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GHG release, as well as sea level rise, will be limited if global
temperature stays within the range of recent interglacial periods.
Ice core data reveal a positive GHG feedback, GHG changes
lagging temperature change, but the feedback magnitude is moderate (CO2, 20 ppm per °C; CH4, 50 ppb per °C) even if the
entire observed gas change is a feedback (44). However, paleo data
do not constrain the magnitude of feedbacks under BAU warming,
which is far outside the range of interglacial temperatures.
Such feedbacks enhance the dichotomy between AS and BAU
scenarios. If global warming is not limited to 1°C, feedbacks may add to BAU emissions, making a ‘‘different planet’’ (17), including
eventual ice-free Arctic, almost inevitable. The AS requires concerted efforts to both slow CO2 emissions and reduce atmospheric
amounts of CH4, O3, and BC (17, 34). Achievement of the AS
should limit positive climate feedbacks. However, continuation of
BAU growth of CO2 emissions ( 2% per year) through 2015 yields
35% CO2 emissions relative to 2000 CO2 emissions and 40%
CO2 emissions relative to AS 2015 CO2 emissions. Given the long
life of CO2 and the impact of feedbacks on the plausibility of CH4
reductions, another decade of BAU emissions probably makes the
Inference of imminent dangerous climate change may stimulate
discussion of ‘‘engineering fixes’’ to reduce global warming (45, 46).
The notion of such a ‘‘fix’’ is itself dangerous if it diminishes efforts
to reduce CO2 emissions, yet it also would be irresponsible not to
consider all ways to minimize climate change. Considering the
evidence that aerosol effects on clouds cause a large negative
forcing (10), we suggest that seeding of clouds by ships plying
selected ocean regions deserves investigation. However, given that
a large portion of human-made CO2 will remain in the air for many
centuries, sensible policies must focus on devising energy strategies
that greatly reduce CO2 emissions. 1. Hansen J, Sato M, Ruedy R, Nazarenko L, Lacis A, Schmidt GA, Russell G,
Aleinov I, Bauer M, Bell N, et al. (September 28, 2005) J Geophys Res
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3. Lea DW (2004) J Climate 17:2170–2179.
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T, Karl T (2001) J Geophys Res 106:23947–23963.
5. Reynolds RW, Smith TM (1994) J Clim 7:929–948.
6. Rayner N, Parker D, Horton E, Folland C, Alexander L, Rowell D, Kent E,
Kaplan, A (July 17, 2003) J Geophys Res 108:10.1029 2002JD002670.
7. Hansen J, Lebedeff S (1987) J Geophys Res 92:13345–13372.
8. Hansen J, Ruedy R, Glascoe J, Sato M (1999) J Geophys Res 104:30997–31022.
9. Comiso JC (2006) Weather 61:70–76.
10. Intergovernmental Panel on Climate Change (2001) Climate Change 2001: The
Scientific Basis, eds. Houghton JT, Ding Y, Griggs DJ, Noguer M, van der
Linden PJ, Dai X, Maskell K, Johnson CA (Cambridge Univ Press, Cambridge,
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This document was uploaded on 03/15/2014 for the course MEA 570 at N.C. State.
- Spring '08