Otto et al. (2012) analyzed a heatwave in Russia over a month, from July to
August 2010 that caused a high loss of lives, a reduction in crop production by
about 25%, and a total economic loss of US$ 15 billion. This phenomenon was
characterized by a level of air temperatures higher by more than 5 °C above the
long-term mean for which natural causes cannot be discarded.
The same authors argue that two apparently distinct narratives for interpreting
extreme events are complementary. These narratives are (i) that natural variability
was the primary cause of this event and (ii) that with a probability of 80%, the 2010
July heat record would have not occurred without climate warming linked to an
external trend that is the anthropogenic influence on greenhouse gas forcing.
In this case, the key point again posed for low probability events is that the
probability of the event occurrence and the fraction of risk attributable to external
forcing are two different questions. They reported that while in the 1960s a
2010-like event could have been expected every 99 years, in the 2000s the
equivalent period was about 33 years.
Thus, the conjugation of the two narratives in the case of the heatwave in Russia
is based on the principle that even a natural extreme event in 2010 would be more
likely under the anthropogenic warming which occurred in this region since the
1960s. Indeed, the empirical analysis showed that under a stationary climate,
without rising in average annual temperature, the observed monthly mean temperatures for July 2010 would be very unlikely, in relation to the distribution
defined over the 1950–2009 period. This, because, in this period, return times
ranged from 250 to 1000 years, implied that without an enhanced warming factor,
the 2010 heatwave would have been a very unusual event.
The average increase in temperature is much smaller than the anomalies
observed during the heatwave, although an increase occurred in a cascading
non-linear effect. Meanwhile, the probability of a heatwave as large as the one
observed in 2010 has increased three- to fourfold. The return time for the 2010 July
temperature was estimated to be 250 years. Given that the area covers less than 1%
of the global land area and was selected a posteriori, a 1/250-year event could
eventually occur every few years. The heat event in Russia can be internally
generated, but mainly externally driven in terms of probability of occurrence.
Lewis and Karoly (2013) analyzed the role of anthropogenic factors in the
hottest Australian summer over the period where records were available, drawing
conclusions like those of the studies already mentioned. Simulations of natural and
anthropogenic forcings for the periods (i) 1976–2005 and (ii) 2006–2020, under an
RCP8.5 scenario simulating a climate of high emission under the AR5 climate
profile, found that the odds of extreme heat due to human influences during the two
periods were 2.5- and five-fold higher. The natural “La Niña” events alone were
unlikely to cause the extreme heatwave, compared with the RCP8.5 high emissions
scenario. A significant decrease in return times of extremely hot summers was also
evaluated for the periods 1976–2005 and 2006–2020 (RCP8.5). Beyond 2020,
extremely hot summers were likely to occur and in the period 2080–2099, with
288
8 Fundamentals of Global Carbon Budgets and Climate Change
August 2010 that caused a high loss of lives, a reduction in crop production by
about 25%, and a total economic loss of US$ 15 billion. This phenomenon was
characterized by a level of air temperatures higher by more than 5 °C above the
long-term mean for which natural causes cannot be discarded.
The same authors argue that two apparently distinct narratives for interpreting
extreme events are complementary. These narratives are (i) that natural variability
was the primary cause of this event and (ii) that with a probability of 80%, the 2010
July heat record would have not occurred without climate warming linked to an
external trend that is the anthropogenic influence on greenhouse gas forcing.
In this case, the key point again posed for low probability events is that the
probability of the event occurrence and the fraction of risk attributable to external
forcing are two different questions. They reported that while in the 1960s a
2010-like event could have been expected every 99 years, in the 2000s the
equivalent period was about 33 years.
Thus, the conjugation of the two narratives in the case of the heatwave in Russia
is based on the principle that even a natural extreme event in 2010 would be more
likely under the anthropogenic warming which occurred in this region since the
1960s. Indeed, the empirical analysis showed that under a stationary climate,
without rising in average annual temperature, the observed monthly mean temperatures for July 2010 would be very unlikely, in relation to the distribution
defined over the 1950–2009 period. This, because, in this period, return times
ranged from 250 to 1000 years, implied that without an enhanced warming factor,
the 2010 heatwave would have been a very unusual event.
The average increase in temperature is much smaller than the anomalies
observed during the heatwave, although an increase occurred in a cascading
non-linear effect. Meanwhile, the probability of a heatwave as large as the one
observed in 2010 has increased three- to fourfold. The return time for the 2010 July
temperature was estimated to be 250 years. Given that the area covers less than 1%
of the global land area and was selected a posteriori, a 1/250-year event could
eventually occur every few years. The heat event in Russia can be internally
generated, but mainly externally driven in terms of probability of occurrence.
Lewis and Karoly (2013) analyzed the role of anthropogenic factors in the
hottest Australian summer over the period where records were available, drawing
conclusions like those of the studies already mentioned. Simulations of natural and
anthropogenic forcings for the periods (i) 1976–2005 and (ii) 2006–2020, under an
RCP8.5 scenario simulating a climate of high emission under the AR5 climate
profile, found that the odds of extreme heat due to human influences during the two
periods were 2.5- and five-fold higher. The natural “La Niña” events alone were
unlikely to cause the extreme heatwave, compared with the RCP8.5 high emissions
scenario. A significant decrease in return times of extremely hot summers was also
evaluated for the periods 1976–2005 and 2006–2020 (RCP8.5). Beyond 2020,
extremely hot summers were likely to occur and in the period 2080–2099, with
288
8 Fundamentals of Global Carbon Budgets and Climate Change