API to calculate carbon (C02) emissions estimates for common C02 emitting activities.
Carbon Interface API is a free Environment API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
The carbon emissions measurement platform for developers. Generate accurate emissions for common carbon emitting activities with an easy-to-use API.
Details from docs.carboninterface.com.

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The Carbon Interface API is provided by Carbon Interface (docs.carboninterface.com). The carbon emissions measurement platform for developers. Generate accurate emissions for common carbon emitting activities with an easy-to-use API.
Yes. The Carbon Interface API is listed in the Environment category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Carbon Interface API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Carbon Interface API supports CORS, so front-end JavaScript can call it directly.
Yes. The Carbon Interface API is served over HTTPS, so it's safe to use in production.