18 September 2008

Global warming and landslide occurrence

Posted by Dave Petley

One of the most vexed questions in landslide science at the moment is that of the potential link between climate change and mass movement occurrence. I have yet to meet a landslide researcher who does not believe in the reality of anthropogenic global warming, so we are all deeply interested in how our particular systems are likely to respond. Unfortunately this is not an easy question to answer for three reasons:

  1. Landslides respond to changes in pore pressure (i.e. groundwater level). Groundwater level is controlled by precipitation input and by evapotranspiration outputs. So, to know how groundwater will respond requires quantification of both of these parameters. It might be expected that in a warmer world on average precipitation will increase (see below), but evapotranspiration will also increase. Understanding the balance between these parameters is at best a challenge.
  2. Landslides are localised phenomena, usually sited in upland areas in which rainfall patterns are complex and variable. Unfortunately, global climate models work at much larger spatial scales (typically 1 or 1.5 degrees of latitude and longitude). This makes it difficult to scale the outputs to an individual landslide.
  3. In many parts of the world, precipitation is controlled by large-scale weather systems, such as ENSO and the Asian SW monsoon. Climate models are struggling to model these systems adequately.

In the last month an interesting paper has been published that starts to take us in the right direction. The paper is this one:

Allan, R.P. and B.J. Soden, 2008. Atmospheric warming and the amplification of precipitation extremes. Science, 321 (5895), 1481-1483. You can download a copy from Brian Soden’s website.

This paper is interesting because it looks at extreme precipitation in the context of climate change. In the last couple of years it has become clear that many of the most damaging landslide events tend to occur as a result of precipitation extremes – i.e. comparatively short duration, high intensity rainfall (the type associated with a particular storm or front) rather than long duration, lower intensity events. Understanding how extreme precipitation will change is thus very helpful.

Allen and Soden have started from the observation that the GCMs all forecast that extreme precipitation events will become more common as the climate warms. They have used a combination of measurements of daily precipitation over the tropical ocean using a NASA satellite instrument called SSM/I and the outputs from global climate models to look at the response of tropical precipitation events to natural changes in surface temperature and atmospheric moisture content. In the context of landslides, a very clear link was observed. In periods when temperatures were high the number of observed extreme rainfall events increased, and vice-versa. What is surprising though is that the response of the natural system seems to be more extreme than that of the climate models – i.e. the climate models are too conservative in terms of their forecasts of these extreme events.

The relevance of these findings for landslides should be quite clear. Increases in the occurrence of extreme precipitation intensities might well be expected to increase the occurrence of landslides. It should be noted though that there is some way to go to really establish this link. For example, this paper is essentially based on a dataset collected over the ocean. There is a need to see whether the same applies on land. However, it is good to see papers being produced that start to answer the questions that we are asking.