Extreme Heat

Extreme heat, or called "hot extremes" is a period of unusually high temperatures that exceed historical norms for a specific location. In most of the United States, it is defined as high heat and humidity with temperatures above 90 degrees lasting for at least two to three days. In extreme heat your body works extra hard to maintain a normal temperature, which can lead to death. Extreme heat is responsible for the highest number of annual deaths among all weather-related hazards.

Extreme Heat (Source: The Conversation)

Changes in extreme heat with global warming

The frequency and intensity of hot extremes (including heatwaves) have increased since 1950 both globally and at regional scale, with more than 80% of AR6 regions showing similar changes. This trend is expected to persist as global warming continues. The IPCC AR6 report summarizes the projected changes in extreme heat with ongoing global warming, stating: "The frequency and intensity of hot extremes will continue to increase at global and continental scales and in nearly all inhabited regions with increasing global warming levels. This will be the case even if global warming is stabilized at 1.5°C. Relative to present-day conditions, changes in the intensity of extremes would be at least double at 2°C, and quadruple at 3°C of global warming, compared to changes at 1.5°C of global warming. The number of hot days and hot nights and the length, frequency, and/or intensity of warm spells or heatwaves will increase over most land areas. In most regions, future changes in the intensity of temperature extremes will very likely be proportional to changes in global warming, and up to two to three times larger. The highest increase of temperature of hottest days is projected in some mid-latitude and semi-arid regions and in the South American Monsoon region, at about 1.5 times to twice the rate of global warming (high confidence). The highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming (high confidence). The frequency of hot temperature extreme events will very likely increase nonlinearly with increasing global warming, with larger percentage increases for rarer events."

Impacts of extreme heat

Extreme heat events, such as heat waves can cause large economic loss via reducing employee's productivity, increasing hospital visits, reducing crop yields, stressing livestocks, and straining infrastructure. For example, the European Environment Agency (EEA) estimates that, between 1980 and 2000, heat waves in 32 European countries cost up to $70 billion euros. The total estimated damages attributed to heatwaves of 2003, 2010, 2015, and 2018 amounted to 0.3–0.5% of European GDP.

Health

• Heat is an important environmental and occupational health hazard. Heat stress is the leading cause of weather-related deaths and can exacerbate underlying illnesses including cardiovascular disease, diabetes, mental health, asthma, and can increase the risk of accidents and transmission of some infectious diseases. Heatstroke is a medical emergency with a high-case fatality rate. Between 1998 and 2017, more than 166,000 people died as a result of heat waves. In Europe, heatwaves accounted for about 90 percent of weather-related mortality between 1980 and 2022, the European Environment Agency (EEA) has reported.

Agriculture and Livelihood

• Extreme heat stress, often accompanied by water stress, can lead to significantly reduced crop yields or even total crop failure. High temperatures can also degrade the nutrient content of crops, diminishing the overall nutritional quality of food products. Prolonged extreme heat and drought can reduce soil fertility and, in severe cases, cause soil erosion and desertification, making the land less productive or even unsuitable for agriculture.
• For livestock, extreme heat impacts include heat stress, water scarcity, and increased vulnerability to diseases and infections. These factors can lead to reduced feed intake, lower milk production in dairy cows, decreased weight gain, reduced reproductive performance, and in severe cases, death.

Infrastructure

• Direct quote from IPCC AR6: "Extreme heat events raise temperatures in buildings and cities already warmed by the urban heat island effect and can induce disruptions in critical infrastructure networks. Heat affects transportation infrastructure by warping roads and airport runways or buckling railways, and high temperatures reduce air density leading to aircraft take-off weight restrictions. Heat extremes increase peak cooling demand and challenge transmission and transformer capacity and may cause transmission lines to sag or fail. Thermal and nuclear electricity plants may be challenged when using warmer river waters for cooling or when mixing waste waters back into waterways without causing ecosystem impacts. Extreme temperature can also reduce photovoltaic panel efficiency".

Ecosystem and Biodiversity

• Direct quote from IPCC AR6: "Heat extremes factor in mortality, morbidity and the range of some thermally sensitive ecosystem species. Combined heat and drought stress can reduce forest and grassland primary productivity and even cause tree mortality at higher extremes".

Heatwaves are a significant climate risk characterized by prolonged periods of excessively hot weather, which can be detrimental to human health and comfort. The ERA5-HEAT dataset provides essential insights into thermal comfort indices that quantify human thermal stress during such events. This dataset is instrumental for research and planning in climatology, urban development, and public health initiatives.

Data

Indices of extreme heat

Impacts and risk assessments utilize a large variety of indices and approaches tailored to evaluate the impacts of extreme heat. Table 1 below listed some of the indices used.

Table 1 Heat-related indices

Indicator	Description	Data Access
Heat index	A combination of temperature and humidity to measure the conditions of human body's comfort.	Calculation
ERA5-HEAT	A complete historical reconstruction for a set of indices representing human thermal stress and discomfort in outdoor conditions. Derived from the ERA5 reanalysis by the European Centre for Medium-Range Weather Forecasts (ECMWF), it merges model data with observations to offer a consistent global climate profile from January 1940 to the present. This dataset represents the current state-of-the-art for bioclimatology data record production.	Access
Wet Bulb Globe Temperature	A useful index that measures heat stress in direct sunlight, taking many factors such as humidity, solar radiation, and wind speed into account.
Heat Risk	A color-numeric-based index that provides a forecast of the potential level of risk for heat-related impacts to occur over a 24-hour period. It utilizes both the high and low temperatures for a location and compares them to historical values at that location to classify those temperatures that are in the top 5% and above levels identified by the CDC heat-health data as excessive for that climate.	View; Access
Heat and Health Index	The Heat and Health Index is the first national tool to incorporate spatially granular heat-related illness and community characteristics data to measure extreme heat vulnerability and help communities prepare for warming temperatures in a changing climate. For more details, please refer to its technical documentation.	View and Access
Temperature Condition Index (TCI)	Using AVHRR thermal bands, TCI is used to determine stress on vegetation caused by temperatures and excessive wetness. Conditions are estimated relative to the maximum and minimum temperatures and modified to reflect different vegetation responses to temperature.	Access
Heat extreme indices used in IPCC AR6 (reproduced from Table AVI.1)	Monthly maximum value of daily maximum temperature Monthly minimum value of daily maximum temperature Monthly minimum value of daily minimum temperature Monthly maximum value of daily minimum temperature Percentage of days when daily maximum temperature is greater than the 90th percentile Percentage of days when daily maximum temperature is less than the 10th percentile Percentage of days when daily minimum temperature is greater than the 90th percentile Percentage of days when daily minimum temperature is less than the 10th percentile Number of icing days: annual count of days when TX (daily maximum temperature) <0°C Number of frost days: annual count of days when TN (daily minimum temperature) <0°C Warm spell duration index: annual count of days with at least six consecutive days when TX >90th percentile Cold spell duration index: annual count of days with at least six consecutive days when TN <10th percentile Number of summer days: annual count of days when TX (daily maximum temperature) >25°C Number of tropical nights: annual count of days when TN (daily minimum temperature) >20°C Daily temperature range: monthly mean difference between TX and TN Growing season length: annual (1 Jan to 31 Dec in Northern Hemisphere (NH), 1 July to 30 June in Southern Hemisphere (SH)) count between first span of at least six days with daily mean temperature TG >5°C and first span after July 1 (Jan 1 in SH) of six days with TG <5°C One-in-20 year return value of monthly maximum value of daily maximum temperature One-in-20 year return value of monthly minimum value of daily maximum temperature One-in-20 year return value of monthly minimum value of daily minimum temperature One-in-20 year return value of monthly maximum value of daily minimum temperature	Some of these indices are included in the Interactive Atlas of IPCC. All of them can be calculated using temperature data listed below; methods of calculation are provided in Annex VI of IPCC AR6.
HadEX3	Land-based surface climate extremes indices covering 1901 to 2018 on a 1.25° x 1.875° grid. It is produced through the coordination of the joint WMO CCl/WCRP/JCOMM Expert Team on Climate Change Detection and Indices (ETCCDI) and the WMO Expert Team on Sector-specific indices (ET-SCI). It currently comprises of over 80 indices of temperature and precipitation, including the indices used in IPCC AR6 listed above.	Access

Temperature datasets

Table 2 Datasets of temperature

Indicator	Description	Temporal Position	Data Access
Berkeley Earth Surface Temperature	Berkeley Earth provides high-resolution land and ocean time series data and gridded temperature data. It incorporates more temperature observations than other available products, and often has better coverage. Global datasets begin in 1850, with some land-only areas reported back to 1750. The newest generation of the dataset is augmented by machine learning techniques to improve the spatial resolution.	Historical	Access
GISTEMP	The GISS Surface Temperature Analysis version 4 (GISTEMP v4) is an estimate of global surface temperature change. Graphs and tables are updated around the middle of every month using current data files from NOAA GHCN v4 (meteorological stations) and ERSST v5 (ocean areas).	Historical	Access
HadCRUT5	HadCRUT is a global temperature dataset that combines the CRUTEM land surface temperature data with the HadSST sea surface temperature data. It does not involve interpolation, which results in significant coverage gaps across certain regions. It is the longest global temperature dataset, extending back to 1850.	Historical	Access
CRUTEM5	CRUTEM is a gridded dataset of observed near-surface air temperature anomalies over land, dating back to 1850.	Historical	Access
NOAAGlobalTemp v6.0	This global temperature dataset integrates long-term sea surface (water) temperature and land surface (air) temperature records. It is used to support climate monitoring activities, including the Monthly Global Climate Assessment, and serves as input data for various climate models.	Historical	Access
ERA5	ERA5 is a reanalysis dataset that integrates extensive historical observations from diverse sources into global, gridded estimates using advanced modeling and data assimilation systems. It delivers hourly estimates for a wide range of atmospheric, land, and oceanic climate variables at a spatial resolution of 31 km.	Historical	Access; AWS access
JRA-55	JRA-55 is also a reanalysis datasets that spans 1958 to present. It offers several versions at different resolution, with the finest as 0.562 degree.	Historical	Access
GHCNv4	The Global Historical Climatology Network (GHCN) is the core global land surface air temperature dataset used for climate monitoring and assessment activities. It contains temperature of over 25,000 stations across the globe.	Historical	Access; AWS link
PRISM	PRISM datasets provide estimates of seven primary climate elements (precipitation, minimum temperature, maximum temperature, mean dew point, minimum vapor pressure deficit, maximum vapor pressure deficit, and total global shortwave solar radiation on a horizontal surface) over the US using climate observations from a wide range of monitoring networks. It is available at both 800 m and 4 km.	Historical	Access
GHCNd	The Global Historical Climatology Network daily (GHCNd) is an integrated database of daily climate summaries from land surface stations across the globe. GHCNd is made up of daily climate records from numerous sources that have been integrated and subjected to a common suite of quality assurance reviews. GHCNd contains records from more than 100,000 stations in 180 countries and territories. NCEI provides numerous daily variables, including maximum and minimum temperature, total daily precipitation, snowfall, and snow depth. About half the stations only report precipitation. Both record length and period of record vary by station and cover intervals ranging from less than a year to more than 175 years.	Historical	Access
EEA data	Projected number of heatwaves (2068-2100; RCP 8.5) over Europe	Future	Access
NEX-GDDP-CMIP6	Global downscaled climate simulations generated from statistically downscaling CMIP6 General Circulation Model runs. It focuses on two "Tier 1" greenhouse gas emissions scenarios known as Shared Socioeconomic Pathways (SSPs). Daily resolution.	Future	Access; AWS link
nClimGrid	The NOAA Monthly U.S. Climate Gridded Dataset (NClimGrid) consists of four climate variables derived from the GHCN-D dataset: maximum temperature, minimum temperature, average temperature and precipitation. Each file provides monthly values in a 5x5 lat/lon grid for the Continental United States. Data is available from 1895 to the present. On	Historical	AWS link