So it's Sunday morning, you're sitting down at the computer with your cup of coffee, and you decide to search for the latest YouTube hit. You groan. That video is taking too long to load. You open a new window and decide to read the morning paper while you wait for the video to download. This scene may seem innocent enough, but were you aware of the fact that your websurfing has an impact on the environment? Would you be surprised to know that it contributes to a greater global carbon footprint than the whole of the aviation industry?

Although they might seem ephemeral, websites have real-world environmental footprints. Their files are stored on servers, viewed by personal computers, and connected via networks. To operate these components, all of which are necessary to create a complete website experience, electricity must be consumed. And to generate much of that electricity, fossil fuels like coal and natural gas are usually being burned.

So, when you are sitting in London viewing a website hosted in California, there are power plants on at least two continents actively pumping carbon dioxide into the atmosphere in order for you to watch that video or read that online newspaper.

Since millions of people are surfing the web every hour of every day, that carbon footprint adds up to an astounding 2% of international emissions each year. In fact, according to the American research firm Gartner, the carbon footprint of information and communications technology exceeded that of the global aviation industry for the first time in 2007. Add to your own footprint those of the billions of existing web searchers, not to mention those now coming online in Asia and Africa, and the numbers are staggering.

Even simple online activities take a toll on the environment. Google does not divulge its energy use or carbon footprint but, based on publicly available information, we have calculated that each Google search generates an estimated 5-10 g of CO2, in part because Google's unique infrastructure replicates queries across multiple servers, which then compete to provide the fastest answer to your query. On the other hand, just browsing a basic website generates about 20 mg of CO2 for every second you view it.

More complex websites with rich animations and video can be responsible for the emission of CO2 at up to 300 mg per second. Where are these emissions coming from? For a typical website experience, the dominant contribution to its footprint comes from the electricity consumed by its visitors' computers, followed by the network infrastructure needed to transmit the website, with the servers and data centers providing the website as the smallest contributor.

Many prominent sites are, however, increasing emissions through software errors and other problems, which increase the time – and energy – needed to access them.

Our analysis of the Prime Minister's site on Saturday, January 10, 2009, showed it was operating at only 64% network energy efficiency, while the Queen’s website was even worse at 54% efficiency. The highly trafficked BBC News website only operates at 49% efficiency.

As a web consumer, though, you can make a difference. Over the years, Internet users have become accustomed to demanding certain levels of service from the websites that they visit. It is now de rigueur for e-commerce sites to present badges certifying that their credit card processing systems are secure. Savvy users even know to watch for certification that sites they visit are safe from hacker attacks and that have audited privacy policies to protect their identities.

Websites now need to be put under pressure to clean up their environmental impacts too and demonstrate that their sites are as green as they can be.

Dr Alexander Wissner-Gross, is a physicist and Environmental Fellow at Harvard University. His website, www.CO2Stats.com, is designed to allow webmasters and bloggers to make their sites greener in an auditable way.

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