Some events such as extreme cold temperatures will decrease but overall, most
events including heatwaves, precipitation episodes, and storms in urban areas are
expected to increase along this century. There is an increasing trend for the
occurrence of global warm spells of average, minimum, and maximum temperatures over a greater area and with higher magnitude than the typical summer
heatwaves, contributing to changing the annual trends (Perkins et al. 2012).
Trends are generally more heterogeneous for daily precipitation extremes. For
example, for the period 1900–2009, a modeling study showed that out of a global
set of about 8320 weather stations, about two-thirds reported increases in extreme
events, and these were mainly located in the tropics and higher latitudes (Westra
et al. 2013; AR5 Report).
Reliable assertions of very heavy and extreme precipitation events are possible
only in areas with dense networks of precipitation recording stations. In the
mid-latitudes, a widespread increase in the frequency of the very heavy precipitation was seen over the twentieth century (Groisman et al. 2005). The later authors
assume that an intense precipitation event has more than 0.3% of the upper daily
rainfall, and that the projections for a greenhouse-enriched atmosphere reveal an
increasing probability of these events. These extreme precipitation events show a
return time of 3–5 years for a daily event for annual and 10–20 years for seasonal
events, depending on the probability of local daily and seasonal rain events.
Other extreme phenomena such as droughts, that are not tightly linked to temperatures, are highly uncertain. These are highly complex processes caused by
anthropogenic and hydrometeorological factors with impacts that vary greatly
among locations. The common view of drought or dryness is the relationship with
rainfall deficit. Additional complexity can be added by socioeconomic and/or
atmospheric factors such as aerosols and land–atmosphere interactions which can
induce or non-linear and self-enforcing events (e.g., Sippel and Otto 2014). Several
studies also reported that in Europe, heatwave intensity, length, and frequency have
increased over the last century and even more so in Southeast Europe over the last
50 years (Della-Marta et al. 2007; Kuglitsch et al. 2010).
It is very likely that several of the unprecedented events of the past decade would
not have occurred without anthropogenic global warming (e.g., Coumou and
Rahmstorf 2012). However, a clear deterministic cause-effect between human
activity leading to warming and the occurrence of extreme events is not possible,
because extreme events can occur by chance under strictly natural conditions (e.g.,
Sippel and Otto 2014; Stott et al. 2004). In this context, Coumou et al. (2013b)
concluded that approximately 80% of recent monthly heat records would not have
occurred without human influence. This share will increase to more than 90% by
2040 under a medium future global warming scenario.
Perkins-Kirkpatrick and Gibson (2017) studied the variation of characteristics of
regional heatwave changes with mean global warming, assuming the 1.5–2 °C
scenarios stipulated by the Paris Agreement as scenarios of warmer thresholds. Two
global climate model ensembles, comprising 27 models, were applied. Basically,
these models were the Coupled Model Inter-comparison Project Phase 5 (CMIP5)
and a 21-member version of the Community Earth System Model (CESM), with
8.4 Occurrence of Extreme Events
283
events including heatwaves, precipitation episodes, and storms in urban areas are
expected to increase along this century. There is an increasing trend for the
occurrence of global warm spells of average, minimum, and maximum temperatures over a greater area and with higher magnitude than the typical summer
heatwaves, contributing to changing the annual trends (Perkins et al. 2012).
Trends are generally more heterogeneous for daily precipitation extremes. For
example, for the period 1900–2009, a modeling study showed that out of a global
set of about 8320 weather stations, about two-thirds reported increases in extreme
events, and these were mainly located in the tropics and higher latitudes (Westra
et al. 2013; AR5 Report).
Reliable assertions of very heavy and extreme precipitation events are possible
only in areas with dense networks of precipitation recording stations. In the
mid-latitudes, a widespread increase in the frequency of the very heavy precipitation was seen over the twentieth century (Groisman et al. 2005). The later authors
assume that an intense precipitation event has more than 0.3% of the upper daily
rainfall, and that the projections for a greenhouse-enriched atmosphere reveal an
increasing probability of these events. These extreme precipitation events show a
return time of 3–5 years for a daily event for annual and 10–20 years for seasonal
events, depending on the probability of local daily and seasonal rain events.
Other extreme phenomena such as droughts, that are not tightly linked to temperatures, are highly uncertain. These are highly complex processes caused by
anthropogenic and hydrometeorological factors with impacts that vary greatly
among locations. The common view of drought or dryness is the relationship with
rainfall deficit. Additional complexity can be added by socioeconomic and/or
atmospheric factors such as aerosols and land–atmosphere interactions which can
induce or non-linear and self-enforcing events (e.g., Sippel and Otto 2014). Several
studies also reported that in Europe, heatwave intensity, length, and frequency have
increased over the last century and even more so in Southeast Europe over the last
50 years (Della-Marta et al. 2007; Kuglitsch et al. 2010).
It is very likely that several of the unprecedented events of the past decade would
not have occurred without anthropogenic global warming (e.g., Coumou and
Rahmstorf 2012). However, a clear deterministic cause-effect between human
activity leading to warming and the occurrence of extreme events is not possible,
because extreme events can occur by chance under strictly natural conditions (e.g.,
Sippel and Otto 2014; Stott et al. 2004). In this context, Coumou et al. (2013b)
concluded that approximately 80% of recent monthly heat records would not have
occurred without human influence. This share will increase to more than 90% by
2040 under a medium future global warming scenario.
Perkins-Kirkpatrick and Gibson (2017) studied the variation of characteristics of
regional heatwave changes with mean global warming, assuming the 1.5–2 °C
scenarios stipulated by the Paris Agreement as scenarios of warmer thresholds. Two
global climate model ensembles, comprising 27 models, were applied. Basically,
these models were the Coupled Model Inter-comparison Project Phase 5 (CMIP5)
and a 21-member version of the Community Earth System Model (CESM), with
8.4 Occurrence of Extreme Events
283