CO2 and Climate Change

Lisiecki & Raymo, 2005

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black body

A theoretical object that absorbs all incoming electromagnetic radiation.

It emits radiation as a function of temperature.

Blackbody temperature

The sun radiates primarily in the visible.Earth radiates in the IR.Earth has a blackbodyTemperature below thefreezing point of water.

Sun (T~ 5780°K)

Earth (T~ 255°K)

Wavelength (microns)

Greenhouse effect

Atmosphere allows visible light to pass, but ‘greenhouse’ gases(H20, CO2, CH4) trap outgoing infrared and warm the Earth.

Siegenthaler et al. (2005)

CO2

(ppmv)

D (‰)Temperature

Epica Dome C ice core (Antarctica)CO2 (greenhouse gas) and D (temperature) vary repeatedly through multiple glacial cycles. The variations are approximately 90 ppmv.

Vostok ice core (Antarctica) CO2 and D are nearly in phase. Both lead 18O atm O2 (ice volume). Both lead Greenland D and 18O ice (Northern hemisphere temperature).

Time

Vostok ice core (Antarctica) CO2 and D are nearly in phase. Both lead 18O atm O2 (ice volume). Both lead Greenland D and 18O ice

(Northern hemisphere temperature).

Phase relationships (relative timing) These allow us to rule out several possible mechanisms for driving the CO2 changes.

Carbon reservoirsMost carbon is in solid earth.Ocean has most of the rest (60:1)Atmosphere small reservoir, but important!

Isotopic fractionation Stable isotope fractionation may be diagnostic tool.

Carbon reservoirs Different carbon pools are isotopically distinct.

Climate, ocean and land CO2

Climate changes themselves cannot account for the observed changes in CO2.

Changes in the terrestrial biosphere, evident in carbon isotopes in the ocean, are too small to explain CO2 change, and they indicate a shift in the wrong direction!

Biological pumpsto change atmosphericCO2

Productivity near the sea surface changes the chemistry of both the surface and deep ocean.

Coral reef hypothesis

Growth of coral reefs on flooded margins as sea level rises would have the effect of increasing atmospheric pCO2, but the timing is wrong.

Phosphate burial hypothesis

Burial of phosphate and other nutrients on shelves as sea level falls, and then release during sea level rise would have the right effect on atmospheric pCO2, but again, the timing is wrong.

Influence of pumps

Various pumps would have differing effects on atmospheric pCO2, and also on the isotopic composition of dissolved inorganic carbon (DIC).

Productivity

Biological activity resultsin systematic changes inconcentration and isotopicratio of bio-limiting and bio-intermediate elements.

Carbon isotopes

Photosynthetic fractionationof organic carbon leavesseawater enriched in heaviercarbon-13. The resultingIsotopic ratio in seawater isthen incorporated in CaCO3,providing a nutrient-tracer.

Isotopic influences

Photosynthetic fractionation results in a strong negative correlation between nutrients and carbon isotopes. Gas exchange, local productivity, and global reservoir shifts can also influence 13C.

So 13C can be used as a tracer for water masses (circulation), although gradients are more reliable than the absolute values.

NADW

GEOSECS

The meridional overturning circulation (MOC) produces North Atlantic Deep Water (NADW).

Evident in salinity and many other properties…

NADW

Kroopnick (1985)

Also evident in carbon isotopes (13C).

The meridional overturning circulation (MOC) produces North Atlantic Deep Water (NADW).

LGM meridional section, western basin

Curry and Oppo (2005)Paleocean circulation

The configuration was different, but not the rate of circulation?

Cadmium as a tracer

Dissolved cadmium isstrongly correlated with phosphate and nitrate.

Cd a “nutrient” tracerHigh Cd = high nutrients

Cadmium as a tracer

Dissolved cadmium in bottom water is reflected in benthic foraminifera shells.

Ocean circulation

‘Biological pump’ and ‘conveyor belt’ combine to distribute nutrients and ‘nutrient proxies’ in ocean.

Combined proxiesBoth carbon isotope and cadmium tracers support repeated glacial to interglacial changes in ocean circulation combine to distribute nutrients and ‘nutrient proxies’ in ocean.

Nutrient changes and pCO2.

Changes in the inventory or whole ocean distribution of nutrients could explain the observed shifts in pCO2.

But there is no evidence for change in whole ocean inventory, and no evidence for widespread oxygen depletion in the deep ocean. Signal is most evident in Atlantic, and is most likely circulation.

Nutrient shiftCarbon isotopes suggest a wholesale shift to lower values.Cadmium harder to discern. If anything, there is a small change to lower values in glacial, opposite required shift.

Ocean carbon shift Mean ocean isotopic ratio changed during ice age.

Focus on Southern OceanIt is the main region where deep ocean and in atmosphere are in nearly direct contact.

Southern OceanNutrient utilization (dust?), stratification (sea ice?), and/or circulation combined with carbonate compensationRemain the leading explanations for CO2 change.

Atmospheric CO2

I Greenhouse gases and the temperature of the Earth

II Ice core evidence that glacial pCO2 was 80 ppm lower.

III Could it be due to terrestrial biosphere change? No!

IV How did the ocean do it?Physical - chemical property changes (T, S)Physical pumpsBiological pumps (nutrients, Corg, alkalinity)