IPCC AR6 Chapter 12, in its section on the Emergence of Climate Impact Drivers, identifies high confidence that increasing mean air temperatures and extreme heat have already emerged in the historical period and medium confidence that decreasing cold spells are beginning to emerge.

The World Meteorological Organization (WMO) defines heat waves and cold waves as follows:

HEAT WAVE: a heatwave can be defined as a period of marked and unusually hot weather persisting for at least two consecutive days.

COLD WAVE: a cold wave can be defined as marked and unusually cold weather, which can be associated with a sharp and significant drop of air temperatures near the Earth’s surface and persisting for at least two consecutive days.

Heat wave definitions vary by country, largely as a function of their climates. For example:

• a period of at least 3 consecutive days in which the average maximum temperature across more than half the country exceeds 28°C (82.4°F) (Denmark)
• at least five days in a row with a daily high exceeding 25°C (77.0°F) (Sweden)
• three consecutive days with temperatures at 39°C (102°F) or higher (Greece)
• a period of at least five consecutive days in which the maximum temperature in De Bilt exceeds 25°C (77°F). During this period the maximum temperature in De Bilt must exceed 30°C (86°F) for at least three days. (Netherlands, Belgium, Luxembourg)
• at least three days above 25°C (77°F) across much of the country. Greater London has a threshold of 28°C (82°F). (UK)
• temperatures exceeding 25°C (77°F) for five or more consecutive days. (Ireland)
• temperatures between 28°C (82°F) in Newfoundland and 35°C (95°F) in interior (British Columbia)
• five consecutive days at or above 35°C (95°F), or three consecutive days at or over 40°C (104°F) (Australia)
• temperature threshold ranging between 27°C (81 °F) in Greymouth and 32°C (90°F) in Gisborne. (New Zealand)
• temperature reaches or exceeds 90°F (32.2°C) for three consecutive days (US Northeast).
• temperature reaches 100°F (37.8°C) for three or more consecutive days over a wide area (California)

Cold wave definitions are less country or region specific. For example:

• a rapid fall in temperature within a 24-hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. (US National Weather Service)
• temperature below 20°F

The relatively mild temperatures which define heat waves in much of Europe and portions of Canada and New Zealand lead to a significant increase in heat waves, whereas the requirement for a rapid decrease in temperature within a 24-hour period and a 20°F temperature lead to a significant decrease in cold waves. There is greater interest in heat waves than in cold waves, since heat waves support the global warming / climate change narrative, despite the fact that cold causes more than an order of magnitude more deaths annually than heat according to data reported by Lancet.

While both heat waves and cold waves are described as extreme weather, temperatures at or below 20°F are far more dangerous (~78° below body temperature) than temperatures at or above 77-95°F (3-20°F below normal body temperature).

Clearly, some extremes are more extreme than other extremes.