The Carbon Cycle:Acceleration of global warming due to Carbon-

Cycle feedbacks in a coupled climate model(Cox et al., 2000)

Soil warming and Carbon-Cycle feedbacks to the Climate System

(Melillo et al., 2002)

By: Karl PhilippoffMajor: Earth Sciences

Why do we care?

• “Man’s greatest geophysical experiment” (Revelle)• Perturbing the carbon cycle• Will it stay the same? (positive/negative feedbacks)

Why do we care? Cont’d

• Releasing ~10 Gt C/yr (2010)• How much is a 1Gt C?_?_ humans _?_ Empire StateBuildings

Where is it going?

• We can only account for ~ 50% of the CO2 we release (via accounting for the use of fossil fuels and deforestation)

• sad

Bathtub analogy

𝑰𝒏𝒑𝒖𝒕=𝑶𝒖𝒕𝒑𝒖𝒕+𝑺𝒕𝒐𝒓𝒂𝒈𝒆Graphic: Nigel Holmes. Sources: John Sterman, MIT; David Archer, University of Chicago; Global Carbon Project

Short-term Carbon Cycle

Numbers in ()’s are storage terms

Input

Output

InputOutput

Large fluxes, with little net flow

Oceanic Carbon Cycle

• ‘Biological’ pump• ‘Solubility’ pump

Low HighRough indication of the productivity of oceans

Due to the fact that warm water cannot hold as much CO2 as cold water

T

solubility

Terrestrial Carbon-Cycle

• Major inputs:– Photosynthesis

• Major outputs:– Respiration

(by plants and microbes)

Cox et al. article

• Used a coupled ocean-atmosphere model and added the oceanic carbon cycle (solubility, exchange, biological pumps) and dynamic vegetation (TRIFFID)(5 functional plant types)

+ +

3 Scenarios

• All used base ocean-atmosphere model• 1)Emissions and fixed vegetation (standard

GCM)• 2)’Interactive’ CO2 and dynamic vegetation but NO

indirect effects of CO2 (Temp, H2O,etc.) only the fertilization component

• **3)Fully coupled simulation (Including all feedbacks)**

• Limitations: aerosols, large-scale ocean, no deforestation

Fully coupled resultsPast Projected

Net source

Net sink0 No change

Cox et al, Fig. 2

Rates from 19502000 are comparable to observations

Results:1) Airborne

fraction increases from ½ ~ ¾

2) Land becomes source ~ 2050

Source and sink values determined with respect to 1860

Wait…what happens around 2050?• Photosynthesis usually increases

when CO2 concentrations increase (fertilization), assuming other resources are not limiting (sink/input)

• Plant maintenance (respiration) and microbial respiration increase with temperature (source/output)

• Around 2050, outputs begin to exceed inputs, reducing terrestrial carbon storage

Before 2050 After

2050

+ -

Results, cont’d

• a

Cox et al., Fig 4

Amazon

Blue arrows show difference between climate feedback due indirect and direct effects of CO2 at GLOBAL scale(model runs 2 and 3)

Green arrows show difference between two model runs for South America

Climate feedbacks completely change the terrestrial carbon cycle, especially in the Amazon and for soil microbes

For scale, the change in soil carbon between the two runs is roughly ~2X our cumulative CO2 emissions (~290Gt C)

Carbon stored in Vegetation

Soil Carbon

Total CO2 emissions(2004)

Oceanic carbon cycle

• Oceans show saturation effect at high CO2• Partially caused by– Non-linear dependence of total ocean carbon

concentration to atmospheric carbon– Slower ocean circulation (-25%)– Thermal stratification reduces upwelling, causing

primary productivity to decrease ~5%

And the results of this are…?

• A

• In 2100, [CO2] = 980 ppmv (250ppm > standard)• Average land temperatures increase 8K (5.5K standard)

1

Cox et al, Fig. 3

2

3

1

2

3

Equivalent of moving from Columbus (11) Gainesville, FL (20) or Houston, TX (21)

Melillo et al. study• Harvard Forest• Took soil CO2 fluxes (91-00)• Nitrogen mineralization (91-98)

6m

6m

Heating cables

Used 6 similar plots

+5C

Results:

𝐶𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑 (𝐻𝑒𝑎𝑡𝑒𝑑−𝐷𝑖𝑠𝑡𝑢𝑟𝑏𝑎𝑛𝑐𝑒𝐷𝑖𝑠𝑡𝑢𝑟𝑏𝑎𝑛𝑐𝑒 )∗100

Increased soil CO2 flux

Large Δ Small to no Δ

Back to normal

Melillo et al, Fig.1

~80% of respiration due to soil microbes

Soil CO2 fluxes

What does that mean?

• Two-pool model

Total amount of carbon stored in soil

Small amount of carbon (~10%) that is easily broken down by microbes (polysaccharides)Sensitive to Temperature

Large amount of carbon (~90%) that is more difficult (aromatic rings)Insensitive to Temperature

Results, cont’d

• ad

Melillo et al., Fig.3

Large increase in usable nitrogen

Large, consistent increase in usable N

This increase had no effect on loss processes (leaching or gaseous) and led to a total increase of 41 g/m^2

Mineralized Nitrogen is in the form NH4+, or NO3-

Many mid-latitude forests are nitrogen-limited

Results, cont’d

Net:

CO2 release due to increase in respiration by

microbes-944 g/m2+ ~1500 g/m2

CO2 uptake due to increased N mineralization

∆𝐶𝑡𝑜𝑡𝑎𝑙=𝐶 𝑓𝑖𝑥𝑒𝑑−𝐶𝑟𝑒𝑠𝑝𝑖𝑟𝑒𝑑= (1500 g/m2) –(944 g/m2)

=556 g/m2 Or ~60% greater than the Δ in respiration

(measured in a different study in the same area)

Caveats to study:• Would also be affected by other quantities tied to

climate change such as: (effect on CO2 flux in())– Water availability (+ with increase, - with decrease)– Temperature effects on plant photosynthesis and

respiration (+/-)– Increase in concentration of CO2 (+)– Also warming will probably have its largest effects

on high-latitude ecosystems (large amounts of C)

In Summary…

• Carbon cycle is complex with many portions, both in the terrestrial and oceanic components

• The presence of a multiplicity positive (decrease in soil carbon) and negative (increase in biomass) feedbacks make it difficult to predict how it will respond in the future

• Not only this, but some signal to noise problems as well

What I think…• As the papers demonstrate, there is still large

uncertainties associated with following our excess carbon after it exits the atmosphere.

• Very interesting to see the 2 papers more or less contradict each other.

• Future directions: – Still have little idea of the controls of the controls on primary

productivity and respiration in global sense (biomes, species) and how they would respond to a change in their environment (and we don’t know how that will change either… (Amazon from paper #1) (like to trying to hit a moving target )

– Explicit modeling of such complexity has only just begun, and with all the feedbacks in play, it will probably take some time to get a good handle on it

Questions?