].

The arrows point from the event or sector that affects another sector or asset to the sector or asset that is affected. The colors of the arrows identify the main driver of the impact and the width of the arrows is representative for the importance of the impact. It reflects the number of associated interconnections or mentions found in the literature.

Table 2 shows the sectors mainly affected (according to the literature) for each of the extreme events assessed. Table 3 contains illustrative examples of impacts reported in the different sectors, that served as a basis for the analysis in this chapter and for the elaboration of Fig 3 . This table is not intended to be exhaustive, but it mainly offers key examples for the purpose of understanding the connections and interdependencies between sectors.

3.2 Impact interaction and cascading effects

Heat and drought extreme events can impact single sectors as well as multiple sectors, and some sectors can be affected by several extreme events. At the same time, impacts in one sector can also propagate and affect other sectors, putting strain not only on one sector but on the whole system. Fig 3 is a simplified visualization of the interconnectedness of the different sectors and impacts based on the assessed historical heat and drought extreme events. For a more detailed version of the scheme consult the appendix (S1 Fig). The most reported direct impacts of heat and drought extreme events affect human health and the agricultural and food production sector. However, almost all the assessed sectors have been affected indirectly through cascading impacts emerging from other sectors, the most affected ones being the sectors of energy, transport and mobility as well as the economy and financial system. Sectors that have been given less importance to but have themselves strong potential to affect other sectors are water resources, ecosystems as well as critical infrastructure and buildings. Eventually all impacts propagating through the system can be expected in one way or another to end up compromising the economy, the state and public services and thus society and culture. The only sector that was not reported to have experienced important failures and losses is ‘internet and communication’. However, the internet and communication sector strongly depends on the availability of electricity which was found to be affected by concurrent heat and drought events. With increasing frequency and magnitude of concurrent heat and drought events, the internet and communication sector can thus be expected to become increasingly affected too.

Heat and drought extremes are documented to have a high impact on health. For instance in the second week of August during the heat wave of 2003, the excess mortality ratio reached values between 21% (Belgium) and >96% (France) in most central and western European countries [93]. These excess deaths are especially important as they happened only within one week putting strain on the health system. For example, it was found for Australia as well as for Europe, that the demand for emergency health services such as ambulance interventions and presentation to emergency departments increases during hot weather [8, 41] and wildfires [96]. In the Australian summer 2009 the dispatch of ambulance service increased by 3326% compared to the same time in the previous year [94]. Apart from the heat and wildfire related excess mortality, [79] also found excess risks for other common causes of death–not directly linked to heat—including cancer and diseases of the digestive system during the heat wave in Moscow in 2010. They hypothesize that this is related to poor conditions in hospitals and other treatment facilities, such as lack of air conditioning [79]. Similar impacts are reported from the COVID pandemic, which also brought the health system in most countries to its very limits, delaying treatment of people with other severe disesases like cancer. Heat waves in combination with other types of ‘extreme events’ can thus even heavily affect and put life in danger of people with severe diseases.

The energy sector suffers concurrent impacts from heat and drought extremes which are cascading through the infrastructure and building as well as the transport sectors. The need for climatization in the short-term and for climate-proving of buildings and infrastructure in the long-term is ever increasing [98] and has implication for the energy sector as well as for the economy on broader terms. The increased electricity demand during hot extremes potentially causes a gap between demand and supply. Especially as the energy sector is already strongly affected by heat and drought extremes, suffering from efficiency loss and reduction in both the generation as well as the distribution of power. Lower supply and higher demand can lead to a rise in energy prices [42, 98], trading restrictions and market contorsions with social consequences (such as fuel poverty [98]) and international consequences as seen in France that cut power exports by more than half due to the unprecedented demand on its domestic energy infrastructure in 2003 [1].

While the electricity system is strongly connected to all relevant sectors of society, its importance has shown especially evident in its interaction with the transport and mobility sector. As seen in Europe as well as in Australia, disruption in the rail and electrical transport system may be significant. Power outages have led to the failure of traffic signals and air conditioning in trains [45] and have the potential to cause major mobility disruptions when relying on electrical transportation. At the same time the energy sector in landlocked countries like Switzerland is found to be strongly dependent on a functioning transport system for import of fossil fuel. In Switzerland, vulnerability arises from the high importance of waterway transportation for the energy supply. Most affected are the reserves of motor and combustible fuel, since 25% of the petroleum used in Switzerland is imported through the Rhine river [67].

While the disruption of road and railway transport caused by power outages, rail buckling and the melting of roads causes problems in the public transport system and creates losses and repair costs, low water levels limit waterway traffic and lead to increased shipping prices and costs for the associated industries and economy. Similar to the import of fossil fuels, waterway transport plays an essential role in the transportation of feeding stuff and comestible goods. In Switzerland a significant portion of animal feeding stuff and comestible goods are imported through the Rhine river [67], affecting the livestock and agriculture sector and eventually impacting food production. This has particularly severe consequences when the agricultural sector already suffers great losses in extreme heat and drought periods and relies even more on import. Extreme heat increases evapotranspiration and the crop’s need for water and drought causes the necessity for crop irrigation potentially depleting natural water resources. Heat stress and insufficient irrigation lead to harvest losses and fodder shortages causing production losses in both the agriculture and livestock sector, again having strong consequences for commodity prices and exporting as well as importing economies. In 2010, Russia’s agricultural sector suffered losses of US$ 1.7 billion which pushed a steep increase in grain prices and widespread market speculation [80]. Hoarding and panic buying led to shortages of buckwheat and higher prices for bread and grain products, as well as for dairy products due to higher feed prices. And as Russia is a significant grain exporter, losses in agriculture also caused losses in exportation and had global-scale impacts on food security [116]. At the same time one of the reasons agricultural losses were so high during 2010, is market-driven changes in crop rotation practices that resulted in a decline in drought resistance in cultivated crops [116].

The increased water use in agriculture stands in direct conflict with environmental needs and the domestic water demand. In 2003 several European regions suffered threats to their water supply systems [41] and during the multiannual extreme drought in South Africa water transfers from the agricultural sector to the urban supply system caused sever financial consequences for farmers [120].

Incompliance with ecological minimum flow standards [82] compromise ecosystems and biodiversity already suffering from the extreme heat and drought. And beside heat-related eutrophication, algae bloom and botulism [82, 119], insufficient dilution of waste water in streams can further degrade the water quality [59]. While this in turn can put more strain on water treatment and affect the public water service [98], it also has severe consequences for the environment. Aquatic organisms loose their habitat and most ecosystems suffer from heat and drought stress weakening flora and fauna. Weak organisms are more vulnerable to pest infestation or extreme weather events and associated impacts as storms and wildfires. Not only leads this to extreme soil erosion and biodiversity loss, it also has cascading impacts on economy and society. Forced wood harvesting and water oxygenation, as well as widespread fish death cause elevated costs and losses in forestry and aquaculture. The increased drying of vegetation makes it easier for wildfires to rapidly spread and not only consume ecosystems and biodiversity, but also cause destruction in urban regions entailing high monetary losses and putting increased strain on insurance companies. Especially in an urban context wildfires can pose a serious threat to human life and health.

The protection of human health in wildfire situations as well as generally in heat and drought extreme events increases the need for public services and often demands for restrictions in cultural and societal opportunities and leisure possibilities. Due to wildfires in the Czech Republic, there were twice as many fire fighter interventions in 2015 than on average [82]. And in Sydney and Canberra the air contamination in summer 2019/2020 was so severe that schools were forced to close, and play and sports activities were cancelled in the former, and public servants were instructed to stay home and the airport of Canberra was closed on two occasions [72]. While wildfires are a recurring and well visible hazard related to extreme heat and drought events, other problems arise for nature and people. Hazardous high water levels in streams with glacier contribution related to extreme heat, drought related stability issues of dike systems or heat and drought related dying of forests can lead to the necessity of closing off certain sectors. Not only has this restriction of access to water bodies, forests and mountains in order to protect human or environmental health affect society and culture. Together with the destruction of the natural environment and the contamination of urban centres it can lead to a potential loss of image and landscape value and to reductions of consumption and tourism yields [72].

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[1] Url: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000057

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