why do climates change ? climate changes over the last millennium
Post on 22-Dec-2015
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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) = T4
Earth’s incoming radiation ( = Earth albedo, or reflectivity)
I incoming = (1-) Tsun4
Our first climate model
Earth incoming radiation ( = Earth albedo, or reflectivity)
I incoming = ((1-) Isolar ) / 4, or ((1-) Tsun4 )/ 4
Earth outgoing radiation
I outgoing = Tearth4
is ~0.3, or 30% (emissivity) = 1 = 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-) Tsun4 ) / 4 = T4
earth
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.”
There are only 3 ways to change Earth’s temperature
Change inputa) Solar variability
b) Earth reflectivity (volcanoes)
Change outputc) Greenhouse gases
Volcanic eruption can change albedo by 1%
= ~30% on average
Teffective = 254.8K
Recalling Iin = (1-) T4
((1-) Tsun4 )/ 4 = T4
earth
Increase 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 2-4 years.
• Warms the Stratosphere by 2-3°C (!)
• Short-term but significant impact
What if the solar radiation changes by +2 W/m2 ?
Recalling Isolar = 1365 W/m2
Set Isolar = 1367 W/m2
= 0.3
Solve for Tearth
((1-) Isolar)/ 4 = T4earth
∆T = 0.1°C (Small !)
Sunspot Cycles
Very weak forcing, but significant climate responses to it.
17001600 1800 1900 2000
- S
un
spo
ts +
~0.1%
Little Ice Age (1500-1850 AD)
LIA
Cooling was the result of lower solar radiation and some big volcanic eruptions
Solar Variability
• Forcing is very weak (in visible spectrum), only ±0.1-0.2%, 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
“Proxies” - getting temperatures from treesa) Measure ring widths b) Calibrate ring widths
0
5
10
15
20
25
30
0 0.1 0.2 0.3 0.4 0.5
Ring Width (mm)
Growth Temp. (°C)
C) Validate and Apply
T (°C)
Year AD
warm
cool
Results from the IPCC AR4 report
Natural forcingonly
…most of modernwarming is due to humans
Natural + Human
Timescales of Natural Climate Variability
A. Short time scales (1-2 years): Random weather-related 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)
Most probable estimate is +2 to +4°C in next 100 years
All scenarios warmer.
What’s in store for the future?
Today
past future
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