"An email written by an AI is a bottle of water"

In September 2024, the Washington Post published, with Shaolei Ren's team at the University of California, Riverside, an estimate of the water used by an email written by AI: "a bottle of water per email", says its headline. The article is paywalled and we were unable to check it: its figure is not used here. The team's method is published: its 2023 study gave 500 mL for 10 to 50 GPT-3 replies, or 10 to 50 mL per reply. It counts the water evaporated to cool the servers and the water consumed by the power stations that produce their electricity.

In August 2025, Google published a production measurement, with its method: 0.26 mL for a median question to Gemini, the water evaporated on its sites only. OpenAI gives 0.32 mL for ChatGPT, with no method. Mistral AI, in its life cycle assessment, counts 45 mL for a reply of 400 tokens, about half a page, adding the water used for electricity and for chip manufacturing.

Water

Water for one question, from 0.26 to 50 mL depending on scope

Water attributed to a written question to an AI, in millilitres, logarithmic scale · 2023 to 2025

Google, Geminion-site cooling, measured0.26mL

OpenAI, ChatGPTdeclaration, no methodology0.32mL

UC Riverside, GPT-3cooling and electricity10 to 50mL

Mistral AI, Le Chatfull life cycle45mL

↑a factor of 170with Google, by scope

0.1110100 mL

Source: Google; OpenAI; UC Riverside; Mistral AI · projetcelsius.com/en/ai · CC BY 4.0

PNGCSV

The 4 figures in the chart are consistent once their scope is set out: the lowest counts only the water evaporated on sites, for a question to a recent model; the highest add the water used for electricity, and for Mistral AI the water used for server manufacturing.

Company reports distinguish water withdrawn, which is pumped and partly returned, from water consumed, which does not return to the environment. In its 2025 financial year, Microsoft withdrew 13.3 million m³ and consumed 8.2; half of its withdrawals take place in water-stressed areas. 1 m³ evaporated in Arizona, under water stress, does not count the same as 1 m³ evaporated in Ireland.

Water

Microsoft's water use, 2020-2025

Water withdrawn and water consumed, in million m³ per financial year · Financial years 2020, 2024 and 2025

Source: Microsoft, 2026 data sheet · projetcelsius.com/en/ai · CC BY 4.0

PNGCSV

At the scale of one person, the water used by a question weighs very little: 0.26 to 45 mL depending on the scope, whereas a French person uses 143 litres of drinking water a day at home, largely returned to the environment. Globally, data centres consumed about 560 billion litres in 2023 according to the IEA, counting cooling, electricity and chip manufacturing, and about 1,200 billion in 2030 in its base case. In our view, water has to be assessed site by site, within its river basin.