the anthropogenic greenhouse effects

ESS 433 Frances Lin

(chiefly of environmental pollution and pollutants) originating in human activity – Google

anthropogenic (definition)

(chiefly of environmental pollution and pollutants) originating in human activity – Google

anthropogenic (definition)

UGH! People..

GHGs Basic

•  absorb and emit radiation within the thermal infrared range •  affect the

temperature within the Earth, therefore responsible for warming

•  H2O vapor •  CO2

•  CH4

•  N2O •  O3

Formula Contribution H2O 36 – 72 % CO2 9 – 26 % CH4 4 – 9 % O3 3 – 7 %

Kiehl, J.T.; Kevin E. Trenberth (1997)

Industrial revolution

The anthropogenic era is generally thought to have begun when the industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere.

Industrial revolution

The anthropogenic era is generally thought to have begun when the industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere.

?

TAKE HOME

Anthropogenic emissions of CH4 and CO2 first altered atmospheric concentrations in pre-industrial centuries. Ruddiman’s explanations:

1)  un-match pattern of CH4 and CO2 cycles 2)  natural forcing is NOT the cause 3)  explanation tied to early agriculture in Eurasia

CH4 & CO2 Cyclicity

Natural (Monsoonal) Source of CH4The orbital monsoon theory of Kutzbach (1981) states that •  ↑summer insolation heats

land and causes air to rise •  rising air ↓Psurface and draws

in moist air from the ocean •  the air rises over high

topography and cools, it drops moisture in heavy rains

•  rains flood wetlands, which release CH4

www.telegraph.co.uk

CH4 :Cyclic Variation

Long-term Vostok CH4 record of Petit et al., 1999

three preceding interglaciations

CH4 :Cyclic Variation•  expected pattern continued

until 5000 years ago •  increase could have come

from natural or human sources, or some combination of the two

CH4 :Cyclic Variation•  ongoing drying trend since

9000 yrs BP across tropical regions (COHMAP, 1988)

•  natural (monsoonal) forcing could NOT be the cause

•  wild rice cultivation started around 7500 yrs BP (Chang, 1976; Glover and Higham, 1996) and

•  rice irrigation, extensive flooding of wetland, began near 5000 yrs BP (Roberts, 1998)

CO2 :Cyclic Variation

•  natural orbital-scale CO2 trends are more complicated •  origins of these CO2 cycles are not yet clear •  why δ18O?

Why 18O? Just FYI•  δ18O ∝ global mean temperature • ↑ δ18O means more liquid water; higher T • ↓ δ18O means less liquid water; lower T

•  record(s) of paleoclimate change

CO2 :Cyclic Variation•  near 8000 years ago, CO2

trend began an anomalous increase that has no counterpart in any of the three preceding interglaciations

Previous Explanation for CO2•  natural loss of biomass •  changes in ocean CaCO3 chemistry

REASON: •  terrestrial carbon has an average δ13C value near –25 ‰; ocean carbon reservoirs close to 0 ‰ •  an atmospheric trend towards

negative δ13C values indicates a growing influx of terrestrial carbon

Natural loss of biomass

Indermuhle et al. (1999) proposed that the 20–25 ppm CO2 increase during the last 8000 years resulted from a slow natural loss of terrestrial biomass.

REASON: •  terrestrial carbon has an average δ13C value near –25 ‰; ocean carbon reservoirs close to 0 ‰ •  an atmospheric trend towards

negative δ13C values indicates a growing influx of terrestrial carbon

PROBLEM:

•  85% of the loss remained unexplained •  the net change simulated by

DEMETER, a process-based ecosystem model, was minimal

Natural loss of biomass

Indermuhle et al. (1999) proposed that the 20–25 ppm CO2 increase during the last 8000 years resulted from a slow natural loss of terrestrial biomass.

REASON:

•  growing forests took CO2 from the ocean-atm system, and caused deposition of extra CaCO3 in the deep ocean •  when forest expansion slowed near

8000 yrs BP, the net extraction of CO2 ended and caused dissolution of the ‘excess’ sedimentary CaCO3 previously deposited •  atm CO2 values gradually risen

Change in ocean CaCO3 chemistry

Broecker et al. (1999) proposed that the ocean could have caused the late-Holocene CO2 increase.

PROBLEM:

•  CO2 ‘rebound’ has been ~4 times the size of the early Holocene CO2 level •  no ↑CO2 occurred in any of the

last three interglaciations

Change in ocean CaCO3 chemistry

Broecker et al. (1999) proposed that the ocean could have caused the late-Holocene CO2 increase.

To conclude.. Natural forcing 1) natural (monsoonal) source of CH4, 2) natural loss of biomass and 3) ocean chemistry hypothesis can be rejected. By process of elimination… ↑CO2 of the last 8000 years points to an anthropogenic origin.

Industrial-era vs. Early-anthropogenic perspective

A. Industrial-era perspective suggests that most land clearance occurred in the last 200 years. B. Early-anthropogenic perspective suggests that much slower but longer pre-industrial land clearance occurred.


PROBLEM with Industrial-era views • neglects the impact of time • an enormous amount of

evidence of human influences on the Eurasian landscape many millennia before the industrial era


Pre-industrial emission Industrial emission

Smaller

Larger

Start early 200 years

7800 yr x 0.04 GtC/yr average = 320 GtC cumulative total

200 yr x 0.08 GtC/yr average = 160 GtC cumulative total

Earlier Agriculture in Eurasia•  land clearance near 8000 years BP •  deforestation

Significant Land Clearance near 8000 years BP

•  Europe between 8000 and 7000 yrs BP

•  China since 9400 yrs BP •  India since 8500 yrs BP

eastern Mediterranean, Zohary and Hopf (1993)

Extensive Deforestation between 8000 yrs BP to 2000 yrs BP

•  heavy deforestation (~50-75%) had occurred in Southeast Asia •  ‘persistent’ deforestation

(~25%) in north-central Europe

This total is ∼85–95% of the target needed to validate the hypothesis that humans caused the rise in CO2 after 8000 yrs BP.

Roberts, 1998; Lewthwaite and Sherratt, 1980

Inconsistency?QUESTION If this estimate is accurate, how could such a large anthropogenic warming have escaped notice?

Inconsistency?QUESTION If this estimate is accurate, how could such a large anthropogenic warming have escaped notice? One reason is that the warming was spread over 8000 years and thus imperceptibly gradual.

Inconsistency?QUESTION If this estimate is accurate, how could such a large anthropogenic warming have escaped notice? One reason is that the warming was spread over 8000 years and thus imperceptibly gradual. The main reason is that the anthropogenic warming has been masked by a larger cooling trend caused by decreasing summer insolation.

Effects on Climate Also, early gas emissions reached a global-mean value of 0.8◦C (2◦C at high altitudes); large enough to have stopped a glaciation of northeastern Canada…

Andrews et al.,1976; Willams 1978

•  A significant part of northeast Canada should then have been glaciated during the last millennium.

•  A significant part of northeast Canada should then have been glaciated during the last millennium.

•  Based on δ18O (‘ice volume’) cycles, northeast Canada is overdue for a glaciation; ice sheets should have begun to grow in the last 3000 to 6000 years (Imbrie and Imbrie, 1980).

RECAP anthropogenic GHGs emissions altered the atmospheric composition during pre-industrial era •  un-match patterns has been observed in CH4

and CO2 cycles •  natural forcing can be rule out •  rice irrigation, land clearance and deforestation

in Eurasia near 8000 yrs ago are responsible for the un-match patterns

warming is large enough to stop a glaciation in northeastern Canada

References Broecker, W. S., Clark, E., McCorckle, D. C., Peng, T.-H., Hajdas, I., and Bonani, G.: 1999, ‘Evidence for a

Reduction in the Carbonate Ion Content of the Deep Sea during the Course of the Holocene’, Paleoceanogr. 3, 317.

Charlson, R. J., Schwarz, S. E., Hales, J. M., Cess, R. D., Coakley, J. A., Hansen, J. E., and Hoffman, D. J.: 1992, ‘Climate Forcing by Anthropogenic Aerosols’, Science 255, 423.

Imbrie, J. and Imbrie, J. Z.: 1980, ‘Modeling the Climatic Response to Orbital Variations’, Science 207, 943.

Indermuhle, A., Stocker, T. F, Joos, F., Fischer, H., Smith, H. J., Wahlen, M., Deck, B., Masttroianni, D., Blunier, T.,

Meyer, R., and Stauffer, B.: 1999, ‘Holocene Carbon-Cycle Dynamics Based on CO2 Trapped in Ice at Taylor Dome, Antarctica’, Nature 398, 121.

Kutzbach, J. E.: 1981, ‘Monsoon Climate of the Early Holocene: Climate Experiment with Earth’s Orbital Parameters for 9000 Years Ago’, Science 214, 59.

Ruddiman, W. F.: 2003, ‘Insolation, Ice Sheets and Greenhouse Gases’, Quat. Sci. Rev. 22, 1597. Ruddiman, W. F. and Raymo, M. E.: 2003, ‘A Methane-Based Time Scale for Vostok Ice: Climatic Implications’,

Quat. Sci. Rev. 22, 141.

Ruddiman, W. F. and Thomson, J. S.: 2001, ‘The Case for Human Causes of Increased Atmospheric CH4 over the Last 5000 Years’, Quat. Sci. Rev. 20, 1769.

QUESTIONS?

the anthropogenic greenhouse effects

Environment