I recently received this exhortation as the footer of an email,
If every adult in the UK sent one fewer "thank you" email a day we would save more than 16,433 tonnes of carbon a year - equivalent to 81,152 flights to Madrid or taking 3,334 diesel cars off the road.' So 'thank you' in advance!
Now there are 52 million adults in the UK approximately, so that's 52 x 365 million emails, or about 19 billion emails. Divide 16,433 million by 19 billion and you get 0.865g of carbon per email. In the UK, on average 270g of CO2 are emitted to make 1kWh of electricity, so that makes 3.2Wh of electricity used. My laptop's battery is 58wh, which can power it for 75 minutes, so it uses 0.77wh/min. That would suggest that the time it takes to compose a thank you email and read it is about 4 mins which seems about right, until you realize that both computers would be on anyway, whether you sent the "thank you" email or not, so the additional electricity used is too small to be measured. This is exactly the same counting overheads as running costs that gives us headlines like "Missed GP appointments costing NHS millions" and just as false.
Lies, damned lies and statistics.