Sending a text message or email, eating an apple or watching TV - each of these activities has a different carbon footprint.
People around the world are getting ready to mark Earth Hour by turning out the lights this Saturday, but a long list of seemingly harmless everyday actions also contribute to emissions of carbon dioxide (CO2) and other planet-harming greenhouse gases.
Total global emissions in 2010 were estimated at 49 gigatonnes (Gt or billion tonnes) of CO2 equivalent (CO2e).
Even a short email is estimated to have a footprint of four grammes (0.14 ounces) of CO2e (gCO2e) - including greenhouse gases produced in running the computer, server and routers and a part of their manufacture.
An email with a large attachment emits about 50g CO2e, and a spam message, not even opened by the recipient, is responsible for 0.3g CO2e.
The annual global footprint of spam is equivalent to 3.1 million passenger cars on the road in a year, using two billion gallons (7.6 billion litres) of gasoline.
A web search on an energy-efficient laptop leaves a footprint of 0.2g CO2e, and on an old desktop computer some 4.5g CO2e.
Find out the environmental costs of daily activities like sending a cellphone text message, drinking a coffee and shopping in the gallery below.