The Holocene Climate Optimum at 8000 yrs. It’s been warmer recently

The Holocene Climate: the place/time where we live. The Last 8000 years.

According to orbital parameters, we should be PAST the present Inter-Glacial Period and approaching another GLACIAL PERIOD (i.e., reduction in Northern Hemisphere sunlight, in the summer).

BUT – we don’t seem to be getting colder… The last remnants of the Laurentide Ice Sheet vanished about 7000 years ago.

The Holocene Climate: the place/time where we live. The Last 8000 years.

Solar Insolation

Solar insolation during the Holocene was about the same as it was during the Last Glacial Maximum; and during the previous glacial/inter-glacial cycles, when the solar insolation changed, the climate changed.

But not now, in the present.

Holocene Features –

1. A mid-Holocene temperature (and CO2) rise that started about 8,000 year BP.

2. Atmospheric methane also started to rise, but later - about 5,000 years BP.

3. We are now here, and large-scale agriculture and anthropogenic de-forestation started about 6,000 years BP.

Things to Explain about the Holocene

1. How does the present inter-glacial period (the Holocene) compare to previous inter-glacial periods?

2. Why aren’t we heading toward another glacial period?

3. Why did the atmospheric CO2 and CH4 start to rise in mid-Holocene?

4. What caused the recent (2000 BP to 1750 AD) cooling?

5. Is any of this anthropogenically-caused (are humans responsible)?

Carbon Dioxide Cycle

CO2 cycles: affected by all orbital frequencies, dominated by 100 Ka cycle.

Greater impact on climate than methane changes.

What should CO2 levels be today?

• Chose a CO2 value which corresponds to early-interglacial CH4 minima 11,000 years following insulation max. (above, Vostok ice core data)

What should CO2 be today?

• Taylor Dome CO2 concentration showing increase at 8 Ka when it should be decreasing.

• Should be 240-245 – is 280-285 ppm

• ~40 ppm excess.

During the last 8,000 years, Antarctic ice cores show an increase in atmospheric CO2 of 20 ppmv that is difficult to explain.

Possible explanations of the rise include

1. Decrease in terrestrial storage;

2. Coral reef re-growth;3. Early Anthropogenic slash

and burn agriculture.

Ice Core Data - 13C and CO2 measured in air trapped in ice from Dome C, Antarctica.

Ruddiman “we-warmed-the-Holocene” Hypothesis

Anthropogenic greenhouse gases began to influence climate 6,000 to 8,000 years ago (4 to 6,000 years BC) due to the rise of agriculture.

• Major destruction of forests caused CO2 to increase in atmosphere – affected climate by 8000 years ago.

• Rice production and irrigation produced enough CH4 by 5000 BP to affect climate.

Secondary Ruddiman Hypothesis

Deaths due to pandemic plagues and subsequent abandonment of farms during the last 2,000 years, and

resulting re-growth of forests,

sequestered CO2 and cooled climate periodically.

Early anthropogenic hypothesis.

Human activities during the late Holocene caused increases in

(a) CH4 and

(b) CO2

That are recorded in ice cores, and this is in contrast to the downward trends in these GHSs during previous interglaciations (during similar periods with the same orbital parameters).

(c) Late Holocene greenhouse gasincreases preventing much of the natural cooling that occurred in previous inter-glaciations.

The closest 65°N insolation analog within MIS stage 11 indicating that the current (Holocene) interglacial period should have ended by now.

Comparison of trends of (left) methane and (right) CO2 during the last five interglaciations based on the Vostok gas timescale of Ruddiman and Raymo [2003] and Ruddiman [2005a].

SUPERIMPOSE ALL THE GLACIAL TERMINATIONS ON TOP OF EACH OTHER, based on orbital parameter-based time scale.

Only the Holocene shows an increase in gas concentrations.

Looking at comparing the Holocene to previous interglacials – just the total summer insolation.

Caloric summer half-year insolation trends during the early parts of the last five interglaciations.

The timescales are aligned on the first precession insolation minimum in each interglaciation.

At and near the point of alignment the insolation values most similar to modern levels occur in stage 9.

Warm enough to prevent glacial expansion

The Impact of a Glacial Threshold

The glacial threshold depends on positions of continents, CO2 levels, ocean circulation rates, etc.

Estimated anthropogenic CH4 anomaly of 230 ppb in the year 1500 accounted for by contributions from several sources:

• emissions from livestock and human waste (both linearly tied to human population),

• large emissions from biomass burning and irrigation, and • climate system feedbacks.

Why excess CO2? Deforestation due to Growth in Agriculture

Europe-Middle EastEurope-Middle East: Significant agriculture 7.8 Ka, : Significant agriculture 7.8 Ka, Ox-drawn Plows at 6 Ka.Ox-drawn Plows at 6 Ka.

ChinaChina: Agriculture at 9.4 Ka, Substantial at 6 Ka: Agriculture at 9.4 Ka, Substantial at 6 Ka

IndiaIndia: Agriculture at 8.5 Ka, Substantial at 8 Ka: Agriculture at 8.5 Ka, Substantial at 8 Ka

Horses Domesticated by 6 Ka.Horses Domesticated by 6 Ka.

Water Buffalo by 5 KaWater Buffalo by 5 Ka

Why Increasing Methane after 5 Ka?

Rice Production• Wild rice cultivation by

7.5 Ka• Irrigation of rice near

5.0 Ka.• China and India had

large-scale irrigation projects.

• Inefficient practices, weeds, no fertilizer, large CH4 / person.

Population scientists estimate that all river valleys and flat lands were in food production by then, because they can date the construction of terraces on valley walls (which are labor-intensive places for food production).

Growth of Agriculture

Pattern of deforestation in England based on historical-ecological data.

Early phase of large-scale land clearance for agriculture (‘‘agricultural deforestation’’) continued until all arable land was in use (soon after 1089 in England).

The later (and final) phase of deforestation of higher, more remote terrain (‘‘resource deforestation’’) was smaller in magnitude. Per capita deforestation peaked near the end of the phase of agricultural deforestation and then fell substantially (use of coal).

Geographical distribution of griculture by 2000 BP (0 AD)

What about lack of CO2 increase during last 2,000 years – seen in ice core data below?

Plagues Reforestation

What about lack of CO2 increase last 2,000? Plagues Reforestation

• Example: Bubonic Plague 540 to 542 AD – Est. 50% of population in England died– 25% in Europe and North Africa died

• Plagues encouraged by Agriculture practices

Bubonic Plague – Rat lice

Smallpox – from Pigs

• Cholera – India, China, Europe

Black Bars, epidemics and population reduction.

Red line; extrapolated CO2 levels from previous data

Circles: CO2 levels measured from Antarctic ice cores

Plague-Induced Reforestation Estimates

• Jostinian Plague 164 to 592 AD - ~22% re-forestation.

• Black Death 1347 to 1352 – 25 to 45% re-forestation.

• Americas – 1550 to 1800 Native American population decimated by 90% (55 million to 5 million) – 5 to 10% of forests re-grew.

Plague versus volcanism and solar forcing; alternative hypotheses

Industrial Era View

Impulse response function (Joos et al. [2004]) showing the effect of carbon system exchanges on the amount of carbon in the atmosphere.

If 10 Gt C were suddenly sequestered in forests, the initially large perturbation(reduction) in atmospheric CO2 concentrations would decrease rapidly for several decades and then much more slowly over centuries and millennia.

Reservoir Exchange Rates

Gigatons C/yr or Pg C/yr

(a) Model simulation of the effects of major depopulation intervals on atmospheric carbon. Solid black lines are carbon lost from the atmosphere and sequestered in forests, based on historical data.

(b) Solid red line shows the simulated effect of reforestation on atmospheric CO2 concentration resulting from exchanges with other carbon reservoirs;

(c) dashed red line shows additional reductions caused by reductions in deforestation and coal burning.

Lower Panel: Composite CO2 target

signal compiled from the well-dated Law and Taylor Dome record.

Net anthropogenic contributions during the Holocene to

(a) atmospheric CH4 concentration,

(b) CO2 concentration, and

(c) global mean temperature.

a decrease in terrestrial carbon storage – fewer trees and vegetation;

increases in sea surface temperature – seawater less soluble to CO2

Ca2+ + 2 HCO3- → CaCO3 + CO2 + H2O  

coral reef regrowth,

Ruddiman [2003] proposed that the CO2 growth was caused by human activities including CO2 emissions due to slash and burn agriculture. Basically, we did it.

Alternative Hypotheses – not everyone buys the Ruddiman theory.

...”Ice core data of atmospheric CO2 between 8 ka BP and preindustrial climate can only be reproduced if CO2 outgassing due to shallow water sedimentation of CaCO3 is considered….

We can therefore explain the late Holocene rise in CO2 by invoking natural forcing factors only, and anthropogenic forcing is not required to understand preindustrial CO2 dynamics.”

How does this work?

Ca++ + 2HCO3- --> CaCO

3 + H2O + CO2 (forams)

and Corals make their skeletons from aragonite; a form of calcite.

If they re-grow vigorously, this formation of calcium carbonate will release CO2 into the atmosphere, which could give the 20 ppm CO2 elevation observed in the mid-Holocene.

During the last 8,000 years, Antarctic ice cores show an increase in atmospheric CO2 of 20 ppmv that is difficult to explain.

Possible explanations of the rise include

1. Decrease in terrestrial storage;

2. Coral reef re-growth;3. Early Anthropogenic slash

and burn agriculture.

Ice Core Data - 13C and CO2 measured in air trapped in ice from Dome C, Antarctica.

So now you have a hypothesis with well-respected climate scientists on both sides.

1. Early Human agriculture (and subsequent plagues) caused elevated (by 20 ppm) CO2 levels in the mid-Holocene – which decreased when the population underwent dramatic reductions.

or

2. The mid-Holocene CO2 increase of 20 ppm was NOT anthropogenically caused, but was (instead) driven by increased calcium carbonate formation in coastal waters.

The jury is still out on this issue.

Climate for the last 1,000 years puts the Holocene warming in perspective.

Anthropogenic sources have raised CO2 levels from 280 ppm to 387 ppm in last 150 years. And increased temperatures by 0.76 °C.