Isotopes
EES 2110
Introduction to Climate Change
Jonathan Gilligan
Class #18:
Monday, February 20
2023
CO2 vs. Methane
-
CO2:
- After 1000 years, around 30% of excess CO2 remains in atmosphere
- After 10,000 years, 13% remains
- After 100,000 years, 6% remains
- Methane (CH4):
- 31 times more powerful (molecule-for-molecule) than CO2
- Reacts with OH− (hydoxyl
radicals) and oxidizes into H2O
and CO2.
- Atmospheric lifetime: 9.6 years:
- After 25 years, 7% remains.
- After 100 years, 0.003% remains.
- When we reduce methane emissions, atmospheric methane drops
quickly
- When we reduce CO2
emissions, CO2 continues rising,
but more slowly
- When we stop CO2 emissions
altogether, CO2 only drops
slowly.
Weathering as
Thermostat
CO2 is balance of volcanic outgassing and chemical
weathering
- Higher temperatures:
- More rain, faster chemical reactions
- Faster weathering
- Atmospheric CO2 falls
- Lower temperatures
- Less rain, slower chemical reactions
- Slower weathering
- Atmospheric CO2 rises
- Net effect:
- Keeps temperature stable near some “set point”
- Set-point is determined by geology
Temperature of Earth
- Weathering acts as thermostat.
- Earth’s temperature has been remarkably stable over time.
- 4 billion years ago, sun was 30% dimmer…
- But there has constantly been liquid water.
- Geologic change alters thermostat “setting”:
- Volcanic outgassing
- Land surface (e.g., mountain ranges)
- Vascular plants
- In the long run, silicate thermostat will fix global warming…
- …but it will take tens to hundreds of thousands of years.
What’s Causing CO2 to Rise?
Carbon Isotopes:
Stable Isotopes: 12C and
13C
12C: 99% of all carbon on earth
-
13C: About 1%. Just like 12C, but
slightly heavier
- Greater mass → slower chemical reactions
- Molecules produced by photosynthesis have slightly less
13C, more 12C than the atmosphere.
-
Notation:
δC13=((C13C12)specimen−(C13C12)reference(C13C12)reference)
Unstable Isotopes:
14C
- 14C: Radioactive, unstable
- Produced in the atmosphere by cosmic rays hitting nitrogen
atoms
- Decays from 14C to 14N over thousands of
years
- Every 5,500 years, half of the 14C turns into
14N
- Measuring the amount of 14C relative to
12C in animal or plant matter tells you about the age since
it died.
Evidence: O2 and
13C
Evidence: 13C and
14C
Fossil Fuels vs. CO2
- Concentrations match 46% of fossil fuel consumption
Assessing the Evidence
- Decreasing O2: CO2 produced by burning or oxidizing.
- Not a mineral source (volcanoes).
- Decreasing δC13: CO2 must have biological origin.
- Decreasing δC14: The fuel must be
thousands of years old.
- Possible sources:
- Where are billions of tons per year of very old organic matter being
burned or oxidized?
- Rate of rise matches fossil fuel consumption.
- Therefore: Dominant source must be fossil
fuels.
Oxygen & Hydrogen Isotopes
and Past
Climates
Oxygen & Hydrogen Isotopes
δO18=((O18O16)sample−(O18O16)ref(O18O16)ref)×1000‰
-
δO18 compares
measured concentration of O18 to the concentration in
a
reference sample.
- Lighter isotopes (1H and 16O) evaporate
faster
- Vapor has less of heavier isotopes (smaller δO18, δH2=δD)
- Ocean is richer in heavier isotopes (greater δO18, δD)
- Warmer → greater
δO18, δD in vapor
Rain, Snow, Ice
Rain, Snow, Ice
- Rain, snow are richer in heavier isotopes
- More precipitation →
less deuterium (D, H2) and O18 left in vapor
- Farther from source region →
smaller δD and δO18.
- Reduction in δD and
δO18 depends on air
temperature.
- Comparing δD and
δO18 can tell us
about both sea-surface temperature and air temperature over
glaciers.
- Higher air temperature over
glacier →
greater
δD and δO18 in glacier
snow/ice.
Inside the Ice Core
Image credit: National Ice Core Laboratory
800,000 years of CO2 and
Temperature
Sediments and History
Bottom → top = oldest
→ youngest
Ocean Cores for Past
Climates
Deep-Sea Sediments
Past Sea Levels
- Water vapor, rain, snow is always isotopically lighter than sea
water
- Snow, ice on land remove light isotopes from ocean
- Bigger glaciers:
- Lower sea-level
- Greater (positive) δO18 in ocean sediments
- Smaller glaciers:
- Higher sea-level
- Smaller δO18 in
ocean sediments
Sediment Climate
Record
Summary of Oxygen Isotopes
- Two different uses:
-
δO18 in
glacial ice tells us about air
temperature:
- Greater δO18
means warmer temperature.
-
δO18 in
sea-floor sediments (skeletons of deep-sea organisms)
tells us about sea level:
- Greater δO18
means lower sea-level.
- During ice-age cycles:
-
cold
temperatures go with low sea-level
-
δO18 is
lower than usual in glaciers, greater in sea-floor
sediments.
-
warm
temperatures go with high sea-level:
-
δO18 is
greater than usual in glaciers, lower in sea-floor
sediments.
- But sea-level changes more slowly than temperature, so changes in
sediments usually lag behind changes in glaciers.
Review
- What do we learn from studying δC13 in the atmosphere?
- What do we learn from studying δC14 in the atmosphere?
- How do we know that the rise in CO2 comes from
burning fossil fuels?
- What do δO18
and δD in glacial ice
tell us about the past?
- What is different between what we learn from δO18 in ice and δO18 in sea-floor
sediments?
- How does the silicate weathering cycle act as a thermostat to
keep earth’s temperature stable?
- How do the lifetimes of CO2 and CH4 compare? Why is this
important?
Isotopes
EES 2110
Introduction to Climate Change
Jonathan Gilligan
Class #18:
Monday, February 20
2023