Page 365 - Earth's Climate Past and Future

CHAPTER 18 • Causes of Warming over the Last 125 Years 341

downwind of the eastern United States, Eastern facts and its assumptions. It would be accurate to con-
Europe, and China (see Figure 18–10). clude that an enormous range of evidence now supports
Current estimates are that sulfate aerosols have the mainstream view that Earth’s climate has warmed in
reduced incoming solar radiation by enough to offset the last 125 years mainly because of humans and to
about 15% of the radiative forcing (and global warming) claim that no serious counterarguments to this view
caused by greenhouse gases (see Box 18–1). Ignoring remain.
this offsetting anthropogenic cooling effect makes the Meanwhile, Earth continues to register the many
climate system appear less sensitive than it actually is. effects of global warming year after year. Most of the
The brown-cloud hazes have the same kind of effect warmest years in the last 125 years have occurred in the
on estimates of Earth’s sensitivity as the sulfate aerosols. 1990s and 2000s. Circum-Arctic snow and sea ice are
In regions of very strong hazes, the carbon-rich brown shrinking to limits that are unprecedented during the
clouds are estimated to have offset as much as 50% of the satellite era. Almost every mountain glacier on Earth is
local radiative forcing from greenhouse, with a smaller melting, and most will disappear within 25 years at their
(and highly uncertain) effect on a global mean basis. current melting rates. The margins of the Greenland
ice sheet are melting at increasing rates. Humanity’s
IN SUMMARY, the combined effects of oceanic thermal great experiment with the climate system is well under-
inertia, sulfate aerosols, and carbon (brown-cloud) way, and the ongoing increases in greenhouse gases
aerosols reconcile the seemingly small amount of promise much more of the same in the future.
global warming measured at present relative to the
considerable greenhouse-gas buildup during the last
century. Key Terms
chlorofluorocarbons equivalent CO (p. 335)
2
Global Warming: Summary (CFCs) (p. 331) radiative forcing
ozone (p. 331) (p. 335)
By 2006, global climate had warmed by 0.7°C above ozone hole (p. 332) enhanced greenhouse
the level in the late 1800s. Climate scientists agree that effect (p. 337)
the atmospheric concentrations of CO and other sulfate acrosols (p. 332)
2
greenhouse gases produced by human activities have brown clouds (p. 334) Intergovernmental Panel
increased markedly in the last century, and they agree global dimming (p. 334) on Climate Change
that the gas increases have caused climate to warm. The 2 × CO sensitivity (IPCC) (p. 337)
2
major disagreement over global warming has focused (p. 335)
on whether the greenhouse-gas increases explain some,
all, or little of the observed warming.
As recently as the late 1990s (when the first edition
of this book was written), it was still possible to make a Review Questions
case that the role of humans in this warming had been
relatively small, but subsequent evidence has seriously 1. What human activities produce CO and how have
2
undercut the central arguments that supported this they changed in the last 200 years?
position. Satellite-measured temperature trends once 2. Where does the CO produced by humans go?
failed to show a warming since 1980, but later investiga- 2
tions found that proper adjustments for a range of com- 3. How high in the atmosphere do sulfate aerosols
plicating factors yielded a warming trend in line with from smokestacks reach?
the one recorded by surface stations. Earlier hypotheses 4. Why do chlorofluorocarbons (CFCs) reach much
that the variability of Sunlike stars might have higher in the atmosphere than sulfate aerosols?
accounted for half or more of the observed warming
were undercut by observations that Sunlike stars do not 5. What are the strongest positive and negative
show the large variations in irradiance proposed. Earlier feedbacks on changes in Earth’s temperature?
speculation that carbon aerosols may have been falsely 6. In a net sense, do feedbacks increase or decrease
boosting the apparent size of the true greenhouse-gas the direct radiative effects of greenhouse gases on
warming were upended by the finding that the carbon global temperature?
aerosols in brown cloud hazes have instead kept Earth’s
surface cooler and have actually countered a significant 7. What factors complicate attempts to estimate
fraction of the true greenhouse-gas warming. Earth’s sensitivity to CO by directly comparing
2
It would be inaccurate to say that this debate is “over” the observed twentieth-century warming to the
because the scientific process constantly reexamines its measured rise in greenhouse gases?