Geoscience Reference
In-Depth Information
time. series. (Figure 1.6,. bottom. panel). shows. no. appreciable. change—even. in. the.
late-twenty-irst.century.
We. note. again. that. this. regionally. averaged,. annually. averaged. view. does. not.
mean.that.precipitation.will.not.change.at.all..The.models.indicate.that.the.character.
of.precipitation.in.the.West.will.change.differently.from.place.to.place,.from.sea-
son.to.season,.in.type.(rain.versus.snow),.in.frequency.(how.often),.and.in.intensity.
(amount.per.unit.time)..These.differences.relect.the.physical.mechanisms.at.work.
and.are.internally.consistent..Such.changes.will.provide.a.wide.variety.of.selection.
pressures.to.which.ecological.systems.can.and.will.respond.in.their.usual.complex.
ways..These.selection.pressures.will.exist.in.a.context.of.other.human-induced.non-
climatic.environmental.changes.that.provide.additional.selection.pressures..Climate.
is.merely.one.of.these.changes,.albeit.a.very.important.one.
We. also. note. an. overall. inding. with. respect. to. temperature. and. precipitation.
changes.. With. one. exception. (the. Parallel. Climate. Model. [PCM]),. in. those. loca-
tions. where. drying. is. projected,. generally. models. that. project. larger. temperature.
increases.also.project.larger.precipitation.decreases..Conversely,.in.those.locations.
where. wetter. conditions. are. projected,. generally. models. that. project. smaller. tem-
perature.increases.also.project.larger.precipitation.increases.
s
TaTisTiCal
p
roperTies
of
C
hange
Christensen.et.al..(2007).also.provide.statistical.summaries.of.change.for.regional.
averages..Their.deinition.of.western.North.America.is.different.from.ours,.extending.
from.30°.to.60°.latitude.and.from.the.Paciic.coast.east.to.100°W.longitude..Table 1.1.
summarizes.the.statistical.properties.of.change.in.this.region.
For.this.domain,.summer.warming.exceeds.that.in.other.seasons.for.the.min-
imum,. 25th,. 50th,. and. 75th. percentiles. but. not. for. the. hottest. model.. For. pre-
cipitation,.the.median.change.is.quite.small,.which.is.consistent.with.our.results.
presented.previously..Extremes.are.a.different.story:.the.vast.majority.of.seasons.
exceed.the.twentieth-century.95th.percentile.of.heat,.and.large.shifts.toward.fewer.
extremely. dry. seasons. and. more. extremely. wet. seasons. occur. in. every. season.
except.summer.
C
hanges
in
V
ariabiliTy
So. far. we. have. described. changes. in. the. irst. moment. of. climate—that. is,. the.
mean—but.one.important.aspect.of.climate,.especially.for.biological.systems,.is.the.
variability.. In. Figure 1.7. we. compare. each. model's. regionally. averaged. change. in.
mean.temperature.and.precipitation.with.its.change.in.variance..For.both.variables,.
some.models.show.a.decrease.in.variance,.but.most.indicate.an.increase..There.is.
little.correspondence.between.change.in.mean.and.change.in.variance.for.tempera-
ture,.but.for.precipitation,.the.models.that.indicate.larger.increases.in.precipitation.
generally.also.experience.an.increase.in.variance..These.clearly.have.implications.
for.ecological.systems.
