At 2 °C warming, the likelihood of most extreme cases (99.99% quantile)
increases by a factor of 1.5–3 depending on the region and the model. Thus, an
event that occurred once every 30 years under pre-industrial conditions is expected
to occur every 10–20 years under a 2 °C warming scenario. The probability of
extreme precipitation increases at the expense of days with moderate, low, or no
rainfall. From this warming scenario, the fraction of precipitation episodes attributable to human influence rises to about 40%. The increased water vapor holding
capacity of warmer air, could be linked to a decrease in the mean precipitation
regimes giving way to the predominance of extreme rainfall events. Overall, more
heavy rain events are expected in a warmer world due to the intensification of the
hydrological cycle. Most models suggested a general increase of 10–30% in the
precipitation intensity at most latitudes, for doubled carbon dioxide levels (Kettenberg et al. 1996).
The probability of extreme hot events under warming of 2 °C is double that for
1.5 °C and fivefold higher than that for the current 0.85 °C increase. This has strong
implications in terms of mitigation targets in climate negotiations, because the
differences of global average temperatures are small but large in terms of the
probability of extreme events. For every single degree of warming, the likelihood of
a rarest and extreme event occurring increases, along with a greater role for
anthropogenic emissions, with higher socioeconomic and environmental impacts.
The anthropogenic contribution toward heatwave and heavy precipitation was
assessed for the European heatwave of 2003 (Stott et al. 2004), for the Russian
heatwave of 2010 (Otto et al. 2012), and for the Australia heatwave in the summer
of 2013 (Lewis and Karoly 2013).
In 2003, the threshold of increase of annual temperatures of 1.6 °C in Europe
was surpassed for the first time, by comparison with the period 1961–90, and 2001
showed the second warm European summer with a correspondent 1.5 °C threshold
(Stott et al. 2004). This record-breaking heatwave in 2003, believed to be the hottest
since AD 1500, arguably caused 70,000 excess deaths and damage to agriculture
and forest amounting to more than €13.1 billion (e.g., Schiermeier 2010).
Stott et al. (2004) argued that asking whether external influences in climate, for
example, increase of GHG emissions, are deterministic in this hot discrete event is
not the right question to ask as such an event could happen by chance in an
unmodified climate. A more relevant question would be whether the likelihood or
the risk of the heatwave occurring would be higher with anthropogenic influence.
The same authors estimated, using a temperature threshold for mean summer
which was surpassed in 2003, and before only once in 1851, that it is very likely
that human activity at least doubled the odds of exceeding that threshold. Their
calculations showed that with a chance higher than 90%, half of the risk of
European summer temperatures to exceed a threshold of 1.6 °C was attributable to
human influence on climate. The fraction attributable risk (FAR) is sometimes a
parameter valid in establishing the liability for compensation for such events with a
value of 0.5 corresponding to doubling the risk over natural conditions.
8.4 Occurrence of Extreme Events
287
increases by a factor of 1.5–3 depending on the region and the model. Thus, an
event that occurred once every 30 years under pre-industrial conditions is expected
to occur every 10–20 years under a 2 °C warming scenario. The probability of
extreme precipitation increases at the expense of days with moderate, low, or no
rainfall. From this warming scenario, the fraction of precipitation episodes attributable to human influence rises to about 40%. The increased water vapor holding
capacity of warmer air, could be linked to a decrease in the mean precipitation
regimes giving way to the predominance of extreme rainfall events. Overall, more
heavy rain events are expected in a warmer world due to the intensification of the
hydrological cycle. Most models suggested a general increase of 10–30% in the
precipitation intensity at most latitudes, for doubled carbon dioxide levels (Kettenberg et al. 1996).
The probability of extreme hot events under warming of 2 °C is double that for
1.5 °C and fivefold higher than that for the current 0.85 °C increase. This has strong
implications in terms of mitigation targets in climate negotiations, because the
differences of global average temperatures are small but large in terms of the
probability of extreme events. For every single degree of warming, the likelihood of
a rarest and extreme event occurring increases, along with a greater role for
anthropogenic emissions, with higher socioeconomic and environmental impacts.
The anthropogenic contribution toward heatwave and heavy precipitation was
assessed for the European heatwave of 2003 (Stott et al. 2004), for the Russian
heatwave of 2010 (Otto et al. 2012), and for the Australia heatwave in the summer
of 2013 (Lewis and Karoly 2013).
In 2003, the threshold of increase of annual temperatures of 1.6 °C in Europe
was surpassed for the first time, by comparison with the period 1961–90, and 2001
showed the second warm European summer with a correspondent 1.5 °C threshold
(Stott et al. 2004). This record-breaking heatwave in 2003, believed to be the hottest
since AD 1500, arguably caused 70,000 excess deaths and damage to agriculture
and forest amounting to more than €13.1 billion (e.g., Schiermeier 2010).
Stott et al. (2004) argued that asking whether external influences in climate, for
example, increase of GHG emissions, are deterministic in this hot discrete event is
not the right question to ask as such an event could happen by chance in an
unmodified climate. A more relevant question would be whether the likelihood or
the risk of the heatwave occurring would be higher with anthropogenic influence.
The same authors estimated, using a temperature threshold for mean summer
which was surpassed in 2003, and before only once in 1851, that it is very likely
that human activity at least doubled the odds of exceeding that threshold. Their
calculations showed that with a chance higher than 90%, half of the risk of
European summer temperatures to exceed a threshold of 1.6 °C was attributable to
human influence on climate. The fraction attributable risk (FAR) is sometimes a
parameter valid in establishing the liability for compensation for such events with a
value of 0.5 corresponding to doubling the risk over natural conditions.
8.4 Occurrence of Extreme Events
287