Climate change has caused public concern over the last several decades and greater attention is now being paid to its impact on the cryosphere ([Lunardini, 1996] and [Nelson, 2003]). As one of the main components of the cryosphere, permafrost is extremely sensitive to climate change at different spatial and temporal scales. The highest and most extensive middle–low latitude permafrost is located on the Qinghai–Tibet Plateau. The permafrost over the Qinghai–Tibet Plateau is very sensitive to climate change because of its high temperature, high soil water content and thin thickness (Cheng, 1979). Climate change has caused extensive permafrost degradation over the Qinghai–Tibet Plateau ([Cheng et al., 1993], [Wu and Liu, 2004] and Wu et al., 2005 T.H. Wu, S.X. Li, G.D. Cheng and Z.T. Nan, Using ground-penetrating radar to detect permafrost degradation in the northern limit of permafrost area on the Qinghai–Tibetan Plateau, Cold Regions Science and Technology 41 (2005), pp. 211–219. Including rising temperature of permafrost, thickening of the active layer, expansion of taliks, and the disappearance of sporadic permafrost.

Most general circulation models (GCM) project that global warming will continue and that its amplitude will increase during the 21st century (IPCC, 2007). The impact of projected climate warming on mean annual air temperatures (MAATs) is estimated to be as much as 4 °C by the year 2100. More conservative estimates suggest a probable warming in the MAATs of about 1.1 °C by 2040 (Jin et al., 2008). Other studies (Li et al., 1996; Li and Cheng, 1999; Nan et al., 2005) have also reported that climate change may significantly affect the permafrost on the Qinghai–Tibet Plateau.

The active layer, which is the layer of soil or other earth materials between the atmosphere and permafrost body subject to freezing and thawing on an annual basis, is an extremely important factor in permafrost regions because most exchanges of heat and moisture between the atmospheric and terrestrial systems occur through it ([Kane et al., 1991], [Nelson and Anisimov, 1993] and [Zhang, 2005]). Permafrost along the Qinghai–Tibet Highway changed greatly because of climate change and human activity. The ground temperature at a depth of 6 m has risen by about 0.1 °C to 0.3 °C and the thickness of the active layer has increased by 0.15 to 0.50 m between 1996 and 2001(Cheng and Wu, 2007).

Thickening of the active layer may affect local hydrology, ecology, engineering infrastructure, and even the climate ([Jin et al., 2000], [Zhang et al., 2005] and [Cheng and Wu, 2007]). The active layer provides a rooting zone for plants and acts as a seasonal aquifer for near-surface ground water (Burn, 1998), so that thickening of the active layer could reduce the water content of the ground surface and affect the soil and vegetation in permafrost regions ([Wang et al., 2002] and [Yang et al., 2004]). At locations where the uppermost permafrost is rich in ground ice, thickening of the active layer could have severe destabilizing effects on engineering works (Wu et al., 2003) and could trigger the release of significant amounts of greenhouse gases to the atmosphere ([Lin et al., 1996], [Michaelson et al., 1996] and [Anisimov et al., 1997]).

The Golmud–Lhasa section of the Qinghai–Tibet Railway is 1142 km in distance, of which 632 km is over permafrost. A large fraction of the permafrost region is warm, ice-rich permafrost, which makes it much more difficult to construct any infrastructure system on it (Zhang et al., 2008). Thickening of active layer may cause serious damage to the Qinghai–Tibet Railway. To deal with the changing permafrost and global warming, a series of proactive roadbed-cooling methods were employed, which include solar radiation control using shading boards, heat convection control using air ducts, thermosyphons, air-cooled embankments, and finally heat conduction control using “thermal semi-conductor” materials. A proper combination of these measures can enhance the cooling effect. All these methods can be used to keep the permafrost from thawing, lower the ground temperature and help stabilize the Qinghai–Tibet Railway (Cheng et al., 2008).

It is necessary to know the current active layer conditions and their potential variations under climate change. Many studies have been carried out to investigate the active layer of permafrost regions over the Qinghai–Tibet Plateau in the last decade, including drilling, soil temperature monitoring and ground-penetrating radar ([Wu and Liu, 2004], [Wu et al., 2005] and [Jin et al., 2008]). Spatial and temporal variability are usually assessed by repeated observation at permanent plots over several years. Most of the observation plots lie along the highways, so little is known about the active layer in the other regions of Qinghai–Tibet Plateau. Models are of great importance in studying the active layer on the Qinghai–Tibet Plateau. In this study, a model based on Kudryavtsev's formulations is used to calculate the ALT of the permafrost regions over the Qinghai–Tibet Plateau.

The rising air temperature and increased human activities have caused permafrost degradation over the Qinghai–Tibet Plateau, including thickening of the active layer. The active layer plays an important role in cold regions. Active layer thickness (hereafter ALT) variation may have profound socio-economic and eco-environmental consequences. Using a climate-driven model based on Kudryavtsev's formulations and data from climate records, snow and vegetation parameters, and soil features, we calculated ALTs of the permafrost regions over the Qinghai–Tibet Plateau. A general agreement was found when comparing the calculated results with measured values at survey sites. A distribution map of ALT over the Qinghai–Tibet Plateau shows that ALTs in the central part of the plateau are generally smaller than those in other regions while ALTs in the northern and western regions are larger than those in the eastern and southern regions of the plateau. ALTs of the permafrost regions along the Qinghai–Tibet Highway are generally larger than 2.0 m. The regions with ALTs less than 2.0 m are mainly in the high-mountain areas.