5.4 Permafrost and tundra travel days
The retreat of the permafrost is serious. Building foundations are collapsing, and there are ‘drunken forests’ as land beneath trees melts, subsides and slumps. Buildings require carefully built foundations, and the Trans-Alaska Pipeline was even designed with refrigerated pillars to prevent pipe fracture through permafrost thaw subsidence.
Activity 13 Tundra travel days
Figure 31 (repeated below) shows the number of days on which travel has been allowed on the tundra (land with underlying permafrost is known as tundra). The best fit line has a value of 203 days in 1970 and 120 days in 2013. Estimate the average rate of change in the number of days over this period, to the nearest whole day per year.
The number of days on which travel has been allowed on the tundra has decreased by an average rate of 2 days per year from 1970–2013.
There is, however, another more worrying problem as the permafrost retreats. As the ground subsides, the depressions usually form lakes because the melt water cannot flow through the frozen ground beneath. Thawing of the permafrost at the lake bottom releases organic matter that is perhaps 30 000–40 000 years old into the water. The organic matter decomposes, giving off methane – a potent global warming gas (Figure 33). The permafrost–methane feedback cycle is another positive feedback in the system.