Why do climates change ?

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

2. 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

3. 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) = s T4 Earth’s incoming radiation (a = Earth albedo, or reflectivity) I incoming = (1-a) s Tsun4

4. Our first climate model Earth incoming radiation (a = Earth albedo, or reflectivity) I incoming = ((1-a) Isolar ) / 4, or ((1-a) s Tsun4 )/ 4 Earth outgoing radiation I outgoing = s Tearth4 • is ~0.3, or 30% • e (emissivity) = 1 • s = 5.67 x 10-8 Wm-2 K-4 Archer Fig. 3.3 (p. 23)

5. Earth’s temperature with no greenhouse effectTeffective = 254.8K (-18°C) At equilibrium, I incoming = I outgoing ((1-a) s Tsun4 ) / 4 = s T4earth Solve for Tearth Eqn. 3.1 in Archer Chapter 3

6. 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.

7. 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.”

8. Many 1000s of stations

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

10. There are only 3 ways to change Earth’s temperature Change input • Solar variability • Earth reflectivity (volcanoes) Change output c) Greenhouse gases

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

13. Volcanic eruption can change albedo by 1%  = ~30% on average Teffective = 254.8K Recalling Iin = (1-a) s T4 ((1-a) s Tsun4 )/ 4 = s T4earth Increase  to 31% New Teffective = 253.9K or -1°C cooler due a volcanic eruption

14. 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

15. Mt. Tambora 1815 eruption “Year Without a Summer” “Poverty Year” “1800-and-froze-to-death”

16. Mt. Merapi (Indonesia)tropical volcanoes cool climate most

17. 2. Solar Variability

19. Sunspots, Facular brightening, and Irradiance

20. 30 years of satellite observations: ±1 W/m2

21. 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-a) Isolar)/ 4 = s T4earth ∆T = 0.1°C (Small !)

23. Sunspot Cycles - Sunspots + ~0.1% 1600 1700 1800 1900 2000 Very weak forcing, but significant climate responses to it.

24. Little Ice Age (1500-1850 AD) LIA Cooling was the result of lower solar radiation and some big volcanic eruptions

25. Little Ice Age (1500-1850) London Frost Fair (1814)

26. 2001 1859 Rhône Glacier

27. 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

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

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

30. b) Calibrate ring widths C) Validate and Apply warm T (°C) cool Year AD “Proxies” - getting temperatures from trees a) Measure ring widths

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

32. Measured CO2 trends since 1958

33. Atmospheric CO2: Last 250 years

34. Natural forcing only Natural + Human …most of modern warming is due to humans Results from the IPCC AR4 report

35. Natural only Natural + GHG

36. Modeling the climate of the last 1000 years…

37. Climate = Solar Irrad. + Volcanoes + GHG

38. 4 lights per square meter * Sum of all forcings (1850-2000) Net = +1.6 W * *

39. Another way to imagine global warming

40. What global warming really looks like Data source: NASA / GISS

41. Timescales of Natural Climate Variability • Short time scales (1-2 years): Random weather-related variations of turbulent, chaotic atmosphere. • Interannual (2-8 years): Primarily ENSO and Volcanic eruptions. • Decadal-to-century scale: • Solar Variability (decades to centuries) • Anthropogenic greenhous gas emissions (decades to centuries)

42. 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

43. Global Temperature in 2050

44. 2010 * +3°C warming Recent warming is unusual, Future warming is “another world”