Page 516 - Environment: The Science Behind the Stories

Figure 18.11 Precipitation
Percent change (June–Aug.) is projected to change
in precipitation for the decade 2090–2099, relative
>20% decrease to 1980–1999. Browner shades indicate
10–20% decrease less precipitation, and bluer shades
5–10% decrease indicate more. White indicates areas
5% decrease where models did not agree. This map
to 5% increase was generated using an intermediate
5–10% increase emissions scenario involving an average
10–20% increase global temperature rise of 2.8°C (5.0°F)
by 2100. Data from IPCC, 2007. Fourth
>20% increase
assessment report. Climate Change 2007:
Synthesis Report, Fig 3.3.

Moreover, a 2011 study by climate scientist James Hansen and became stuck in place (see Figure 18.12b). Atmospheric block-
others revealed that summer temperatures since the 1950s have ing patterns also were associated with the 2011 drought in Texas,
become not only warmer, but also more variable. As a result, the 2012 wildfires in Colorado, both floods and heat waves in
extreme summers (some of them unusually cool, more of them Europe, and various other extreme weather events. Additional
unusually warm) have occurred more and more frequently. independent research has supported Francis and Vavrus’s jet
Scientists are not the only ones to notice the increase stream hypothesis, and if it continues to be strengthened by fur-
in extreme weather events. The insurance industry is finely ther data, it would provide a valuable explanatory mechanism
attuned to such patterns, since insurers are the ones paying out for how climate change brings extreme weather to North Amer-
money each time a major storm, drought, or flood hits. The ica and Europe.
major German insurer Munich Re calculated that from 1980
to 2011, extreme weather events causing losses increased
fully 5 times in North America. They rose by 4 times in Asia,
2.5 in Africa, 2 in Europe, and 1.5 in South America. And this
was not even counting the events of 2012.
Researchers have long conservatively stated that although
climate trends influence the probability of what the weather Cool
may be like on any given day, no single particular weather weather
event can reliably be attributed to climate change. However,
as we gain a better understanding of what causes extreme
weather events, it is starting to become possible to link certain Warm Typical jet stream
Typical jet stream
weather events to climate change. weather
In 2012, a research paper by Jennifer Francis of Rutgers
University and Stephen Vavrus of the University of Wisconsin Weather systems move
west to east at normal rate
outlined a mechanism that may explain how global warming
may commonly lead to more extreme weather. Their analysis (a) Normal jet stream
indicated that because warming has been greater in the Arctic
than at lower latitudes, this has weakened the intensity of the
Northern Hemisphere’s polar jet stream. This jet stream is a
high-altitude air current that blows west-to-east and meanders
Looping jet stream stuck in blocking pattern
north and south, influencing much of the weather from day to
day across North America and Eurasia. Persistent
cold weather
As the jet stream slows down, its meandering loops Looping jet stream stuck in blocking pattern
become longer. These long lazy loops move west to east Persistent
more slowly, and may get stuck in a north–south orientation hot weather
for long periods of time. This is what meteorologists call an CHAPTER 18 • Glob al Cli M aT e Chan G e
atmospheric blocking pattern, because it can block the east- Weather systems are held in
place, creating prolonged
ward movement of weather systems and hold them in place bouts of extreme weather
(Figure 18.12). When this happens, a rainy system that would
normally move past a city in a day or two might instead be (b) Jet stream in March 2012
held in place for several days, resulting in flooding. Or dry
conditions over a farming region might last two weeks instead Figure 18.12 Changes in the jet stream can cause extreme
weather events. When Arctic warming slows the jet stream, it
of two days, causing a drought. Cold spells last longer, and departs from its normal configuration (a) and goes into a blocking
hot spells last longer, too. pattern (b) that stalls weather systems in place, leading to extreme
Indeed, the record-breaking heat wave that roasted the east- weather events. The blocking pattern shown here brought record-
ern United States in March 2012 resulted after the jet stream breaking heat to the eastern United States in March 2012. 515

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