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

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 State Buildings

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)

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

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

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

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 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

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

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

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…? 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

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

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

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

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

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

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?