This has become a bit of a mission at Buttercup Learning in the last 6 months. I've collected some links to read on the topic, support and tools.
While one person’s digital carbon footprint may be quite small, the collective amount of the world’s digital usage is huge and already accounting for approximately 3% of the global greenhouse emissions - more than the global aviation industry!
Business Declares - Digital Declutter Toolkit
Sustainable Web Design | Mightybytes
Website Carbon calculators and green graders
Website Carbon Calculator | How is your website impacting the planet?