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Biol384_Exam_160204 Name _____KEY________________________ 1) Briefly explain two distinct lines of evidence discussed in class that global warming is caused by
humans. (2 points)
1) The coincidence of increasing temperature, greenhouse gas levels, and human population growth provides strong circumstantial evidence. (1 point) 2) Climate models (shown in IPCC reports) demonstrate that recent and current climate trends can only be explained if you include human activities; without the human contribution, no significant warming occurs. (1 point) 2) For the Keeling Curve shown here, explain the
primary and secondary cause of the secular trend (the
long-term rise) and explain the cause of the annual
oscillation (periodic ripple). Finally, what
mechanism(s) discussed in class might explain the
increasing amplitude of this ripple in recent years? (3
points)
The primary cause of the secular trend in atmospheric [CO2] is fossil fuel combustion; the
secondary cause is land use/land cover change. (1 point)
The annual oscillation is due to the seasonal patterns of photosynthesis and respiration (the
“breathing of the biosphere”) in the northern hemisphere (since this station is Mauna Loa),
drawing down [CO2] in the spring and summer, and increasing it in the fall and winter.
(1point)
The increasing amplitude is due to altered rates and seasonality of photosynthesis and
respiration. In other words, the Earth’s metabolism is changing. (1point)
1 Biol384_Exam_160204 Name _____KEY________________________ 3) What does the satellite record tell us about vegetation change in terrestrial arctic regions?
Specifically, what satellite index (or metric) is being used, what have been the trends in this index
in recent years, what independent field evidence supports these trends, and what are the
implications of these trends for changing ecosystem function and feedback effects? (Discuss at 2
alternate scenarios, and be sure to state the nature and direction of any feedback effects you
discuss) (5 points)
The increase in the satellite-­‐derived Normalized Difference Vegetation Index (NDVI) suggests that the Arctic has been greening, implying increased photosynthetic activity and primary productivity (Myneni et al. 1997). [Depending upon how, when and where NDVI is measured, it might also indicate an earlier snowmelt or less cover by snow, ice and standing water] (2 points) Independent evidence, including field studies and photographic comparisons going back to WWII, document an increase in shrub growth over this period (e.g. Sturm et al. 2001), consistent with the primary interpretation above. Also, evidence from warming experiments (e.g. Hudson et al. 2011) support the shrub expansion hypothesis. (1 point) One implication of arctic greening is that arctic regions are taking up more carbon via photosynthesis, increasing primary productivity, which would slow down climate change by reducing atmospheric [CO2] (negative climate feedback). (e.g. Myneni et al. 1997, Goetz et al. 2005) (1 point) Alternatively, decreased albedo due to shrub expansion could alter energy balance, warming the surface (positive climate feedback). (e.g. Chapin, 2005) (1 point) Possible additional relevant points regarding feedbacks & implications: If surface drying or extensive permafrost melting occurs with warming, ecosystem respiration and decomposition could exceed photosynthesis (positive climate feedback). (e.g. Huemmrich et al. 2010, Schurr et al. 2011). – [See also question 5] On balance, these processes suggest a positive feedback to climate, accelerating warming. Depending upon how these are implemented in models, one scenario that results is a “runaway greenhouse” (rapid and accelerating climate warming) due to the large reservoir of stored carbon locked up in arctic soils. Thus a major question is whether we will reach a tipping point, altering the biospheric and global carbon balance and climate. [See also question 5] 2 Biol384_Exam_160204 Name _____KEY________________________ 4) How has the boreal forest in western North America been responding to climate change in recent
decades and what causes these trends? Specifically, what ecosystem processes or properties have been
changing, how, and why? (Discuss mechanisms of change.) What are some climate feedback and
socioeconomic implications for these trends? (Discuss at least two different feedbacks and their possible
climate effects over time) (5 points)
In recent years, according to the satellite record, the western boreal forest has been “browning” (Goetz et al. 2005). This browning is most likely due to a combination of drought (due to warming), increased insect infestations, and increased wildfires (Kurz et al. 2008, Kasischke & Turetsky, 2006). (2 points for a good description of these trends) Address 2 or more of the three feedbacks discussed below (2 points): In the short term, these effects act as a positive feedback by reducing CO2 uptake and causing CO2 release from terrestrial ecosystems (Kurz et al. 2008). Alternatively, the increased winter albedo associated with loss of forest cover could be a negative feedback (Randerson et al. 2006) Also, as succession proceeds, the net effect in climate becomes less clear, so successional processes (Brown et al., 2012), and fire return cycles (Randerson et al. 2006) are important in evaluating the overall feedback effects on climate over time. The net effect of altered carbon balance and energy balance on climate feedbacks of the boreal browning is less clear than the individual component processes, [which is one reason why NASA is returning to study the forest via the ABoVE project]. Socioeconomic implications: Presumably, they also affect income from timber, tourism, and alter wildlife habitat and hunting/fishing opportunities. Note that direct anthropogenic disturbance (e.g. LULCC), and attendant effects on carbon balance and feedbacks, is not really the point of the question, which specifically asked about the forest’s response to climate. 3 Biol384_Exam_160204 Name _____KEY________________________ 5) How is arctic surface hydrology changing in response to climate warming and permafrost degradation
and what are the ecosystem implications? Specifically, what are the likely effects on the carbon cycle
(Discuss at least two different possible effects.) Use the concept of a “tipping point” to speculate on one
possible outcome of hydrological change for both climate and the biosphere (5 points)
Due to permafrost melting, in some places, the arctic surface is getting wetter due to thermokarsting and lake formation (Walter et al. 2006). In other places, the arctic surface is getting drier, as evidenced by shrinking ponds (Smith et al., 2005, Andresen and Lougheed, 2015) and the recent Alaskan tundra wildfire (Mack et al. 2011). (2 points) Address two of the following points regarding different carbon cycle scenarios resulting from hydrological changes (2 points): These changes are likely to release carbon previously stored in soils and frozen permafrost. Wetter surfaces are more likely to release methane, and drier surfaces are more likely to release carbon dioxide. The source/sink carbon status of the tundra seems to vary with moisture (moisture acts as a “switch” on the carbon cycle according to Huemmrich et al. 2010) Over time, a warming Arctic could also have a rather different biotic composition. For example, unless things get too dry, we might expect more shrub expansion (and perhaps tree expansion; Chapin et al. 2005), which could encourage carbon uptake to balance the loss of carbon from melting permafrost (e.g. Myneni et al 1997). The balance of these processes remains unclear, but one prediction is that extensive hydrological change due to permafrost degradation could turn the Arctic from a small sink to a large source of carbon to the atmosphere, due to decomposition, respiration, and/or fire, increasing the rate of climate warming (possibly leading to a “tipping point” in the form of a “runaway greenhouse”) (Schurr et al. 2011). (1 point for a plausible “tipping point” scenario) Note that discussions of energy balance (e.g. albedo) and other non-­‐carbon cycle effects of hydrology or warming may not be directly relevant to this question. 4