# Why do climates change ? Climate changes over the last millennium.

## Presentation on theme: "Why do climates change ? Climate changes over the last millennium."— Presentation transcript:

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/m 2 ) =  T 4 Earth’s incoming radiation (  = Earth albedo, or reflectivity) I incoming = (1-  )  T sun 4

Our first climate model Earth incoming radiation (  = Earth albedo, or reflectivity) I incoming = ((1-  ) I solar ) / 4, or ((1-  )  T sun 4 )/ 4 Earth outgoing radiation I outgoing =  T earth 4  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 T effective = 254.8K (-18°C) At equilibrium, I incoming = I outgoing ((1-  )  T sun 4 ) / 4 =  T 4 earth Solve for T earth 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)Solar variability b)Earth reflectivity (volcanoes) Change output c) Greenhouse gases

1. Volcanoes cool climate, briefly Mt. Tambora - 1815Mt. Pinatubo - 1994

Volcanic eruption can change albedo by 1%  = ~30% on average T effective = 254.8K Recalling I in = (1-  )  T 4 ((1-  )  T sun 4 )/ 4 =  T 4 earth Increase  to 31% New T effective = 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. 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

30 years of satellite observations: ±1 W/m2

What if the solar radiation changes by +2 W/m 2 ? Recalling I solar = 1365 W/m 2 Set I solar = 1367 W/m 2  = 0.3 Solve for T earth ((1-  ) I solar )/ 4 =  T 4 earth ∆T = 0.1°C (Small !)

Sunspot Cycles Very weak forcing, but significant climate responses to it. 17001600180019002000 - Sunspots + ~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 2001 1859

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 trees a) Measure ring widthsb) Calibrate ring widths C) Validate and Apply T (°C) Year AD warm cool

Temperatures over the last 2,000 years * 2005 Date (AD)

Measured CO 2 trends since 1958

Atmospheric CO 2 : Last 250 years

Results from the IPCC AR4 report Natural forcing only …most of modern warming 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 per square 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 pastfuture

Global Temperature in 2050

2010 * Recent warming is unusual, Future warming is “another world” +3°C warming

Similar presentations