A carbon footprint is a "measure of the impact human activities have on the environment in terms of the amount of greenhouse gases produced, measured in units of carbon dioxide".[1] It is meant to be useful for individuals and organizations to conceptualize their personal (or organizational) impact in contributing to global warming. A conceptual tool in response to carbon footprints are carbon offsets, or the mitigation of carbon emissions through the development of alternative projects such as solar or wind energy or reforestation. A carbon footprint can be seen as a subset of earlier uses of the concept of ecological footprints, as said in the US.