L24 - Biogeochemical cycling in a human dominated world Flashcards
Outline the role of vegetation in the carbon cycle
How long can carbon remain stored in vegetation?
How can vegetation be converted into other major types of carbon stores?
- Photosynthesis fixes atmospheric CO2 as simple sugars
- Simple sugars often metabolised into complex organic molecules
- Much C respired within days
- Starch lasts for weeks to months
- Polymers e.g. cellulose, lignin, chitin lasts for decades to centuries
Dead tissue preserved under hypoxic conditions decomposes slowly to form:
- Peatland (<10,000 years old)
- Coal (ancient buried peatland)
- Oil (buried bodies of algae + zooplankton)
Rank the major carbon stocks in order of amount of carbon stored
Approximately equal and low:
- Plants (560 Pg)
- Surface ocean (725 Pg)
- Atmosphere (750 Pg)
- Soils (1,500 Pg)
Larger Stocks:
- Fossil fuels (5,000 - 10,000 Pg)
- Deep oceans (40,000 Pg)
- Earth’s crust (100,000,000 Pg, much in calcareous rocks)
Briefly describe the role of GHGs in energy fluxes
- Incoming and outgoing fluxes of solar radiation virtually equal
- Incoming flux dominated by visible light, outgoing flux by longwave radiation
- GHGs absorb longwave radiation and partly reradiate this back to Earth
What causes the main circulation of heat and water across the globe? What is the significance of this?
- Radiant energy flux greatest at tropical latitudes
- Air temperature (= air pressure) gradient drives global wind + precipitation patterns
- Majority of water returns to ocean but some precipitated on land
- These fluxes of water necessary for life + for interior vegetation
How much of the world’s water is freshwater?
What percentage of fresh water is held in lakes and rivers? Where is the remainder held?
- 2.5% is freshwater
- Only 0.3% of freshwater held in lakes and rivers
- 31% held as groundwater
- 69% held in glaciers + permanent snow
How long is the residency time of water in the atmosphere and organisms, oceans and ground water respectively?
When did many aquifers form and why is this significant?
- Water passes through organisms + atmosphere in a few days
- 4,000 years in oceans
- Generally much longer in glaciers + groundwater
- Many aquifers formed during cool, wet periods of Quaternary + Holocene
- Tapping into “fossilised” water is unsustainable
What are the main two ways in which humans are influencing the global carbon cycle?
Why is atmospheric C not rising as rapidly as expected from the two factors above?
- Oxidising fossilised C for fuel + cement
- Land use change influencing soil + vegetation C stores
- Atmospheric increase of 46% total emissions because:
a) Carbonic acid formed, accumulates in oceans + acidifies
b) Forests gaining C causes a sink
What is radiative forcing?
List some of the factors influencing radiative forcing and how they may change due to humans?
- A measure of how greatly an earth-system component affects the energy budget
GHGs have a positive radiative forcing effect
Albedo has a generally negative effect
- Earlier snow melt and vegetation changes may increase (make more positive) albedo
What proportions of extracted water are used for what functions globally?
- 69% agriculture
- 19% industry
- 11% homes + offices
Give two case studies that describe the effect of irrigation in seasonally dry regions on aquifer water supply
How is aquifer size measured?
Ogallala Aquifer, Central USA:
- 0.5 million km^2
- Over 200,000 wells drilled since 1940s
- Accounted for 20% of irrigated farmland
- Lost 5% depth in one decade
- Predicted to be depleted in one century
Saudi Arabia:
- Aquifer below desert from Pleistocene
- 20 trillion litres water exported per year in mid-1990s peak
- Saudi became 6th largest exporter of wheat!
- Now largely depleted, farming industry contracted, deep bore holes needed
- Springs dried up, e.g. around ancient settlements
GRACE mission measures anomalies in gravitational field using satellites
Give an example of an aquifer that can be recharged by rain water?
What effect does irrigation have here?
Aquifer under the Central Valley of California
- Recharged by rainwater
- Major agricultural region for almonds
- During drought, irrigation vastly outweighs replenishment
- Falling water table = compaction of ground, reducing capacity to refill
Give a classic example of the effects of unsustainable exploitation of rivers and lakes
The Aral Sea:
- Rivers flowing into lake diverted to irrigate cotton crops in USSR
- Sea shrank to 10% original size + salinified
- Thriving fishing industry destroyed
- Eastern region renamed Aralkum Desert
How much do evaporation and transpiration contribute separately to “evapotranspiration”
80 - 90% of water flux is transpiration (Jasechko et al. 2013)
- Based on analyses of ratio of stable isotopes of O2 and H2O in lakes + rivers
- Discrimination against heavier isotopes much stronger during evaporation
- Shows efficiency of plant water transport + small amount of freshwater lakes + rivers
Describe the theoretical expected trend in evapotranspiration (ET) with global temperature increase and the factors that may affect this
Describe the measured trend in ET since the 1980s
- POTENTIAL ET increases as air temp. increases
- ACTUAL ET only possible if there’s adequate water in soil + if leaves keep stomata open under stress
- ET measured using network of “eddy covariance towers” that measure vertical H20 and CO2 fluxes
- ET increased as expected through in 80s and 90s
- Post-1998 limited soil moisture prevented further increase
- Superficial soil moisture closely correlates w/ ET
What effect can land use change have on evapotransipration?
Converting natural vegetation to cropland has two main effects:
1) Water retaining capacity and permeability of soil reduced
2) Reduced vegetation cover for periods of the year from harvest or ploughing, reducing evapotranspiration
But evapotranspiration depends on the crop:
- E.g. perennial crops like oil palm and coffee have transpiration rates of a rainforest
Why is evapotranspiration important?
- Recycles precipitation across continents
- Cools air above land
- Increases the planet’s albedo through cloud formation
- Has knock on effect in carbon and energy cycle (and vice versa)
