1. By: Karl Philippoff Major: Earth Sciences
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 Philippoff
Major: Earth Sciences

2. Why do we care? “Man’s greatest geophysical experiment” (Revelle)
Perturbing the carbon cycle
Will it stay the same? (positive/negative feedbacks)

3. Why do we care? Cont’d Releasing ~10 Gt C/yr (2010)
How much is a 1Gt C?
_?_ humans
_?_ Empire State
Buildings

4. 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)

5. Bathtub analogy sad 𝑰𝒏𝒑𝒖𝒕=𝑶𝒖𝒕𝒑𝒖𝒕+𝑺𝒕𝒐𝒓𝒂𝒈𝒆
Graphic: Nigel Holmes. Sources: John Sterman, MIT; David Archer, University of Chicago; Global Carbon Project

6. Short-term Carbon Cycle
Input
Input
Output
Output
Large fluxes, with little net flow
Numbers in ()’s are storage terms

7. Oceanic Carbon Cycle ‘Biological’ pump ‘Solubility’ pump Low High
Due to the fact that warm water cannot hold as much CO2 as cold water
solubility
Rough indication of the productivity of oceans
Low
High
 T

8. Terrestrial Carbon-Cycle
Major inputs:
Photosynthesis
Major outputs:
Respiration (by plants and microbes)

9. 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) + +

10. 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

11. Fully coupled results 0 No change Past Projected Results:
Airborne fraction increases from ½  ~ ¾
Land becomes source ~ 2050
Rates from 19502000 are comparable to observations
Net source
0 No change
Net sink
Source and sink values determined with respect to 1860
Cox et al, Fig. 2

12. 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

13. Carbon stored in Vegetation
Results, cont’d
Blue arrows show difference between climate feedback due indirect and direct effects of CO2 at GLOBAL scale(model runs 2 and 3) a
Carbon stored in Vegetation
Green arrows show difference between two model runs for South America
Total CO2 emissions(2004)
Amazon
Soil Carbon
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)
Cox et al., Fig 4

14. 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%

15. And the results of this are…?3 A
In 2100, [CO2] = 980 ppmv (250ppm > standard)
Average land temperatures increase 8K (5.5K standard) 1 3 2 1 2
Equivalent of moving from Columbus (11) Gainesville, FL (20) or Houston, TX (21)
Cox et al, Fig. 3

16. Melillo et al. study Harvard Forest Took soil CO2 fluxes (91-00)
Nitrogen mineralization (91-98)
Used 6 similar plots
+5C
Heating cables 6m 6m

17. Results: Soil CO2 fluxes ~80% of respiration due to soil microbes
Increased soil CO2 flux
Large Δ
Small to no Δ
Back to normal
𝐶 𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑 𝐻𝑒𝑎𝑡𝑒𝑑 −𝐷𝑖𝑠𝑡𝑢𝑟𝑏𝑎𝑛𝑐𝑒 𝐷𝑖𝑠𝑡𝑢𝑟𝑏𝑎𝑛𝑐𝑒 ∗100
Melillo et al, Fig.1

18. What does that mean? Two-pool model
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
Total amount of carbon stored in soil

19. Results, cont’d ad Mineralized Nitrogen is in the form NH4+, or NO3-
Many mid-latitude forests are nitrogen-limited
Large, consistent increase in usable N
Large increase in usable nitrogen
This increase had no effect on loss processes (leaching or gaseous) and led to a total increase of 41 g/m^2
Melillo et al., Fig.3

20. Results, cont’d Net: =556 g/m2 ∆ 𝐶 𝑡𝑜𝑡𝑎𝑙 = 𝐶 𝑓𝑖𝑥𝑒𝑑 − 𝐶 𝑟𝑒𝑠𝑝𝑖𝑟𝑒𝑑
CO2 uptake due to increased N mineralization
CO2 release due to increase in respiration by microbes
-944 g/m2
(measured in a different study in the same area)
+ ~1500 g/m2
∆ 𝐶 𝑡𝑜𝑡𝑎𝑙 = 𝐶 𝑓𝑖𝑥𝑒𝑑 − 𝐶 𝑟𝑒𝑠𝑝𝑖𝑟𝑒𝑑
= (1500 g/m2) –(944 g/m2)
=556 g/m2
Or ~60% greater than the Δ in respiration

21. 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)

22. 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

23. 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

24. Questions?