EVIDENCE

Throughout the description and analysis of climatic regimes in mountain areas, the question of climatic change has so far been ignored. There is now overwhelming evidence of important climatic fluctuations on time scales of human significance and, since the mountain environment is, in many respects, marginal for human activities, it is necessary to understand and take account of such fluctuations (Barry, 1990).

The documented increases in greenhouse gases (carbon dioxide, methane, chlorofluorocarbons and nitrous oxide) since 1958 represent a global increase in heating of just over 1 W m− 2 and the accumulated increases since the beginning of the Industrial Revolution (c. ad 1800) have contributed in excess of 2 W m− 2 i.e. 1 percent of the average absorbed solar radiation of 240 W m− 2 (Hansen et al., 1990). Some offsetting cooling may have occurred during the 1970s–80s as a result of increasing levels of tropospheric aerosols, predominantly sulfur dioxide; these represent a net cooling effect through the increase in planetary albedo, particularly over the oceans. The period from about 1950–90 saw a reduction in solar radiation at the Earth's surface, especially in northern mid-latitudes and in urban areas, averaging 0.5 W m− 2 per year, or 2.7 percent per decade. This has been referred to as “global dimming” (Gilgen et al., 1998; Stanhill and Cohen, 2001). The effect is attributed to both increased aerosol load blocking sunlight, and increased cloud reflectivity caused by the increase in condensation nuclei provided by the aerosols (Crutzen and Ramanathan, 2003).