Large-Scale Temperature Changes During the Past

Millennium

Michael E. Mann, Department of Environmental Sciences

University of Virginia

Smithsonian Environmental Research CenterEdgewater Maryland

March 15, 2000

OVERVIEW

•Climate Change

OVERVIEW

•Climate Change

•Reconstructing the climate in previous centuries with “proxy” climate records

OVERVIEW

•Climate Change

•Reconstructing the climate in previous centuries with “proxy” climate records

•Diagnosing the factors influencing climate variations

OVERVIEW

•Climate Change

•Reconstructing the climate in previous centuries with “proxy” climate records

•Diagnosing the factors influencing climate variations

•Conclusions

GREENHOUSE EFFECT?

GREENHOUSE EFFECT

GLOBAL RADIATION BALANCE

ENHANCED GREENHOUSE EFFECT?

CO2 Concentration

Measurements of CO2 in parts per million (ppm) at Mauna Loa Observatory.

GLOBAL TEMPERATURE TREND SINCE MID 19th CENTURY

Greenhouse Gases and Warming

CO2

Related?

MANTRA OF PALEOCLIMATOLOGY

ASTRONOMICAL CLIMATE FORCING

Eccentricity

Obliquity

Precession

ASTRONOMICAL CYCLES AND ICE AGESASTRONOMICAL CLIMATE FORCING

ICE CORES

CORALS

TREE RINGS

HISTORICAL DOCUMENTS

VARVED LAKE SEDIMENTS

Map showing global proxy climate network. All of the records date back to at least 1820, most to the early 18th century, and many back to 1600. Before 1400, the network is too sparse to constrain large-scale patterns of variability without greatly expanded uncertainties.

INSTRUMENTAL TEMPERATURE

RECORD

GLOBAL PROXY

CLIMATE RECORDS

Five leading patterns of global temperature variation during the 20th century.

Calibration (1902-1980)

Verification (1854-1901)

Reconstructions of time histories of 5 leading temperature patterns. Note that fewer eigenvectors are reconstructed further back in time.

1791

RECONSTRUCTED GLOBAL

TEMPERATURE PATTERNS

RECONSTRUCTED GLOBAL

TEMPERATURE PATTERNS

NORTHERN HEMISPHERE MEAN ANNUAL DURING THE PAST MILLENNIUM

El Niño and La Niña

1791

El Niño

“Niño3”

CLIMATE FORCINGS

CLIMATE FORCINGS

Relationship of variations in Northern Hemisphere mean temperature reconstruction to estimates of three candidate forcings

Volcanism

Greenhouse Gases

Solar Irradiance

Temperature

CONCLUSIONS

•1990s likely the warmest decade in past 1000 years

CONCLUSIONS

•1990s likely the warmest decade in past 1000 years

•Recent El Ninos among the largest of the past few centuries

CONCLUSIONS

•1990s likely the warmest decade in past 1000 years

•Recent El Ninos among the largest of the past few centuries

•Natural (volcanic and solar) forcings have detectable influence during past several centuries

CONCLUSIONS

•1990s likely the warmest decade in past 1000 years

•Recent El Ninos among the largest of the past few centuries

•Natural (volcanic and solar) forcings have detectable influence during past several centuries

•Anthropogenic likely the dominant forcing during the late 20th century