Global Warming and the Climate System

1. Global Warming and the Climate System Kim Cobb EAS http://shadow.eas.gatech.edu/~kcobb

4. 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:

5. Temperature is a state variable of the Climate System. Changes in temperature are associated with Climate Change Understand processes that control TEMPERATURE changes

6. 5ºC A paleo perspective CO2 range = 200 to 280ppm 80 ppm Temperature range = 5ºC

7. What’s the greenhouse effect? 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!)

8. A moderate greenhouse: On Earth, the atmosphere contains about 360 parts per million CO2, and the surface temperature is about 16°C. Without greenhouse gases, the Earth would be a frozen snowball! A runaway greenhouse: On Venus, the atmosphere is nearly all carbon dioxide, and the temperature is about 400°C. The greenhouse effect occurs naturally...

9. Mainly, we burn fossil fuels - coal, oil, and gas. 80% of the CO2 increase Secondarily, we cut down forests (particularly in the tropics) 20% of the CO2 increase Changes in atmospheric CO2induced by humans

10. CO2 changes in the last 300 yr 380 100 ppm • 379 ppm • 381 ppm • 383 ppm • 385 ppm • 473 ppm • 2100 573 ppm Industrial Revolution Theory predicts that increasing CO2 will warm the planet – what do we see?

11. from thermometers from paleo records 1ºC

12. 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.

13. Other greenhouse gases are also increasing

14. Evidence of Global Warming in the Climate System

15. Glacier melting • Kilimanjaro: ice caps have melted back by 80% since early 1900’s; will be gone by 2020 • All glaciers in tropics are melting rapidly • Impacts: water supply, power generation, tourism, local climate and ecology

17. Arctic - the most sensitive ecosystem? • Arctic sea ice has shrunk by 1 million sq km and thinned from 3.1m to 1.8m average • More freshwater, reduced ability to travel over ice • All summer ice gone in this century • Ecological consequences huge!

18. Arctic warming • Sea ice melting • Key ALBEDOfeedback! • Reduces albedo (reflectivity) of earth, allowing more radiation to be absorbed http://www.gsfc.nasa.gov/topstory/2003/1023esuice.html • Warming temperature • Melts permafrost • Ice sheet stability? Key sea level question! • Rate of warming 8x faster in last 20 years than in last 100 1979 2003

19. Arctic ecosystems impacts • Reduced ice: less algal production under ice = undermines base of food chain • Seal pups emerge just when ice is melting - earlier melt means they are exposed before ready to thrive • Caribou need ice to island-hop; they are falling through as ice thins • Polar bears hunt on ice in winter, retreat to land in summer. Less ice forces them onto land earlier

20. More intense storms… Hurricanes get their energy and staying power from warm water in the tropical oceans. As waters get warmer, we expect that hurricanes will become more intense. Increase in Category 4 & 5hurricanes observed. Katrina does not prove this! A weak 2006 season does not disprove this!

21. Higher sea level… • As water warms, it expands (“thermal expansion”). • Glaciers are melting • Observation: 3mm/yr in past few decades • Prediction: ~0.5m rise by the end of this century, 2-4m in 500 years • This will have a major impact on • Developed coastal regions • Low-lying island nations • Intensity of coastal flooding during storm surges • Coastal ecosystems (e.g. mangroves, estuaries)

22. What is the role of human emissions? Are we responsible or is it just a natural cycle of the climate system?

24. The temperature in the climate system is controlled by many different processes, however anthropogenic forcing by CO2 represents one of the major contributors

26. What will happen in the future?

27. 70 ppm

28. CO2 TEMPERATURE 4.5 C

29. Warmer temperatures… Prediction: Doubled CO2 will warm the planet by 1.8-5.8°C (before 2100) Some areas warm up more than others. Continents warm faster than oceans. Higher latitudes warm more than low. Map of predicted temperature change for a doubling of atmospheric CO2

30. Can we do anything about it?

31. Who is responsible for CO2 emissions?(these data are relatively correct even for today) • Mainly the U.S.: 25% • Other industrialized countries: 25% • Developing world: 30% • Remaining 20% split: Mideast, former Soviet Union, E. Europe. • The U.S. also has the largest per capita carbon emissions

32. Personal choices make a difference! • About one third of all GHG emissions in the US are related to personal choices • How you travel • Home energy use • Waste production • The remaining 2/3 is related to factors like industry, agriculture, and business uses 3.1 tons/yr CO2 14.3 tons/yr CO2!!

33. Is human-caused global warming for real? The evidence • Greenhouse gases are rising due to human activity • The “greenhouse effect” is real (past climate example) • The Earth is warming up • No other obvious causes are seen • This point is no longer debated by climate scientists (IPCC) http://www.ipcc.ch/ What can be done? • Personal choices • Political actions (e.g. a successor to the Kyoto treaty capping carbon emissions) • Industry initiatives (e.g. hybrid gas-electric cars)

34. What can or should be done about global warming? • Potential for dramatic irreversible consequences • Evidence that changes are beginning • But there is uncertainty about details of future change What gets in the way of action? • Uncertainty • (don’t we always make decisions in the face of uncertainty?) • Economics • (is this really an issue? How are estimates of costs made, and what are the costs of doing nothing? What are the benefits of acting? • Politics • Do governments follow scientific advice?

35. Are more dramatic effects possible? How about the movie The DAY after TOMORROW FACT: Some computer models suggest that the thermohaline circulation could abruptly shut down as a result of global warming. FICTION: .

36. The Thermohaline circulation today Responsible for heat transport to North Atlantic

37. Slow/shut down thermohaline circulation? • Future climate ==> more precipitation over North Atlantic • Surface ocean there will become fresher, less dense. • Leads to weaker or failed thermohaline circulation Predicted cooling of Europe directly related to thermohaline collapse. Perhaps more complicated effects that we cannot predict, but NYC will NOT freeze over!

38. End of lecture

41. Predicted vegetation ranges for future climate Modern vegetation ranges Changing where plants and animals can live Plant and animal species are best adapted to certain climate conditions. • General pattern: northward migration of ecosystems • Also some areas get wetter (shrinking grasslands) • Extinctions likely as new stresses are added to threatened species

44. Climate surprises…we don’t know enough about climate to predict it perfectly Even relatively simple things are hard to predict accurately. Best guess is that the globe will warm by about 3°C, but… • Will El Niño’s become more frequent? • Will another Dust Bowl-scale drought occur? • Will the ocean’s thermohaline circulation collapse? • Will the west Antarctic or Greenland ice sheets melt? (125,000 years ago - melting ice = sea level 5 m above present)