Geoscience Reference
In-Depth Information
Through. technology. and. sheer. numbers,. humans. have. acquired. the. ability. to.
modify.climate.in.many.ways..Chief.among.these.are.the.production.of.(1).green-
house. gases,. such. as. carbon. dioxide,. ozone,. nitrous. oxide,. methane,. chloroluoro-
carbons;. and. (2). aerosols. that. originate. from. disturbed. soils,. soot,. ash,. pollution,.
and.gases.transformed.through.photochemistry.to.particles..All.of.these.affect.the.
low.of.radiant.energy.through.air..Deforestation,.irrigation,.agricultural.practices,.
paving,.and.other.kinds.of.development.change.land.surface.properties.and.inluence.
the.dynamics.of.energy.exchange,.heat.transfer,.and.surface.winds..Recent.indings.
indicate.that.changes. in.atmospheric.particle.concentration. can.greatly.alter.cloud.
properties.and.reduce.precipitation.eficiency.and.amount.(e.g.,.Forster.et.al..2007)..
Anticipated.changes.in.temperature.may.also.affect.precipitation.type.(rain.or.snow)..
Changes.in.atmospheric.CO
2
,.ozone,.and.other.gaseous.and.aerosol.constituents.have.
direct.but.differential.physiological.effects.on.vegetation,.species.competitiveness,.
and. amount. and. quality. of. light,. which. in. turn. affect. soil. moisture. and. recharge.
budgets,.plant.species.composition,.and.community.properties.
Assessing. historical. and. future. biological. responses. to. global. climate. change.
requires. an. understanding. of. two. causal. connections.. The. irst. is. the. connection.
between.the.local.climate.variables.and.the.biological.system.of.interest..Because.so.
many.factors.are.at.work.simultaneously,.this.determination.is.seldom.straightfor-
ward..For.instance,.temperature.may.affect.tree.growth;.however,.other.climatic.and.
nonclimatic.factors.like.competition.for.light.or.other.resources.might.be.as.impor-
tant..Put.differently,.if.we.knew.perfectly.how.climate.would.change.in.the.immedi-
ate.vicinity.of.an.ecological.community,.how.well.could.we.predict.the.ecological.
response?. The. second. is. the. connection. between. the. biologically. important. local.
climate. variables. and. global. climate. drivers. like. greenhouse. gases.. In. the. ield. of.
climate.research,.this.connection.is.called.“detection.and.attribution.”.These.involve.
whether. a. change. has. actually. been. observed,. within. measurement. error. (detec-
tion),.and.whether.any.such.change.(e.g.,.in.global.average.temperature).could.have.
occurred. naturally. or. can. conidently. be. attributed. to. human. activity. (attribution;.
Stott.et.al..2000;.Broccoli.et.al..2003;.Meehl.et.al..2004)..Attribution.is.most.success-
ful.when.the.signal-to-noise.ratio.is.high,.that.is,.when.the.response.of.the.variable.
in.question.to.greenhouse-gas.forcing.is.large.relative.to.natural.variability..Keeping.
the.signal-to-noise.ratio.large.typically.requires.considerable.spatial.averaging.and.
a.long.period.of.record.(>50.yr).for.analysis..The.two.causal.connections.are.in.ten-
sion,.owing.to.the.inherently.conlicting.spatial.and.temporal.scales..Detection.and.
attribution. are. clearest. at. the. global. scale. over. multiple. decades,. but. responses. of.
ecosystems.or.species.to.climate.are.often.clearest.at.the.local.scale.and.at.shorter.
time.periods.
In. this. chapter. we. highlight. the. meteorological. and. physical. background. of.
observed. climate. variability. and. change,. and. recent. attribution. efforts. related. to.
contemporary.climate.change..We.also.describe.scenarios.of.future.climate.for.the.
western.United.States..Climate.is.a.principal.driver.of.the.natural.and.managed.envi-
ronmental.systems.of.the.western.United.States,.and.is.such.a.pervasive.inluence.
that.its.properties.and.behavior.in.space.and.time.must.be.taken.into.account.and.
factored.into.the.management.of.western.lands.and.resources.(Redmond.2007).
