# Climate Change over Recent Millennia

```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:
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
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&deg;C)
At equilibrium, I incoming = I outgoing
((1-a) s Tsun4 ) / 4 = s T4earth
Solve for Tearth
Eqn. 3.1 in Archer Chapter 3
Galileo’s Thermoscope (1500s)
Daniel Fahrenheit (1714) closed
thermometer
First weather stations
established ~250 years ago
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&deg;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
• Cools the troposphere by up to 0.5-1.0&deg;C for only 24 years.
• Warms the Stratosphere by 2-3&deg;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,
30 years of satellite observations: &plusmn;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&deg;C (Small !)
- Sunspots +
Sunspot Cycles
~0.1%
1600
1700
1800
1900
Very weak forcing, but significant climate responses to it.
2000
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
Rh&ocirc;ne Glacier
Solar Variability
• Forcing is very weak (in visible spectrum), only &plusmn;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 (&plusmn;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. (&iexcl;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 (&deg;C)
cool
Temperatures over the last 2,000 years
*
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.
-
Anthropogenic greenhous gas emissions (decades to
centuries)
What’s in store for the future?
Most probable estimate
is +2 to +4&deg;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&deg;C warming
```