This is a reprint of the article that describes how I assigned a relative weighting for the different types of impacts for the EVO Tree.
There are 4 main types of environmental impacts:

    * CO2 and CH4 (main greenhouse gases)

    * Land Uses (including agriculture, building, timber, etc.)

    * Water & Air Pollution (not including CO2 and CH4)

    * Fresh water consumption

How are the different types of impact weighted in the EVO model?

The weighting of each impact in Total Environmental Impact Model framework is derived by integrating data from two important impact models. The first is the Ecological Footprint model used by The World Wildlife Fund and developed by which quantifies seven types of impacts (including CO2) in terms of land use. The second is the Union of Concerned Scientists Consumers Guide to Effective Environmental Choices, which incorporates data on pollution and water impacts. EVO’s model creates a single framework that accounts for all four types of environmental impacts in one framework. This allows us to a weighting, “efficacy” value for each of the EVO questions.

Greenhouse gases, predominantly CO2 and CH4 (Methane) are clearly the largest problem the world faces. There is only a finite amount of forested land available to absorb the ever-increasing amount of CO2 emissions released by modern civilization (and that land is shrinking rapidly). The research groups Redefining Progress and Global Footprint Network have quantified the amount of forest land that we require to absorb our CO2emissions. In addition, they look at other types of land use impacts – crop, pasture, forest (for lumber), built-up land and fisheries. By doing so they create a relative proportion of CO2 versus Land impacts, which breaks down our total consumption into units of land (m2) required to both provide the material resources and subsume the resulting wastes of all the products and services we require for our daily lives. This is called the “ecological footprint” model.

This is what the map of the world looks like if you weight different countries by their per capita consumption of natural resources:

What about Pollution and Water impacts?

The Ecological footprint model does not account for air & water pollutants or water extraction. Though these impacts are extremely significant, they cannot be quantified in units of land the way that CO2 and land impacts can be. So in order to give some weighting to these impacts in the EVO question framework, we turn to the Union of Concerned Scientists Guide to Effective Environmental Choices. This guide measures the amount of pollutants and water used for every type of consumable good – from shoes to automobiles.

The EVO Tree takes into account all types of pollution by using a proxy or “placeholder” value for all pollutants in the Total Environmental Impact framework. This proxy value is modest - 2% for each pollution type - but it nevertheless helps us weight the importance of certain questions, such as “organic cotton” which in the Ecological Footprint model would not be weighted very heavily, even though cotton uses 25% of all pesticides in the world.

In the EVO model, this question is weighted more heavily to account for the large amount of pollution associated with cotton production. Similarly, water extraction is a highly variable impact. In some parts of the country water is scarce and in others abundant, so it is typically left out of the Ecological footprint.

According to the Living Planet Report (PDF), the US in total is consuming 16% of its available freshwater per year. So while significant, it is currently not considered to be a critical impact. EVO adds a 2% proxy value for water impacts, in order to help weight certain questions, such as “lawn irrigation,” highlighting the importance of water conservation in the US.

Republished from my original 2007 post on

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