The Exceptional 2018 European Water Seesaw Calls for Action on Adaptation

Abstract

Temperature and precipitation are the most important factors responsible for agricultural productivity variations. In 2018 spring/summer growing season, Europe experienced concurrent anomalies of both. Drought conditions in central and northern Europe caused yield reductions up to 50% for the main crops, yet wet conditions in southern Europe saw yield gains up to 34%, both with respect to the previous 5‐years' mean. Based on the analysis of documentary and natural proxy based seasonal paleoclimate reconstructions for the past half millennium, we show that the 2018 combination of climatic anomalies in Europe was unique. The water seesaw, a marked dipole of negative water anomalies in central Europe and positive ones in southern Europe, distinguished 2018 from the five previous similar droughts since 1976. Model simulations reproduce the 2018 European water seesaw in only four years out of 875 years in historical runs and projections. Future projections under the RCP8.5 scenario show that 2018‐like temperature and rainfall conditions, favourable to crop growth, will occur less frequent in southern Europe. In contrast, in central Europe high‐end emission scenario climate projections show that droughts as intense as 2018 could become a common occurrence as early as 2043. Whilst integrated European and global agricultural markets limited agro‐economic shocks caused by 2018's extremes, there is an urgent need for adaptation strategies for European agriculture to consider futures without the benefits of any water seesaw.

Full text

The Exceptional 2018 European Water Seesaw Calls for
Action on Adaptation
Andrea Toreti1, Alan Belward1, Ignacio Perez-Dominguez2, Gustavo Naumann1,
Jürg Luterbacher3, Ottmar Cronie4, Lorenzo Seguini1, Giacinto Manfron1,
Raul Lopez-Lozano1, Bettina Baruth1, Maurits van den Berg1, Frank Dentener1,
Andrej Ceglar1, Thomas Chatzopoulos2, and Matteo Zampieri1
1European Commission, Joint Research Centre (JRC), Ispra, Italy, 2European Commission, Joint Research Centre
(JRC), Seville, Spain, 3Department of Geography, Climatology, Climate Dynamics and Climate Change, Centre for
International Development and Environmental Research, Justus-Liebig University of Giessen, Giessen, Germany,
4Department of Mathematics and Mathematical Statistics, Umeå University, Umeå, Sweden
Abstract Temperature and precipitation are the most important factors responsible for agricultural
productivity variations. In 2018 spring/summer growing season, Europe experienced concurrent anomalies
of both. Drought conditions in central and northern Europe caused yield reductions up to 50% for the main
crops, yet wet conditions in southern Europe saw yield gains up to 34%, both with respect to the previous
5-year mean. Based on the analysis of documentary and natural proxy-based seasonal paleoclimate
reconstructions for the past half millennium, we show that the 2018 combination of climatic anomalies in
Europe was unique. The water seesaw, a marked dipole of negative water anomalies in central Europe and
positive ones in southern Europe, distinguished 2018 from the five previous similar droughts since 1976.
Model simulations reproduce the 2018 European water seesaw in only 4 years out of 875 years in historical
runs and projections. Future projections under the RCP8.5 scenario show that 2018-like temperature and
rainfall conditions, favorable to crop growth, will occur less frequent in southern Europe. In contrast, in
central Europe high-end emission scenario climate projections show that droughts as intense as 2018 could
become a common occurrence as early as 2043. While integrated European and global agricultural markets
limited agro-economic shocks caused by 2018's extremes, there is an urgent need for adaptation strategies
for European agriculture to consider futures without the benefits of any water seesaw.
1. Introduction
Climate change poses particular challenges for agricultural production systems as plant growth is affected
by climate conditions (e.g., Gray & Brady, 2016; Lobell & Gourdji, 2012; Porter & Semenov, 2005). Rising
temperatures, changes in precipitation regimes, and increasing frequency, duration, and intensity of extreme
events negatively affect crop yields and fodder production (e.g., Asseng et al., 2014; Toreti, Bassu, et al., 2019;
Webber et al., 2018; Zhao et al., 2017). The adverse impacts of climate extremes on the main crops in the
last decades have been addressed in numerous studies (e.g., Deryng et al., 2014; Fontana et al., 2015; Lesk
et al., 2016; Rezaei, Siebert, Manderscheid, et al., 2018; Zampieri et al., 2017). Besides heat stress, drought
and water excess have been shown to trigger losses when occurring in critical phenological phases. Thus,
to reduce the impacts associated with these extreme events at the local scale, agricultural management and
planning need to consider them in the development and implementation of risk reduction strategies.
At the regional scale, the push/pull of droughts in one region and the absence of water stress elsewhere
(i.e., the water seesaw) can translate into crop yield differentials. Thus, it is key to estimate how often water
seesaw conditions have occurred and will occur and to understand if climate change adaptation strategies for
agriculture can count on recurrent water seesaws. The extreme climate conditions experienced by Europe
in 2018 have triggered all these questions.
2. Data and Methods
Starting from the 2018 event as a reference, we investigate past, current, and future water seesaw events.
The following data sets are used to perform such analysis: daily climate observations from ground weather
RESEARCH ARTICLE
10.1029/2019EF001170
Key Points:
• Unique concurrent spring and
summer climatic anomalies affected
Europe in 2018
• 2018-like droughts could become a
common occurrence as early as 2043
• Climate change adaptation strategies
for agriculture in Europe cannot
count on recurrent water seesaws
Supporting Information:
• Supporting Information S1
Correspondence to:
A. Toreti,
andrea.toreti@ec.europa.eu
Citation:
Toreti, A., Belward, A.,
Perez-Dominguez, I., Naumann, G.,
Luterbacher, J., Cronie, O., et al.
(2019). The exceptional 2018
European water seesaw calls for action
on adaptation. Earth's Future,7,
652–663. https://doi.org/10.1029/
2019EF001170
Received 29 JAN 2019
Accepted 7 MAY 2019
Accepted article online 15 MAY 2019
Published online 19 JUN 2019
©2019. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and
distribution in any medium, provided
the original work is properly cited, the
use is non-commercial and no
modifications or adaptations are made.
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stations covering the last decades from 1976, remote sensing data, atmospheric reanalysis, paleoclimate
reconstructions covering the last ∼500 years, and climate projections till 2100. The observational climate
gridded data set (MarsMet) used is maintained by the Joint Research Centre of the European Commission
(Toreti, Maiorano, et al., 2019). This data set covers the European Union and its neighboring countries at a
spatial resolution of 25 km.
The remote sensing analysis of vegetation status is based on the fraction of absorbed photosynthetically
active radiation data (fAPAR; an indicator related to biomass; Verger et al., 2014) obtained from the
Copernicus Global Land Service, more specifically, the fAPAR version 2 at 1 km (Verger et al., 2014).
The large-scale atmospheric circulation during the spring-to-summer period is analyzed by using geopoten-
tial height values at 500 hPa from the ERA-Interim reanalysis (Dee et al., 2011).
Gridded multiproxy, documentary and natural proxy-based paleoclimate reconstructions of seasonal tem-
perature (Luterbacher et al., 2004) and precipitation (Pauling et al., 2006) covering the period back to
1500 CE are also used. The climate model projections under the high-end emission scenario RCP8.5 come
from the FP7-project HELIX (High-End cLimate Impacts and eXtremes; www.helixclimate.eu). They have
been obtained with the atmospheric model EC-EARTH3-HR v3.1 (Hazeleger et al., 2012) at 0.35◦(with an
improved dynamics and parameterization) having prescribed Sea Surface Temperature and Sea Ice concen-
tration originating from seven independent CMIP5 GCMs (see Table S1 in the supporting information and
Naumann et al., 2018). The higher spatial resolution, compared to CMIP5 model runs, can positively affect
the representation of the hydrological cycle and the atmospheric blocking condition (Berckmans et al., 2013;
Dawson & Palmer, 2015; Wyser et al., 2017).
The 2018 drought affected central and northern Europe; however, in this study we focus only on central
Europe being the geographic core of the event and to facilitate the spatial analysis and comparison with the
other observed, reconstructed, and projected drought events. Central Europe is here defined as the region
spanning the following latitude/longitude limits: 3–20◦E, 46–56◦N. While southern Europe is defined by
10◦W–30◦E, 36.5–45.5◦N.
The water seesaw is characterized by using the Standardized Precipitation Evapotranspiration Index
(SPEI; Vicente-Serrano et al., 2010) computed by using precipitation data and the FAO-recommended
Penmann-Monteith evapotranspiration function (Allen et al., 1998). Furthermore, these anomalous cli-
mate conditions are also characterized and analyzed by studying concurrent seasonal (spring and summer)
temperature and precipitation extremes.
The spatial distributions of the temperature and precipitation quantiles, as well as of the SPEI values, are
compared with the ones in 2018 by using the Kullback-Leibler (KL) divergence. Given distributions Pand
Qto be compared, which have probability densities pand q, respectively, the KL divergence is defined as
D(P||Q)=∫p(x)log p(x)
q(x)dx.(1)
Here Qis for instance the spatial distribution of the 2018 SPEI values in central Europe, while Pis the spatial
distribution of the SPEI values of any other year to be compared with 2018. Given observations x1,…,xn,
we here estimate the KL divergence by means of (Perez-Cruz, 2008):
̂
D(P||Q)=−1+1
n
n
∑
i=1
log ΔPc(xi)
ΔQc(xi),(2)
where ΔPc(xi)=Pc(xi)−Pc(xi−𝜖)for any 𝜖<mini{xi−xi−1}and Pcis the continuous piecewise extension
of the stepwise empirical cumulative distribution function.
The spatiotemporal intensity functions of the drought events and the anomalous wet conditions are esti-
mated with a resample-smoothed Voronoi estimator (Moradi et al., 2019). Let Y={x1,…,xN}⊂[0,T]be
the collection of random time points associated to the events occurring in the time interval [0,T], and denot-
ing the associated spatial extents with s≤si≤̄
s,i=1,…,N, we obtain the spatiotemporal point process
X= {(x1,s1),…,(xN,sN)} ⊂[0,T]×[s,̄
s]. Let further X1,p,…,Xm,pbe m≥1independent p-thinnings
of X,0≤p≤1; we obtain each collection Xi,pby running through the points of Xand independently
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throwing out each of its points with probability 1−p. Then, the resample-smoothed Voronoi estimator
(Moradi et al., 2019) is given by
̂𝜌p,m(t,v)= 1
mp
m
∑
i=1
̂𝜌i(t,v)=
m
∑
i=1
∑
(x,s)∈Xi,p
1{(t,v)∈(x,s)(Xi,p)}
mp|(x,s)(Xi,p)|,(t,v)∈[0,T]×[s,̄
s],(3)
where for any (x,s)∈Xi,p,|(x,s)(Xi,p)|is the size of
(x,s)(Xi,p)={u∈[0,T]×[s,̄
s]∶||u−(x,s)|| ≤||u−(x′,s′)|| for any (x′,s′)∈Xi,p⧵{(x,s)}},(4)
which is the Voronoi cell consisting of all points u∈[0,T]×[s,̄
s]closer to (x,s)than any (x′,s′)∈Xi,p⧵{(x,s)}
in terms of the Euclidean distance. It has been suggested (Moradi et al., 2019) to choose pand msuch that
0<p≤0.2and m≥400. The interval [0,T]is here chosen empirically by letting 0 represent the first
observed event time minus half its distance to the second smallest observed event time. Tis chosen similarly
but considering the largest observed event, and this allows to correct for the edge effects/bias (Moradi et al.,
2019). The same procedure is applied to the spatial extension.
To identify when the 2018-like drought events will become a common occurrence in the climate projections,
we use the intensity function thresholded at 0.5; that is, we identify the year when the estimated intensity
function exceeds and remains above 0.5. This implies that in a given year it is more likely to observe an
extreme drought event, such as the one in 2018, than not.
3. Results
3.1. The 2018 Climate Extreme
In 2018 drought affected central and northern Europe, with an exceptionally negative spring/summer water
balance. At its geographic core, this deficit also affected the first months of the year (Figure S1). According
to the SPEI, the 2018 drought event can be classified as severe to extreme both at 3-month (June to August)
and 6-month (March to August) time scales (Figure 1). In central Europe over 34%of the landmass is used
for agriculture, and 52%of the entire region suffered severe-to-extreme drought (SPEI-6 less than −1.5;
Figure 1), while 20%was affected by extreme drought (SPEI-6 less than −2; Figure 1). Meanwhile, southern
Europe experienced wetter than usual spring (March to May) conditions, and to a certain extent also summer
(June to August), with large areas characterized by SPEI-6 higher than 1.5 and 2 (Figure 1). The exceptional
wet conditions in March 2018 over the Iberian Peninsula brought to an end the 2016–2017 drought and
were induced by an exceptional planetary wave activity, followed by a sudden stratospheric warming and a
subsequent persistent negative North Atlantic Oscillation anomaly (Ayarzagüena et al., 2018).
The 2018 drought was characterized by (i) a relatively dry spring (March, April, and May) with 3-month
total precipitation being in the lower percentiles of the 1976–2005 distribution (with the median of central
Europe equal to the 40th percentile; Figure S2). (ii) Exceptionally high mean spring temperatures were also
recorded, all in the highest percentiles of the 1976–2005 distribution (spatial median greater than the 99th
percentile; Figure S2). Finally, (iii) a dry summer (June, July, and August) with total precipitation in the low-
est percentiles (i.e., median equal to the 17th percentile) associated with (iv) very warm mean temperatures
in the highest percentiles of the 1976–2005 distribution (spatial median equal to the 97th percentile; Figure
S2) were observed. This comparison in terms of percentiles based on the 1976–2005 distribution helps to
understand and quantify how anomalous the 2018 conditions were in a climatological perspective.
The large-scale atmospheric circulation was characterized by pronounced positive geopotential height
anomalies in April (Figure 2), covering a large area centered over eastern Europe and extending to the
Mediterranean in the South and to central Europe in the West. These atmospheric conditions persisted in
subsequent months and moved northward bridging toward the North Atlantic. In May, the geopotential
height anomalies were covering a large area stretching from the Scandinavian Peninsula to the Atlantic. In
August, this anomaly moved South and elongated from the North Atlantic to western Russia (Figure 2). The
complex evolution of these blocking conditions highlights both the higher intensity of the anomalies and
the broader spatial extension (e.g., compared to the classification of Stefanon et al., 2012), which contribute
to explain the exceptional observed temperature anomalies. The occurrence and persistence of atmospheric
blocking conditions are key factors in the development of large-scale heat wave and drought, and to
trigger soil-moisture temperature feedbacks (Brunner et al., 2018, 2017; Miralles et al., 2014; Luterbacher
et al., 2004).
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Figure 1. (a) 2018 estimated SPEI-6 (March–August) and (b) associated spatial extent (km2·105) of the severe-to-extreme (orange bars) and extreme (red bars)
drought events in central Europe from 1976 to 2018. The white boxes in (a) indicate the two regions of interest: southern Europe and central Europe. (c) 2018
estimated SPEI-3 (June–August) and associated (d) spatial extent (km2·105) of the severe-to-extreme (orange bars) and extreme (red bars) drought events in
central Europe from 1976 to 2018. SPEI = Standardized Precipitation Evapotranspiration Index.
The severity of the 2018 drought as well as the pronounced water seesaw can be visualized in the anoma-
lous fAPAR values from March to August (spring and summer) compared with the entire measurement
period 1999–2017 (Figure 3). The drought event impacted plant growth in large areas of central and north-
ern Europe and exceptionally reduced biomass accumulation, as suggested by fAPAR anomalies of 15–25%
(compared to 1999–2017; Figures 3 and S3 on agricultural areas) even exceeding 25% locally. In these
regions, the 2018 summer biomass accumulation in the agricultural areas (as inferred by the fAPAR) was
the lowest of the entire observational record since 1999 (Figure 3). In contrast, southern Europe experienced
above-normal biomass accumulation (Figure 3) sustained by the exceptional spring precipitation.
3.2. The 2018 Extreme Event in a Paleoclimate Perspective
By using the SPEI-6 spatial distribution applied to the available observational records since 1976, only five
events resemble the 2018 drought in central Europe: 1976, 1990, 1992, 2003, and 2015 (Figure S4). However,
none of these years was characterized by a water seesaw as/as pronounced as the one in 2018. The 2018
drought in central Europe compares well in extent with the drought in 1976 and 2003 (Figures 1 and S4).
The 1976 drought has often been considered a benchmark due to its severity (Burke et al., 2010; Briffa et al.,
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Figure 2. Anomalies (with respect to the available data period) of the 2018 monthly (from March to August)
geopotential height at 500 hPa as represented by ERA-Interim (values in meters).
2009), with one of the longest heat waves (in the observational records) from June to August and negative
rainfall anomalies from May to August. The 2003 event and its impacts have been described and analyzed in
several studies (Ciais et al., 2005; Fink et al., 2004; Garcia-Herrera et al., 2010). The 1990 and 1992 droughts
were among the biggest events in Europe since 1950 (Spinoni et al., 2015), while the 2015 event in central
Europe was even drier in summer than 2003 (Orth et al., 2016b). Concerning the summer heat waves in 1976,
1992, and 2003, the contribution of Mediterranean dry springs was highlighted by Zampieri et al. (2009).
The combination of dry spring, the exceptionally warm spring/summer temperatures, and the dry summer
makes the 2018 event unique in a longer climatological perspective. Studying seasonal European tempera-
ture and precipitation gridded paleo-reconstructions back to 1500 CE reveals no events similar to 2018 in
central Europe in terms of concurrent spring-to-summer temperature and precipitation quantiles spatial
distribution (Figure 4). Focusing on summer months alone (June to August), a few events appear to be sim-
ilar to 2018, notably the 1540 and 1947 droughts. According to reconstructed temperature and precipitation
fields, the 1540 event was characterized by a very dry and warm summer (similar to the 2018 one) though
spring conditions were even drier than in 2018 (Figure 4). However, though drier, the extreme 2018 spring
temperatures were not reached. The 1540 event is also unique from a climate perspective (Orth et al., 2016a;
Pfister, 2016; Wetter et al., 2014). In 1540, chronicles narrate of people taking refuge in cellars during the
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Figure 3. (Left panel) Anomalies (percent) of the fAPAR accumulated from March to August 2018 with respect to 1999–2017. (Right panel) Regions where the
2018, 2015, and 2003 summer (June to August) fAPARs (calculated on agricultural land) were the lowest, second lowest, and third lowest since 1999.
fAPAR = fraction of absorbed photosynthetically active radiation data.
day in France, of autumn-like trees and forest fires in many areas of Europe (Pfister, 2016), while the entire
agricultural sector was heavily affected with cattle dying of thirst and hunger and complete loss of spring
grains, legumes, and fruits (Pfister, 2016).
By looking only at the summer temperature conditions being as extreme as the 2018 one (or more), 14 events
can be identified since 1500 CE, 5 of them being in the 16th century and 3 in the 20th century and the 2003.
While 2018-similar extreme spring conditions are more rare with only five events detected in the last 500
years: 1794, 1822, 1994, 2000, and 2007. This points again to the importance and uniqueness of the concur-
rent extreme conditions both in spring and summer that characterized the 2018 event. The contribution of
the exceptionally and rare warm spring conditions can be evaluated by performing an idealized experiment:
Figure 4. (a) 2018-like summer drought events in central Europe identified by using as metric the concurrent spatial
distribution of spring and summer temperature and precipitation quantiles. (b) Estimated spatial probability density
functions of spring and summer temperatures (red for summer and violet for spring) and precipitation (yellow for
spring and brown for summer) in 1540 (dashed lines) and 2018 (bold lines).
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Figure 5. Estimated spatiotemporal (time of occurrence in the xaxis and spatial extent ×105in the yaxis) frequency of
extreme drought events (Standardized Precipitation Evapotranspiration Index-6 <−2) in central Europe as identified in
the seven climate model simulations from 1976 to 2100 under the high-end emission scenario RCP8.5. Values in the
frequency legend are ×10−2.
the SPEI-6 (from March to August) and its spatial distribution in central Europe are rederived by replacing
the extreme spring temperatures with average values, while all the other factors (i.e., drier spring precipita-
tion, very warm, and dry summer) are kept as they were observed in 2018. The spatial distribution of this
idealized SPEI still points to severe drought conditions but not to extreme ones (not shown).
3.3. Future Projections
To understand how the frequency and severity of the 2018 water seesaw and its drought component
might change in the coming decades over Europe, climate projections till 2100 are analyzed. As done
for the period covered by observational gridded data (1976–2018), we derive SPEI-3 (June–August) and
SPEI-6 (March–August). Then, we investigate their changes and identify events similar to 2018 (i.e., with
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Figure 6. (a) 2018-like drought events in central Europe in the seven climate model simulations from 1976 to 2100
under the high-end emission scenario RCP8.5. The events have been identified by using the spatial distribution of the
estimated Standardized Precipitation Evapotranspiration Index-6. (b) Estimated frequency of occurrence of the
2018-like drought events in central Europe in the seven climate model simulations from 1976 to 2100. The dashed lines
indicate when these events will become the norm in each simulation.
similar spatial distribution of SPEI values). The combined spatiotemporal SPEI analysis (investigat-
ing both the occurrence and the spatial extent) of extreme drought events (SPEI less than −2) in
central Europe reveals an increased frequency toward the end of the century in all seven climate
realizations (Figure 5). However, the magnitude of these changes (especially concerning the spa-
tial extent) varies among the seven climate model projections (Figure 5). Similar findings character-
ize the severe-to-extreme drought events (SPEI less than −1.5; Figure S5). Regarding the 2018-like
drought events in central Europe (Figure 6), all seven climate model simulations show a fre-
quency similar to the one derived from observations in the historical period (1976–2005). Projec-
tions for the next decades (2006–2100) show a remarkable increase in the frequency of occurrence of
2018-like drought events (considering both the 6-month period from March to August and the sum-
mer months). All seven model simulations are coherent and consistent in this projected increase,
which can be better evaluated through the estimated nonstationary intensity functions (see section 2)
describing the frequency of occurrence of these large-scale drought events (Figure 6). The frequency of
occurrence also reveals how these events could become the norm (see section 2) as early as 2043 (Figure 6).
Three model runs show that these droughts will become the norm 13–23 years after global mean warm-
ing reaching 3◦(Table S1), while in other two model runs, this will happen exactly when 3◦of global mean
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Figure 7. Estimated spatiotemporal (time of occurrence in the xaxis and spatial extent ×105in the yaxis) frequency of
spring-to-summer anomalous wet events (Standardized Precipitation Evapotranspiration Index-6 >1.5) in southern
Europe as identified in the climate model simulations from 1976 to 2100 under the high-end emission scenario RCP8.5.
Values in the frequency legend are ×10−2.
warming will be reached. Only two model runs show these events being the norm already 1-6 years after
having reached 2◦of global mean warming (Table S1).
To better understand the projected drought events in central Europe as well as the uniqueness of the 2018
one, concurrent very warm and dry springs and very warm and dry summers are analyzed in the model pro-
jections (as done for the observational period and the paleoclimate reconstructions). Only a few events like
the 2018 one can be identified (Figure S6). This lack of 2018-like concurrent event is mainly due to projected
changes in spring precipitation regimes that reduce the frequency of as dry as 2018 conditions. Therefore,
future drought events in central Europe will be mainly associated with extreme summer conditions and very
warm springs.
Despite models' ability to reproduce drought events in central Europe during the historical simulations
(1976–2005; Figure 6), the 2018 water seesaw (with concurrent drought and anomalous wet conditions over
Europe) is reproduced only twice in two realizations, that is, in 4 years out of 875 simulated years. Fur-
thermore, model simulations reproduce large-scale wetter conditions, such as those observed in southern
Europe in 2018, very rarely. Projections show a decrease in the frequency of occurrence and spatial exten-
sion of anomalous wet conditions over those regions (Figure 7), with all model simulations pointing to
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almost no events by mid-21st century (already by 2030s in a couple of simulations). These results imply the
exceptionality of the 2018 compensation, which is not likely to occur again in the future.
4. Discussion
The 2018 drought had severe impacts in a number of socioeconomic sectors (beAWARE news, 2018; WMO
news, 2018), for example, higher than usual death rates among elderly people, difficulties in power plant
cooling, stability issues in The Netherlands' dike system due to lack of freshwater, extremely low river lev-
els with negative impact on the transport sector, and industries dependent on water way transport, forest
fires, and notably serious impacts on agriculture. Although official production and yield estimates are not
yet available, preliminary wheat production estimates (Eurostat, 2018) in the main affected countries report
losses from −9% to −50% with respect to the mean of the previous 5 years. Barley production dropped by −1%
to −27%. In Germany, a maize production reduction of −25% has been reported (Eurostat, 2018). Drought
also heavily affected pasture (generally not irrigated) with detrimental effects on the livestock/dairy sector.
However, the positive effects of Europe's 2018 water seesaw were manifest in favorable conditions in south-
ern Europe. Preliminary wheat production estimates report an increase of 19% in Spain and Portugal and
24% in Romania. Thus, market cooperation across the European Union could, in this instance, act as a form
of adaptation to the climatic extremes experienced preventing higher volatility and price spikes.
Observations and paleoclimate reconstructions have shown the uniqueness of the 2018 event, characterized
by concurrent seasonal anomalies in spring/summer temperature and precipitation. It is worth to point out
that the exceptionally warm spring of 2018 turned a severe drought into an extreme drought, while it is
virtually certain that the Northern Hemisphere heat events in 2018 have been caused by human-induced
climate change (Vogel et al., 2019).
In the future projections, similar drought conditions in central Europe will become a common occurrence,
but the 2018 event will remain unique in terms of concurrent spring and summer climate anomalies.
Drought events will indeed be mainly associated with extreme temperature conditions (both in spring and
summer) and very dry summer. Disentangling and quantifying the effects of mean warming conditions and
atmospheric dynamics on the identified future drought events is not straightforward and requires dedicated
analysis. However, an idealized experiment, obtained by rederiving the SPEI-6 from March to August (dur-
ing the identified drought events) considering only the temperature component due to the mean warming
(while keeping unchanged all the other factors), points to a time-dependent response. Up to a certain degree
of warming (e.g., more than 3◦for the first model simulation), the atmospheric dynamics seems to act by
enhancing the mean temperature warming effects in the drought conditions. Afterward, a change in this
mechanism seems to appear pointing to modified drought dynamics. Dedicated in-depth analyses are of
course needed to better understand these changes in very high warming scenarios.
The projected favorable change in the spring precipitation regime in central Europe could be seen as an
opportunity for adaptation, but it also points to issues that could affect agricultural risk assessments. Crop
growth models (often used in such assessments) could be indeed too positively responsive to future spring
precipitation, and thus, the need of having realistic representation of heat stress and soil-atmosphere fluxes
in these models is essential to avoid underestimating the impacts of future drought events.
5. Conclusions
The findings of this study point to the urgent need to identify realistic adaptation pathways to minimize
risks and losses induced by large-scale drought events in key and complex sectors such as the agricul-
ture. European resilience analysis for the coming decades needs to consider the projected reduced/lack of
compensation given by water seesaw. Adaptation strategies for the agricultural sector will need to address
concurrent water-stress conditions throughout Europe. Negative effects of the projectedincrease in extremes
might be only partially limited by shorter phenological cycles, new varieties, and fertilization effects of
increased atmospheric CO2concentration (Challinor et al., 2016; Kimball, 2016; Obermeier et al., 2017;
Parkes et al., 2018; Rezaei, Siebert, Hging, et al., 2018; Trnka et al., 2011). Crops, such as maize, could be
more affected by projected increases in severe drought events (Webber et al., 2018; Zampieri et al., 2019),
and as seen in 2018, the livestock sector will also be negatively impacted due to the lack of fodder crops.
Besides the local impacts, it will be important to define strategies to limit the propagating effects of economic
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shocks induced by these extremes at the European level. While market forces can play a role in mitigat-
ing the adverse effects of extreme events (e.g., global cereal prices have stabilized after the summer peak in
2018, thanks to good forecasts for wheat production in the United States and Russia; European Commission,
2018) balancing higher than expected yields from one set of European countries against losses elsewhere
may not be a viable option in the future. The results presented here show that the push and pull of droughts
in one region and the absence of water stress elsewhere do translate into crop yield differentials, but they
also show that such water seesaw conditions are rare, and going to get rarer still, while drought occurrence
will increase. Furthermore, global concurrent climate extremes must be taken into account for robust risk
assessments (Toreti, Cronie, et al., 2019).
Climate change adaptation strategies for agriculture in Europe cannot count on recurrent water seesaws. It
may have taken more than 500 years to reach the concurrent extreme conditions experienced in 2018, but
the next 50 years will see similar conditions replicated many times over. 2018 should serve as a warning call.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration—Guidelines for computing crop water requirements.
FAO Irrigation and drainage paper 56.
Asseng, S., Ewert, F., Martre, P., Rötter, R. P., Lobell, D. B., Cammarano, D., et al. (2014). Rising temperatures reduce global wheat
production. Nature Climate Change,5, 143.
Ayarzagüena, B., Barriopedro, D., Garrido-Perez, J. M., Abalos, M., de la Camara, A., Garcia-Herrera, R., et al. (2018). Stratospheric
connection to the abrupt end of the 2016/2017 Iberian drought. Geophysical Research Letters,45, 12,639–12,646. https://doi.org/10.1029/
2018GL079802
beAWARE news (2018). Extreme weather events in the summer of 2018 in Europe. H2020-beAWARE project https://beaware- project.eu
Berckmans, J., Woollings, T., Demory, M.-E., Vidale, P.-L., & Roberts, M. (2013).Atmospheric blocking in a high resolution climate model:
influences of mean state, orography and eddy forcing. Atmospheric Science Letters,14, 34–40.
Briffa, K. R., van der Schrier, G., & Jones, P. D. (2009). Wet and dry summers in Europe since 1750: evidence of increasing drought.
International Journal of Climatology,29, 1894–1905.
Brunner, L., Hegerl, G. C., & Steiner, A. K. (2017). Connecting atmospheric blocking to European temperature extremes in spring. Journal
of Climate,30, 585–594.
Brunner, L., Schaller, N., Anstey, J., Sillmann, J.,& Steiner, A. K. (2018). Dependence of present and future European temperature extremes
on the location of atmospheric blocking. Geophysical Research Letters,45, 6311–6320. https://doi.org/10.1029/2018GL077837
Burke, E. J., Perry, R. H. J., & Brown, S. J. (2010). An extreme value analysis of UK drought and projections of change in the future. Journal
of Hydrology,388, 131–143.
Challinor, A. J., Koehler, A. K., Ramirez-Villegas, J., Whitfield, S., & Das, B. (2016). Current warming will reduce yields unless maize
breeding and seed systems adapt immediately. Nature Climate Change,6, 954–958.
Ciais, P., Reichstein, M., Viovy, N., Granier, A., Ogée, J., Allard, V., et al. (2005). Europe-wide reduction in primary productivity caused by
the heat and drought in 2003. Nature,437, 529–533.
Dawson, A., & Palmer, T. N. (2015). Simulating weather regimes: Impact of model resolution and stochastic parameterization. Climate
Dynamics,44, 2177–2193.
Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., et al. (2011). The ERA-Interim reanalysis: Configurationand
performance of the data assimilation system. Quarterly Journal of the Royal Meteorological Society,137, 553–597.
Deryng, D., Conway, D., Ramankutty, N., Price, J., & Warren, R. (2014). Global crop yield response to extreme heat stress under multiple
climate change futures. Environmental Research Letters,9, 34011.
European Commission (2018). Short-term outlook for EU agricultural markets in 2018 and 2019. DG Agriculture and Rural Development,
Short-term outlook 22.
Eurostat (2018). ec.europa.eu/eurostat.
Fink, A. H., Brücher, T., Krüger, A., Leckebusch, G. C., Pinto, J. G., & Ulbrich, U. (2004). The 2003 European summer heatwaves and
drought synoptic diagnosis and impacts. Weather,59, 209–216.
Fontana, G., Toreti, A., Ceglar, A., & De Sanctis, G. (2015). Early heat waves over Italy and their impacts on durum wheat yields. Natural
Hazards and Earth System Sciences,15, 1631–1637.
Garcia-Herrera, R., Diaz, J., Trigo,R. M., Luterbacher, J., & Fischer, E. (2010). A review of the European summer heat wave of 2003. Critical
Reviews in Environmental Science and Technology,40, 267–306.
Gray, S. B., & Brady, S. M. (2016). Plant developmental responses to climate change. Developmental Biology,419, 64–77.
Hazeleger, W., Wang, X., Severijns, C., ¸Stef˘
anescu, S., Bintanja, R., Sterl, A., et al. (2012). Ec-Earth v2.2: Description and validation of a
new seamless earth system prediction model. Climate Dynamics,39(11), 2611–2629.
Kimball, B. A. (2016). Crop responses to elevated CO2 and interactions with H2O, N, and temperature. Current Opinion in Plant Biology,
31, 36–43.
Lesk, C., Rowhani, P., & Ramankutty, N. (2016). Influence of extreme weather disasters on global crop production. Nature,529, 84.
Lobell, D. B., & Gourdji, S. M. (2012). The influence of climate change on global crop productivity. Plant Physiology,160, 1686–1697.
Luterbacher, J., Dietrich, D.,Xoplaki, E., Grosjean, M., & Wanner, H. (2004). European seasonal and annual temperature variability, trends,
and extremes since 1500. Science,303, 1499–1503.
Miralles, D. G., Teuling, A. J., van Heerwaarden, C. C., & Vilá-Guerau de Arellano, J. (2014). Mega-heatwave temperatures due to combined
soil desiccation and atmospheric heat accumulation. Nature Geoscience,7, 345–349.
Moradi, M. M., Cronie, O., Rubak, E., Lachieze-Rey, R., Mateu, J., & Baddeley, A. (2019). Resample-smoothing of voronoi intensity
estimators. Statistics and Computing. https://doi.org/10.1007/s11222-018- 09850-0
Naumann, G., Alfieri, L., Wyser, K., Mentaschi, L., Betts, R. A., Carrao, H., et al. (2018). Global changes in drought conditions under
different levels of warming. Geophysical Research Letters,45, 3285–3296. https://doi.org/10.1002/2017GL076521
Acknowledgments
We thank L. Panarello for his support
with some of the Figures and K. Wyser
for useful HELIX documentation. J.
Luterbacher acknowledges the DAAD
(German Academic Exchange Service)
project “The Mediterranean Hot-Spot:
Challenges and Responses in a
Changing Environment”. MarsMet
data can be retrieved at http://
agri4cast.jrc.ec.europa.eu/DataPortal/
Index.aspx. Paleoclimate data are
available at https://www.ncdc.noaa.
gov/data-access/paleoclimatology
-data/datasets/climate- reconstruction.
The SPEI data derived with HELIX
projections can be downloaded at
http://data.europa.eu/89h/
jrc-climate- spei-drought
-helix-ec-earth- 1975-2100, while the
raw HELIX projections can be
requested by contacting the HELIX
Project Manager https://helixclimate.
eu/.
TORETI ET AL. 662

Earth’s Future 10.1029/2019EF001170
Obermeier, W.A., Lehnert, L. W., Kammann, C. I, Müller, C., Luterbacher, J.,Erbs, M., et al. (2017). Reduced CO2 fertilization in temperate
C3 grasslands towards more extreme weather conditions. Nature Climate Change,7, 137–141.
Orth, R., Vogel, M. M., Luterbacher, J., Pfister, C., & Seneviratne, S. I. (2016a). Did European temperatures in 1540 exceed present-day
records? Environmental Research Letters,11, 114021.
Orth, R., Zscheischler, J., & Seneviratne, S. I. (2016b). Record dry summer in 2015 challenges precipitation projections in central Europe.
Scientific Reports,6, 28334.
Parkes, B., Sultan, B., & Ciais, P. (2018). The impact of future climate change and potential adaptation methods on maize yields in west
Africa. Climatic Change,151, 205–217.
Pauling, A., Luterbacher, J., Casty, C., & Wanner, H. (2006). Five hundred years of gridded high-resolution precipitation reconstructions
over Europe and the connection to large-scale circulation. Climate Dynamics,26, 387–405.
Perez-Cruz, F. (2008). Kullback-Leibler divergence estimation of continuous distributions. In 2008 IEEE international symposium on
information theory, pp. 1666–1670.
Pfister, C. (2016). The black swan of 1540: Aspects of a European megadrought. In K. Leggewie, & F. Mauelshagen (Eds.), Climatic change
and cultural transition in Europe (pp. 156–196). Boston: Leiden Brill.
Porter, J. R., & Semenov, M. A. (2005). Crop responses to climatic variation. Philosophical Transactions of the Royal Society B: Biological
Sciences,360, 2021–2035.
Rezaei, E. E., Siebert, S., Hüging, H., & Ewert, F. (2018). Climate change effect on wheat phenology depends on cultivar change. Scientific
Report,8, 4891.
Rezaei, E. E., Siebert, S., Manderscheid, R., Müller, J., Mahrookashani, A., Ehrenpfordt,B., et al. (2018). Quantifying the response of wheat
yields to heat stress: The role of the experimental setup. Field Crops Research,217, 93–103.
Spinoni, J., Naumann, G., Vogt, J. V., & Barbosa, P. (2015). The biggest drought events in Europe from 1950 to 2012. Journal of Hydrology,
3, 509–524.
Stefanon, M., D'Andrea, F., & Drobinski, P. (2012). Heatwave classification over Europe and the Mediterranean region. Environmental
Research Letters,7, 14023.
Toreti, A., Bassu, S., Ceglar, A., & Zampieri, M. (2019). Climate change and crop yields. In P. Ferranti, J. R. Anderson, & E. M. Berry (Eds.),
Encyclopedia of food security and sustainability (Vol. 1, pp. 223–227). Cambridge: Elsevier.
Toreti, O., Cronie, A., & Zampieri, M. (2019). Concurrent climate extremes in the key wheat producing regions of the world. Scientific
Reports,9, 5493.
Toreti, A., Maiorano, A., De Sanctis, G., Webber, H., Ruane, A. C., Fumagalli, D., et al. (2019). Using reanalysis in crop monitoring and
forecasting systems. Agricultural Systems,168, 144–153.
Trnka, M., Olesen, J. E., Kersebaum, K. C., Skjelvag, A. O., Eitzinger, J., Seguin, B., et al. (2011). Agroclimatic conditions in Europe under
climate change. Global Change Biology,17, 2298–2318.
Verger, A., Baret, F., & Weiss, M. (2014). Near real-time vegetation monitoring at global scale. IEEE Journal of Selected Topics in Applied
Earth Observations and Remote Sensing,7, 3473–3481.
Vicente-Serrano, S. M., Beguera, S., & Lopez-Moreno, J.I. (2010). A multiscalar drought index sensitive to global warming: The standardized
precipitation evapotranspiration index. Journal of Climate,23, 1696–1718.
Vogel, M. M., Zscheischler, J., Wartenburger, R., Dee, D., & Seneviratne, S. I. (2019). Concurrent 2018 hot extremes across Northern
Hemisphere due to human-induced climate change. Earth's Future. https://doi.org/10.1029/2019EF001189
WMO news (2018). July sees extreme weather with high impacts. https://public.wmo.int
Webber, H., Ewert, F., Olesen, J. E., Müller, C., Fronzek, S., Ruane, A. C., et al. (2018). Diverging importance of drought stress for maize
and winter wheat in Europe. Nature Communications,9, 4249.
Wetter, O., Pfister, C., Werner, J. P., Zorita, E., Wagner, S., Seneviratne, S. I., et al. (2014). The year-long unprecedented European heat and
drought of 1540—A worst case. Climatic Change,125, 349–363.
Wyser, K., Strandberg, G., Caesar, J., & Gohar, L. (2017). Documentation of changes in climate variability and extremes simulated by the
HELIX AGCMs at the 3 SWLs and comparison in equivalent SST/SIC low-resolution CMIP5 projections. Helix-deliverable D3.1.
Zampieri, M., Ceglar, A., Dentener, F., Dosio, A., Naumann, G., van den Berg, M., & Toreti, A. (2019). When will current climate extremes
affecting maize production become normal? Earth's Future,7, 113–122.
Zampieri, M., Ceglar, A., Dentener, F., & Toreti, A. (2017). Wheat yield loss attributable to heat waves, drought and water excess at the
global, national and subnational scales. Environmental Research Letters,12, 64008.
Zampieri, M., D'Andrea, F., Vautard, R., Ciais, P., de Noblet-Ducoudré, N., & Yiou, P. (2009). Hot European summers and the role of soil
moisture in the propagation of Mediterranean drought. JournalofClimate,22, 4747–4758.
Zhao, C., Liu, B., Piao, S., Wang, X., Lobell, D.B., Huang, Y., et al. (2017). Temperature increase reducesglobal yields of major crops in four
independent estimates. Proceedings of the National Academy of Sciences,114(35), 9326–9331.
TORETI ET AL. 663