What drives emissions up the most
- Page weight — uncompressed images and video, autoplay, unused JavaScript.
- Hosting powered by fossil fuels (verify via the Green Web Foundation).
- No caching — every returning user downloads resources from scratch.
- Unnecessary requests: trackers, marketing tags, redundant fonts.
Ecology goes hand in hand with performance
Almost every emissions optimization — smaller images, less JavaScript, better caching, lower TTFB — is simultaneously a performance and Core Web Vitals optimization. Reporting a site's carbon footprint is also becoming part of ESG: the W3C is developing the Web Sustainability Guidelines as a common reference point.
Example
Reducing page weight from 4.2 MB to 1.1 MB lowers data transfer and estimated emissions per visit — without changing the SWDM model, the direction is the same: less data = less energy.
Example
Before: 4.2 MB / visit → ~0.7 g CO₂e (SWDM estimate)
After: 1.1 MB / visit → ~0.2 g CO₂e (SWDM estimate)
At 500,000 visits/month:
savings of ~250 kg CO₂e/month (estimate, not an ESG audit guarantee)How much CO₂e does an average page emit?
Based on SWDM data, an average page view is in the range of 0.3–0.8 g CO₂e. At a million views per month, the difference between a light and a heavy page can reach tonnes of CO₂e per year.