Geoscience Reference
In-Depth Information
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
1900
1920
1940
1960
1980
2000
FIGURE 1.3
Values.of.annual.mean.temperature.over.the.western.United.States.calculated.
from. the. CRUv2.1. dataset. (dashed). and. the. PRISM. dataset. (solid).. Linear. trends. (straight.
lines).and.slow.variations.(curves).were.calculated.using.locally.weighted.regression.(loess;.
Cleveland.1995)..For.both.curves,.the.mean.over.the.entire.record.is.subtracted.
with.elevation.(Vuille.et.al..2003;.Pepin.and.Seidel.2005;.Liu.et.al..2006;.You.et.al..
2008).. Pepin. and. Seidel. (2005). noted. that. the. correspondence. between. surface. and.
free-air. variability. and. trends. depended. on. the. convexity. (hilltop. vs.. valley. or. frost.
hollow).of.the.terrain.in.which.the.station.was.situated,.such.that.variability.at.stations.
higher.than.surrounding.terrain.more.closely.resembled.that.of.the.free.air,.while.sta-
tions.in.valley.bottoms.were.more.likely.to.differ.from.the.free.air.
As.illustrated.by.Pepin.and.Seidel,.the.key.question.in.evaluating.the.dependence.of.
trends.on.elevation.is.the.extent.to.which.surface.temperatures.are.determined.by.local.
energy. balance. as. opposed. to. mere. exposure. to. free. air.. Local. advection. (drainage.
winds,.upslope.winds).can.exert.signiicant.inluence.as.well..Some.areas.may.remain.
for.some.time.as.cold-air.pools.and.in.effect.serve.as.“climate.refugia”.(Ashcroft.2010;.
Petit. et. al.. 2003;. Bennett. and. Provan. 2008).. Another. approach. to. evaluating. trends.
that.brought.the.surface-energy.balance.into.clearer.focus.was.the.work.of.Pepin.and.
Lundquist. (2008).. Examining. trends. globally,. they. noted. that. the. largest. warming.
trends. were. found. at. locations. whose. mean. annual. temperature. was. near. 0°C,. sug-
gesting.a.strong.role.for.snow-albedo.feedback..This.observational.result.was.largely.
corroborated. by. the. regional. modeling. work. of. Salathé. et. al.. (2010),. who. noted. the.
largest.warming.trends.in.a.future.scenario.in.montane.areas.presently.near.snowline..
In.short,.there.are.ample.reasons.to.believe.that.topographic.complexity.may.produce.
considerable.small-scale.variability.in.change.rates.that.could.rival.or.exceed.(posi-
tively.or.negatively).regionally.averaged.rates.(Daly.et.al..2009).
V
ariabiliTy
and
C
hanges
in
p
reCipiTaTion
Although.trends.in.temperature.are.positive.almost.everywhere.in.the.western.United.
States,.trends.in.precipitation.are.far.more.diverse..Linear.its.are.a.poor.description.
