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) = 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.”

Many 1000s of stations

http://data.giss.nasa.gov/gistemp/

There are only 3 ways to change Earth’s temperature

Change inputa) Solar variability

b) Earth reflectivity (volcanoes)

Change outputc) Greenhouse gases

1. Volcanoes cool climate, briefly

Mt. Tambora - 1815 Mt. Pinatubo - 1994

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

Mt. Tambora1815 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

= 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

Little Ice Age (1500-1850)

London Frost Fair (1814)

Rhône Glacier

2001200118591859

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

http://data.giss.nasa.gov/gistemp/

How do we estimate past climate change, before there were thermometers

“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

Temperatures over the last 2,000 years

* 2005

Date (AD)

Measured CO2 trends since 1958

Atmospheric CO2: Last 250 years

Results from the IPCC AR4 report

Natural forcingonly

…most of modernwarming is due to humans

Natural + Human

Natural only

Natural + GHG

Modeling the climate of the last 1000 years…

Climate = Solar Irrad. + Volcanoes + GHG

Sum of all forcings (1850-2000)

4 lights persquare meter

Net = +1.6 W

* **

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 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

Global Temperature in 2050

2010 *

Recent warming is unusual, Future warming is “another world”

+3°C warming