Geoscience Reference
In-Depth Information
Initial. detection. and. attribution. efforts. focused. on. global. mean. temperature.
(Hasselmann. 1997),. but. more. recent. studies. have. looked. at. regional. temperature.
(Stott.2003),.sea.level.pressure.(Gillett.et.al..2003),.global.mean.precipitation.(Gillett.
et.al..2004),.zonal.mean.precipitation.(Zhang.et.al..2007),.and.others..For.the.west-
ern.United.States,.the.most.relevant.results.are.those.of.Stott.(2003),.who.attributed.
warming.to.rising.greenhouse.gases.over.western.North.America,.and.Barnett.et.al..
(2008),. who. attributed. snowmelt-related. hydrologic. changes. to. rising. greenhouse.
gases.over.the.western.United.States..They.concluded.that.“up.to.60%.of.the.climate-
related. trends. of. river. low,. winter. air. temperature. and. snow. pack. between. 1950-
1999. are. human-induced.”. In. a. set. of. companion. papers. focusing. on. western-US.
mountain.regions,.Barnett's.colleagues.attributed.changes.in.temperature.(Bonils.et.
al..2008),.spring.snowpack.(Pierce.et.al..2008),.and.streamlow.(Hidalgo.et.al..2009).
to.rising.greenhouse.gases.
MODELED FUTURE CHANGE
Envisioning.western-US.climate.in.a.future.with.much.higher.greenhouse.gas.con-
centrations. requires. the. use. of. physically. based. numerical. models. of. the. ocean,.
atmosphere,.land,.and.ice,.often.called.global.climate.models.(GCMs).or.climate.sys-
tem.models..A.common.set.of.simulations.using.21.GCMs.was.coordinated.through.
the. Intergovernmental. Panel. on. Climate. Change. (IPCC),. described. in. the. IPCC.
2007.report.(Randall.et.al..2007),.with.results.archived.by.the.Program.for.Climate.
Model.Diagnostics.and.Intercomparison.(PCMDI)..These.models.typically.resolve.
the.atmosphere.with.between.6,000.and.15,000.grid.squares.covering.the.surface,.
and.with.between.12.and.56.atmospheric.layers..All.GCMs.in.the.PCMDI.archive.
include.a.fully.resolved.global.ocean.model,.usually.with.higher.resolution.than.the.
atmospheric.model,.and.nearly.all.include.models.of.sea.ice.dynamics.and.models.of.
the.land.surface..By.calculating.energy.luxes.between.the.sun,.atmosphere,.and.sur-
face,.these.models.compute.surface.temperature.distributions.that.compare.well.with.
observations..Details.of.the.models,.as.well.as.references,.can.be.found.in.Table 8.1.
of.Randall.et.al..(2007).
Simulations.of.twenty-irst-century.climate.require.projections.of.future.green-
house. gas. concentrations. and. of. sulfate. aerosols,. which. relect. sunlight. and. also.
promote. cloud. droplet. formation,. thereby. offsetting. the. effects. of. greenhouse.
gases. locally.. More. than. 40. such. projections. were. produced. under. the. auspices.
of. the. IPCC. (Special. Report. on. Emissions. Scenarios. [SRES];. Nakićenović. and.
Swart. 2000). after. considering. a. wide. range. of. future. socioeconomic. changes..
Three.of.these.“SRES”.scenarios.were.commonly.chosen.for.forcing.the.GCMs:.
B1,.A1B,.and.A2..The.climate.forcing.of.all.scenarios.is.similar.until.about.2020..
A2. produces. the. highest. climate. forcing. by. the. end. of. the. twenty-irst. century,.
but.before.mid-century,.none.of.the.scenarios.is.consistently.the.highest..Because.
more.modeling.groups.ran.A1B.than.A2,.and.because.our.focus.for.this.study.was.
on.mid-century.change,.we.chose.A1B.as.the.higher.emissions.scenario.and.B1.as.
the.low.emissions.scenario.for.a.regional.analysis.of.twenty-irst-century.Paciic.
Northwest.(PNW).climate..Although.B1.is.the.lowest.of.the.IPCC.illustrative.sce-
narios,.it.still.produces.changes.in.global.temperature.in.excess.of.2°C..That.value.
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