• Mike Berners-Lee (the inventor of the World Wide Web) in 2010 has published carbon emissions associated with e-mails, which have been widely used for measuring the carbon foot of sending an e-mail. A spam e-mail emits 0.3 gram of CO2, a normal/standard e-mail emits 4 gram of CO2, and an e-mail with large attachments emits 50 grams of CO2. These figures take into account the carbon emission from powering the data centers to computers/servers that send, filter, and read e-mails, whether the e-mail recipients open the e-mail or not.
• A recent study by OVO reveals that over 64 million normal e-mails were sent by people in the UK, which emits 16,433 tonnes of carbon in 2019. This indicates less than 256.8 grams of CO2 per e-mail per year (16,433 tonnes/64 million x 1,000,000), equivalent to the emission of 81,152 flights to Madrid or 3,334 diesel-powered cars.
• Based on 121 e-mails received by an average office worker per day and approximately half of these e-mails are spam, a day's e-mail per person emits 1,652 grams of CO2, equivalent to 603,393 grams of CO2 a year. For example, the annual CO2 emission of an average person in India is estimated to be 1,500,000 grams.
• The percentage of spam e-mails have seen a declining trend, from 69% of total e-mails in 2012 to 55% in 2018. In March 2020, the percentage stands at 53.95%.
• The average daily cost of energy to store 1 GB of data is approximately 32kWh and the average size of an e-mail is 75kb, which results in the consumption of 0.0023 kWh per e-mail [(32/1024/1024) x 75] and 1 kWh of energy emits 707.2 grams of CO2 per day (39,035 million grams/55.2 million kWh). Hence, the amount of carbon emission for storing an average e-mail per day is 1.6 grams of CO2 (0.0023kWh x 707.2 grams) or 584 grams per year (1.6 grams x 365).
• In 2019, 4.13 trillion kWh of electricity emitted 1.72 billion metric tons of CO2 in the US. This results in 416.5 grams of CO2 emission per kWh (1.72 billion metric tons/4130 billion kWh x 1,000,000). If it applies to the above calculation, the CO2 emission of an average e-mail per day amounts to 0.958 grams (0.0023kWh x 416.5 grams) or 350 grams per year (0.958 x 365).
• In comparison to the total carbon emission (435.2 million tonnes) in the UK, the amount of CO2 from e-mails (16,433 tonnes) is negligible. As most personal devices, networks, and data centres run daily, the reduced carbon emission by removing few e-mail servers from data centres is expected to be far less than 1 gram per e-mail.

Summary of Initial Findings

• The initial hour of research identifies some statistics that help estimate the carbon emission of 15,000 unread e-mails. The benchmarks released in 2010 by Mike Berners-Lee are still widely referenced in the public domain, which are based on the number of e-mails. In comparison, some recent study provides an indication on the daily or annual emission of an average/normal e-mail, ranging between 256 grams and 584 grams per year.
• There is limited information that specifically addresses CO2 emission associated with unread e-mails, but e-mails sent from, stored and hosted in data centers in general.
• Further research will focus on some insights surrounding the equivalent measures used to assess e-mail-induced CO2 emissions and the trends on the emissions.