Why do climates change ?
Climate changes over the last millennium
Take away ideas and understandings
• What factors influence climate change over decadal
to century timescales?
• Solar variability, volcanic eruptions, and greenhouse
gases.
• How we can quantify their effects ?
• Climate history of the last millennium
Our first climate model
Recall how to calculate Earth’s effective temperature, or the temperature the
Earth would be without an atmosphere.
The Stefan-Bolzmann equation:
Blackbody radiation:
I (w/m2) = s T4
Earth’s incoming radiation (a = Earth albedo, or reflectivity)
I incoming = (1-a) s Tsun4
Our first climate model
Earth incoming radiation (a = Earth albedo, or reflectivity)
I incoming = ((1-a) Isolar ) / 4, or ((1-a) s Tsun4 )/ 4
Earth outgoing radiation
I outgoing = s Tearth4
a is ~0.3, or 30%
e (emissivity) = 1
s = 5.67 x 10-8 W m-2 K-4
Archer Fig. 3.3 (p. 23)
Earth’s temperature with no greenhouse effect
Teffective = 254.8K (-18°C)
At equilibrium, I incoming = I outgoing
((1-a) s Tsun4 ) / 4 = s T4earth
Solve for Tearth
Eqn. 3.1 in Archer Chapter 3
Surface temperature readings
Galileo’s Thermoscope (1500s)
Daniel Fahrenheit (1714) closed
thermometer
First weather stations
established ~250 years ago
Widespread measurements for
last 150 years only.
Stockholm Observatory
31 December, 1768:
“No one can recall such a mild
Autumn: the ground is as green
as in the Spring, and today I
have picked sufficient young
nettles, dandelions, and other
herbs to cook green cabbage
tomorrow, which is New Year's
day.”
Many 1000s of stations
http://data.giss.nasa.gov/gistemp/
There are only 3 ways to change
Earth’s temperature
Change input
a)
b)
Solar variability
Earth reflectivity (volcanoes)
Change output
c) Greenhouse gases
1. Volcanoes cool climate, briefly
Mt. Tambora - 1815
Mt. Pinatubo - 1994
Volcanic eruption can
change albedo by 1%
a = ~30% on average
Teffective = 254.8K
Recalling Iin = (1-a) s T4
((1-a) s Tsun4 )/ 4 = s T4earth
Increase a to 31%
New Teffective = 253.9K
or -1°C cooler due a volcanic eruption
Climate Impacts of Volcanic Eruptions
• Volcanoes inject aerosols and particles into the
stratosphere which can scatter and/or absorb
incoming solar radiation.
• Cools the troposphere by up to 0.5-1.0°C for only 24 years.
• Warms the Stratosphere by 2-3°C (!)
• Short-term but significant impact
Mt. Tambora
1815 eruption
“Year Without a Summer”
“Poverty Year”
“1800-and-froze-to-death”
Mt. Merapi (Indonesia)
tropical volcanoes cool climate most
2. Solar Variability
Sunspots, Facular brightening,
and Irradiance
30 years of satellite observations: ±1 W/m2
What if the solar radiation changes
by +2 W/m2 ?
Recalling Isolar = 1365 W/m2
Set Isolar = 1367 W/m2
a = 0.3
Solve for Tearth
((1-a) Isolar)/ 4 = s T4earth
∆T = 0.1°C (Small !)
- Sunspots +
Sunspot Cycles
~0.1%
1600
1700
1800
1900
Very weak forcing, but significant climate responses to it.
2000
Little Ice Age (1500-1850 AD)
LIA
Cooling was the result of lower solar radiation
and some big volcanic eruptions
Little Ice Age (1500-1850)
London Frost Fair (1814)
2001
1859
Rhône Glacier
Solar Variability
• Forcing is very weak (in visible spectrum), only ±0.10.2%, so climate response should be weak.
• Climate response is actually quite high - still not sure
why.
• One possibility is UV part of spectrum - much greater
changes (±10%)
… suggests that global climate is very sensitive
http://data.giss.nasa.gov/gistemp/
How do we estimate past climate change,
before there were thermometers
“Proxies” - getting temperatures from trees
a) Measure ring widths
b) Calibrate ring widths
Growth Temp. (¡C)
30
25
20
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
Ring Width (mm)
C) Validate and Apply
warm
T (°C)
cool
Year AD
Temperatures over the last 2,000 years
*
Date (AD)
2005
Measured CO2 trends since 1958
Atmospheric CO2: Last 250 years
Results from the IPCC AR4 report
Natural forcing
only
Natural + Human
…most of modern
warming is due to humans
Natural only
Natural + GHG
Modeling the climate of the last 1000 years…
Climate = Solar Irrad. + Volcanoes + GHG
Sum of all forcings (1850-2000)
Net =
+1.6 W
4 lights per
square meter
*
*
*
Another way to imagine global warming
What global warming really looks like
Data source: NASA / GISS
Timescales of Natural Climate Variability
A. Short time scales (1-2 years): Random weatherrelated variations of turbulent, chaotic atmosphere.
B. Interannual (2-8 years): Primarily ENSO and
Volcanic eruptions.
C. Decadal-to-century scale:
-
Solar Variability (decades to centuries)
Anthropogenic greenhous gas emissions (decades to
centuries)
What’s in store for the future?
Most probable estimate
is +2 to +4°C in next
100 years
All scenarios warmer.
past
future
Today
Global Temperature in 2050
Recent warming is unusual,
Future warming is “another world”
2010
*
+3°C warming