01-12-2000 1

Detection of anthropogenic climate change

Gabi Hegerl,

Nicholas School for the Environment and Earth Sciences,

Duke University

01-12-2000 2

Temperature trend 1901-2000

01-12-2000 3

Fingerprint methods: lin. regression

Estimate amplitude of model-derived climate change signals X=(xi),i=1..n

from observation y

Best Linear Unbiased Estimator

u: noise residual(Hasselmann, 79 etc, Allen + Tett, 99)Vector: eg Temperature(space,time), scalar product: Inverse

noise covariance

Signal pattern from model, amplitude from observation!

uxay ii

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June-July-August Greenhouse gas + sulfate aerosol

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uncertainty range Estimated from coupled model internal

variability

Safety checks: – Use model with strong variability – test consistency with observed noise

residual u

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Contribution of greenhouse gas and sulfate aerosols to to trend 1949-98

o: Greenhouse gas + sulfate aerosol simulation

+: Greenhouse gas only

o/+ inconsistent with observation

Ellipse: 90% uncertainty range in obs. Signal estimate

from: Hegerl and Allen, 2002

01-12-2000 7

reconstruction of NH warm season temperature

Forced componentFat: best fit to paleoThin: 5-95% range*: significant

The longer perspective

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Conclusions global/NH SAT

Significant climate change observed Uncertainty in distinction between

forcings, but: “Most of the recent (last 50 yrs) global

warming is likely due to greenhouse gases”

Significant and consistent climate signals in long temperature records

01-12-2000 9

Towards detection of anthropogenic changes in climate extremes

How to compare course-grid model with station data?

Can daily data be substituted by monthly/annual and shift in distribution => no

Which index to use for early detection (avoid baseball statistics!) that is moderately robust between models? Change in once/few times/yr events robust and strong

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Changes in precipitation extremes stronger

01-12-2000 11

Change in rainfall wettest day/yr NAmerica

Consensus

Observations show overall increase, too

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Annual mean precip changes consistent between two models

Wettest day/yr

Wettest 5 consecutive days

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Results: Anthropogenic vs natural

signals, time-space

Bars show 5-95% uncertainty limitsAllen et al, 2002

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Annual mean rainfall change NAmerica

consensus