  In the past decade, global warming made several headlines and turned the
attention of the whole world to it. Carbon footprint is the main factor that
drives greenhouse emissions up and results in the temperature increase of the
planet with dire consequences. While the attention of the public is turned to
reducing carbon emissions by transportation, food consumption and household
activities, we ignore the contribution of CO2eq emissions produced by online
activities. In the current information era, we spend a big amount of our days
browsing online. This activity consumes electricity which in turn produces
CO2eq. While website browsing contributes to the production of greenhouse gas
emissions, the impact of the Internet on the environment is further exacerbated
by the web-tracking practice. Indeed, most webpages are heavily loaded by
tracking content used mostly for advertising, data analytics and usability
improvements. This extra content implies big data transmissions which results
in higher electricity consumption and thus higher greenhouse gas emissions. In
this work, we focus on the overhead caused by web tracking and analyse both its
network and carbon footprint. By leveraging the browsing telemetry of 100k
users and the results of a crawling experiment of 2.7M websites, we find that
web tracking increases data transmissions upwards of 21%, which in turn implies
the additional emission of around 11 Mt of greenhouse gases in the atmosphere
every year. We find such contribution to be far from negligible, and comparable
to many activities of modern life, such as meat production, transportation, and
even cryptocurrency mining. Our study also highlights that there exist
significant inequalities when considering the footprint of different countries,
website categories, and tracking organizations, with a few actors contributing
to a much greater extent than the remaining ones.
