ERTH 535: Planetary Climate Change (Spring 2018) Reading Questions (Responses due Monday, May 21) Dr. Dave Dempsey Dept. of Earth & Climate Sci., SFSU

## "Climate Modeling", Schneider, May 1987,Scientific American  "The Physical Science behind Climate Change", Collins et al., August, 2007, Scientific American   Also Recommended: "Climate Change 2013: The Physical Science Basis Summary for Policy Makers", Intergovernmental Panel on Climate Change (IPCC), 2013

Respond briefly to each of the "Reading Questions" below about the two required articles above, in the manner outlined by the assignment "Reading, Discussing, and Writing about Articles from the Literature". We will have a round-table discussion of the topic of this reading in class Monday, May 21. Completing this assignment in advance should help prepare you to participate actively. (Your response to the reading questions is worth 10 pts. Your participation in the discussion is worth 5 pts.)

### "Climate Modeling" (Schneider, 1987)

1. On what are mathematical models of planetary climate based? Can physically complete models be constructed in this way?
2. Basic Elements

3. How do models that are designed to simulate and forecast the weather differ in some ways from models designed to simulate and forecast climate (even though they are based on the same fundamental physical principles)?

4. What is meant by the number of dimensions that a model simulates? What is meant by the resolution of a model? What is a general circulation model? [Note: General circulation models have since evolved further into even more sophisticated global climate models (GCMs), which can be coupled with ocean models (CGCMs).]

5. What cost do general circulation models [and global climate models] incur for their greater comprehensiveness than simpler models?
6. Grids and Parameters

7. Climate models can't calculate the state of the atmosphere (and ocean and land surface and ice surface) at every point—the task would be overwhelming even for the fastest and most powerful computers on the planet. What do models do instead to address this problem?

8. What problem does limited spatial resolution in climate models create? How do clouds illustrate the problem?

9. How do models try to compensate for their inability to represent explicitly the effects of physical phenomena smaller than the spatial resolution of the model? [This strategy is called "parameterization".]

10. To fully simulate climate, a model must take into account feedback mechanisms that influence climate and make it complex. What problem do clouds present when climate modelers try to parameterize some of their feedback effects?
11. Climate Sensitivity

12. The author says (in 1987, at least) that reliable forecasting of future climate is not yet realizable. What two reasons does he state as the basis for his assertion? [Note that although climate models have advanced greatly since 1987, one of these reasons applies just as much today as in 1987 and probably always will, and the other still applies significantly today, if not to the same degree as in 1987.]

13. The author makes a distinction between uncertain and unpredictable variables. To help understand the impact of unpredictable variables, modelers construct scenarios. What does the author meantby a scenario? How do modelers try to understand the impact of uncertain variables?

14. Do climate model forecasts tell human societies what they must do in response?

15. The author cites three methods of checking or verifying climate models so that we can trust them enough to make decisions about social policy. What are they? Are any one of these methods sufficient to verify the reliability of climate models to forecast future climate? From what shortcoming(s) does each suffer?
16. Recent History

17. What were two successful simulations of past climate performed using general circulation models? Why do model predictions, and disagreements between models, sometimes stimulate field-based or other observational (i.e., non-modeling) investigations?
18. The Cretaceous Period

19. During the middle of the Cretaceous Period (about 100 million years ago), middle and high latitudes were much warmer than today. Paleoclimatologists hypothesized that the different distribution of continents at that time modified the ocean currents so they transported heat poleward more efficiently than today. How did climate modelers test this hypothesis? What were the results, and how did they explain the results produced by the model?
20. The Modern Greenhouse

21. The earth's surface temperatures lie between the very hot temperatures on the surface of Venus and the very cold temperatures on the surface of Mars. What accounts for these differences, which far exceed what we'd expect based solely on the relative distances of these planets from the sun?

22. What two aspects of the greenhouse effect in the near future are uncertain? Is the ongoing and likely future enhancement of the greenhouse effect due predominantly to increases in carbon dioxide in the atmosphere?

23. Rather than changes in global average climate, what consequences of ongoing and likely future changes in climate do we really need to know about?

24. Climate modelers first tried to assess the impact of increases in atmospheric carbon dioxide on climate by instantaneously doubling the amount of carbon dioxide in the model atmosphere, running the model until it reached thermal equilibrium, and comparing the new simulated climate to today's. What does the author mean by thermal equilibrium? What shortcoming does this approach have (independent of shortcomings of the particular models used to implement this approach)? How did the author highlight and begin to address these shortcomings in his own modeling efforts? What sort of model does he say is needed to assess the impact of increases in greenhouse gases more realistically? [More than a dozen versions of the sort of model he describes have since been developed by different modeling groups around the world and serve as the primary tool for simulating future climate today. These models are now referred to as global climate models.]
25. Nuclear Winter

Uncertainties

26. The author refers to climate models as "a dirty crystal ball". Why does he say this? What dilemma do climate models therefore pose for us? What ultimate test of climate model forecasts does the author describe?

### "The Physical Science behind Climate Change" (Collins et al., 2007)

1. [For this question, see side notes on p. 71.] What is the IPCC, what does it do, how often has it done it, who does it, and when did it do it most recently?

2. What effect did the evidence about climate change accumulating over the 20 years before the most recent IPCC report, have on scientists' confidence about the causes of climate change and the likelihood of it continuing?

3. What four aspects of climate change does the IPCC's physical science assessment address?
4. Drivers of Climate Change

5. What four gases or types of gases have increased in the atmosphere as a result of human activities and contribute to the greenhouse effect? How do concentrations of carbon dioxide and methane in 2007 or so compare with preindustrial concentrations? How do we know?

6. How do we know that human activities have been responsible for much of the increase in these gases?

7. What do climate scientists mean by radiative forcing, a measure of the impact of greenhouse gases?

8. What are examples of "natural drivers" of climate change? In addition to greenhouse gases, what are examples of other significant, human-induced drivers of climate change? Compared to greenhouse gases, how well do we understand the impact of these other human-induced drivers?

9. Starting with the third IPCC report in 2001, scientists have estimated the uncertainty in the various radiative forcing mechanisms quantitatively. Based on this work, how much do scientists estimate that human-induced drivers have contributed to climate change compared to natural drivers? Is the sign (positive or negative) of the net radiative forcing due to all drivers clear? [See figure: "Influences on Climate", on p. 67.]
10. Observed Climate Changes

11. Since reliable instrumental records began in 1850 or so, how did the annual, global average temperatures during the decade or so leading up to 2007 compare with previous years? How rapidly did global average temperature increase during the 20th Century (1901 to 2000)? How did that compare to the period from 1906 to 2005 (a shift of only five years)? Was the trend a steady one, or was the warming uneven during that period?

12. What has been happening to the frequency of relatively extreme highs and extreme lows of temperature (heat waves, cold snaps)? Is this observed pattern consistent with a warming trend?

13. In addition to familiar measures of climate such as average temperatures and precipitation measured directly by instruments, what other kinds of evidence for global warming do the authors cite?

14. What proportion of the heat added to the climate system has been absorbed by the ocean? What consequence does adding heat to the ocean have on sea levels? (Why?) At what rate has sea level risen in recent decades?

15. What indirect measures of changes in precipitation have been observed? (These changes might result in part from greater evaporation associated with global warming.)

16. How do (global average) temperatures in recent decades compare to reconstructed global temperatures during other periods over the last 1,300 years? [Note: This record has since been extended.]

18. What is the question of attribution? What three broad categories of causes might be linked to the observed global warming? (What do you suppose is meant by "spontaneous variability", as opposed to "natural forcing"?)

19. According to the 2007 IPCC report, how likely is it that most of the warming since the mid-20th century is attributable to humans? What are four reasons why that likelihood increased significantly since the 2001 IPCC report? [Note: This level of confidence has since increased significantly.]

20. Why are climate models so important for studying attribution and climate change more generally?

21. What two important advances since the previous IPCC report have increased confidence in the use of models for both attribution and projection of climate changes?

22. What modeling experiments do climate scientists perform to study attribution? What are the results do these studies show?

23. What two patterns of climate change are consistent with human-driven greenhouse gas warming? Which one is inconsistent with an increase in solar output?

24. Why is it harder to attribute the cause(s) of variations in climate over smaller (e.g., regional) scales of time and space than it is to attribute variations averaged over the whole globe? On how small a spatial scale can scientists say with some confidence that humans have significantly influenced temperature? [Note: The size of at least some regions over which scientists can say that humans have significantly influenced temperature has since decreased.]

25. Why can't individual extreme weather events be attributed to human-caused global warming? What can we say about such events instead? [Note: The science of discerning contributions of anthropogenic warming to at least some extreme weather events has since progressed rapidly, and such contributions can now be detected with some confidence for some such events.]
26. Projections of Future Changes

27. Would global warming and consequent auxiliary changes in climate cease if humans reduced greenhouse gas emissions immediately to levels that would stabilize greenhouse gases at current concentrations? Why or why not?

28. For a range of plausible emissions scenarios over the next 20 years (that is, regardless of what humans do during the next 20 years, within reason), what do the ensemble models project will happen to global average temperatures? What is mean by a "commitment" to future climate change, which we've already made? How much of the projected 20-year (that is, "near term") warming will be due to such commitment?

29. Unlike shorter-term climate change (i.e., over the next several decades), longer-term warming depends strongly on future emission rates. For a plausible range of emissions scenarios, what range of global average warming outcomes does the model ensemble project is likely? What do the models say about longer term changes over regions (rather than the global average)

30. What positive feedback in the carbon cycle is projected to make greenhouse warming more acute in the future? Do we understand the likely strength of this feedback well?

31. What two major impacts on the ocean is global warming projected to have?

32. What impacts on low latitudes is global warming projected to have?

33. What is perhaps the most prominent remaining uncertainty in the physical processes represented in climate models?

34. For how long are living things on earth likely to have to cope with the ongoing change in climate? How certain are we about the particulars?
35. Facing Our Future: Notes from the Editors

The Consequences of Ongoing Warming

36. What sorts of additional evidence of ongoing global warming is cited in this section? What are some of the consequences for humans that continued warming would make the most likely? On whom would these consequences likely have the greatest impact?

37. Would there be any positive consequences of global warming?
38. What Needs to Be Done

39. The authors describe two categories of response to climate change available to us. What are they called and what does each mean? Are we in a position to chose between them?

[Note that a third category of response, called "geoengineering", is also available in principle. Geoengineering tries to reverse global warming by creating new sinks of atmospheric carbon dioxide or stimulating existing ones, or by reducing the absorption of solar radiation by increasing the earth's albedo. Geoengineering is more speculative and risky than the other two alternatives and is hence controversial. It could become a relatively desperate fallback option if global warming becomes worse than expected.]

40. To avoid some of the worst consequences of global warming, one subgroup of IPCC scientists recommended a threshold concentration of carbon dioxide that the atmosphere should not be allowed to exceed. What was that level? What was the latest date by which they recommended that carbon dioxide levels be stabilized at that level?

41. What technical strategies and regulatory and incentive policies can we adopt that might allow us to achieve this target? Are we on track to achieve the target?

### Recommended: "Climate Change 2013: The Physical Science Basis: Summary for Policy Makers" [PDF file].

The Fourth IPCC report on the physical basis of climate change, published in 2007, was based on research published through about 2005. In the years since then, our understanding of climate change has continued to advance. The Fifth IPCC report, "Climate Change 2013: The Physical Science Basis", now stands as the most authoritative statement about the state of climate science available. However, like all of the main IPCC reports, the fifth report is long, detailed, and a slog to read. In contrast, the Summary for Policy Makers is a much more concise summary of 2013 IPCC report's key conclusions. The Sixth IPCC report, on which work is currently underway, is expected in 2021.

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