Respond briefly to each of the following "Reading Questions" about portions of the four 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 addressed by this reading in class Friday, May 11. (Your response to the reading questions is worth 10 pts. Your participation in the discussion is worth 5 pts.)
[Note that this article was published well before September 2012, when the extent of Arctic sea ice reached its lowest point since the satellite record began in 1979. The record second lowest extent occurred in 2016, and the third-lowest extent occurred in 2007 and the fourth lowest in 2011. However, a better measure of the state of Arctic sea ice is ice volume. There is no way to measure ice volume directly, but in September 2012 the Arctic ice volume calculated by the Pan-Arctic Ocean Modeling and Assimilation System (PIOMAS) (from satellite and other data) reached the lowest value since the satellite record began.]
- What observations collectively suggest that the Arctic has been warming?
- Why should Arctic warming have a disproportionate influence on the climate of the rest of the planet?
- What is one reason why predicting the magnitude of future climate change in the Arctic is so difficult?
- What are two positive feedbacks that could amplify Arctic warming?
- What are several other examples of the kinds of observations of Arctic warming that have been gathered?
Shrinking Glaciers, Thawing Permafrost
- What has been happening to Alaskan glaciers, and for how long? How about the Greenland Ice Sheet?
- Why do borehole records of Arctic permafrost temperatures reflect changes in amount and timing of winter snowfall as well as air temperatures? What has been the temperature trend measured in boreholes on Alaska's North Slope?
- What are several direct and indirect consequences, both observed and hypothetical, of melting permafrost in the Arctic?
Greening of the Arctic
- What changes to vegetation in the Arctic are occurring as a result of warming? What feedback(s) in the earth's heat budget (via transfer of carbon stored in permafrost into the atmosphere) might be occurring as a result of these changes?
Melting Sea Ice
- What changes have been observed in the extent (i.e., area spanned by the edges) of Arctic sea ice? What about the thickness of the sea ice? How have these observations been made?
- What consequences should significant decreases in Arctic sea-ice cover have? (Do these consequences include a rise in sea level? Why or why not?)
A Complex Web
- How does ice-albedo feedback work? How does cloud feedback work? Is cloud feedback positive or negative? [Note that the dependence of cloud feedback on the type of cloud is opposite in the Arctic from what it is at lower latitudes.]
Is Greenhouse Warming the Culprit?
- Broadly speaking, what competing hypotheses have been posed to account for the observed warming of the Arctic? Are they mutually exclusive?
- The authors' present a disturbed-rock-in-uneven-terrain analogy of Arctic climate, and assert that cessation of whatever is disturbing Arctic climate would not cause the changes to halt. Based on their statements, do you think the authors consider Arctic climate to be stable or unstable? (Why?)
- Other than confirming further the observations of Arctic warming and perhaps helping identify the cause with more certainty, what would be the point of gathering more and better data about Arctic climate?
Winds of Change
- What is the North Atlantic Oscillation (NAO)? How does it affect weather patterns in regions around the North Atlantic over periods of decades (and hence the climate in those regions)? What effect has it been having in recent decades? (How does this complicate the hypothesis that Arctic warming has been due to anthropogenic greenhouse warming?)
[The following terms appear in the abstract:
Additional terms that appear in the body of the article, in the order in which they appear:
- C.E. ("Christian Era", which starts with the year 1 A.D. ["Anno Domini"])
- first millennium C.E. (the first 1000 years of the Christian era, from 1 A.D. to 1000 A.D.)
- decadally resolved (detailed enough to show variations occurring over periods of around 10 years or longer, but not detailed enough to show variations over shorter periods)
- proxy temperature records (temperature records deduced from evidence that depends in some known way on temperature, as opposed to direct instrumental measurements of temperature)
- transient climate simulation (a computer model simulation that can account for continuous changes over time, as opposed to a steady-state simulation, which assumes steady forcing and hence doesn't account for changes over time)
- inference (deduction; conclusion based on strict reasoning based on evidence)
- Holocene (the geological epoch covering the most recent 12,000 or so years of the earth's geologic history; it is the current interglacial period, a relatively warm period since the end of the last glacial period, sometimes called an "ice age")
- decadal scale (time periods on the order of ten years to several tens of years; refers to variations in some quantity occurring over periods of about that long)
- preindustrial (before the Industrial Revolution began in the early 1700s)
- climate perturbation (a departure of the climate from the prevailing climate before the perturbation, particularly if it is temporary)
- Little Ice Age (a relatively cold period in much of the Northern Hemisphere from roughly the 1600s to the mid 1800s)
- Middle Ages (a period in human history about 1000 years long [a millennium], from roughly the 5th through the 16th centuries)
- spatial coherence (degree of similarity across different places at the same time)
- sensitivity limits of the proxies (the smallest differences [in temperature] that the indirect measures (proxies) [of temperature] are able to show)
- resolved to an annual or decadal level (can show changes over a single year or over around ten years)
- published (this probably means that it was subjected to peer review by other scientists, thereby meeting a higher standard of credibility than non-peer-reviewed published work)
- terrestrial records (proxy records retrieved from land areas, vs. ocean [marine] sediments, say)
- varve (an annual layer of sediment [for example, from a lake bottom] or sedimentary rock)
- observed temperature (temperature measured directly by instruments)
- oxygen isotopes (oxygen with different numbers of neutrons in the atomic nucleus, which changes the weight of the atom but not it's characteristic chemical properties)
- average series (sequence of 10-year averages, arranged in order of decreasing age)
- standardized (each member of a group, or in this case series, has subtracted from it the mean of the group/series, and the difference is then divided by the standard deviation [the square root of the variance] of the group/series; the resulting set of numbers has a mean of zero and a standard deviation [and hence variance] of 1.0)
- composited (values from different data sets are combined, averaging them where they apply at the same spatial location at the same time)
- least-squares linear regression (the relationship between two quantities, if it exists, modeled as a line with a slope and intercept [intersection with the vertical axis] that minimizes the sum of the squared differences between the quantity plotted on the vertical axis and the value on the regression line)
- r2 (The fraction of the variation in the quantity plotted along the vertical axis that can be accounted for by a linear relationship between that quantity and the quantity plotted along the horizontal axis)
- P (the probability, expressed as a fraction, that the least-squares linear relationship between two quantities is just an accident of random chance)
- principle components (PC) analysis (a mathematical technique that tries to represent the variations in some quantity in terms of several independent "components" of variation; in the case of a time series, the components could be periods of different length)
- precession of the solstices (gradual shift in the two points along the earth's elliptical orbit at which the December and June solstices, respectively, occur)
- tundra (plant community in which growth is severely stunted by low temperatures and short growing seasons)
- orbital forcing (changes in insolation at the top of the earth's atmosphere caused by changes in the earth's orbital parameters)
- centennial-scale (variations that occur over time periods on the order of a century to several centuries)
- anthropogenic (caused by human activities)
- millennial-scale (variations that occur over time periods on the order of 1000 years to several thousands of years)
- instrumental record (measurements of air temperature recorded directly using instruments, such as liquid-in-glass thermometers; this record dates from about 1850 to the present)]
- Why do the authors think there is a need for a relatively long-term reconstruction of the temperature record in the Arctic? (This is what motivated their study.)
- This article reports the results of a "reconstruction" of a temperature history of the Arctic region. How long a period does the reconstruction cover (back from the present time)? How long was the previous longest reconstruction for this region? What three types of "proxy" temperature data did the authors use to create the reconstruction?
- The authors assert that it's important to know the degree of spatial coherence of warm periods in the Arctic before the Little Ice Age, in early parts of the period between 1,000 and 2,000 years ago as well as during the Middle Ages, if we are to understand the causes of those warm periods. (Although the authors don't say so, one implication of this statement is that understanding the cause(s) of those warm periods can help us better to understand the cause(s) of the currently observed warming in the Arctic.) Temperature records prior to the period of instrumental record (which began around 1850) are deduced from proxy records. According to the authors, in what way might proxy records from the Arctic be superior to those from lower latitudes, and hence provide a clearer reconstruction of the temperature history than elsewhere?
- What criteria did the authors use to select the proxy data that they used in their analysis? How many locations (from which proxy data have been extracted and analyzed previously) met their criteria? Where do most of the proxy data come from (see Figure 1)?
- The authors assert that the locations from which the proxy data come, collectively represent the Arctic-wide mean temperature accurately. What is the basis for this assertion? During what period of the year does the assertion apply?
- The authors focus their analysis on temporal and spatial patterns of 10-year average temperatures. What three reasons do they cite for this?
- How many of the sources (locations) of proxy data that the authors analyzed extended into the late 20th century, when direct instrumental measurements of temperature are available? How did the authors take advantage of the overlap between these proxy records and the instrumental record? (What does Figure 2 show, and at least visually, does it support the statistical results that the authors claim for the overlap?)
- What striking, Arctic-wide-average temperature trend from 1 C.E. to 1900 C.E. emerged from the analysis? Is this result consistent with other proxy temperature data for the period from 6,000 to 10,000 years ago (the first half of the present interglacial period, the Holocene epoch)? In what way?
- What has the average change in temperature per 1000 years been over the period from 1 C.E. to 1900 C.E.? To what do the authors attribute this millennial-scale trend?
- The authors note that, although changes in insolation were the likely, ultimate cause of the reconstructed temperature trend between 1 C.E. and 1900 C.E., the changes in insolation by themselves were too small to account for the deduced changes in temperature, so (positive) feedback(s) must have amplified the temperature changes. What two positive feedbacks involving sea-ice cover likely contributed? What land-based positive feedback(s) likely contributed?
- What additional support for the connection between changes in insolation and the reconstructed temperature record do the authors cite?
- What three centennial-scale variations (departures from the millennial-scale linear trend) in Arctic temperatures do the authors describe in their analysis that are consistent with results reported by previous investigators? In what important way do the results reported by the authors differ from what earlier investigators reported? To what causes do the authors attribute the centennial-scale variations in reconstructed temperatures?
- How did observed Arctic-wide mean summer temperatures in 1950 differ from what a simple linear extrapolation of trends from 1 C.E. to 1900 C.E. would have predicted? To what cause(s) do the authors attribute this departure from the trend?
- How did proxy-inferred temperatures in the last half of the 20th century (1950-2000 C.E.) compare to temperatures extrapolated from the long-term trend and, for that matter, to temperatures inferred from any time during the analyzed period? The authors say, "[i]n recent years, the magnitude of the warming seems to have emerged above the natural variability". What do you suppose they mean by that?
- How does the Arctic-wide average temperature recorded during 1998-2008 compare with 10-year average temperatures during the analyzed period (1 C.E. to 2000 C.E.) and with temperatures extrapolated from the long-term trend?
- What is permafrost? What questions are the author's research expeditions trying to answer? Why are the answers potentially important?
Leaving the Freezer Door Open
- What proportion of land surface area on the earth comprises permafrost? Where does the carbon stored in permafrost come from?
- How is the carbon stored in permafrost converted into methane (CH4)? (Under other conditions, carbon stored in permafrost converts into carbon dioxide (CO2). What is the difference in the conditions that lead to production of methane vs. carbon dioxide?)
- What is the sequence of events that leads to the formation of small lakes, which promote the production of methane? (Why does lake formation promote methane production?)
- What does the trend in permafrost temperatures appear to be in recent years? What are the sources of evidence?
- How much might increases in atmospheric methane (and carbon dioxide) concentrations from melting permafrost add to greenhouse warming already projected to occur from other sources of greenhouse gases by the year 2100? According to the author, would this increase be significant?
The Mother Lode in Siberia
- Why is Siberia of particular interest to the author and her colleagues? What proportion of Siberia is now covered by lakes (and hence important for methane production)?
Blown Away by Bubbles
- How much has atmospheric methane concentration increased since preindustrial times? How does this change compare to fluctuations that occurred during the previous 650,000 years? [Note: the evidence comes from the analysis of the composition of air bubbles trapped in the Antarctic and Greenland ice caps and retrieved by coring deep into the ice.] How much of this increase can be attributed directly to anthropogenic sources vs. natural sources?
- How did the author figure out how to measure the production of methane from thawing permafrost in Siberia more accurately? What evidence pointed toward carbon stored in permafrost as the source?
- What evidence from the beginning of the Holocene epoch (10,000-11,000 years ago) suggests that the positive feedback between increased surface temperatures, melting permafrost, and methane production can be very significant? Why is that evidence possibly relevant today?
- How much methane do the author and her colleagues project might be released from thawing permafrost in the coming decades and centuries, relative to the amount present in the atmosphere today?
Fine-tuning the Models
- What negative feedback (considered by itself) tends to reduce the impact of permafrost thawing on methane production?
- In broad terms, what strategy is the author, her colleagues, and others pursuing to try to determine the net effect of positive and negative feedbacks acting simultaneously? What is the ultimate goal of that strategy?
- What solutions have been proposed to reduce the impact of positive feedback between surface temperatures, thawing permafrost, and methane concentrations in the atmosphere? Ultimately, what solution does the author argue is the only one likely to make a significant difference?
- According to this news article, according to some researchers, what level of alarm does the current scientific understanding of the threat posed by release of methane into the Arctic atmosphere seem to warrant?
Sure looks scary
- Where are the two great, potentially catastrophic sources of methane?
Here it comes?
- This news article cites three research reports about methane release from the ocean floor that were published in the preceding year. What did they say?
Not so fast
- How have concentrations of methane in the atmosphere varied since 1983, when a global network of 46 measuring sites was established?
- According to Dlugokencky and colleagues, what accounted for the increase observed by this network in 2007?
- How much warming (on top of warming from other greenhouse gases) does the model of David Archer (University of Chicago) estimate might be forthcoming due to methane release from these sources?
- What is Archer's message about (a) catastrophic warming due to sudden methane release from melting methane hydrates and permafrost; and (b) global warming more generally?
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