Kim M. Cobb kcobb@eas.gatech

1. The Science of Climate Change Kim M. Cobb kcobb@eas.gatech.edu

2. Which of the following are scientific statements? • Reducing CO2 emissions would hurt the economy. • 2) Improved technology is the best way to slow global • warming. • 3) A warming of 1ºC over the next 50yrs is dangerous. • 4) Global temperatures were 5ºC colder during the Last • Glacial Maximum. • 5) Hurricane Katrina was caused by global warming.

3. Which of the following are scientific statements? • Reducing CO2 emissions would hurt the economy. • 2) Improved technology is the best way to slow global • warming. • 3) A warming of 1ºC over the next 50yrs is dangerous. • 4) Global temperatures were 5ºC colder during the Last • Glacial Maximum. • 5) Hurricane Katrina was caused by global warming.

4. greenhouse gases in the atmosphere trap heat at the Earth’s surface and prevent it from escaping. • These gases include: • Carbon dioxide CO2 • Methane CH4 • Nitrous oxide N2O • Chlorofluorocarbons • Water vapor H2O • (this is the most important one, by far!) 4 1 without greenhouse gases average temp of Earth would be -18ºC instead of 15ºC 3 2

5. greenhouse gases trap heat because they absorb radiation in the infrared range, according to specific bond geometries and vibrational modes (ex CO2 below)

7. ice core CO2 records confirm that the CO2 trend began in the 1800’s • clear land for agriculture • Industrial Revolution

8. Who is responsible for CO2 emissions?

9. Total CO2 emissions (in thousands metric tons/yr)

10. Per capita CO2 emissions (in tons/per capita per yr)

12. The ‘instrumental’ record of climate shows a ~1ºC warming over the last century Source: Hadley CenterUK Met OfficeJones et al., 1999

13. 5ºC A paleo perspective: glacial-interglacial cycles 80 ppm CO2 range = 200 to 280ppm Temperature range = 5ºC

14. But why doesn’t an 80 ppm change in CO2 correspond to a 5 C change? The climate system does not reach equilibrium instantaneously Other processes can change the equilibrium temperature.

15. Why do 99.999% of climate scientists believe that CO2 is warming the planet? • Theory predicts that increasing atmospheric CO2 should warm the planet. • Geologic evidence links CO2 and temperature in the past. • The warming is unprecedented in the most recent centuries (dwarfs natural variability). • Climate models show that rising CO2 is necessary to simulate • 20th century temperature trends (solar and volcanic minor players).

16. Ice core climate and CO2 records tiny gas bubbles in the ice trap ancient air samples

17. #2 Atmospheric CO2 and temperature over the past 650 thousand years CO2 and temperature are closely linked on geologic timescales

18. To understand how climate has changed in the past, we need to use records of climate preserved in ice cores, ancient tree rings, coral bands, and other “paleoclimatic” sources: key is to CALIBRATE to temperature records

19. #3 The “Hockey Stick” Key Points: error bars increase as you go back in time natural variability accounts for <0.5ºC over the last millennium late 20th century temperature trend is unprecedented

20. #4 Intergovernmental Panel on Climate Change (IPCC) 2001

21. Consequences of Global Warming (IPCC SPM-AR4) • Reduced uptake of CO2 by land and ocean in warmer climate • Rising sea levels (0.3 to 0.6m by 2100)…. at a minimum • Ocean pH will decrease by 0.14 to 0.35 (already down 0.1) • Snow cover will decrease, permafrost melt, sea ice melt • Extreme events (temperature and precipitation) will become more frequent • Tropical cyclones will become more intense • Storm tracks will move poleward • Rainfall will increase in the high latitudes, decrease in the subtropics • Meridional overturning of Atlantic ocean will decrease

22. The uncertain climate future Range of CO2 emissions scenarios: Strict international agreements  CO2 at 600ppm by 2100 *390ppm today Mid-ground  850ppm by 2100 280ppm 1800 Business as usual  1550ppm by 2100 Lower limit: 1°C by 2100 Upper limit: 6.5°C by 2100

23. uneven warming will shift rainfall patterns Projected temperature change: global view Take-homes: -poles warm more -land warms more -ocean warming patchy and complex

24. Regional models use global model output, run at high-resolution (5km) grid Length of heat waves increase (# days/event) Peak temperatures increase Diffenbaugh et al, 2005 US http://www.usgcrp.gov/usgcrp/nacc/se-mega-region.htm

25. Projected precipitation change: global view white = models disagree color = models mostly agree stippled = models agree Projecting precipitation is VERY uncertain business, yet extremely critical to human impacts.

26. Projected precipitation change: regional view change in yearly average precipitation # dry days # heavy rain days mm/day days/yr days/yr Diffenbaugh et al, 2005

27. IPCC says increase in hurricane intensity “likely” (66%)

28. Arctic Summer Sea Ice reducing over 30 years of record

29. New Radar Data Show ice streams That penetrate Deep into Antarctica Rignot et al., Science 2011

30. Mass balance estimates (blue/black = traditional; Red = satellite gravity) For GREENLAND ANTARCTICA BOTH Can we detect melting of the polar ice sheets? YES Rignot et al., GRL 2011

31. CERTAIN Warming of 1-6°C by 2100. Sea levels will rise by 6 to 30 inches by 2100. Precipitation patterns will change. More erratic precipitation. Extreme events will increase, hurricanes more intense. Prospect of abrupt climate change. UNCERTAIN

32. Can we do anything about it?

33. What is a country to do? • There are only three (prudent) options: • use less energy • - drive less, drive smaller (plug-in?) cars • - conserve electricity • - recycle, reuse • 2) make “clean” energy • - solar power, wind power, nuclear energy • 3) take CO2 out of the sky (much more difficult) …. but how much at what cost?

35. Some clear lessons: • 1) efficiency makes $$ • gains from cellulosic • biofuels likely (not • corn ethanol!) • 3) taking CO2 out of the sky • is more costly than • not putting it in the • atmosphere to begin with