Global Carbon Cycling Where does it all go?

Main Concepts Pre-anthropogenic CO 2 fluxes in and out Current CO 2 fluxes What are C reservoirs? Carbon Residence time? Timescales of carbon removal from the atmosphere.

IPCC AR5 (2013) Carbon: Ins and Outs

Atmospheric CO 2 What are the major sources of C emissions? How unique are modern CO 2 levels? Where does it all go? How long will it stick around?

Fossil fuel CO 2 emissions: Burning buried sunshine

Carbon emissions rising faster than estimates

Global C emissions map Where emissions come from

Atmospheric CO 2 :Last 50 years (2.0 ppm/year increase, or 0.5%) 400 ppm

It’s alive! Seasonal cycle

CO 2 growth rates

CO 2 growth rates

What do we know about greenhouse gases and past climate change?

Glacial ice “traps” ancient air Snow accumulates… Snow becomes ice Pore spaces are sealed and they trap ambient air. Up to 800,000 year old ice… with ancient trapped air bubbles! Free air Trapped air

Atmospheric CO 2 : Last 250 years

Atmospheric CO 2 : last 400,000 years!

Atmospheric CO 2 : Last 50 MILLION years How unusual are modern CO2 levels?

Carbon fluxes (in Gt/yr), reservoirs (bold, Gt), and residence times (years) Note: 2010 emissions were 9 Gt / year 1990s data

How much is a gigaton (Gt)? One billion metric tons (10 12 kg) It is about 2750 Empire State Buildings. Global C emissions are about 9 Gt as of How much does global population weigh? 7 x 10 9 people x 10 2 kg/person 7 x kg = 0.7 Gt

AR5 Observed carbon fluxes ReservoirPre-Ind Fluxes (Gt/year) Current Flux (Gt/year) Photosynthesis Respiration Ocean Fossil fuels emissions +7.8 Land Use changes +1.1 “Other” (volcanoes, lakes, rivers) Atmosphere CO 2 increase Negative (positive) means removed from (added to) the atmosphere; IPCC AR5 data)

Carbon ins and outs Source: Carbon Emissions 7.8 Gt/year Deforestation 1.1 Gt/year Sink: Obs. Atm increase -4.0 Gt/year Ocean uptake -2.3 Gt/year “missing sink” -2.6 Gt/year IPCC AR5 data

Human Carbon emissions 2012 emissions are ~9 Gt… were about 6 Gt when I started teaching this course !

Deforestation accounts for an additional +1.1 Gt / year Deforestation - Mainly tropical rainforests - Cutting down forests to make agricultural land is a net source of carbon to the atmosphere. CH 2 O + O 2  CO 2 + H 2 O Bolivia ( )

Where do our carbon emissions go? Ocean takes up about -2.3 Gt / year Roughly one-third of our fossil fuel emissions Air (CO 2 ) Sea (CO 2 ) CO 2 + H 2 O  H + + HCO 3 - Oceanic “Buffer reaction”

Why does the ocean take up CO 2 ? CO 2 gas is soluble in the ocean -Gas solubility is highest in colder water -CO 2 enters the oceans at the poles -CO 2 is converted to HCO 3 - by “buffer reaction” -The ocean acidifies as a direct result Ocean “buffer chemistry” can take up only a finite amount of CO 2.

Air-Sea CO 2 fluxes Ocean uptake Ocean release Gases are more soluble in COLD water Ocean uptake Ocean release Net: -2 Gt/yr

Where is our carbon in the oceans ? Vertical Sections through the oceans Total ocean uptake is about -2.5 Gt / year

Carbon ins and outs Source: Carbon Emissions 7.8 Gt/year Deforestation 1.1 Gt/year Sink: Obs. Atm increase -4.0 Gt/year Ocean uptake -2.3 Gt/year “missing sink” -2.6 Gt/year IPCC AR5 data

What is the “missing sink” The “missing sink” is the amount of carbon required to balance sources and sinks. It is a big number: -2.6 Gt Carbon / year ! What is it ???

The Missing Sink (history)

Missing C sink: 1-2 Gt CO 2 fertilization “CO 2 fertilization” of high-latitude forests Plants grow faster/better at higher CO 2 But … the effect is assymptotic (not linear) Atm CO 2 level Plant C uptake

Other things we need to know Not only Fluxes of carbon in/out (Gt / year) Sizes of the carbon reservoirs Residence Time of carbon in each reservoir These additional factors determine who the biggest players are and how quickly they will act.

Why these things matter What would happen to CO 2 levels if we stopped all emissions today? What if the ocean warms up a lot? What if deep ocean circulation were to change ? Does Arbor day matter ?

Ocean and Atmoshere C reservoirs Atmosphere: 1580 Gt (as CO2) Ocean C: 39,000 Gt (as HCO3-, CO32-) Ocean has 50x more carbon than the atmosphere.

Residence time Residence time is a “replacement time”: time required to affect a reservoir given a certain flux.  (years) = reservoir / input rate Example: Residence time of a CU undergrad Reservoir: Size of Columbia’s UG Student Body? Input rate: Incoming 1st-year class size

Calculating residence time of Carbon due to air-sea exchange Ocean uptake rate: -2.0 Gt / year Total Ocean C reservoir : 39,000 Gt Surface Ocean C reservoir : 600 Gt C residence time (surface only) = ? C residence time (whole ocean) = ?

The fate of fossil fuel CO 2 Q: How quickly will the planet take up our CO 2 ? A: Not very quickly… Fast: “solubility pump” Air-Sea CO 2 exchange (centuries) Moderate: “Deep ocean acid neutralization” (tens of thousands of years) Really slow: “Weathering of continental rocks” (millions of years)

Fastest response (decades to centuries): The CO 2 solubility pump Air-Sea gas exchange

Medium response time (10 4 years): Neutralize ocean acidity Neutralize deep ocean acidity by Dissolving ocean CaCO 3 sediments CaCO 3  Ca 2+ + CO 3 2-

Really Slow response time (10 6 years) Continental weathering (dissolves mountains!) “Urey reaction” - millions of years CaSiO 3 + CO 2 --> CaCO 3 + SiO 2

75% in 300 years 25% “forever” Time of removal

Bottom Line Human C Emissions are large Nature can’t keep up Natural C sinks are diminishing Lifetime of CO 2 from your tailpipe: “300 years, plus 25% that lasts forever”

Radiative Forcing Helps us quantify how global climate responds to an imposed change (“forcing”).

What is Radiative Forcing? Radiative forcing: An imposed change in Earth’s radiative energy balance. Measured in Watts per square meter (W/m 2 ) “Radiative” because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance sets the Earth’s surface temperature. “Forcing” indicates that Earth’s radiative balance is being pushed away from its normal state. Examples: Solar variability, volcanic emissions, greenhouse gases, ozone, changes in ice cover (albedo), land use changes.

Our first climate model Recall how to calculate Earth’s effective temperature The Stefan-Bolzmann equation: Blackbody radiation I (w/m 2 ) =  T 4 Earth incoming radiation (  = Earth albedo, or reflectivity) I incoming = (1-  ) I solar = (1-  )  T sun 4  Is ~0.3, or 30%

Our first climate model Earth incoming radiation (  = Earth albedo, or reflectivity) I incoming = ((1-  ) I solar ) / 4, or ((1-  )  T sun 4 )/ 4 Earth outgoing radiation I outgoing =  T earth 4

Earth’s temperature with no greenhouse effect T effective = 254.8K (-18°C) At equilibrium, I incoming = I outgoing Set Sunlight = Earthlight Solve for T earth Eqn. 3.1 in Archer Chapter 3

Volcanic eruption can change albedo by 1%  = ~30% on average T effective = 254.8K Increase  to 31% New T effective = 253.9K or -1°C cooler due a volcanic eruption Recalling I = (1-  )  T 4

Adding an atmosphere

Greenhouse gases are “selective absorbers”of outgoing long wavelength radiation (Earthlight) Spectrum of IR light emitted from earth to space

Water Vapor Molecule (H 2 O) Vibrational modes H 2 O bend H 2 O stretch

Carbon Dioxide Molecule (CO 2 ) Vibrational mode (~15µm) CO 2 bend

Natural CO 2 radiative forcing Makes Earth habitable Pre-Industrial CO2 level of ~280 ppm Increases surface temperature from -18°C (effective temperature) to +15°C (Water vapor is also important)

CO 2 “Band Saturation” More CO2 warms the Earth less and less 10 ppm 1000 ppm100 ppm No CO2 Notice the CO 2 absorption band

CO 2 and surface warming just due to radiation changes - no feedbacks About 1°C per 100 ppm Pre-Industrial = 280 ppm Today = 390 ppm So, w/o feedbacks: ~1.2°C With feedbacks: ~3°C (Feedbacks include water vapor and sea ice changes) CO2 ppm Temp (K)

Atmospheric CO 2 CO2 has increased by about +40% Long term average growth rate is +1.4% per year Last decade growth rate is +2.0% per year CO 2 (ppm)

All Radiative Forcing factors ( ) Sum = +1.6 W/m 2

Radiative Forcing Contributions GHGs warm (CO2, CH4, N2O) H2O (vapor) warms Tropospheric O 3 warms, Strat O 3 cools Human and natural Aerosols cool Solar irradiance warms Net Effect: +1.6 W/m 2