Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 129

can be mistaken for climate trends. An often-cited example is the question of whether
the current global temperature trend is the result of recovery from the Little Ice Age or
the result of anthropogenic greenhouse gas increases (Meier et al. 2003).
Mountain and glacier environments are especially sensitive to climate changes and
variability (Meier et al. 2003; Kohler et al. 2010; Kaltenborn et al. 2010). Many climate
changes have been detected in mountain records (e.g., Cayan et al. 2001; Pepin and
Losleben, 2002; Huber et al. 2005; Kaser et al. 2004; Hastenrath 2010). Climate
changes are well documented to have occurred in the geologic past, as illustrated by the
glacial and interglacial climates of the Pleistocene and over the period of instrumental
records (Beniston et al. 1997; Thompson et al. 2003). Current scientific consensus holds
that the climate is, in fact, in the process of changing, primarily warming because of an-
thropogenic inputs of greenhouse gases to the atmosphere (IPCC 2007). Different mag-
nitudes of warming, and even cooling, are predicted for different mountainous regions
of the world (Thompson et al. 2003; IPCC 2007). Landmasses are expected to warm
more than the oceans, and northern, middle, and high latitudes more than the tropics
(Lean and Rind 2009). Precipitation, in particular, is predicted to both increase and de-
crease depending upon region because of changes in general circulation (IPCC 2007).
Climate models in mountainous regions, however, tend to be rather poor because of
coarse spatial resolution, topographic smoothing, and local effects not captured by the
models (Beniston 2003; Suklitsch et al. 2010). In mountains, higher temperatures would
cause both a higher percentage of annual precipitation to fall as rain (i.e., higher snow-
lines) and an acceleration of summer ablation (Stewart 2009). Longer snow-free periods
will increase evaporative demands and lower soil moisture, increasing the dominance of
drought-tolerant species (Erschbamer 2007). Characterizing the exact climatic impacts
on any mountain site is difficult. However, past and likely future climatic changes and
variations are likely to have major impacts in mountain environments.
Mountains provide freshwater to half of the global population, and climate change
will affect its availability (Kohler et al. 2010; Kaltenborn et al. 2010). Changes in winter
precipitation and summer temperatures will alter the rate and extent to which snow-
lines migrate up or downslope, and contribute to glacier mass balance and runoff (Clare
et al. 2002; Diaz et al. 2003). Seasonal snowpacks in the northern hemisphere have sig-
nificantly declined over recent years (Diaz et al. 2003; Pielke et al. 2004; Mote et al.
2005; Adam et al. 2009; Stewart 2009). Glaciers are likely to experience loss of mass,
which will contribute more water to melt-season runoff and cause the glacier to thin
and retreat. Glacier recession will have an impact on local climatic conditions, such
as energy and moisture exchanges and the generation of local winds. Measurements
of alpine glacier mass balances globally have documented retreats in recent decades
(Pelto 1996; Meier et al. 2003, Oerlemans 2005; Dyurgerov and McCabe 2006; Hoelzle
et al. 2007; Barry 2008; Nesje et al. 2008; Zemp et al. 2009; Hastenrath 2010). A com-
prehensive survey of global glacier coverage is not yet possible due to limited spatial
and temporal observations (Barry 2008; Zemp et al. 2009). However, the vast majority
of studies demonstrate a major trend of glacier recession, with global estimates of glaci-
er area (and volume) lost ranging from 0.9 to 1.5 percent during the period 1961-1990
and accelerated loss since then (Dyurgerov and Meier 1997; Meier et al. 2003; Barry
2008; Zemp et al. 2009). The recession rates observed in most mountains are consider-
ably greater, with some experiencing 60-80 percent loss in small glacier cover over the