11/16/10. Why do climates change? Take away ideas and understandings. Our first climate model. Climate changes over the last millennium

Transcription

1 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 ) = σ T 4 Earth s incoming radiation (α = Earth albedo, or reflectivity) I incoming = (1-α) σ T sun 4 1

2 Our first climate model Earth incoming radiation (α = Earth albedo, or reflectivity) I incoming = ((1-α) I solar ) / 4, or ((1-α) σ T 4 sun )/ 4 Earth outgoing radiation I outgoing = σ T 4 earth α is ~0.3, or 30% ε (emissivity) = 1 σ = 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-α) σ T sun 4 ) / 4 = σ 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. 2

3 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 3

4 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 Mt. Pinatubo

5 α = ~30% on average T effective = 254.8K Volcanic eruption can change albedo by 1% Recalling I in = (1-α) σ T 4 ((1-α) σ T sun 4 )/ 4 = σ T 4 earth Increase α 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 C for only 2-4 years. Warms the Stratosphere by 2-3 C (!) Short-term but significant impact 1815 eruption Mt. Tambora Year Without a Summer Poverty Year 1800-and-froze-to-death 5

6 11/16/10 Mt. Merapi (Indonesia) tropical volcanoes cool climate most 2. Solar Variability 6

7 Sunspots, Facular brightening, and Irradiance 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 α = 0.3 Solve for T earth ((1-α) I solar )/ 4 = σ T 4 earth T = 0.1 C (Small!) 7

8 Sunspot Cycles ~0.1% - Sunspots Very weak forcing, but significant climate responses to it. Little Ice Age ( AD) LIA Cooling was the result of lower solar radiation and some big volcanic eruptions 8

9 11/16/10 Little Ice Age ( ) London Frost Fair (1814) Rhône Glacier Solar Variability Forcing is very weak (in visible spectrum), only ± %, 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 9

10 11/16/10 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) Ring Width (mm) C) Validate and Apply warm T ( C) cool Year AD 10

11 Temperatures over the last 2,000 years * 2005 Date (AD) Measured CO 2 trends since 1958 Atmospheric CO 2 : Last 250 years 11

12 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 12

13 Climate = Solar Irrad. + Volcanoes + GHG Sum of all forcings ( ) Net = +1.6 W 4 lights per square meter * * * Another way to imagine global warming 13

14 11/16/10 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. C. Interannual (2-8 years): Primarily ENSO and Volcanic eruptions. E. 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 14

15 Global Temperature in 2050 Recent warming is unusual, Future warming is another world 2010 * +3 C warming 15