a) types of climate change hazard events

It is important to note that acute climate weather hazard events can occur because of natural climate processes, as well as be exacerbated by human influences on the climate, so anthropogenic. Heatwaves, extreme precipitation, and flooding associated with tropical storm events are affected by anthropogenic climate change with high confidence. At the same time, there are no clear conclusions for droughts and hurricane or storm activities thus far (WMO, 2023).

Furthermore, most infrastructure assets are built based on historical climate conditions. As anthropological climate change results in more extreme climate conditions and higher intensity and frequency of climate hazard events, most infrastructure assets are exposed to higher physical risks than before. The following segment introduces major physical risk events and their corresponding impact on various infrastructure types.

Floods and storms are the most common types, accounting for 44 and 28 per cent of all climate events from 2000 to 2019 (UNDRR, 2020). While floods, storms, and heat are well-studied and offer much information, data for other hazard events are hardly available. Therefore, the current iteration of quantified physical risk metrics focuses on flood-, storm-, and heat-related physical risks as these are the most material today.

Floods

A flood refers to the phenomenon where a stream or another water body surpasses its ordinary boundaries, leading to the inundation of normally dry regions. This encompasses various types of floods (e.g., fluvial, pluvial, flash, urban, coastal, glacial lake outbursts, etc.).

Besides being the most common climate hazard, floods often accompany storms and cyclones as a secondary hazard. Floods damage infrastructure assets based on duration, depth of inundation, and the material and structure of the asset itself. Accordingly, some asset classes, like roads, commercial buildings, and other transport sectors, could experience severe damage caused by floods, while other sectors (e.g., power generation) would hardly be affected, independent of the flood depth (Huizinga et al., 2017).

However, even within the same sector, the damage caused by floods can vary tremendously. Depending on regional differences in assets’ immediate environment, building structure, materials used, and safety standards, a flood of the same depth can damage roads to different extents. For example, road damage in the Netherlands is relatively low because of the country’s high flood protection standards. On the other hand, the Western Alps in France, Switzerland, and Italy face severe risks from floods due to the motorways' location in plain unprotected valleys along rivers (Van Ginkel et al., 2021). The metro network in New York City experienced the financial impact a (minor) difference in materiality can cause: In 2012, Hurricane Sandy flooded 17 per cent of the city and nine out of 14 underground tunnels. The flood did not only cause general water damage, but the saltwater also corroded all aspects of the aged metro facilities, resulting in at least USD 5 billion of flood-damaged stations, tunnels, and electrical signalling systems (Sneider, 2016). Our physical risk models include such regional differences when calculating the potential damage and physical risk.

Extratropical and tropical storms

Storms and cyclones are used interchangeably for weather hazard events where strong winds result from a low-pressure atmosphere system.

_Table:_{Definitions of the various types of storms}
_{Classification}	_Description_{(based on NOAA, 2023)}
_{Atmospheric disturbance}	_{Refers to the circular/cyclonic wind flows and thunderstorm activities resulting from a region where the atmospheric pressure is lower than the surrounding. The wind flows lack a closed circulation.}
_{Atmospheric depression}	_{Refers to a cyclonic wind flow with a closed circulation and a wind speed under 39 miles per hour.}
_{Storm/ Cyclone}	_{Refers to a cyclonic wind flow with a closed circulation and a wind speed between 39 and 74 miles per hour. Depending on the location, the National Oceanic and Atmospheric Administration differentiates between four types of cyclones:} _{Sub-tropical: Refers to a cyclone that occurs between latitudes 25°N to 35°N} _{(or °S). Such cyclones are either asymmetrical with a warm core or symmetrical with a cold core.} _{Tropical: Refers to a cyclone that occurs between latitudes 25°S to 25°N. Tropical cyclones derive their energy from vertical temperature differences, are symmetrical, and have a warm core.} _{Extra-tropical: Refers to a cyclone that occurs between latitudes 35°N to 65°N} _{(or °S). Such cyclones obtain energy from the horizontal temperature contrasts that exist in the atmosphere and are asymmetrical with a cold core.} _{Post-tropical: Refers to a former tropical cyclone that no longer possesses sufficient characteristics to be considered a tropical cyclone, such as convection at its centre, but can continue to generate strong winds and heavy rains.}
_{Hurricane/ Typhoon}	_{Refers to a cyclonic wind flow with a closed circulation and a wind speed exceeding 74 miles per hour. Known as a hurricane in the Atlantic and East Pacific Oceans or a typhoon in the Northern West Pacific.}

To avoid confusion due to different terminologies, we use storm as the standardised term to refer to weather hazard events with strong winds of at least 74 miles per hour. If a storm occurs in latitudes between 25 degrees south and 25 degrees north, it is called a tropical storm that hits countries near the equator. Extratropical storms occur in latitudes between 35 degrees north and 65 degrees north (or the equivalent latitudes in the south) in the northern and southern hemispheres. Accordingly, the main difference in such storms is the location, and most countries would be affected by either tropical or extratropical storms. However, one exception is the Philippines, where both types of storms occur on a regular basis.

Storms typically yield both direct and indirect consequences. Direct impacts encompass the meteorological conditions accompanying the storm, including increased precipitation, escalated wind speeds, and extreme wave action. These consequences can uplift roofs and other objects and result in dangerous projectiles from loose items, further damaging assets. On the other hand, indirect consequences include subsequent phenomena from heightened meteorological activity, such as floods, landslides, coastal erosion, alterations to ecosystems, and coastal flooding (Kurniawan et al., 2021).

Thermal stress

Thermal stress, in the context of climate change, refers to the adverse impacts on living organisms and systems resulting from excessive temperature conditions. The rise in global temperatures due to climate change is projected to worsen thermal stress globally (Copernicus, 2023). Thermal stress is further classified into varying degrees of heat or cold stress.

Thermal stress’s impact is two-fold, with both short- and long-term consequences on humans and physical assets. Very high or low temperatures affect a person’s operational capacity and can lead to (chronic) health risks for workers (Cheung et al., 2016). Accordingly, international standards ensure workers' safety by defining a temperature range where people can work safely (WHO, 2011). When extreme temperatures breach such thresholds, businesses face operational losses due to employees’ inability to conduct their work (in time) and more extended absences to recover from related health issues.

Additionally, assets have physical safety and operating temperature thresholds. Extreme heat or cold cases could alter and compromise the integrity of materials and structures, resulting in safety issues. For example, rail tracks can buckle and dislodge at very high temperatures. Once certain temperature thresholds are exceeded, companies stop operations and incur additional maintenance costs to repair and replace damaged assets, further contributing to a business’ operational loss during extreme temperatures (Mulholland & Feyen, 2021).

Extreme heat stress occurs with heat wave periods that bring consistent abnormal high temperatures and – similarly to floods and storms – impacts asset classes and individual assets differently. Excessively high temperatures have the potential to disrupt air travel operations, as they can cause runway damage, and the reduced air density makes take-offs more challenging. Similarly, roads are susceptible to extreme heat. While asphalt roads tend to soften and become uneven (due to a process called rutting, where repeated heavy pressure deforms the road), concrete roads buckle as high temperature causes the material to expand (Chiu, 2022). Accordingly, asphalt roads in the United Kingdom started to sink when temperatures reached 40 degrees Celsius (ITV, 2022). On the other hand, Germany’s concrete roads buckled at air temperatures of about 30 degrees Celsius (DW, 2018). Again, it must be noted that even for assets in similar geographical regions, the associated physical risks can vary drastically due to a myriad of confounding factors, such as materials used and construction methods.

Extreme cold stress occurs with periods of consistent abnormal low temperatures. Cold snaps can wreak havoc on power stations when key transformers, electricity transmitters, and pipelines freeze (CISA, n.d.). In 2022, a winter storm and related cold froze coal and gas power plants and pipelines in the United States, which exacerbated the lack of energy supply in areas predominately reliant on gas power (Leber, 2022). The same storm also grounded 6,900 flights (Yousif, 2022), as extreme cold and snowfall can reduce visibility and the ability to land safely. Furthermore, extremely low temperatures can damage aircraft water systems and lead to other mechanical issues (CISA, n.d.).

_{Cheung, S.S., Lee, J.K.W., & Oksa, J. (2016). Thermal stress, human performance, and physical employment standards.}_{Applied Physiology, Nutrition, and Metabolism, 41}_{, S148-S164.}_{https://doi.org/10.1139/apnm-2015-0518}

_{Chiu, A. (2022, July 20). With extreme heat, we can’t build roads and railways as we used to.}_{The Washington Post}_._{https://www.washingtonpost.com/climate-environment/2022/07/20/heat-wave-road-railway-buckling/}

_{CISA (n.d.). Extreme cold.}_{Cybersecurity and Infrastructure Security Agency.}_{https://www.cisa.gov/topics/critical-infrastructure-security-and-resilience/extreme-weather-and-climate-change/extreme-cold}

_{Copernicus. (2023). How changes in thermal stress will impact lives in the future.}_{Copernicus Climate Change Service}_._{https://climate.copernicus.eu/how-changes-thermal-stress-will-impact-lives-future}

_{DW (2018, February 6). Germany’s autobahns crumble in early summer heat.}_{Deutsche Welle.}_{https://www.dw.com/en/germanys-autobahns-crumble-in-early-summer-heat/a-44050774}

_{Huizinga, J., de Moel, H., Szewczyk, W. (2017). Global flood depth-damage functions: Methodology and the database with guidelines.}_{Joint Research Centre Technical Report}_._{https://data.europa.eu/doi/10.2760/16510}

_{ITV. (2022, July 18). UK heatwave: Lincolnshire roads 'melt' as surface temperatures hit 54C.}_{ITV News}_._{https://www.itv.com/news/calendar/2022-07-18/roads-melt-under-us-as-temperatures-hit-54c}

_{Kurniawan, R., Harsa, H., Nurrahmat, M.H., Sasmito, A., Florida, N., Makmur, E., … Adrianita, F. (2021). The impact of tropical cyclone Seroja to the rainfall and sea wave height in East Nusa Tenggara.}_{IOP Conference Series: Earth and Environmental Science, 925}_{, 012049.}_{https://doi.org/10.1088/1755-1315/925/1/012049}

_{Mulholland, E., & Feyen, L. (2021). Increased risk of extreme heat to European roads and railways with global warming.}_{Climate Risk Management, 34}_{, 100365.}_{https://doi.org/10.1016/j.crm.2021.100365}

_{NOAA (2023). Hurricanes Frequently Asked Questions.}_{Atlantic Oceanographic & Meteorological Laboratory}_._{https://www.aoml.noaa.gov/hrd-faq/#what-is-a-hurricane}

_{Sneider, J. (2016, November). Hurricane Sandy: Four years later, New York City Transit is still fixing, fortifying the rail system.}_{Progressive Railroading.}_{https://www.progressiverailroading.com/passenger_rail/article/Hurricane-Sandy-Four-years-later-New-York-City-Transit-is-still-fixing-fortifying-the-rail-system--49988}

_{UNDRR. (2020). The human cost of disasters: An overview of the last 20 years (2000-2019).}_{United Nations Office for Disaster Risk Reduction}_._{https://www.undrr.org/publication/human-cost-disasters-overview-last-20-years-2000-2019}

_{Van Ginkel, K., Dottori, F., Alfieri, L., Feyen, L., & Koks, E. (2021). Flood risk assessment of the European road network.}_{Natural Hazards and Earth System Sciences, 21}_{, 1011-1027.}_{https://doi.org/10.5194/nhess-21-1011-2021}

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_{WMO. (2023, May 22). Atlas of mortality and economic losses from weather, climate and water-related hazards (1970-2021).}_{World Meteorological Organization}_._{https://public.wmo.int/en/resources/atlas-of-mortality}

_{Yousif, N. (2022, December 23). Deadly winter storm knocks out power for 1.5m in US and Canada.}_{BBC News}_._{https://www.bbc.com/news/world-us-canada-64061588}