Certain terms and expressions that relate to the environment are problematic in that they vary widely in usage within and between disciplines, and several have been used as synonyms. In an attempt to correct the problem and standardize usage, this paper defines or redefines 10 of the most common environmental terms. Two of these terms, natural environment and environmental change, are defined on the basis of what is meant by natural as reflected by common usage and dictionary entries. The three degradation terms, environmental degradation, land degradation, and soil degradation, are defined as any change or disturbance to the environment, land, or soil perceived to be deleterious or undesirable. A sixth term, soil regeneration, is defined as the reformation of degraded soil through biological, chemical, and/or physical agencies. The remaining four terms, environmental quality, air quality, soil quality, and water quality, are defined as measures of the condition or state of each relative to the requirements of one or more biotic species and/or to any human need or purpose.

As quantitative or spatially distributed studies of environmental change over truly long-term periods of more than 100 years are extremely rare, we re-photographed 361 landscapes that appear on historical photographs (1868-1994) within a 40,000 km² study area in northern Ethiopia. Visible evidence of environmental changes apparent from the paired photographs was analyzed using an expert rating system. The conditions of the woody vegetation, soil and water conservation structures and land management were worse in the earlier periods compared to their present conditions. The cover by indigenous trees is a notable exception: it peaked in the 1930s, declined afterwards and then achieved a second peak in the early 21 st century. Particularly in areas with greater population densities, there has been a significant increase in woody vegetation and soil and water conservation structures over the course of the study period. We conclude that except for an apparent upward movement of the upper tree limit, the direct human impacts on the environment are overriding the effects of climate change in the north Ethiopian highlands and that the northern Ethiopian highlands are currently greener than at any other time in the last 145 years.

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