HON 301 Surviving the Anthropocene V. Global Warming and Climate Change Profs. E. Mendieta & F. Walter 6 March 2015 Contents I.  What sets the temperature of the Earth? II.  Earth’s atmosphere and the Greenhouse III.  Sunspots, TSI, and Climate IV.  Evidence for global warming V.  What causes global warming VI.  Consequences of global climate change The Temperature of the Earth
The Earth is in equilibrium with the Sun - on
average it is neither heating nor cooling.
The equilibrium temperature is set by equating
–  the heat absorbed from the Sun with
–  the heat radiated by the Earth.
Heat in = heat out
Energy Balance
(1-­‐a) πR⊕2 (L¤ / 4π d2) ⇒ ⇓ 4πR⊕2σT⊕4 Heat In
•  Energy input comes from the Sun
(internal heat is negligible)
•  The Solar brightness = L¤ / 4π d2 (solar constant)
–  L¤ is the solar luminosity
–  d is the distance from the Earth to the Sun, 1AU
–  The solar constant is 1.36 x 106 erg/cm2/s, or
1361 W/m2/s.
Heat Out
•  Approximate the Earth as a blackbody.
Black Bodies
Opaque objects in thermal equilibrium
–  Neither heating up nor cooling down
•  Spectral shape depends only on temperature
•  Wien's law: T = 2.9 x 106 /λmax K/Å
Peak of the spectrum gives the temperature.
•  Stephan-Boltzmann law: power emitted per unit
area = σT4. σ is the Stephan-Boltzmann constant,
5.67 x 10-5 erg/cm2/K4.
The brightness of an object and its temperature
determine its angular size (radius/distance).
Continuous Spectra
Black Body Spectra
Generated by hot
opaque gas
• Peak λ ∝ T-1
• Brightness ∝T4
• Luminosity ∝area x T4
Heat Out
•  Approximate the Earth as a blackbody.
•  Output: LE = 4πr2σTE4 (Stefan-Boltzmann law)
•  Input: Lin = A πr2 (L¤ / 4π d2)
•  A: albedo: fraction of light reflected back to
space (≈ 0.39)
•  πr2: area of Earth intercepting sunlight
Balance
Earth is in equilibrium
•  Output = input
(if not, the mean temperature would change)
•  πr2σTE4 = A πr2 L¤ / d2
Solve for TE.
For the Earth, TE = 247K
In general, Tp ~ (L*/d2)1/4
Spectrum of a Planet
Reflected Sunlight
Thermal Emission
Mars
Earth is Not a Blackbody
Energy Balance
(1-­‐a) πR⊕2 (L¤ / 4π d2) ⇒ ⇓ 4πR⊕2σT⊕4 Levels of the Atmosphere •  Troposphere: •  temperature falls with height •  Heated from below: Unstable to convec\on •  Stratosphere: •  temperature rises with height •  Heated in-­‐situ by solar UV •  Exosphere: •  essen\ally the vacuum of space •  Heated by X-­‐rays •  Includes the ionosphere Troposphere •  Sunlight heats Earth's surface •  Earth re-­‐radiates in IR •  greenhouse gases absorb IR –  T decreases with height. –  Convec\on and weather Stratosphere •  O3 absorbs UV photons •  Top of stratosphere absorbs more UV than boaom •  T increases with al\tude •  No convec\on •  Stagnant Thermosphere •  All gases absorb X-­‐
rays •  Solar X-­‐rays → absorbed by top of thermosphere –  T increases with al\tude •  Gas: (mostly) ions + free electrons –  Ionosphere reflects radio broadcasts Exosphere •  High T, low density gas •  Some ar\ficial satellites orbit in Earth's exosphere •  Atmospheric gases escape from Earth's exosphere Greenhouse Effect
In equilibrium, TE = 247K
In actuality, TE = 287K (14C)
The 40K difference is due to the greenhouse effect.
–  At 247K, the Earth tries to radiate in the IR (Wien‘s law)
–  The atmosphere is not transparent in the IR
–  ∴ Heat is trapped
Greenhouse Effect
The blackbody is the most efficient radiator possible
The Earth is not exactly a blackbody
It must heat up to compensate
Greenhouse gases include
–
–
–
–
–
carbon dioxide
Methane
water vapor
nitrous oxide
chlorofluorocarbons,
These all absorb infrared light.
Atomic Spectroscopy Electrons in atoms can absorb photons to jump to higher energy levels -­‐> line spectrum •  Electronic transi\ons: –  typical energies: 0.1-­‐1 eV –  Typical wavelengths: UV, op\cal, near-­‐IR 1 eV = 1.6 x 10-­‐12 erg; λν=c; E = hν Molecular Spectroscopy Molecules have more degrees of freedom: •  Vibra\ons: –  ω~ √(k/meff) k: spring constant, meff: reduced mass –  E = (ν+½)hω/2π typical λ 2-­‐20 µm (IR) •  Rota\on: –  E = (h/2π)2/2I J(J+1) I: moment of iner\a, J: 0,1,2,… –  Typical wavelengths: sum-­‐mm to mm •  1 eV = 1.6 x 10-­‐12 erg; λν=c; E = hν Equilibrium
Other things being equal, the Greenhouse Effect
keeps the Earth in equilibrium, but at a hotter
temperature than in the absence of the
greenhouse.
Is the Earth really in equilibrium?
Equilibrium
Is the Earth in equilibrium?
•  There has been liquid water on Earth for at least
3.8 billion years
•  ∴ the surface temperature has been between
273 and 373 K.
•  Fossils suggest a much narrower T range
A Delicate Balance Life has adapted to condi\ons on Earth •  For metazoa: –  273 <T(K) < 313 –  0.15 < ρ(O2) < 0.26 –  Ocean pH stable within ranges Snowball - Oxygen
1kPa ~ 0.01 atm Source: Harrison et al. 2010, Proc. R. Soc. B The Faint Young Sun Problem
Maunder Minimum
Little Ice Age
"Sports on a Frozen River" by Aert van der Neer
Coincidence?
Correlation
≠
Causation
The Magnetic
Cycle
Spot cycle ~11 years
Magnetic cycle ~22 yrs
The Maunder Minimum
•
•
•
•
~1645 – 1715
Few Sunspots seen
Few aurorae seen
Coincides with coldest part of the “Little
Ice Age” in Europe and North America
The Little Ice Age
•  ~1350 – 1850
•  Follows Medieval Warm Period
•  Overlaps 3 solar activity minima
–  Sporer
–  Maunder
–  Dalton
Other Grand Minima
Today’s Sunspot Cycle
“It’s tough to make predictions,
especially about the future.”
Yogi Berra
Spots Are Weakening
Polar Fields are Weakening
De Toma, G. 2011, Solar Physics, 274, 195
Subsurface flow patterns are
changing
Recent Data vs Predictions
Predicting the Future
Abdussamatov, I. 2011, Applied Physics Research, 4, 178
Predicting the Future
Abdussamatov, I. 2011, Applied Physics Research, 4, 178
Will we see sunspots after 2020?
•  And if not, what are the consequences?
–  To space weather
–  To terrestrial climate
Solar Irradiance
TSI / Sunspot correlation
Sunspot Numbers
Reconstructed Total Solar Irradiance
Predicted Change in Irradiance
•  1 W/m2 is about 0.15% of L⊙
•  Change in equilibrium T⊕ is <0.05% (0.1K)
•  Significant?
•  But spectral energy distribution changes more
•  Note: Schrijver et al (GRL 38, L06701) dispute
extrapolation to low TSI
SED Variability
J. Lean NASA/NRL
How Might the Sun Affect
Terrestrial Climate?
•  ACTIVITY: Weak heliospheric B field lets
in more galactic cosmic rays:
–  generating NOx,
–  destroying ozone,
–  reducing stratospheric heating
–  affecting global circulation.
•  TSI: Change in TSI may be up to 6 W/m2
(depends on evolution of quiet sun ephemeral regions)
How Might TSI Drive Climate? •  Not through total irradiance. •  Most variability in UV –  UV is absorbed in stratosphere –  Can this affect global circula\on paaerns? Solar Irradiance?
Terrestrial
temperatures
are not
tracking the
Solar
luminosity
TSI is a Red Herring! •  Recent changes in global temperature swamp any possible Solar effect. Global Temperatures,
1850 - 2006
Source:
http://www.ncdc.noaa.gov/
img/climate/globalwarming
Temperatures through 2013
Note: this excludes the internal oceanic hea\ng Mean global
temperature,
700 - 2000 CE
Source:
http://
www.ncdc.noaa.gov/
img/climate/
globalwarming
Change in
mean
sea level
since 1870
Source:
http://
www.ncdc.noaa.gov/
img/climate/
globalwarming
Predicted global
temperatures in
the next century
Source: http://www.ncdc.noaa.gov/img/climate/globalwarming
If not the Sun, Then What?
Terrestrial Changes?
Terrestrial Changes?
What Causes Global Warming?
•  Not the Sun
•  Our atmosphere.
–  Most likely anthropogenic factors
Has Warming Stopped?
No
•  Earth Just Finished Its Warmest
Quarter-Year Ever
–  http://www.slate.com/blogs/future_tense/2014/07/15/
april_may_and_june_2014_is_the_warmest_three_month_period_ever.html
•  Global warming slowdown answer
lies in depths of Atlantic, study finds
•  Excess heat being stored hundreds of metres down in
Atlantic and Southern oceans – not Pacific as
previously thought
–  http://www.theguardian.com/environment/2014/aug/21/global-warmingslowdown-answer-lies-in-depths-of-atlantic-study-finds
–  Science 22 August 2014: Vol. 345 no. 6199 pp. 897-903
No. Definitely not!
The annual anomaly of the global average surface temperature in 2014 (i.e. the average of the near-­‐surface air temperature over land and the SST) was +0.27°C above the 1981-­‐2010 average (+0.63°C above the 20th century average), and was the warmest since 1891. On a longer \me scale, global average surface temperatures have risen at a rate of about 0.70°C per century.
Five Warmest Years (Anomalies) 1st. 2014(+0.27°C), 2nd. 1998(+0.22°C), 3rd. 2013,2010(+0.20°C), 5th. 2005(+0.17°C) JMA: hap://ds.data.jma.go.jp/tcc/tcc/products/gwp/temp/ann_wld.html Oceans as Buffer: The Pacific Decadal Oscilla\on •
•
•
•
Measure of temperatures in North Pacific el Nino-­‐like oscilla\on, but 20-­‐30 years long Poorly characterized May influence hemispheric temperatures No
The Future
•  Irreversible Damage Seen From
Climate Change in UN Leak
–  http://www.bloomberg.com/news/2014-08-26/
irreversible-damage-seen-from-climatechange-in-un-leak.html
•  Hundreds of Methane Plumes
Erupting Along East Coast
–  http://news.sciencemag.org/climate/2014/08/
numerous-methane-leaks-found-atlantic-seafloor
–  https://news.yahoo.com/hundreds-methaneplumes-erupting-along-eastcoast-170504645.html
–  Nature Geoscience 2014 Aug 24
•  Mysterious Siberian crater
attributed to methane
–  http://www.nature.com/news/mysterioussiberian-crater-attributed-to-methane-1.15649
Consequences of Global Climate Change •  Global circula\on paaerns change Basic Atmospheric Circula\on – no rota\on (Hadley cells) Terrestrial Winds Consequences of Global Climate Change •  Global circula\on paaerns change •  Decrease in equatorial-­‐polar gradient will weaken trade winds and jet stream. Increasing Temperatures •  In oceans •  On land Upsala Glacier, Argen\na Consequences of Increasing Temperatures •  In oceans –  Anoxia –  Sea life migrates towards poles –  Weakening of conveyor system •  On land –  “habita\on zones” migrate poleward –  Flora/fauna must adjust Oceanic Acidity •  Ocean water is naturally basic, with pH=8.2 (averaged over the past 300 million years) •  Atmospheric CO2 is in chemical equilibrium with the oceans, meaning some CO2 is absorbed. –  About 25% of anthropogenic CO2, 22 megatons/day, is absorbed •  CO2 absorbed in H2O forms carbonic acid, OC(OH)2 •  Current ocean pH has decreased to 8.1 –  This is a 25% increase in pH in 200 years –  pH could decrease to 7.7 by 2100 Source: Smithsonian Ocean Portal Consequences of Ocean Acidity •  Reduces concentra\on of CO3-­‐ -­‐ •  Calcium carbonate (CaCO3) is the principle ingredient in shells (limestone) •  Carbonic acid dissolves CaCO3 •  Affects metabolism of marine organisms Most mass ex\nc\ons coincide with \mes of oceanic acidifica\on Sea Level Rise •  3.5 mm/yr •  Causes: –  Thermal expansion –  Mel\ng glaciers/ice caps •  Rate expected to con\nue and accelerate •  Total increase: 0.8 – 2m –  up to 7m if Greenland ice cap melts Consequences of Sea Level Rise •  Flooding of ci\es/low lying land –  5 million Americans live within 4 • of high \de line –  634 million live within 9m of sea level •  Increased coastal erosion •  Increased storm-­‐surge flooding in storms •  Disappearance of low-­‐lying island na\ons •  Note: con\nents are rising too Effects on Weather •  A warmer atmosphere holds more water. •  Energy release from H2O condensa\on drives weather. •  Atmospheric circula\on paaerns change Consequences of Extreme Weather •  More severe storms –  Increased erosion •  Longer droughts •  Damage from storms •  More wildfires Effects on Agriculture •  Rain and temperatures affect crop yields •  Irriga\on raises surface salt levels Economic Effects •  Flood mi\ga\on •  Reloca\ng agriculture •  fisheries Human Costs •
•
•
•
Disrup\on of socie\es Reloca\on from low-­‐lying areas Starva\on Extreme weather What is Cause and What is Effect?