Long-Term Climate Regulation (Ch.12) The Faint Young Sun Paradox Revisited (P )

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1 Long-Term Climate Regulation (Ch.12) The Faint Young Sun Paradox Revisited (P )

2 Dating with radioactive elements Carbon 14 (read Ch.5, P97-98)

3 Natural radioactivity Atomic nuclei are composed of p protons and n neutrons Stable atoms have p ~ n

4 Natural radioactivity Atomic isotopes are atoms with the same p but different values of n Carbon has 6 p (atomic # = AN) C 12 has 6 n (atomic wt = AW = 12) C 13 has 7 n (AW = 13) C 14 has 8 n (AW = 14) N 14 has 7 p and 7 n (AN = 7 and AW = 14)

5 Natural radioactivity C 12 and C 13 are stable (most C is C12) C 14 is unstable too much mass (14) for the amount of charge (6) C 14 decays spontaneously by converting one n to a p (net +) and spitting out an electron (charge conservation)

6 Natural radioactivity Cannot predict when a particular C 14 atom will decay, BUT statistically For any sample of C 14 atoms, ½ will decay in 5730 years (half life)

7 Provides an atomic timer If you know how many C 14 you have in a sample and how many you started with, you can date the sample Natural radioactivity

8 Two questions Where does the C 14 come from? Shouldn t it all have decayed long ago? How do you know how much was in the sample when you started?

Cosmic rays produce C 14 Atmospheric concentration is roughly 1 C 14 atom for every 1 trillion C 12 atoms For all living organisms, the amount of C 14 in their

9 Cosmic rays produce C 14 Atmospheric concentration is roughly 1 C 14 atom for every 1 trillion C 12 atoms For all living organisms, the amount of C 14 in their bodies is pretty much the same as the atmospheric concentration As soon as the organism dies, the amount of C 14 begins to decay with a half-life of 5730 years

10 In class activity 1. If the ratio of C 14 to C 12 in a sample is 25% of the current atmospheric ratio of C 14 to C 12, how old is the sample? 2. What big assumption did we make in determining the age of the sample? (Think immutable laws and cosmic rays) 3. If I told you the carbon dated age of a sample is 60,000 years, would you believe me? Why or why not?

In class activity 1. If the ratio of C 14 to C 12 in a sample is 25% of the current atmospheric ratio of C 14 to C 12, how old is the sample? 2. What big assumption did we make in determining the age of C14/C12 the ratio sample?

11 In class activity 1. If the ratio of C 14 to C 12 in a sample is 25% of the current atmospheric ratio of C 14 to C 12, how old is the sample? 2. What big assumption did we make in determining the age of C14/C12 the ratio sample? (Think immutable laws and cosmic rays) constant in time 3. If I told you the carbon dated age of a sample is 60,000 years, would you believe me? Why or why not?

12 Conclusions Carbon 14 dating is the best technique we have for dating samples from the present back to about 50,000 years C 14 amounts can be measured very accurately, roughly equivalent to an accuracy of 20 years The largest uncertainty is that cosmic rays are not constant in time, so the production of C 14 is not constant varies with factors like the earth magnetic field This affects the exact age, but we can try to correct for these factors; the relative age of samples is unaffected

13 Faint young sun Over time, H in sun is converted by fusion to He He burns hotter than H solar output increases with time Rate of change = 10-4 W / m 2 / 1000 years

14 Te = [1368*(1-0.3)*RSL/4σ]1/4

15 Simple model of surface and atmosphere Range of earth temperatures Solar energy Model assumes: -- same CO2 as current earth -- same albedo as current earth Is this model right?

16 Critters from Warrawoona formation in Australia Age 3.5 b. y. Archean life Requires water!

17 Paradox Sun was less luminous (less energy reaching earth) But life is evident Life requires liquid water But planet was too cold for liquid water Conceptual framework: T sfc = f( S 0, Albedo, ΔT g )

18 So, what are the possible explanations? (How can we keep earth warm?) T sfc = f( S 0, Albedo, ΔT g ) Solar model is wrong Not likely solar physics models working well Albedo was lower Not likely clouds; ice-albedo feedback, not standing water, etc. Geothermal energy Not likely too small to play much of a role Greenhouse gases?

19 So, what are the possible explanations? (How can we keep earth warm?) T sfc = f( S 0, Albedo, ΔT g ) Greenhouse gases? H 2 0 no: saturation vapor pressure limits amount; it acts as a feedback rather than as a forcing H 2 O Positive Feedback Loop Surface temperature (+) Greenhouse effect Atmospheric H 2 O

20 The Faint Young Sun Problem More H 2 O Less H 2 O

21 Snow/ice albedo feedback Surface temperature (+) Snow and ice cover Planetary albedo

22 Both of these feedback loops are positive, and hence destabilizing The climate system must also contain negative (stabilizing) feedback loops as well

Radiative Balance and the Faint Young Sun Paradox

Radiative Balance and the Faint Young Sun Paradox Solar Irradiance Inverse Square Law Faint Young Sun Early Atmosphere Earth, Water, and Life 1. Water - essential medium for life. 2. Water - essential