From The Earth System, 3rd Ed., Chapter 15:
"Global Warming, Part 1: Recent and Future Climate
(pp. 295-319)

• Key Questions: all (p. 295)
  
  
• CHAPTER OVERVIEW (p. 295)
  
  
• INTRODUCTION (pp. 295-296)
  
  
  1. The authors raise two questions: (a) how do we know if the observed global average temperature changes in the last century have been caused by humans (mostly by release of greenhouse gases into the atmosphere), or if they simply represent natural fluctuations in the climate system; and (b) how do we know if the recent increase in atmospheric CO₂ (which itself is not disputed) will continue into the future? What do the authors say we have to do to answer these questions?
  
  
• HOLOCENE CLIMATE CHANGE (pp. 296-303)

  Proxy Climate Data

  2. How far back in time do our temperature records on land (recorded by weather stations) and on the ocean surface (recorded by ships and buoys) extend? How do we fill spatial gaps in coverage in modern times? How far back in time do consistent and reliable satellite observations (which now provide the bulk of climate observations) extend?
     
     
  3. What are some examples of proxy temperature data used to infer temperatures before the "instrumental record" began in the mid-1800s? How far back do some of these methods provide evidence of temperatures?

  Holocene Warm and Cold Periods

  4. How large have global temperature fluctuations been in the last 10,000 years (the Holocene, the current interglacial period), compared to the transition to the Holocene?
     
     
  5. How large do global temperature variations have to be to produce large changes in the physical environment?
     

  The Holocene Climatic Optimum

  6. During the Holocene Climatic Optimum, a period of relatively consistent climate, global average temperatures in the summer appear to have been a little higher than they were in the 20th century. When was this period? Why are analogs from the past, such as this one, of limited use for predicting the future?

  The Medieval Warm Period

  7. When was the European Medieval Warm Period (MWP)? What was the climate like in northern Europe during the MWP, compared to the 20th century?
     
     [Note that there have been significant questions about whether the MWP was a global or regional phenomenon. Recent research (Mann, 2009) suggests that although temperatures in some regions, such as northern Europe and perhaps several other regions, reached maxima that were higher than in the same regions today, global mean temperatures during the MWP were slightly lower than they have been in recent decades.]

  The Little Ice Age

  8. When did the Little Ice Age (LIA) begin? Was it global in scope? What climatic changes occurred during that period that left evidence?

  Volcanoes and Climate

  9. How, and by what mechanism, do major volcanic eruptions typically affect global temperatures? How long do their effects typically last?
     
     
  10. What was the level of volcanic activity during the MWP and during the LIA? Does volcanic activity explain what happened to global average temperatures during these periods?

  Solar Variability

  11. What are the greatest changes in solar output on time scales of a decade to a century associated with?
      
      
  12. How far back to reliable sunspot data exist? What happens to solar output when sunspots increase in number?
      
      
  13. Over the course of a typical 11-year sunspot cycle, by how much does solar output vary? How large a direct effect would such a difference have on climate? Are any feedbacks commonly accepted that could amplify the solar output variability associated with sunspots? Are there as-yet untested hypotheses about how sunspots might significantly affect the earth's surface temperature?
      
      
  14. What, and when, were the Maunder Minimum, the Spörer Minimum, and the Wolf Minimum? What were global temperatures like during these periods? What about the period of greatest sunspot activity in the record (both proxy data and direct observations), during the 12th and 13th centuries? Is there a demonstrated physical connection between sunspot numbers and global climate?

  The Last 150 Years

  15. By how much have global average surface temperatures recorded by thermometers increased since 1850? Has this observed increase occurred at a uniform rate? If not, what has the pattern of variation been? What is the tentatively accepted explanation for the halt in global warming, and possible slight cooling, between 1940 and 1970 (or so)?
      
      
  16. To try to address the question of how much of the warming observed from 1850 to today has occurred naturally (a rebound from the Little Ice Age) and how much has been caused by human activities (mostly greenhouse gas emissions), climatologists have used global climate models (GCMs). How have these models been used to address the question, and what are the results? How well do the climate models reproduce global temperatures during the 20th century?
      
      
  17. What does the performance of climate models in simulating global mean temperature patterns in the last century do for our confidence in their ability to simulate prospective temperature changes in the current century?
      
      
• CARBON RESERVOIRS AND FLUXES (pp. 303-306)
  
  
  18. By how much have atmospheric CO₂ concentrations increased since the beginning of the 19th century? How does this compare to natural fluctuations since the retreat of the glaciers 11,000 years ago? To what do we attribute the increase, and with what degree of certainty?

  Natural Reservoirs and Fluxes

  19. What is the most abundant type of fossil fuel? Are the extent of fossil fuel deposits well known?
      
      
  20. What is as important for understanding the atmospheric carbon budget as the size of various carbon reservoirs? Does reservoir size necessarily dictate the size of the flux of carbon into it or out of it? How important on scales of a few centuries are carbon fluxes associated with processes important on geologic time scales, such as weathering of silicate rocks and volcanism?
      
      
  21. What are the largest shorter term fluxes of carbon into and out of the atmosphere? Do they differ from each other very much (especially when averaged over a full seasonal cycle—that is, a year or more)?
      
      
  22. How does the amount of carbon stored in marine organisms compare to the amount stored in terrestrial forests?
      

  Rates of Fossil-Fuel Burning and Deforestation

  23. How do we know, and how accurately do we know, the rate at which humans have been burning fossil fuels? What was this rate in 2004 (the most recent year for which data were available to the authors, at least from one source)? How does this rate compare to the rate of exchange with the terrestrial biosphere and the rate of production by volcanoes? Why is its effect disproportionately large?
      
      
  24. At current rates of consumption by humans (that is, assuming no increase in consumption rate), how much longer would "recoverable" reservoirs of coal, "conventional" oil (that is, not counting hard-to-extract shale oils, for example), and natural gas last?
      
      
  25. In what regions of the world are fossil fuel consumption the greatest? What is the per capita (that is, per person) rate of consumption by residents of the U.S.? How does this compare to the world average? Among countries, why is China now the single largest consumer of fossil fuels in the world (as of 2006 or 2007)? [Note that this is the total, not the per capita, consumption, where the U.S. still has a large "lead".]
      
      
  26. How did global CO₂ emissions change from 1990 to 2006? How did this change compare to predictions made in 2001?
      
      
  27. During the 2000-2006 period, what was the average CO₂ emission rate due to (a) fossil fuel consumption and cement production (another major CO₂ producer) and (b) deforestation? (Where does most of the deforestation occur?)
      
      
  28. How much of the CO₂emitted to the atmosphere by human activities accumulated in the atmosphere? Where did the rest of it go? Why is the answer, in part, surprising?
      
      
• CO₂ REMOVAL PROCESSES AND TIME SCALES (pp. 306-309)
  
  
  29. How do scientists predict future CO₂ concentrations in the atmosphere?

  Northern Hemisphere Reforestation

  30. What was responsible for most of the increase in atmospheric CO₂ between 1800 and 1850? What is one reason why the biosphere is a net sink for CO₂ today, in spite of ongoing deforestation in the tropics? How much longer can we expect some of the individual biospheric CO₂ sinks to act as sinks? How much of the overall biospheric sink is due to reforestation in North America?

  CO2 and Nitrogen Fertilization

  31. What might constitute the bulk of the rest of the biospheric sink for CO₂? What is CO₂ fertilization? Do scientists expect CO₂ fertilization to continue to be a major CO₂ sink in the future? What is nitrogen fertilization? How does it affect the atmospheric carbon budget?

  Dissolution in the Oceans

  32. What is the expected lifetime of CO₂ released into the atmosphere today? (That is, how long does the average molecule of CO₂ released into the atmosphere stay in the atmosphere before being removed?) Why is this "residence time" expected to increase in the future?
      
      
  33. What has been happening to the pH (a measure of acidity and alkalinity) of the surface layer of the ocean? Why? If it continues, what consequences might this have for ocean ecosystems?
      
      
• PROJECTIONS OF FUTURE ATMOSPHERIC CO₂ CONCENTRATIONS AND CLIMATE (pp. 309-318)
  
  
  34. What assumptions have to be made to estimate future atmospheric concentrations of CO₂?
      
      

  Atmospheric CO₂ Levels for Different Emission Scenarios

  35. The Intergovernmental Panel on Climate Change, in its 2007 report, described global climate model projections of 21st century climate under a number of CO₂ emission "scenarios". What assumptions does the "low" emissions scenario make about CO₂ emissions through the end of the current century? What is the projected concentration of CO₂ by 2100 under this scenario? [The global average CO₂ concentration in 2018 is about 389 parts per million (ppm), and the pre-industrial concentration was about 280 ppm.] Will it have stabilized by then?
      
      
  36. What assumptions does the IPCC's "medium" emissions scenario make about CO₂ emissions through the end of the 21st century? What is the projected CO₂ concentration by 2100? By what year would CO₂ concentrations reach twice their pre-industrial levels?
      
      
  37. What assumptions does the IPCC's "high" emissions scenario make about CO₂ emissions through the end of the 21st century? (How much of the known "recoverable" reserves of fossil fuels would have been consumed by then?) What is the projected CO₂ concentration by 2100? By what year would CO₂ concentrations reach twice their pre-industrial levels?
      
      
  38. The IPCC considered a scenario with even higher emission rates than the "high" scenario. Over the last 5-6 years of the first decade of the 21st century, how have actual CO₂ emissions (so far) compared to this most "pessimistic" scenario?
      
      
  39. Based on simple, one-dimensional, radiative-convective climate model simulations (which provide a rough first estimate but are not as sophisticated as global climate models), what is the range of projected global average temperature increases by the end of the 21st century (relative to the 1850 baseline value) in response to the range of CO₂ emission scenarios considered by the IPCC? How much of this projected increase has already occurred? Based on these projections, how would the rate of temperature increase compare to what has already occurred?
      
      
  40. Do the projections described above, made in response to various CO₂ emission scenarios, account for contributions from other greenhouse gases?
      
      Increases in Other Trace Gases
      
      
  41. How have concentrations of methane, nitrous oxides, and chlorofluorocarbons (all greenhouse gases with at least some sources related to human activities) varied over the last several decades?

  The Concept of Radiative Forcing

  42. What is meant by radiative forcing? What is the radiative forcing associated with a doubling of CO₂ concentrations? How large a surface temperature increase does this produce in one-dimensional radiative-convective climate models? How have the radiative forcings for methane, nitrous oxide, and halocarbons each increased between 1750 and 2005? What has contributed to negative radiative forcing (which produces cooling) since 1750? Are these as well understood?
      
      
  43. What has been the net effect of the factors above on radiative forcing? Is there significant uncertainty about the sign of the net radiative forcing?
      
      
  44. What proportion of the radiative forcing from all greenhouse gases combined has been offset by the effects of aerosols (which increase atmospheric albedo in two different ways)? What has (probably) been the most visible effect of aerosols on the history of global temperature changes since 1850? Why is the offsetting effects of aerosols likely to have less impact in the future?

  AOGCM Predictions of Global Warming

  45. What is meant by thermal drag exerted by the ocean? What effect does thermal drag have on the response of atmospheric temperatures to increases in greenhouse gas concentrations?
      
      
  46. How do the global average temperature projections made by coupled atmospheric-ocean global climate models (AOGCMs) compare to those made by the simple, one-dimensional radiative equilibrium models?
      
      
  47. What is the temperature change projected by AOGCMs if concentrations of aerosols and greenhouse gases are held constant at their levels in the year 2000? Why should the temperature continue to rise at all?
      
      
  48. What is the range of uncertainty in global average temperature projections for the year 2100, due to differences among AOGCMs and among CO₂ emission scenarios?

  Long-Term Climate Warming

  49. CO₂ concentrations are projected to continue to rise beyond the year 2100. By how much might they eventually rise, compared to the preindustrial value?
      
      
  50. What increase in temperature do AOGCMs project as a result? How does this change in temperature compare to surface temperature change during the last 100 million years?

  Possible Changes in the Thermohaline Circulation

  51. In some scenarios, why might northern Europe experience significant cooling while the rest of the world undergoes significant warming?
      
      
• Chapter Summary (all) (p. 318)
  
  
• Review Question: (all) (p. 319)