The concept of ecological footprint captures what an individual is responsible for in terms of portion of the earth’s resource capacity used.
The basic idea is that the earth’s natural systems capacities are measured in gross terms – total amount of fish catchable, amount of carbon absorbable, amount of arable land – and divided across users and groups of users. An individual’s footprint is the far end of this process – each person, based on their average use, can calculate how much of the oceans they need for their fish-eating, how much of the atmosphere or carbon-capturing plants in the ecosystem would absorb the carbon they are responsible for releasing from fossil sequestration, how much arable land they need in order to grow the food they eat.
There are many good resources for learning about the ecological footprint concept and measuring your own. The Global Footprint Network has great resources to help you figure it out.