1. NATS 101-06 Lecture 20 Anthropogenic Climate Change

2. Our changing climate: Key Questions
Climate modelers have predicted the Earth’s surface will warm because of manmade greenhouse gas (GHG) emissions
So how much of the warming is manmade?
How serious are the problems this is creating?
What, if anything, can and should we do?

3. What is Climate Change?
Climate change - A significant shift in the mean state and event frequency of the atmosphere.
Climate change has been a normal component of the Earth’s natural variability.
Climate change occurs on all time and space scales.
A plethora of evidence exists that indicates the climate of the Earth has changed.
Humans are radically altering the concentrations of greenhouse gases in the atmosphere which is likely causing a new type of climate change

4. Climate Change: Moving out of Energy Balance
Changing the
Incoming solar energy
Outgoing energy (IR emitted to space)
Internal transport of energy within the Earth;s atmosphere and oceans
will cause a shift in climate

5. In a stable climate, Solar Energy IN = IR Energy OUT
Global Energy Balance
In a stable climate, Solar Energy IN = IR Energy OUT
IR Out
Ahrens, Fig. 2.14
Solar in

6. Global Energy Imbalance
Increasing GHG concentrations decrease the IR Energy out
So Energy IN > Energy OUT and the Earth warms
IR Out
is reduced
Ahrens, Fig. 2.14
Solar in
Atmosphere

7. Change in IR Emission to Space
Notice that because of the greenhouse gases in Earth’s atmosphere, 91% (=64/70) of the IR emitted to space comes from the atmosphere and only 9% (=6/70) comes from Earth’s surface
When additional GHG’s are added to the atmosphere, the altitude of IR emission to space rises and less IR from the surface makes it to space
Since air temperature in the troposphere decreases with altitude, the temperature of the emission to space decreases
Therefore Earth’s energy emission to space decreases because the IR energy flux emitted by an object decreases with decreasing temperature
Therefore total Energy flux IN > Energy flux OUT
Earth warms until Energy flux IN = Energy flux OUT

8. Change in IR Emission to Space
BEFORE GHG increase: IN=OUT AFTER GHG increase
IR emission to space
3. IR emission to space decreases
because of colder emission temperature SH NH
Altitude of IR emission to space
Ahrens, Fig. 2.21
1. Altitude of IR emission to space rises
Altitude
Temperature of IR emission to space
2. Temperature of IR emission to space decreases
Temperature
Temperature

9. Change in IR Emission to Space (cont’d)
AFTER GHG increase IN>OUT Eventual solution IN=OUT
6. IR emission to space increases until it matches the original IR emission before GHG increases
3. IR emission to space decreases
because of colder emission temperature SH SH
Ahrens, Fig. 2.21
1. Altitude of IR emission to space rises
Ahrens, Fig. 2.21
4. Atmosphere warms until…
2. Temperature of IR emission to space decreases
5. Temperature of IR emission to space increases to original temperature
Temperature
Temperature

10. Change: Atmo IR Emission to Surface
BEFORE GHG increase: AFTER GHG increase
Energy DOWN=UP at surface more IR emission into surface
Atmospheric IR emission to surface SH NH
Ahrens, Fig. 2.21
1. Altitude of IR emission to surface decreases
Altitude
Altitude of IR emission to surface
Temperature
Temperature
2. Temperature of IR emission to surface increases
Temperature of IR emission to space
3. Atmo IR emission to surface increases
because of warmer emission temperature

11. Out-of-Equilibrium Issues
Land temperature increases faster than Ocean temperature
Atmospheric lifetime of CO2 is ~50 years
System response is NOT instantaneous
System takes a while to heat up in response to increase in CO2
Initial temperature increase is smaller than final

12. Anthropogenic Climate Change
The data indicate that global-mean land and sea-surface temperatures have warmed about ~0.6oC during the last 35 years.
Is this the early stages of a man-made global warming?
Two main anthropogenic forcing mechanisms:
Greenhouse gas concentrations => rising.
Aerosol concentrations => also increasing.
We will focus attention on CO2 increases.

13. Greenhouse Gas Concentrations
The two dominant greenhouse gases are H2O and CO2.
There are several other GHGs with smaller concentrations: O3, CH4, N2O
H2O vapor is categorized separately because it is controlled indirectly by evaporation and condensation and changes in response to other changes in climate (temperature, circulation)

14. Absorption 20% of incident Visible (0.4-0.7 m) is absorbedIR
20% of incident Visible ( m) is absorbed
O2 an O3 absorb UV (shorter than 0.3 m)
Infrared (5-20 m) is selectively absorbed
H2O & CO2 are strong absorbers and emitters of IR
Little absorption of IR around 10 m – atmospheric window
Ahrens, Fig. 2.9

15. Global Warming Potential (GWP)
Different gases has different warming potentials which are defined relative to the warming effect of CO2
Gas GWP
Carbon dioxide (CO2) 1
Methane (CH4) 21
Nitrous oxide (N2O) 310
Hydrofluorocarbons ,100
Perfluorocarbons 6,000-9,200
Sulfur hexafluoride 23,900
Ahrens, Fig 2.10

16. CO2 makes the biggest contribution to the climate forcing

17. Ice Core from Vostok, Antarctica
During last ice age (>18,000 years ago)
Temps 6oC colder
CO2 levels 30% lower
CH4 levels 50% lower
H2O levels were lower
than current interglacial.
135,000 years ago it was a bit warmer than today
50% increase in CO2 was associated with 6-8oC increase in temperature
6-8oC decrease in temperature produced incredibly different climate: Ice Age

18. Changing CO2 concentrations
CO2 concentrations have varied naturally by ~30-50% over the past few hundred thousand years (ice ages)
Fossil fuel burning since the industrial revolution has created a recent sharp increase in CO2 concentrations
CO2 concentrations are now higher than at any time in past few hundred thousand years
And concentrations are increasing faster with time
Last 4 Ice Age cycles:
400,000 years
Man made
You are here
See

19. Changing CO2 concentrations
CO2 concentrations measured very precisely since 1958
Over past 45 years they’ve increased by ~21%
Present 0.6%/yr
increase
You are here
See

20. US (5% of world population) now causes 24% of total pollution.
See

21. Currently, 7 gigatons per year of CO2 are injected into the air by burning fossil fuels (80%) and forests (20%).
Half accumulates in atmosphere, where it resides for 50+ years.
If the burning fossil fuels and forests totally ceased, it would still take 50 years for CO2 levels to return to 50% above pre-industrial levels.

22. Missing Carbon Sink
CO2 is accumulating in the atmosphere more slowly than expected (believe it or not)
Based on our understanding of CO2 emissions and ocean and atmosphere uptake, there is a missing sink/uptake of about 25%
NASA OCO mission

23. Natural Variations in Climate

24. Milankovitch Theory of Ice Ages
Attempts to explain ice ages by variations in orbital parameters
Three cycles:
Eccentricity (100,000 yrs)
Tilt (41,000 yrs)
Precession (23,000 yrs)
Changes the latitudinal and seasonal distributions of solar radiation.

25. Milankovitch Theory of Ice Ages
Most recent Ice Ages have occurred for past 2 million years
Ice Ages occur when there is less radiation in summer to melt snow.
Partially agrees with observations, but many questions unanswered.
What caused the onset of the first Ice Age?

26. Milankovitch Theory
Change in daily solar radiation at top of atmosphere at June solstice
Changes as large as ~15% occur

27. Global Temperatures and CO2
There is a very strong relationship between CO2 levels and past global temperatures.
CO2 levels are now higher than during any period of the past 450,000 years.
Will global temperatures responding accordingly?

28. Long-Term Climate ChangeNA
E-A Af SA
India
Ant
Aus
180 M BP
Today
Ahrens, Fig 13.6
250 million years ago, the world’s landmasses were joined together and formed a super continent termed Pangea.
As today’s continents drifted apart, they moved into different latitude bands.
This altered prevailing winds and ocean currents.

29. Long-Term Climate Change
Circumpolar ocean current formed around Antarctica MY ago once Antarctica and Australia separated.
This prevented warm air from warmer latitudes to penetrate into Antarctica.
Absence of warm air accelerated growth of the Antarctic ice sheet.

30. Long-Term Climate Change
Circumpolar seaway leads to large latitudinal temperature gradient.
Circum-equatorial seaway leads to small latitudinal temperature gradient

31. Climate System Feedbacks

32. CO2 and the Greenhouse Effect
If the atmosphere were dry, we could predict with high confidence that a doubling of CO2 (likely before 2100) would increase the global mean surface temperature by ~2C.
The presence of oceans, ice, water vapor and clouds complicates the analysis significantly.
Ahrens, Fig 2.10

33. Complexity of Climate System
The climate system involves numerous, interrelated components.

34. Closer Look at Climate System

35. Climate Feedback Mechanisms

36. Climate System Feedbacks
Feedbacks are what makes the climate problem so tricky
Positive: Increase the impact of the original change
Negative: Reduce the impact of the original change
EXAMPLES
Ice albedo (Positive Feedback):
Temperatures increase because of increased CO2
Ice melts
Reduces Earth’s reflectivity (albedo)
Increases sunlight absorbed by the Earth
Increases warming of the Earth

37. Positive and Negative Feedbacks
Assume that the Earth is warming.
- Warming leads to more evaporation from oceans, which increases water vapor in atmosphere.
-More water vapor increases absorption of IR, which strengthens the greenhouse effect.
-This raises temperatures further, which leads to more evaporation, more water vapor, warming…
“Runaway Greenhouse Effect”
Positive Feedback Mechanism

38. Positive and Negative Feedbacks
Again assume that the Earth is warming.
- Suppose as the atmosphere warms and moistens, more low clouds form.
- More low clouds reflect more solar radiation, which decreases solar heating at the surface.
- This slows the warming, which would counteract a runaway greenhouse effect on Earth.
Negative Feedback Mechanism

39. Cloud Feedback Clouds affect both solar & IR radiation
Clouds increase solar reflection (albedo) cooling the Earth
Clouds absorb and emit IR radiation like a GHG
Currently their net effect is to cool the Earth
Solar effect > IR effect
Not clear what they will do in the future
If the amount of low clouds increases, the albedo effect wins and they cool the Earth (Negative feedback)
If the amount of high clouds increases, the IR effect wins and the Earth warms (Positive feedback)
There are other subtler cloud feedbacks as well involving aerosols in particular

40. Positive and Negative Feedbacks
Atmosphere has a numerous checks and balances that counteract climate changes.
All feedback mechanisms operate simultaneously.
All feedback mechanisms work in both directions.
The dominant effect is difficult to predict.
Cause and effect is very difficult to prove at the “beyond a shadow of a doubt” level.

41. Climate Model Predictions

42. Projections of Global Warming
Atmosphere and coupled Atmo-Ocean models are run for hundreds of years to simulate future climates
They assume continued increases in the levels of greenhouse gasses in the atmosphere.
The models have sophisticated physics…
But they have coarse spatial grid separations!
Atmosphere: 250 km horizontal, 30 levels vertical
Ocean: km horizontal, m vertical

43. How Well Do Models Capture Current Observed Climate?
Performance Varies by Weather Element
In general…
Excellent for Surface Temperature
Skillful for Sea-Level Pressure (SFC Winds)
Marginal Skill for Precipitation
Fig. 8.4 IPCC Report

44. Global Temperature Outlook
Assume CO2 levels rise at current rate of 1% per year until 2070.
Good agreement for past climate and CO2 levels leads to high confidence.
Rather close agreement among models.
Consensus of several model runs indicates an average warming of 2oC
Fig. 9.3 IPCC Report

45. Global Precipitation Outlook
Marginal performance for past climate and CO2 levels means low confidence in outlook.
Large differences exist among models.
Consensus of several model runs indicates an average increase of 2% in global precipitation
Fig. 9.3 IPCC Report

46. Regional Consistency: Warming
Fig IPCC Report
A + or - symbol denotes 7 out of 9 models agree.
A2: no sulphate aerosols; B2: has sulphate aerosols

47. Regional Consistency: Rainfall
Fig IPCC Report
A + or - symbol denotes 7 out of 9 models agree.
A2: no sulphate aerosols; B2: has sulphate aerosols

48. GDFL Model
Lets look at some details from a 500-year simulation by a specific climate model.
Examine Two Scenarios:
1% CO2 increase per year for 70 years 2X Total Increase in CO2
1% CO2 increase per year for 140 years 4X Total Increase in CO2

49. Good agreement for CO2 levels of the past 150 yrs
Mandatory before use as global warming model

50. CO2 Increases 1% per Year
Average Global Surface Temperatures… Warm 2oC for 2X CO2 Warm 4o C for 4X CO2
Sea Level Rises… Equilibrium Not Reached until after 500 years
North Atlantic Ocean… Circulation Weakens

51. CO2 Increases 1% per Year Surface Temperatures for GFDL Model
2X CO2 Temperature Animation Remote
2X CO2 Temperature Animation Local Disk
BIG File (8.2 MB)!!!

52. Climate response is out of equilibrium for a while
Continental temperatures rise faster than ocean temperatures
Water has much larger heat capacity than land
Evaporation cools the ocean surface
Ocean temperatures take long time to reach a new equilibrium
Warming we have seen thus far is NOT the total warming even if GHG concentrations stop increasing
Water saturation vapor pressure over land increases faster than that over the oceans
Most of the atmospheric water vapor ultimately comes from the ocean
Therefore expect the relative humidity over land to decrease in general, at least until ocean temperatures catch up with the land temperatures.
2. Continental interiors will get drier in general

53. Radiative Warming
Radiative Cooling
Radiative Cooling
Annual Energy Balance NH SH
Ahrens, Fig. 2.21
Latitudinal imbalance between visible in and IR out is compensated by meridional energy fluxes
Heat transfer from low latitudes to high latitudes is accomplished by winds and ocean currents
Differential heating and spinning Earth drive winds and currents
This horizontal energy transfer will likely change as climate changes

54. Mid-Latitude Cyclones
Winter storms move tropical air poleward and polar air toward the tropics.
Net result is to transport energy poleward mP cP mT
Ahrens, Meteorology Today, 5th Ed.

55. Ocean Currents of World
Ahrens Fig. 7.24

56. 4X CO2 Sea Ice Animation
4X CO2 Sea Ice Local Link

57. CO2 Increases 1% per Year July Temperature over Southeastern U.S.
Warms 5-9oC
July Heat Index over Southeastern U.S.
Rises 7-14oC due to increase H2O vapor
Consistent Signal
Warmer Southern U.S.

58. CO2 Increases 1% per Year Soil Moisture Much Drier over U.S.
60% Soil Moisture Decrease
Higher Evapo-Transpiration
Altered Balance between Evaporation-Precipitation
Agricultural Implications
How do we feed ourselves?

59. Increasing CO2 concentrations
How high will they go? How warm will it get???
If CO2 concentrations stay within factor of 2 of pre-industrial, then warming of 3+1oC is expected
If concentrations go still higher => larger uncertainty because the climate is moving into unprecedented territory
See
You are going to be
somewhere in here
Last 4 Ice Age cycles:
400,000 years
Man made
You are here
Ice age CO2 range