Heat

Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided
Figure 16: Excess deaths observed during the 2003 heat wave in
France. O= observed; E= expected.
(Duffy and Tebaldi 2012; Jones, Lister, and Li 2008; Hansen et al.
2012; Stott et al. 2011). In the 1960s, summertime extremes of
more than three standard deviations warmer than the mean of the
climate were practically absent, affecting less than 1 percent of
the Earth’s surface. The area increased to 4–5 percent by 2006–08,
and by 2009–11 occurred on 6–13 percent of the land surface. Now
such extremely hot outliers typically cover about 10 percent of the
land area (Figure 18) (Hansen et al. 2012).
The above analysis implies that extremely hot summer months
and seasons would almost certainly not have occurred in the absence
of global warming (Coumou, Robinson, and Rahmstorf, in review;
Hansen et al. 2012). Other studies have explicitly attributed individual
heat waves, notably those in Europe in 2003 (Stott, Stone,
and Allen 2004), Russia in 2010 (Otto et al. 2012), and Texas in
2011 (Rupp et al. 2012) to the human influence on the climate.
Source: Fouillet et al. 2006.
and drought period (NOAA 2012, 2012b). On August 28, about 63
percent of the contiguous United States was affected by drought
conditions (Figure 17) and the January to August period was the
warmest ever recorded. That same period also saw numerous
wildfires, setting a new record for total burned area—exceeding
7.72 million acres (NOAA 2012b).
Recent studies have shown that extreme summer temperatures
can now largely be attributed to climatic warming since the 1960s
Figure 17: Drought conditions experienced on August 28 in the
contiguous United States.
Source: “U.S. Drought Monitor” 2012.
Drought and Aridity Trends
On a global scale, warming of the lower atmosphere strengthens
the hydrologic cycle, mainly because warmer air can hold more
water vapor (Coumou and Rahmstorf 2012; Trenberth 2010). This
strengthening causes dry regions to become drier and wet regions
to become wetter, something which is also predicted by climate
models (Trenberth 2010). Increased atmospheric water vapor
loading can also amplify extreme precipitation, which has been
detected and attributed to anthropogenic forcing over Northern
Hemisphere land areas (Min, Zhang, Zwiers, and Hegerl 2011).
Observations covering the last 50 years show that the intensification
of the water cycle indeed affected precipitation patterns
over oceans, roughly at twice the rate predicted by the models
(Durack et al. 2012). Over land, however, patterns of change are
generally more complex because of aerosol forcing (Sun, Roderick,
and Farquhar 2012) and regional phenomenon including soil,
moisture feedbacks (C.Taylor, deJeu, Guichard, Harris and Dorigo,
2012). Anthropogenic aerosol forcing likely played a key role in
observed precipitation changes over the period 1940–2009 (Sun
et al. 2012). One example is the likelihood that aerosol forcing
has been linked to Sahel droughts (Booth, Dunstone, Halloran,
Andrews, and Bellouin 2012), as well as a downward precipitation
trend in Mediterranean winters (Hoerling et al. 2012). Finally,
changes in large-scale atmospheric circulation, such as a poleward
migration of the mid-latitudinal storm tracks, can also strongly
affect precipitation patterns.
Warming leads to more evaporation and evapotranspiration,
which enhances surface drying and, thereby, the intensity and
duration of droughts (Trenberth 2010). Aridity (that is, the degree
to which a region lacks effective, life-promoting moisture) has
increased since the 1970s by about 1.74 percent per decade,
but natural cycles have played a role as well (Dai 2010, 2011).
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