According to a recent study published in the journal Public Library of Science, glacial melt is taking a backseat in the Himalayas to permafrost melt as a central driver of alpine lake expansion and related environmental hazards. This finding is of great importance to policy-makers and communities, who must prepare for flooding and other hazards which can be caused by the expansion of high-altitude lakes.
The study, led by Yingkyui Li of the University of Tennessee, Knoxville in partnership with the Chinese Academy of Sciences, Beijing, determined that patterns of lake changes in the Tibetan Plateau from 1970 to 2010 were more closely associated with changes in permafrost degradation patterns than glacial retreat patterns. This conclusion suggests, at least for this region, the influence of melting glaciers on lake dynamics is outweighed by other environmental processes.
Permafrost is an ecologically important element of high-latitude and high-altitude ecosystems. Permafrost is defined as “perennially frozen ground remaining at or below 0°C for at least two consecutive years,” according to a document on the policy implications of warming permafrost, released by UNEP (United Nations Environment Programme). This frozen soil comprises about 24 percent of the exposed land area in the Northern Hemisphere, and is also found in mountainous regions of South America and ice-free regions of Antarctica. The thickness of permafrost is determined by the distance between the top of the permafrost layer, known as the permafrost table, and the bottom, also called the permafrost base. There may be an active layer above this, which thaws and freezes seasonally. The most robust type of permafrost is continuous coverage, where the permafrost table is very thick and extends for many meters into the soil. Areas with larger gaps in the permafrost can be called discontinuous permafrost zones, or sporadic permafrost.
At the outset of the study, researchers did not hypothesize that permafrost would play an active role in Tibetan Plateau lake dynamics. In order to determine the factors which influenced lakes, Li et al. gathered two sorts of data to assess fluctuations in the elevation of lakes. They used historical altimetry data for 94 lakes across the plateau for 2003-2009, and Landsat imagery data for 25 lakes across five different regions in the plateau for1972-2010. They correlated spatio-temporal patterns of lake change with various climate and environmental variables such as precipitation, evapotranspiration, glacier coverage, permafrost coverage, and daily mean temperature trends.
The analysis revealed clear spatio-temporal patterns. Lakes in the southern and western plateau showed continuous shrinkage or stable levels except for slight expansion from 2000-2004. Lakes closest to the Himalayas showed evidence of continuous shrinkage. Lakes located in the central and northern plateau seemed to experience rapid expansion after 2000, though data showed slowed expansion after 2006 in the central region. These expansion trends have been confirmed by other studies, including an article published in April 2014; however, the study led by Yingkyui Li is unique in its long time scale and fine-grained analysis of spatio-temporal patterns.
The researchers found, “[there is] no statistically significant correlation between changes in lake levels (2003-2009) and glacier coverage in each lake’s drainage basin.” On the other hand, they were able to conclude, “[the] plateau-wide pattern of lake changes is consistent with the distribution of permafrost on the Tibetan Plateau.”
The mechanism that links permafrost melt with lake expansion rests on temperature regimes in the region. When the ground temperature is lower than the melting point of frozen soil, water contribution of permafrost to lakes is limited because the soil remains frozen. However, higher temperatures accelerate permafrost melt, which contributes to lake expansion. An interesting aspect of this mechanism is when temperatures continuously increase and remain above the melting point; in this case, water contribution once again becomes limited because all water held in the frozen soil has been released. This phenomenon would explain stability in lake levels after rapid expansion such as in the central region.
Along with the effects on alpine lakes, there are other serious ramifications of permafrost degradation. By releasing water and changing the structure of soils, permafrost degradation can lead to high-altitude lake outburst floods. In mountainous areas, soils can lose their stability as they thaw, creating landslides. Moreover, as shown by a 2010 study, ecosystems which have had historically robust and continuous permafrost can experience reduced productivity and function associated with permafrost loss due to the decreases in soil moisture content and soil nutrients. In addition, the UNEP News Centre has highlighted the permafrost-carbon feedback, in which permafrost loss is associated with emissions of carbon dioxide into the atmosphere; this process could exacerbate rising temperatures.
With these effects in mind, it is important to take into account permafrost changes projected for the future. The UNEP Policy Implications of Warming Permafrost Guide indicates that in the near future “active layer thickness will increase and the areal extent of near surface permafrost will decrease” in most regions. Yet, these changes are contingent upon soil and snow processes, future scenarios of anthropogenic greenhouse gas emissions, and the warming response to increased atmospheric carbon dioxide. If warming continues, eventually the active layer of permafrost, in any region, can become so deep that it does not fully refreeze in winter; this creates a talik, or an area of permanently unfrozen ground within an area of permafrost. In extreme cases, the permafrost can completely thaw and disappear. Clearly, it is important for researchers, policy-makers, and practitioners to take permafrost processes into account if they want protect alpine communities and prevent environmental hazards such as landslides and high-altitude lake outburst floods.