Heat

Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided
• United States: In the United State, significant nonlinear effects
are observed above local temperatures of 29°C for maize, 30°C
for soybeans, and 32°C for cotton (Schlenker and Roberts 2009).
• Australia: Large negative effects of a “surprising” dimension
have been found in Australia for regional warming variations
of +2°C, which Asseng, Foster, and Turner argue have general
applicability and could indicate a risk that “could substantially
undermine future global food security” (Asseng, Foster, and
Turner 2011).
• India: Lobell et al. 2012 analyzed satellite measurements
of wheat growth in northern India to estimate the effect of
extreme heat above 34°C. Comparison with commonly used
process-based crop models led them to conclude that crop
models probably underestimate yield losses for warming of
2°C or more by as much as 50 percent for some sowing dates,
where warming of 2°C more refers to an artificial increase of
daily temperatures of 2°C. This effect might be significantly
stronger under higher temperature increases.
High impact regions are expected to be those where trends in
temperature and precipitation go in opposite directions. One such
“hotspot” region is the eastern Mediterranean where wintertime
precipitation, which contributes most to the annual budget, has
been declining (Figure 19), largely because of increasing anthropogenic
greenhouse gas and aerosol forcing (Hoerling et al. 2012).
At the same time, summertime temperatures have been increasing
steadily since the 1970s (Figure 19), further drying the soils
because of more evaporation.
Figure 19: Observed wintertime precipitation (blue), which
contributes most to the annual budget, and summertime temperature
(red), which is most important with respect to evaporative drying, with
their long-term trend for the eastern Mediterranean region.
These climatic trends accumulated to produce four consecutive
dry years following 2006 in Syria, with the 2007–08 drought being
particularly devastating (De Schutter 2011; Trigo et al. 2010). As the
vast majority of crops in this country are nonirrigated (Trigo et al.
2010), the region is highly vulnerable to meteorological drought. In
combination with water mismanagement, the 2008 drought rapidly
led to water stress with more than 40 percent of the cultivated land
affected, strongly reducing wheat and barley production (Trigo et
al. 2010). The repeated droughts resulted in significant losses for
the population, affecting in total 1.3 million people (800,000 of
whom were severely affected), and contributing to the migration
of tens of thousands of families (De Schutter 2011). Clearly, these
impacts are also strongly influenced by nonclimatic factors, such
as governance and demography, which can alter the exposure
and level of vulnerability of societies. Accurate knowledge of the
vulnerability of societies to meteorological events is often poorly
quantified, which hampers quantitative attribution of impacts
(Bouwer 2012). Nevertheless, qualitatively one can state that the
largely human-induced shift toward a climate with more frequent
droughts in the eastern Mediterranean (Hoerling et al. 2012) is
already causing societal impacts in this climatic “hotspot.”
Extreme Events in the Period 2000–12
Recent work has begun to link global warming to recent recordbreaking
extreme events with some degree of confidence. Heat
waves, droughts, and floods have posed challenges to affected
societies in the past. Table 1 below shows a number of unusual
weather events for which there is now substantial scientific evidence
linking them to global warming with medium to high levels of confidence.
Note that while floods are not included in this table, they
have had devastating effects on human systems and are expected
to increase in frequency and size with rising global temperatures.
Possible Mechanism for Extreme Event
Synchronization
The Russian heat wave and Pakistan flood in 2010 can serve as an
example of synchronicity between extreme events. During these
events, the Northern Hemisphere jet stream exhibited a strongly
meandering pattern, which remained blocked for several weeks.
Such events cause persistent and, therefore, potentially extreme
weather conditions to prevail over unusually longtime spans. These
patterns are more likely to form when the latitudinal temperature
gradient is small, resulting in a weak circumpolar vortex. This is just
what occurred in 2003 as a result of anomalously high near-Arctic
sea-surface temperatures (Coumou and Rahmstorf 2012). Ongoing
melting of Arctic sea ice over recent decades has been linked to
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