August 1, 2023

Sea surface temperature

ERA5 extrapolar sea-surface temperature reaches 20.96°C

Copernicus dates the 20.96°C mark to 31 July 2023, not the 1 August that got repeated everywhere — and it beat the old record by one-hundredth of a degree.

The date is 31 July, not 1 August

Copernicus's finalized account places the daily mean at 20.96°C on 31 July 2023, one day earlier than the listing states. Early news reports attributed the value to 1 August, apparently working from preliminary data or a different data cut.

The figure narrowly exceeded the previous ERA5 maximum of 20.95°C, set in March 2016 — a margin of one-hundredth of a degree Celsius.

Copernicus announced the result on 8 August, alongside confirmation that July 2023 was the warmest month of any name in ERA5's global surface-air-temperature record.

What the number actually measures

"The world's oceans reached 20.96°C" is shorthand. The principal series is the ERA5 daily mean sea-surface temperature across the extrapolar ocean, between 60°S and 60°N.

ERA5 is a reanalysis, so values can be revised as observations are assimilated, and different sea-surface datasets, measurement depths and spatial masks yield different dates and rankings.

Sea-surface temperature is also not ocean heat content: it says nothing on its own about the average temperature through the ocean's full depth.

The 32.5°F that isn't a conversion

The listing renders July's 0.3°C margin over the previous warmest month as "0.3 °C (32.5 °F)", converting a difference as though it were an absolute temperature. A difference of 0.3°C is 0.54°F.

The measured margin was 0.33°C above July 2019, with July 2023 running 0.72°C above the 1991–2020 July average.

Nothing discontinuous happened to the climate system at 20.96°C; the significance was statistical, and the mark stood for less than a month before ERA5 reached 21.02°C on 23 and 24 August 2023.

Sources

Researched 24 Aug 2026 7 sources not yet audited Date disputed

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

Date disputed. Sources disagree; the research places it at 2023-07-31.

What the sources leave uncertain

The listed 1 August date conflicts with Copernicus's finalized account, which places 20.96°C on 31 July. Some early news reports attributed the value to 1 August, apparently using preliminary data or a different data cut.
ERA5 is a reanalysis, so values may be revised as observations are assimilated. Different sea-surface datasets, measurement depths and spatial masks can produce different dates and rankings.
The record applies primarily to the ERA5 extrapolar series. It is not a direct thermometer average of every ocean area or the full water column.