Reducing atmospheric CO2 by a deliberate carbon dioxide removal (CDR) strategy is one option being considered for combating global warming caused by the current high levels of atmospheric CO2. Before implementation can occur, it is important to have a reference goal for a planned reduction of the global CO2 concentration against which any specific strategy can be assessed. Scientists agree that a modest aim would be to seek to reduce the CO2 concentration by 50 ppm, which is the equivalent of removing 400 gigatons of CO2,or an average of 4 GtCO2/yr over 100 years. At this substantial level of reduction, it is still expected that dangerous levels of atmospheric CO2 would persist.
Even in the limiting case in which all emissions of CO2 cease, it is not straightforward to estimate how much CO2 must be removed from the atmosphere to lower its content by 4 GtCO2 annually. During any period of zero emissions but no CDR implementation, the atmospheric CO2 concentration will fall. Ocean currents continually bring deep ocean water to the surface that was last at the surface hundreds of years before, when the atmospheric CO2 concentration was at its preindustrial value. As equilibrium is established at the ocean surface between CO2 in the air and dissolved in the ocean, such ocean water scavenges CO2 from the atmosphere. So, with a time scale of hundreds of years, some atmospheric CO2 reduction is done naturally (at the expense of growing acidity in the ocean).
The same requirement for equilibrium at the ocean surface governs the reverse effect that would happen if the CO2 concentration of the atmosphere were lowered someday by deliberate removal of CO2. There would be an immediate compensating transfer of CO2 from the ocean to the atmosphere, as a result of which it would be necessary to remove more than one unit of CO2 from the atmosphere to reduce the atmospheric CO2 content by one unit. Forest responses add complications, as do deep ocean currents. Today, the combined effect of ocean and land CO2 “sinks” results, for a time-scale measured in decades, in roughly half of the CO2 remaining in the atmosphere that was emitted to the atmosphere by the burning of fossil fuel, but the future magnitudes of these sinks are uncertain.
1. Which of the following is listed as a natural phenomenon that would require a CDR strategy to remove CO2by an amount greater than its desired CO2 reduction target?
A) the presence of land CO2 “sinks”
B) the presence of ocean CO2 “sinks”
C) equilibrium at the ocean surface
D) deep ocean water moving to the surface
E) scavenging of CO2 by the ocean surface
2. The author discusses ocean and land CO2 “sinks” in order to:
A) explain how much of the CO2 emitted by the burning of fossil fuels remains in the atmosphere
B) highlight how uncertain the future is regarding the magnitude of CO2 emissions from fossil fuels
C) highlight the challenges in determining natural effects on CO2 levels when implementing a CDR strategy
D) clarify why it would be necessary to remove more than one unit of CO2 from the atmosphere to reduce the atmospheric CO2 content by one unit
E) emphasize that there are many natural means by which CO2 is removed from the atmosphere.
3. The author uses the content in the parentheses at end of the 2nd paragraph to:
A) provide additional information about how the natural process of atmospheric CO2 reduction occurs
B) emphasize the importance of a negative effect that would result from a natural process
C) highlight one result from the many natural processes related to CO2 removal
D) present a negative effect that is not directly related to the primary purpose of the paragraph
E) show the importance of natural removal of CO2 in the atmosphere
4. All of the following are discussed in the passage EXCEPT:
(A) details on how CO2 will be removed from the atmosphere by a CDR strategy
(B) difficulties in measuring the effects of a CDR strategy on CO2 reduction
(C) increases in atmospheric CO2 levels resulting from natural processes
(D) decreases in atmospheric CO2 result resulting from natural processes
(E) the risk of high atmospheric levels of CO2 in the atmosphere
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