A tonne per terabyte

A conversation with Jon Pincus inspired me to wonder what the ecological impact of massive data storage might be. As I’ll outline below, I reckon a terabyte of storage generates a tonne of carbon dioxide every year, the same as the per-passenger cost of a flight from New York to LA, and about 5-10% of what a typical first-worlder generates in a year.

Working off Barroso, Dean and Hölzle’s paper on the Google cluster architecture, I infer that the energy consumption for storage is approximately 1W/GB: a dual 1.4-GHz Intel Pentium III server with a two 80GB drives draws 120W per server. Adding Google’s estimate of about 40% for cooling, gives 165 W per 160 GB, or 1 W/GB (watt/gigabyte). [1]

The environmental advocacy group SEEN estimates carbon dioxide production for a variety of power plant types. For a 1 megawatt (MW) plant the numbers range from 7,900 tonnes/year for coal to 4,000 tonnes/year for a gas-fired plant. I’ll pick 5,000 tonnes per MW.year for lack of knowledge about average capacity. That’s to say: 5,000,000 kg per 1,000,000 W.year, or 5 kg/W over a year.

Combining the two: 1 W/GB times 5 kg/W = 5 kg of CO2 per GB. To simplify further, and to be conservative, let’s say I’m off by a factor of five; that is, either servers or power plants are five times more power efficient than I’m estimating. That gives 1 kg/GB.

Since one metric tonne is 1,000 kg, and a terabyte is (roughly)1,000 gigabytes, we get to a nicely memorable number: a tonne of CO2 emitted per terabyte for data storage per year.

For context, first worlders generate about 20 tonnes of CO2 per year each [2]. Most people aren’t storing anything like a terabyte of personal data yet, and so the load their storage places on the atmosphere is relatively small. However, once we all start using up our TB of gMail storage, that tonne/terabyte will become a significant part of a first worlder’s the personal CO2 emissions.

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[1] As a reality check, a Maxtor 320GB NAS is spec’d at a power consumption of 150 watts, or about 0.5 W/GB. Peter Harrison suggests a rule of thumb that each watt of power consumed requires a watt of cooling, again taking us to about 1 W/GB.

[2] One activist site estimates that a typical family of three with two cars, who flies to an annual vacation, might produce 50 tonnes of CO2 a year; the same family, living in a small, efficient house with no car, and no annual flight, might produce 10 tonnes. Jerry Hannan gives different figures of the same order: A car and driver produce about 5.5 tons of CO2 per year; when all fossil fuel is considered, every man, woman, and child can be said to be responsible for 18.7 tons of CO2 per year. Air travel generates a lot of carbon dioxide. The City of Seattle uses a figure of figure of 0.34 kg per passenger air mile. It’s 2,800 miles from LA to New York, which uses or 0.95 tonnes of CO2 per passenger.