12 Topics from Chapter 12: Climate Change Information for Regional Impact and for Risk Assessment

Katherine Sievers

Introduction: Chapter 12 of the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6) consolidates core physical knowledge from IPCC AR5 and recent literature concerning global and regional risk from shifts in climactic conditions. This chapter uses climate impact-drivers (CIDs) to assess changing climate impacts on sector risks. CIDs are physical climate system conditions generalized in system means, number of events, or extremes that directly impact elements of society and ecosystems. Examples include mean air temperature, number of storm events, or extreme levels of drought. CIDs and their impacts are region specific. The impacts in society and ecosystems can be detrimental, beneficial, neutral, or a combination across interacting system elements.[1]

Project Objective: Explain risks to systems from CIDs due to changes in water quantity and water quality using concepts from the course Fundamentals of Climate Change (ATM S 487/587) at the University of Washington. Our group chose to discuss CIDs most likely to impact human health.

Reference: This paper uses Chapter 12 from the IPCC AR6 Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. The author will refer to the report as IPCC AR6 in this paper. Footnotes will direct you to the applicable section in Chapter 12. At the time of writing the report was not finalized and this paper will therefore refer only to section headers, not page numbers.

Notes: The IPCC has different levels of confidence, which aggregate strength of evidence and model agreement, for the processes described in this paper. Levels of confidence are not specified, please refer to the main report if you have questions about the levels of confidence for a specific region or process. CIDs are highlighted in blue when relevant.

Water Quantity: Climate change is projected to shift both the average amount and temporal distribution of water released from the atmosphere and stored across the globe. This section discusses three CIDs, flooding, aridity and drought. These processes described are not all inclusive and vary widely across regions. If you have specific questions about a region, please refer to the regional information on changing climate in Chapter 12.[2]

Water Distribution

Flooding

Many regions of the world are expected to experience increases in the severity of their floods. Flooding can occur in all regions of the world, regardless of annual average precipitation. Floods have a variety of impacts including direct loss of life through drowning, cause an increase in vector borne disease, damage infrastructure, lower crop yields through water logging and change certain pathogen distributions.

CID: Fluvial floods (river floods) occur when the water level in a river, lake or stream rises and overflows onto the surrounding banks, shores, and neighboring land. The severity of a river flood is primarily determined by the duration and intensity (volume) of rainfall in the catchment area. Rainfall intensity is expected to increase with climate change.

Explanation: The increase in the total volume of rain per event is primarily driven by the relationship between humidity and temperature. The saturation specific humidity (qs) is the maximum amount of water that can exist in air before saturation occurs. Precipitation occurs when the ambient humidity equals the saturation specific humidity. As temperature increases, the air’s capacity to hold water (q_s) increases. Equation 1 defines the saturation specific humidity, where p is pressure, L_v is the latent heat of vaporization (2.56e6 J/kg), R_v is the gas constant for water vapor (461.5 J/kg/K) and T is temperature. Subscripts 0 indicate values at T equals 0°C, q_s0 equals 3.8 grams of water per kilogram of air, and p₀ equals 1000 hPa.

Equation 1                 

The increase is in capacity is close to exponential, with a growth rate of 7% increase in humidity per degree Celsius. This means as the temperature increases, there needs to be more water in the air for precipitation to occur. Therefore, when it rains in a warmer world, the total amount of rain per event is likely to be more. Globally precipitation is expected to increase by about 2% per degree warming. For the strongest precipitation events, the rain increase can be up to 7% per degree warming or more.

CID: Pluvial floods (flash floods) occur when a extreme rainfall event creates a flood independent of an overflowing body of water or when a storm’s volume of rainfall exceeds the infrastructure’s ability to handle the rate of water accumulation in the system. Pluvial floods are driven by extreme precipitation, topography and land use characteristics.

Explanation: In addition to increased precipitation discussed above, pluvial floods are projected to increase in intensity due to the way infrastructure was developed. Floods are often described by terminology such as a “once in a 100 year flood.” This is abbreviated into a 1:100 yr event. Many institutions rely on correct projections of events, for example, building is prohibited in areas with frequent flooding, insurance prices are calculated based on the probability of flooding, and stormwater infrastructure is often built to manage run off from a 1:100 yr event[3]. Models used in the IPCC project the present day 1:100 yr event may shift to between a 1:30 yr event (RCP 4.5) or a 1:20 yr event (RCP 8.5) by 2050, and a 1:5 year event (RCP 4.5) or more than once a year (RCP 8.5) by 2100.[4] This means historical and current infrastructure capacities are not prepared to handle future rainfall events intensified by climate change.

Pluvial flooding may also increase in arid areas or areas experiencing drought due to the intensity of rain events and changing soil characteristics. Many arid areas experience infrequent, intense rainfall events. Depending on the soil characteristics and the surface’s ability to absorb water, these events may cause an increase in overland flow. Increased flood intensity is expected in these areas due to climate change. The causes of drought and increased aridity are discussed below.

CID: Coastal floods occur when land areas are inundated by seawater. Coastal floods are most commonly caused by higher than normal tides, but can also result from storm surges or tsunamis. Coastal floods are often exacerbated by storms and can occur with fluvial floods.[5] Coastal flooding is expected to increase due to rising sea levels associated with climate change.

Explanation: Tides occur in coastal areas due to gravitational forces exerted by the moon on the earth. The absolute height of the tide relative to a fixed point on land is caused by a combination of forces in the ocean and the sea level. Sea levels are expected to rise as the global average mean air temperature increases, and therefore tides are expected to increase relative to their current averages. [6]

Warming temperatures cause thermal expansion of sea water and melting of land ice, resulting in sea level rise. While current sea level rise is primarily driven by thermal expansion, future projections of sea level rise are primarily caused by melting of land ice from two locations, the Greenland and Antarctic ice sheet. If the Greenland ice sheet fully melted, it would contribute 7.2 meters (23.6 ft) of sea level rise and the Antarctic ice sheet would contribute 61.1 meters (200.5 ft) of sea level rise. Currently the Northern Hemisphere is warming at a faster rate than the Southern Hemisphere, which contributes to the observation of the Greenland ice sheet contributing more to sea level rise than the Antarctic ice sheet. The Antarctic ice sheet is not warming evenly, with the East Antarctic gaining a small amount of ice while the West Antarctic has contributed more melt than the East Antarctic gained. Sea level rise from the East Antarctic and Greenland ice sheets are projected to contribute at least 1.5 meters (4.9 ft) by 2100. 1.5 meters of sea level rise is enough to permanently inundate some coastal communities and increases the overall risk of flooding to coastal communities. Figure 1 (Figure 12.4 in IPCC AR6) helps visualize where changes in extreme total water level are expected under different emission scenarios.[7]

Figure 1 (to be added) – Median projected changes in total water level based on CMIP6 models (IPCC AR 6 Fig 12.4)

Aridity and Drought

Changes in aridity and drought conditions can change ecosystem processes, alter suitable climate zones, shift agriculture cultivation regions, and the distribution and prevalence of pests and disease carrying vectors. Reductions in water availability can challenge water supplies required for municipal, industrial, agricultural and hydropower usage. Droughts can lead to crop failure, increase infection rates from vector-borne diseases, increase fire risk, concentrate pollutants and decrease water available for consumption and infrastructure needs. Climate change is expected to increase drought and aridity in many areas, particularly in the mid-latitudes.[8]

CID: Aridity is a nature driven permanent imbalance in the water availability characterized by low annual precipitation or a low ratio of precipitation to evaporation. This means arid regions are dry regions. Aridity is increasing due to climate change, particularly in the mid-latitudes, and causing large changes in water availability and ecosystems relative to historical norms.[9]  Droughts occur when there is a prolonged period of abnormally low precipitation. Droughts can be exacerbated through an increase in aridity.

Explanation: Aridity and droughts can best be understood through the moisture budget in the equation 2 below, where P is precipitation, E is evaporation, and C is moisture convergence. Moisture convergence can be thought of as moisture transport via air currents.

Equation 2                

Globally, precipitation must equal evaporation. However, some regions are wetter than others due to moisture transport from one region to another. Climate change impacts the magnitude and distribution of each term in the equation.

Evaporation is directly influenced by temperature. As temperature increases, the air’s capacity to hold water increases and causes more evaporation from the surface. The increase in capacity is close to exponential, with a growth rate of 7% increase in humidity per degree Celsius, yet global precipitation is limited by energetics and is only increasing by about 2% per degree. This means as temperature increases, more water will be pulled from the surface and stored in the atmosphere. Total evaporation will increase everywhere temperature increases. In arid environments, where precipitation is less than evaporation, this means less water is stored on the surface and soil moisture decreases. Figure 2 (Figure 12.4 (j-l) in AR6) helps visualize where soil moisture is increasing and decreasing. It is important to note the scale in figure 12.4 is percent change which makes some very dry areas appear to be experiencing a large increase in moisture, when the absolute change is quite small.[10]

Figure 2 (to be added) – Median projected changes in soil moisture based on CMIP6 models (IPCC AR 6 Fig 12.4)

Moisture convergence patterns can lead to an increase or decrease of moisture in an area. Hadley cells drive moisture convergence patterns near the equator and mid-latitudes. Hadley cells are characterized by warm, moist air rising at the equator and cool dry air falling at the mid latitudes (roughly 30N and 30S). The dry air then pulls more moisture from the surface and convects moisture towards the equator. Rain is caused by rising motion, so these patterns of convection explain why more rain occurs at the equator and less rain occurs in the mid-latitudes. Outside of this region, moisture is transported poleward from the mid latitudes. Precipitation at higher latitudes is partially explained by storm tracks, which are convergence regions influenced by the jet streams. Precipitation occurs towards the poleward side of storm tracks and drying occurs on the equatorward side. Poleward precipitation is also driven through eddies, which are caused by mixing of the air in the subtropics with the high latitude air. Hadley cells are expected to grow (ie., expand above and below 30N and 30S) with climate change. As the Hadley cells expand, the jet streams and storm tracks are expected to shift poleward. This will contribute to an overall expansion of arid regions in the mid-latitudes.

Patterns of precipitation are expected to change with climate change, leading to less frequent, but more intense events. Models project the number of severe droughts to increase in some areas, and to decrease in others. Figure 3 (Figure 12.4 (g-i) in IPCC AR6) helps visualize the projected change in number of severe droughts per decade.[11]

Figure 3 (to be added) – Median projected changes in number of droughts based on CMIP6 models (IPCC AR 6 Fig 12.4)

Many arid regions currently rely on groundwater as their primary resource. Groundwater recharge occurs in specific zones due to precipitation or the presence of surface water. These zones can be hundreds of miles from where the water is being extracted. The age of groundwater can be relatively young in shallow unconfined aquifers, or up to thousands of years old in deep, confined aquifers. Extraction of groundwater in many arid areas exceeds the current recharge values and this imbalance will be exacerbated with longer periods of drought.

Water Quality: Climate change is projected to impact water quality through a variety of mechanisms. This section discusses two CIDs, changes in dissolved oxygen and saltwater intrusion. These are not all inclusive and vary widely across regions. If you have specific questions about a region, please refer to the regional information on changing climate in Chapter 12.[12]

Changes in Dissolved Oxygen

Dissolved oxygen is a critical component of aquatic ecosystems and is expected to decrease as global temperatures increase. This could impact a wide range of aquatic organisms, habitats, exacerbate anoxic “dead zones” and cause an expansion of oxygen minimum zones in the open.[13] Some of the processes that deplete oxygen also result in poor drinking water quality and can indicate contamination. These impacts may result in decreased food supply in areas relying on fisheries and unsafe surface water.

CID: Decreases in dissolved oxygen are projected and have been observed globally due to a rise in the mean air temperature.

Explanation: In an ideal system the concentration of any gas in water, absent a chemical or biological reaction, is governed by Henry’s law. Henry’s law states that the concentration of a dissolved gas is proportional to a gas’s partial pressure. Henry’s Law is below in Equation 3 where C_i is the dissolved concentration of the gas, P_i is the partial pressure of the gas and k is Henry’s constant.

Equation 3                 

The partial pressure of a gas is related to its gaseous concentration, so when the atmospheric pressure equals 1 atm and the atmosphere is comprised of 21% oxygen, the partial pressure equals 0.21 atm. The percentage of oxygen in the atmosphere is not expected to change significantly with climate change. However, Henry’s constant, k, is highly dependent on temperature. As temperatures increase, Henry’s constant decreases. This means as atmospheric temperatures increase due to climate change, average dissolved oxygen levels will decrease. There are many other processes that influence dissolved oxygen, but this relationship is helpful to understand water’s capacity to hold dissolved oxygen with increasing temperature.

CID: A combination of other factors including drought, flooding and eutrophication can contribute to a decrease in dissolved oxygen. Eutrophication occurs when a body of water has excess nutrients that promotes rapid growth of organisms. Drought concentrates water constituents, including excess nutrients, exacerbating eutrophication. [14] Flooding can provide a rapid impulse of nutrients to a system.

Explanation: Excess nutrients in an aquatic system cause an increased growth rate of plants and organisms, most commonly algae. Plants and algae begin to grow rapidly in a system, this stage is often referred to as an “algal bloom.” The growth begins to create a physical barrier to sunlight. In the absence of sunlight, or as nutrients become limiting, the plants and algae begin to die off. The decomposition of organisms by bacteria requires oxygen and leads to a rapid decrease in dissolved oxygen. Nutrients enter systems through a variety of natural and anthropogenic mechanisms that are exacerbated by climate change. Fertilizer run-off from agricultural practices commonly enters surface water sources when it rains. The more intense the storm, the more likely the water will reach a water way instead of being absorbed in the soil. As storm intensity increases with climate change, more agricultural run-off is possible.

Nutrients also enter systems when flooding overwhelms infrastructure. For example, many sewer systems are combined stormwater and wastewater systems. Figure 4 shows a combined sewer system. During dry weather, sewage is routed to a wastewater treatment plant (Publicly Owned Treatment Works (POTW) in the image below), however, when a precipitation event occurs that exceeds the capacity of the system, the combined sewage and stormwater are designed to run off into a body of water. This can cause eutrophication by adding nutrients to the system as well as introducing pathogens to the environment. Many sewage systems were designed to only overflow when a certain storm intensity occurred, like a 1:5 year event. As climate change increases the frequency of these precipitation events, release from combined sewage systems will increase.

Figure 4 – Combined Sewer System [15]

Saltwater Intrusion

Many coastal communities rely on shallow, unconfined aquifers for their primary water source. Saltwater intrusion occurs when ocean water begins to mix with the freshwater and contaminate aquifers and ecosystems. Saltwater intrusion can have significant impacts on water resources and agriculture.[16] This may pose an especially serious risk for island nations who have no alternate, reliable, water source.

CID: Saltwater intrusion of coastal aquifers is expected to increase with rising sea levels.

Explanation: There are two primary types of groundwater systems, confined and unconfined aquifers. An aquifer is an area below the land space saturated with water, meaning all the pore spaces in the earth are filled with water. A confined aquifer occurs when the saturated media is between layers of impermeable media, such as clay. A confined aquifer is under pressure, so when a well is drilled, water will begin to rise to the surface based on pressure gradients. An unconfined aquifer is not bound by an upper impermeable layer and is at atmospheric pressure. Flow in an unconfined aquifer is based solely on water elevation. Unconfined aquifers are closer to the earth’s surface and in coastal communities these aquifers may have contact with saltwater. The freshwater floats atop the salt water due to differences in density. Saltwater is on average 1.04 times as dense as freshwater. As sea levels rise, the saltwater system gains in elevation and may cause flow into the aquifer’s previous area, leading to saltwater intrusion. This process can also be exacerbated when communities withdraw more water from the system than is recharged.

References

EPA. “Impacts and Control of CSOs and SSO. Document No. EPA 833-R-04-001.” 2004.

NOAA. Severe Weather 101 – Floods. n.d. <https://www.nssl.noaa.gov/education/svrwx101/floods/types/>.

Consequences to Agriculture and Human Health

Kristin Hayman

Objective

Chapter 12 summarizes climate impact-drivers (CIDs), which the IPCC defines as climate conditions that directly affect elements of society or ecosystems and can lead to positive, negative, or inconsequential outcomes.[1] Chapter 12 explains heat and cold, wet and dry, wind, snow and ice, coastal, oceanic, and other CIDs. The goal of this report is to provide information related to the fundamentals of climate change to explain how climate change will cause the CIDs that are expected to have significant consequences for agriculture and, by extension, human health. Along the way, this section will summarize some of the ways in which CIDs are expected to impact human health by impacting agriculture.

Mean Air Temperature

Section 12.3.1.1

The Earth gains energy only through shortwave radiation from the Sun (known as insolation or incoming solar radiation) and loses energy only through longwave radiation.[2] The state of equilibrium in which insolation is equal to outgoing longwave radiation is called “energy balance.” Burning of fossil fuels, deforestation, cement production, refrigerants, electricity transmission, production and application of nitrogen fertilizers, and many other activities[3] release trace gases known as greenhouse gases[4], into the atmosphere. These small, polyatomic molecules make up only a small fraction of the mass of the atmosphere but are very effective at trapping radiation and heating the Earth[5]. As such, increases in the atmospheric concentration of greenhouse gases enhances the atmosphere’s absorption of shortwave radiation and inhibits loss of longwave radiation from the Earth’s atmosphere. As such, the Earth gains more energy than it loses, causing a positive radiative forcing [6] that results in global warming[7].

The IPCC’s AR6 recognizes that the increase in mean air temperature associated with global warming dictates many aspects of crop cultivation, including crop productivity and suitability of cultivation zones for crop species. The report specifically notes that, in some cases, an early season warm spell may reduce plant hardiness or induce fruit tree flowering that exposes plants to devastating subsequent frost impacts. Furthermore, AR6 ties decreases in livestock production, agroforestry, and output from freshwater aquaculture and fisheries, as well as increases the potential for food contamination, food-borne diseases, spoilage, and waste to increases in mean air temperature.

12.3.2.1. Precipitation

Section 12.3.2.1

            Currently, there is some uncertainty about how precipitation will change in response to continued global warming. However, there is relative confidence that global warming will cause regions that are currently wet to get wetter, and dry regions will persist, expand poleward, and, in some cases get drier.

Regions that are currently wet are expected to get wetter with global warming because saturation vapor pressure, defined as how much water vapor can exist in the air before condensation occurs, will increase with global warming. Specifically, air can hold about 7% more water vapor for every 1 degree Celsius increase in temperature.[8] Because precipitation is the result of water vapor and rising motion, precipitation will increase (for a given amount of rising motion) when there is more water vapor in the air.[9] Global circulation patterns will cause this precipitation to occur in regions that currently experience significant precipitation, like the tropics[10] and even high-to-mid latitudes[11], as well as where monsoons and storm tracks occur.

The IPCC’s AR6 recognizes that the increase in precipitation associated with climate change will cause wetter growing season conditions, which increase the chance of water logging and, thus, can damage planted seeds or delay planting. The IPCC also recognizes that wetter conditions may increase canopy moisture, creating environments that are more conducive to crop pathogens. Finally, the IPCC AR6 reports that “the proportion of total precipitation that falls in heavy events also affects the percentage that is retained in the soil column, altering groundwater recharge and deep soil moisture content for agricultural use.”

Drought

Sections 12.3.2.5 & 12.3.2.6

In a warmer climate, droughts will be more frequent[12] because potential evaporation (PET)[13] will increase and precipitation will decrease in drier places. Potential evaporation is defined as the amount of evaporation that would occur under given atmospheric and radiative conditions if water were available.[14] Potential evaporation is determined by radiative energy input[15] and the drying capacity of air[16]. Radiative energy input is largely a function of shortwave radiation, and the drying capacity of air is a function of wind speed and vapor deficit, with the latter being equal to the saturation vapor pressure multiplied by the (positive) difference between one and the relative humidity[17].[18] In a warmer climate, saturation vapor pressure will increase because warmer air can hold more moisture, as stated by the Clausius-Clapeyron equation. Likewise, the energy content of water molecules will increase, simply because molecules have more energy at higher temperatures. Therefore, the increase in mean air temperature associated with global warming will increase potential evaporation.

In a warmer climate, one reason that precipitation will generally decrease in places that are already dry is that the Hadley cells will expand poleward, as they have been doing for decades. Causing a poleward shift of the jet stream[19] that will push mid and extratropical storm tracks poleward. Expansion of Hadley cells and the poleward shift of the jet stream will flux moisture away from the subtropics, thus, contributing to decreased precipitation in regions that are already relatively dry.

The IPCC’s AR6 recognizes that many agricultural systems require minimum rainfall totals. As such, increases in drought frequency or severity (caused by the increase in potential evaporation and the decrease in precipitation in relatively dry regions) may lead to crop failure, particularly when droughts persist for extended periods or occur during key stages of plant development. Finally, the IPCC reports that the changes in soil moisture and surface water can alter suitable climate zones for agricultural cultivation, pests, and pathogen-carrying vectors.

Flooding

Sections 12.3.2.2 & 12.3.2.3

The IPCC projects that, as climate change becomes increasingly severe, flooding will become more frequent and severe.[20] Specifically, increased precipitation in regions that already experience high rainfall, as well as increases in the frequency and severity of heavy rainfall events (as described previously) contribute to flooding. Additionally, drier soils that result from increased potential evaporation (also described previously) increase the likelihood and severity of flooding, because dry soils are often compacted and, thus, unable to readily absorb water.

The IPCC reports that floods can knock down, drown, or wash away crops and livestock. Additionally, the IPCC’s AR6 recognizes that partially submerged plants can have reduced yields, depending on water turbidity and the stage of development.

Consequences to Human Health

Sections Section 12.3.1.1, 12.3.2.1, 12.3.2.2, 12.3.2.3, 12.3.2.5 & 12.3.2.6

Because climate change will impact agriculture, it will significantly impact human health. Specifically, increases in mean air temperature, changes in surface water, and changes in groundwater recharge and deep soil moisture content caused by increases in heavy rain events will alter the suitability of cultivation zones for crop species. In response, farmers may have to switch crops or relocate crop production. This presumably stressful and taxing process will likely take a significant toll on farmers’ mental and physical health. More broadly, changes to the suitability of cultivation zones for crop species will alter availability of certain foods, potentially forcing some groups of people to transition from consuming traditional foods to eating less nutrient-dense, processed foods. It is likely that this will more severely impact the health of more vulnerable communities, like Indigenous communities.

In addition, changes in mean air temperature and the hydrologic cycle are expected to decrease crop productivity, livestock production, agroforestry, and output from freshwater aquaculture and fisheries. For example, early season warm spells may reduce plant hardiness or induce fruit tree flowering that may expose plants to devastating subsequent frost impacts. Likewise, waterlogging from increased precipitation can damage planted seeds or delay planting. The IPCC’s AR6 also recognizes that many agricultural systems require minimum rainfall totals and, as such, increases in drought frequency or severity may lead to crop failure, particularly when droughts persist for extended periods or occur during key stages of plant development. The IPCC reports that floods can knock down, drown, or wash away crops and livestock. Additionally, the IPCC’s AR6 recognizes that partially submerged plants can have reduced yields, depending on water turbidity and the stage of development. These decreases to crop productivity, livestock production, agroforestry, and output from freshwater aquaculture and fisheries may create or exacerbate food shortages, famine, and malnutrition.

Climate change is also expected to increase the potential for food contamination and food-borne diseases. Specifically, the IPCC recognizes that wetter conditions may increase canopy moisture and, thus, create environments that are more conducive to crop pathogens. The IPCC’s AR6 also recognizes that the changes in soil moisture and surface water can alter suitable climate zones for agricultural cultivation, pests, and pathogen-carrying vectors. Increases in food contamination and food-borne diseases may cause an increase food-borne illnesses and death.

Footnotes

Infrastructure

Annie Doubleday 

Infrastructure, including transportation, buildings, housing, cities, and energy infrastructure, will be impacted by climate change in several ways. Three key characteristics of climate change – warming, increased precipitation, and drought – account for the majority of expected impacts. The goal of this section is to elucidate the basics of the climate science behind these impacts, and discuss the types of impacts that can be expected. The level of risk and confidence in the expected impacts varies by geographic region. For further details, please refer to IPCC AR6 WGI Chapter 12.4. The content covered in this section is shown in table 12.2 under the Cities, settlements, and key infrastructure section, which is also covered further in Chapter 6 of the report.

Heat

Climate impacts are fundamentally driven by a warming of the planet caused by an imbalance in earth’s energy budget. Earth gains energy through shortwave radiation, or incoming solar radiation (insolation) from the sun, and loses energy through outgoing longwave radiation. Earth’s energy is balanced because outgoing longwave radiation is equal to the incoming solar radiation. However, since industrialization, there has been a rapid uptick in emissions of heat-trapping gases, or greenhouse gases (GHGs), including carbon dioxide (CO₂) and methane, that create an imbalance in earth’s energy. GHGs come from a variety of processes, including coal burned for electricity, primarily for industrial use; oil, used primarily for transportation; methane, used for heating, electricity, and industry; cement production; and land use. GHGs trap some of the outgoing longwave radiation and absorb some of the incoming shortwave radiation, resulting in an energy imbalance. This leads to a positive radiative forcing that results in an overall increase in the global average temperature. GHGs exist naturally in the atmosphere; however, human activity has caused a significant increase in GHG emissions, and are responsible for the warming effect.

As a result of the rapid increase in human-produced greenhouse gases, the global average temperature increases over time in order to balance the earth’s total energy. This increase in global average temperature has a series of cascading effects on infrastructure (Figure 12.3). For example, the overall warming impacts buildings and other built infrastructure in areas where there is ice-rich permafrost (soil that is frozen year-round), such as in northern latitudes (section 12.3.4.2). Ice expands upon freezing; thus, upon thawing due to warmer temperatures, potholes and sinkholes can form, causing damage to roadways, buildings, and general infrastructure.

In addition, warming results in more frequent high daytime and nighttime temperatures. The annual mean number of days with high daytime temperatures exceeding 35 degrees C is expected to increase (Figure 12.4a-c and d-f). Extreme heat events are magnified in urban environments due to the urban heat island effect (section 12.3.1.2). The urban heat island effect is the phenomenon where temperature is magnified in urban areas due to the high prevalence of pavement covering the earth. This pavement radiates heat and increases the temperature in urban areas relative to what it would be without pavement. Extreme heat can cause disruptions in infrastructure not engineered for high heat (Figure 12.3). For example, extreme heat can result in buckling of roadways, malfunctioning of public transit systems, and decreases in the efficiency of energy transmission lines.

Water

An increase in global average temperature due to an increase in heat trapping gases in the atmosphere has major impacts on the global hydrologic cycle. The saturation vapor pressure, or the amount of water vapor that can exist in air, increases with temperature. For each 1-degree Celsius increase in air temperature, there is a 7% increase in water vapor in the air. However, this does not result in a 7% increase in precipitation. Instead, there is about a 2% increase in global precipitation per 1 degree Celsius of warming. This slower increase in precipitation relative to the increase in water vapor in air is due to constraints on global energetics: precipitation releases heat (due to condensation of water from gas to liquid). For this to occur, the atmosphere must also be able to shed that heat. However, the total energy available in the atmosphere is constrained and thus, the atmosphere cannot cool quickly. The increase in global precipitation cannot keep up with the increase in water vapor.

One result of the increase in water vapor in the air is an increase in severe storms and heavy precipitation events. This includes tropical cyclones, hurricanes, and rain storms (sections 12.3.2.3, 12.3.3.2, 12.3.3.3). In other words, the presence of more water vapor provides more fuel for large storms, such as hurricanes and cyclones. Thus, these storms become more powerful and more destructive. Increasingly large and powerful coastal storms can severely impact coastal communities. Coastal storms can cause erosion and flooding in coastal communities, threatening critical infrastructure in these areas (sections 12.3.5.2 and 12.3.5.3). This includes major damages to buildings, housing, and transportation, as well as major disruption to energy infrastructure, wastewater treatment plants, and electricity and phone networks.

Increased water vapor and heavy precipitation events can also result in flooding, and in particular, more severe flooding, often as a result of major storms (sections 12.3.2.2 and 12.3.2.3). Flooding impacts infrastructure and transportation, particularly in urban areas, as well as in coastal and low-lying communities. Flooding can cause damage to buildings, block transportation routes, damage housing, and damage energy infrastructure. Flooding can also cut off coastal and low-lying communities from receiving goods and services if major roadways and trucking routes are not passable. Flooding may also impact delivery of electricity to impacted communities.

Major precipitation events marked by flooding often include landslides, mudslides or rock falls (section 12.3.2.4). With a heavy precipitation event, or series of precipitation events, the soil absorbs more moisture. However, at a certain point, the soil can no longer absorb more water. This is particularly the case in urban areas with a high proportion of land covered by pavement. Highly saturated soil and flooding can destabilize soil, particularly in areas without significant trees to aid in stabilizing the soil. This can then result in landslides. Landslides near buildings, roads, and other infrastructure can result in irreparable damage. Destabilized soil from significant rainfall events can be devastating for coastal communities in particular, with coastal erosion resulting in loss of land, buildings, and infrastructure into the ocean.

An increase in water vapor and precipitation can also lead to a change in the frequency and size distribution of hail (section 12.3.4.5). Large hail is one way in which heavy precipitation events can manifest. Large hail can cause a significant amount of damage, including severe impacts on buildings, transportation infrastructure, vehicles, solar panels, and other energy infrastructure.

Another result of both an increase in the global average temperature and increased precipitation is a shift in the season and frequency of ice and snow-related events (section 12.3.4.4). In particular, an increase in global average temperature leads to a later start of snowfall in high latitude regions, and thus, a later or more variable start to the avalanche season (section 12.3.4.6). The change in season, predictability, severity, and frequency of avalanches will negatively impact nearby roadways by making them impassable at unpredictable and different times. This may damage nearby buildings and energy infrastructure. An increase in global average temperature may also lead to wetter and thus heavier snow, or to more mixed precipitation events. Heavier snow can increase the uncertainty, severity, and frequency of avalanches in areas with mountains and seasonal snowfall. Additionally, snow followed by rain followed by freezing temperatures can create highly unsafe and unpredictable mountain conditions. This can impact infrastructure in these areas through the creation of potholes and sinkholes, destabilizing buildings, and damaging pipelines and energy infrastructure.

Dry

An increase in the saturation vapor pressure and the water vapor content of air with increased warming also leads to an increase in evaporation. As the air warms, due to an increase in heat-trapping gases, the amount of water vapor it can hold increases significantly. Thus, the air can take more water out of the land. Increased evaporation of water from land into the atmosphere leads to a drying out of land surfaces. The potential evaporation of air is the amount of water that would evaporate if water were present. Potential evaporation is greater in drier areas, as they do not have a lot of moisture available; thus, their potential to hold water in air is greater.

As some areas become wetter, other areas become more arid. To understand this, we consider the earth’s moisture budget, where precipitation (P) must equal evaporation (E) globally (P = E). Thus, as some areas become wetter due to an increase in water vapor in the air with warming, some areas also dry more. Drying occurs particularly in areas that are already dry, around 30 degrees latitude, N and S. These particular areas are dry due to the Hadley cells. Hadley cells are responsible for moving moisture towards the equator. Air rises above the warmest ocean surface, near the equator. This brings moisture toward the equator and pulls moisture away from the major global deserts around 30 degrees N and S. A consequence of climate change is the widening of the Hadley cells, increasing the region that experiences dry conditions.

Drying out of land, particularly in areas that are already dry, results in drier soil. Thus, sand and dust can be more easily disturbed, and more easily travel with wind. Sand and dust storms can create problems for transportation infrastructure, mechanical equipment, and buildings (section 12.3.3.4). Further, arid land leads to less water available for municipal use, which can lead to water shortages, for both drinking water as well as for use in hydropower, a component of the energy infrastructure in many regions (section 12.3.2.5). Additionally, drought conditions can lead to water quality problems for drinking water. Hydrological drought is typically characterized by a lower flow of water and warmer water, which often has a higher concentration of pollutants and lower dissolved oxygen. This impacts drinking water quality for municipal use (section 12.3.2.6).

Infrastructure and health

The impacts discussed in this section focus on climate impacts to infrastructure. Infrastructure, including buildings, transportation, and energy infrastructure, is directly related to human health. Human health encompasses morbidity and mortality as well as access to food, transportation, safe places to live and work, and electricity to live a healthy life. Damaged infrastructure from climate impacts, as discussed above, can have significant negative consequences to human health through lack of access to basic necessities.

Air Quality

Air quality is, and will continue to be, impacted by climate change. The three main drivers of worsening air quality due to climate change are dust, pollen, and smoke from fires. The goal of this section is to convey the climate science behind these impacts, and discuss air quality impacts on human health. The level of risk and confidence in the expected impacts varies by geographic region. For further details, please refer to IPCC AR6 WGI Chapter 12.4.

One key impact on air quality is increased dust from dust storms (section 12.3.3.4). Warming of the atmosphere due to an increase in heat trapping gases increases the water vapor content of the atmosphere, as warmer air holds 7% more water vapor per 1 degree Celsius of warming. This means there is more evaporation, as the air can essentially hold more water vapor. This is also conveyed through the global moisture budget, where total precipitation (P) must equal total evaporation (E) globally (P=E). Increased evaporation leads to drying out of soil, making the top soil easier to transport. This leads to an increase in dust storms in arid regions, which worsens air quality. More dust storms can also redistribute soil-based fungus associated with Valley Fever, and can change the dispersal and distribution of pollen.

Pollen is impacted by an increase in mean air temperature (sections 12.3.1.1 and 12.3.7.1). An increase in greenhouse gases in the atmosphere from human activity traps some of the outgoing longwave radiation, causing a positive radiative forcing, which results in an increase in the global average temperature. This warming leads to changing seasonal patterns, which vary regionally. In many regions, increased temperatures lengthen allergy season, resulting in more pollen present in the air, and thus worse impacts on seasonal allergies. This leads to increased morbidity from seasonal allergies.

A final impact on air quality due to climate change is increased smoke from wildland fires. As noted previously, water vapor in air increases by 7% for each degree Celsius of warming. This increased water vapor in air translates to an increase in evaporation, resulting in a drying out of soil. Further, an increase in global average temperature results in more hot days (Figure 12.4a-c and d-f); this also contributes to the drying out of forests. Dry forests provide fuel for large wildland fires. These climate change impacts, in combination with several anthropogenic factors (e.g., poor forest management, an increase in development on the wildland-urban interface) increase the frequency and severity of wildland fires. Larger and more frequent wildfires emit more smoke, which has a large negative impact on local, regional, and often, continental air quality, with documented impacts on morbidity and mortality (section 12.3.2.8).

Heat impacts

Climate change directly impacts global temperature. This has direct consequences on human morbidity and mortality. The goal of this section is to convey the climate science behind these impacts, and discuss the specific impacts of heat on human health. The level of risk and confidence in the expected impacts varies by geographic region. For further details, please refer to IPCC AR6 WGI Chapter 12.4.

An increase in greenhouse gases results in an energy imbalance due to the trapping of outgoing longwave radiation. This causes a positive radiative forcing that results in an increase in the global average temperature as well as more frequent extreme heat events (Figure 12.4a-c and d-f).

Warmer air also holds more water vapor, with a 7% increase in water vapor per one degree Celsius of warming. This results in more hot and humid heat events. Heat events with high humidity can be deadlier than dry heat events. This is in part due to an increase in nighttime temperatures, that can be attributed to humid air (more water vapor in air) holding heat in at night. During extreme heat events, with higher nighttime temperatures, the body cannot easily cool down overnight, and thus cannot easily get relief from heat stress. Extreme heat events see an increase in hospitalizations, and are dangerous for outdoor workers in particular. Extreme heat also worsens other chronic conditions, including asthma and other respiratory conditions (section 12.3.1.2).

References:

Ranasinghe, R., A.C. Ruane, R. Vautard, N. Arnell, E. Coppola, F.A. Cruz, S. Dessai, A.S. Islam, M. Rahimi, D. Ruiz Carrascal, J. Sillmann, M.B. Sylla, C. Tebaldi, W. Wang, and R. Zaaboul, 2021: Climate Change Information for Regional Impact and for Risk Assessment. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press.