Climate Dynamics manuscript No.(wil

Climate Dynamics manuscript No.
(will be inserted by the editor)
Global Warming and Drought in the 21st Century
Benjamin I Cook · Jason E Smerdon ·
Richard Seager · Sloan Coats
Received: date / Accepted: date
Abstract Global warming is expected to increase the frequency and intensity of
droughts in the 21st century, but the relative contributions from changes in moisture
supply (precipitation) versus evaporative demand (potential evapotranspiration;
PET) have not been comprehensively assessed. Using output from a suite of
general circulation model (GCM) simulations from version 5 of the state-of-the-art
Coupled Model Intercomparison Project (CMIP5), projected 21st-century drought
trends are investigated using an offline calculated index of soil moisture balance
(the Penman-Montieth based Palmer Drought Severity Index; PDSI). The PDSI
calculations are used to quantify the respective contributions of precipitation and
PET to projected drought trends. PDSI projections incorporating both precipitation
and PET changes from the GCMs vary regionally, with robust cross-model
drying in western North America, Central America, the Mediterranean, southern
Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere
high latitudes and east Africa. These regional changes largely reflect the spatially
heterogeneous response of precipitation in the models, although drying in the
PDSI fields extends beyond the regions of reduced precipitation. This expansion
of drought areas is attributed to globally widespread increases in PET, caused by
increases in surface net radiation and the vapor pressure deficit. Increased PET
not only intensifies drying in areas where precipitation is already reduced, it also
drives areas into drought that would otherwise experience little drying or even
wetting from precipitation trends alone. This PET amplification effect is largest
Benjamin I Cook
NASA Goddard Institute for Space Studies
2880 Broadway, New York, NY 10025
Tel.: [phone]
E-mail: [e-mail]
Jason E Smerdon
Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964
Richard Seager
Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964
Sloan Coats
Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964
2 Benjamin I Cook et al.
in the Northern Hemisphere mid-latitudes, and is especially pronounced in western
North America, Europe, and southeast China. Compared to PDSI projections
accounting for changes in precipitation only, the additional effect of increased PET
expands the percentage of global land area projected to experience significant drying
(PDSI≤ −1) by the end of the 21st-century from 23% to 43%. This integrated
accounting of both the supply and demand sides of the surface moisture balance
is therefore critical for characterizing the full range of projected drought risks tied
to increasing greenhouse gases and associated warming of the climate system.
1 Introduction
Extreme climate and weather events have caused significant disruptions to modern
and past societies (Coumou and Rahmstorf, 2012; Ross and Lott, 2003; Lubchenco
and Karl, 2012), and there is concern that anthropogenic climate change will increase
the occurrence, magnitude, or impact of these events in the future (e.g.,
Meehl et al, 2000; Rahmstorf and Coumou, 2011). Drought is one such extreme
phenomenon, and is of particular interest because of it’s often long-term impacts
on critical water resources, agricultural production, and economic activity (e.g.,
Li et al, 2011; Ding et al, 2011; Ross and Lott, 2003). Focus on drought vulnerabilities
has been intensified due to a series of recent and severe droughts in
regions as diverse as the United States (Hoerling et al, 2012a, 2013; Karl et al,
2012), east Africa (Lyon and DeWitt, 2012), Australia (McGrath et al, 2012), and
the Sahel (Giannini et al, 2003). Recent work further suggests that global aridity
has increased in step with observed warming trends, and that this drying will
worsen for many regions as global temperatures continue to rise with increasing
anthropogenic greenhouse gas emissions (Burke et al, 2006; Dai, 2013; Sheffield
and Wood, 2008).
There are significant uncertainties, however, in recent and projected future
drought trends, especially regarding the extent to which these trends will be forced
by changes in precipitation versus evaporative demand (also known as potential
evapotranspiration; PET) (Hoerling et al, 2012b; Sheffield et al, 2012). Drought is
generally defined as a deficit in soil moisture (agricultural) or streamflow (hydrologic);
as such, it can be caused by declines in precipitation, increases in evapotranspiration,
or a combination of the two. In the global mean, both precipitation
and evapotranspiration are expected to increase with warming, a consequence of
an intensified hydrologic cycle in a warmer world (Allen and Ingram, 2002; Huntington,
2006). The characteristics of changes in precipitation and PET trends at
the regional level, and the dynamics that drive such changes, are nevertheless more
uncertain, despite the fact that these changes are perhaps of greatest relevance to
on-the-ground stakeholders.
Precipitation projections in general circulation models (GCMs) have large uncertainties
compared to other model variables, such as temperature (e.g., Knutti
and Sedlacek, 2013). The most confident estimates indicate that precipitation will
increase in mesic areas (e.g., the wet tropics, the mid- to high latitudes of the
Northern Hemisphere, etc) and decrease in semi-arid regions (e.g., the subtropics).
This is generally referred to as the ‘rich-get-richer/poor-get-poorer’ mechanism,
and is caused by both thermodynamic (warming and moistening of the
atmosphere) and dynamic (circulation) processes (Chou et al, 2007, 2013; Held
GLOBAL WARMING AND DROUGHT 3
and Soden, 2006; Neelin et al, 2003; Seager et al, 2010).
Evapotranspiration includes both the physical (evaporation) and biological
(transpiration) fluxes of moisture from the surface to the atmosphere. Evapotranspiration
is expected to increase in the future due to increased evaporative demand
by the atmosphere, driven by increases in energy availability at the surface (surface
net radiation) and vapor pressure deficits (the difference between saturation
and actual vapor pressure; VPD). Increased radiative forcing from anthropogenic
greenhouse gases (GHG) is expected to increase surface net radiation in most areas
by inhibiting longwave cooling, while GHG-induced warming of the atmosphere
is expected to increase the VPD. Importantly, VPD increases with warming, even
at constant relative humidity (e.g., Anderson, 1936). Given the fact that the wellmixed
nature of GHGs will drive widespread patterns of global warming, shifts in
evaporative demand are likely to be more spatially homogenous and widespread
than precipitation changes.
The idea that increased evaporative demand in a warmer world will enhance
drought is not new (e.g., Dai, 2011), but it is important to understand where precipitation
or evaporation changes will be dominant individual drivers of drought
and where they will work in concert to intensify drought. To date, however, little
has been done to quantify and explicitly separate the relative contribution
of changes in precipitation versus evaporative demand to the magnitude and extent
of global warming-induced drought. To address this question, we use output
from a suite of 20th and 21st-century GCM simulations, available through the
Coupled Model Intercomparison Project version 5 (CMIP5, Taylor et al, 2012) to
calculate an offline index of soil moisture balance (the Penman-Monteith based
Palmer Drought Severity Index). This index provides an ideal and flexible estimation
of surface moisture conditions, allowing us to vary inputs such as model
precipitation, temperature, and surface energy availability, which in turn allows
us to separate and quantify the influence of specific variables on future drought
projections. Our analysis thus addresses three questions: 1) What are the relative
contributions of changes in precipitation and evaporative demand to global and
regional drying patterns?, 2) Where do the combined effects of changes in precipitation
and evaporative demand enhance drying?, 3) In which regions, if any, are
increases in evaporative demand sufficient to shift the climate towards drought
when precipitation changes would otherwise force wetter conditions?
2 Data and Methods
2.1 CMIP5 Model Output
We use GCM output available from the CMIP5 archive, the suite of model experiments
organized and contributed from various modeling centers in support
of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate
Change (IPCC). Output from the historical and RCP8.5 model scenarios is
used. The historical experiments are run for the years 1850–2005 and are forced
with observations of transient climate forcings over the last 150 years (e.g., solar
variability, land use change, GHG concentrations, etc). These experiments are
initialized in 1850 using output from long, unforced control runs with fixed preindustrial
climate forcings. The RCP8.5 scenario (2006–2099) is one of a suite of
4 Benjamin I Cook et al.
future GHG forcing scenarios; RCP8.5 is designed so that the top of the atmosphere
radiative imbalance will equal approximately +8.5 W m-2 by the end of
the 21st-century, relative to pre-industrial conditions. The RCP8.5 scenario runs
are initialized using the end of the historical runs. Our analysis is restricted to
those models (Table 1) with continuous ensemble members spanning the historical
through RCP8.5 time periods.
2.2 Penman-Monteith Palmer Drought Severity Index
Simulated soil moisture within the GCMs is not easily separated into contributions
from precipitation or PET, making it difficult to identify the extent to which soil
moisture trends in the models are driven by changes in supply and/or demand.
Moreover,

Kết quả (Việt) 1:

Khí hậu động lực bản thảo số(sẽ được chèn vào bởi các biên tập viên)Sự nóng lên toàn cầu và hạn hán ở thế kỷ 21Benjamin tôi nấu ăn · Jason E Smerdon ·Richard Seager · Áo khoác bành tô SloanNhận được: ngày / chấp nhận: ngàyTrừu tượng nóng lên toàn cầu dự kiến sẽ tăng tần số và cường độ củahạn hán ở thế kỷ 21, nhưng sự đóng góp tương đối của các thay đổi trong độ ẩmCác nguồn cung cấp (mưa) so với nhu cầu evaporative (tiềm năng evapotranspiration;PET) không có được đánh giá toàn diện. Sử dụng các đầu ra từ một bộlưu thông nói chung mô hình (GCM) mô phỏng từ phiên bản 5 của nhà nước-of-the-nghệ thuậtCùng mô hình Intercomparison dự án (CMIP5), dự kiến thế kỷ 21 hạn hánxu hướng đang điều tra bằng cách sử dụng một chỉ số tính diễn đàn của sự cân bằng độ ẩm của đất(Penman-Montieth dựa trên chỉ số mức độ nghiêm trọng của Palmer hạn hán; PDSI). PDSItính toán được sử dụng để định lượng sự đóng góp tương ứng của mưa vàVật nuôi với xu hướng khô hạn dự kiến. PDSI dự kết hợp cả hai mưavà PET thay đổi từ các GCMs thay đổi khu vực, với mạnh mẽ cross-người mẫusấy khô ở Tây Bắc Mỹ, Trung Mỹ, địa Trung Hải, phía namChâu Phi, và Amazon và mạnh mẽ ướt xảy ra ở Bắc bán cầuvĩ độ cao và Đông Phi. Những thay đổi khu vực chủ yếu là phản ánh các trong không gianCác phản ứng không đồng nhất của mưa trong các mô hình, mặc dù làm khô trong cácPDSI lĩnh vực mở rộng vượt ra ngoài vùng mưa giảm. Việc mở rộng nàytrong khu vực hạn hán là do tăng phổ biến rộng rãi trên toàn cầu trong PET, gây ra bởigia tăng bề mặt bức xạ ròng và áp suất hơi thiếu. Tăng PETkhông chỉ tăng cường sấy khô trong khu vực nơi mà mưa đã giảm xuống, nó cũngổ đĩa khu vực vào hạn hán sẽ kinh nghiệm nếu không chút sấy hoặc thậm chílàm ướt từ xu hướng mưa một mình. Hiệu ứng khuếch đại con vật CƯNG này là lớn nhấtBenjamin tôi nấu ănNASA Goddard Space Viện nghiên cứu2880 Broadway, New York, NY 10025Điện thoại: [phone]Thư điện tử: [e-mail] E SmerdonLamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964Richard SeagerLamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964Áo khoác bành tô SloanLamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 109642 Benjamin tôi Cook et al.trong các Bắc bán cầu giữa-vĩ độ, và đặc biệt là phát âm trong TâyBắc Mỹ, Châu Âu và đông nam Trung Quốc. So với dự báo PDSIkế toán cho những thay đổi trong mưa chỉ, có hiệu lực bổ sung tăng thú cưngmở rộng phần trăm dự kiến sẽ có kinh nghiệm làm khô đáng kể diện tích toàn cầu(PDSI≤ −1) vào cuối tháng 21 thế kỷ từ 23% đến 43%. Điều này tích hợpkế toán của cả hai bên cung cấp và nhu cầu của sự cân bằng độ ẩm bề mặtdo đó rất quan trọng cho characterizing đầy đủ các rủi ro dự kiến khô hạn ràng buộcđể gia tăng khí nhà kính và liên quan đến sự nóng lên của hệ thống khí hậu.1 IntroductionExtreme climate and weather events have caused significant disruptions to modernand past societies (Coumou and Rahmstorf, 2012; Ross and Lott, 2003; Lubchencoand Karl, 2012), and there is concern that anthropogenic climate change will increasethe occurrence, magnitude, or impact of these events in the future (e.g.,Meehl et al, 2000; Rahmstorf and Coumou, 2011). Drought is one such extremephenomenon, and is of particular interest because of it’s often long-term impactson critical water resources, agricultural production, and economic activity (e.g.,Li et al, 2011; Ding et al, 2011; Ross and Lott, 2003). Focus on drought vulnerabilitieshas been intensified due to a series of recent and severe droughts inregions as diverse as the United States (Hoerling et al, 2012a, 2013; Karl et al,2012), east Africa (Lyon and DeWitt, 2012), Australia (McGrath et al, 2012), andthe Sahel (Giannini et al, 2003). Recent work further suggests that global aridityhas increased in step with observed warming trends, and that this drying willworsen for many regions as global temperatures continue to rise with increasinganthropogenic greenhouse gas emissions (Burke et al, 2006; Dai, 2013; Sheffieldand Wood, 2008).There are significant uncertainties, however, in recent and projected futuredrought trends, especially regarding the extent to which these trends will be forcedby changes in precipitation versus evaporative demand (also known as potentialevapotranspiration; PET) (Hoerling et al, 2012b; Sheffield et al, 2012). Drought is
generally defined as a deficit in soil moisture (agricultural) or streamflow (hydrologic);
as such, it can be caused by declines in precipitation, increases in evapotranspiration,
or a combination of the two. In the global mean, both precipitation
and evapotranspiration are expected to increase with warming, a consequence of
an intensified hydrologic cycle in a warmer world (Allen and Ingram, 2002; Huntington,
2006). The characteristics of changes in precipitation and PET trends at
the regional level, and the dynamics that drive such changes, are nevertheless more
uncertain, despite the fact that these changes are perhaps of greatest relevance to
on-the-ground stakeholders.
Precipitation projections in general circulation models (GCMs) have large uncertainties
compared to other model variables, such as temperature (e.g., Knutti
and Sedlacek, 2013). The most confident estimates indicate that precipitation will
increase in mesic areas (e.g., the wet tropics, the mid- to high latitudes of the
Northern Hemisphere, etc) and decrease in semi-arid regions (e.g., the subtropics).
This is generally referred to as the ‘rich-get-richer/poor-get-poorer’ mechanism,
and is caused by both thermodynamic (warming and moistening of the
atmosphere) and dynamic (circulation) processes (Chou et al, 2007, 2013; Held
GLOBAL WARMING AND DROUGHT 3
and Soden, 2006; Neelin et al, 2003; Seager et al, 2010).
Evapotranspiration includes both the physical (evaporation) and biological
(transpiration) fluxes of moisture from the surface to the atmosphere. Evapotranspiration
is expected to increase in the future due to increased evaporative demand
by the atmosphere, driven by increases in energy availability at the surface (surface
net radiation) and vapor pressure deficits (the difference between saturation
and actual vapor pressure; VPD). Increased radiative forcing from anthropogenic
greenhouse gases (GHG) is expected to increase surface net radiation in most areas
by inhibiting longwave cooling, while GHG-induced warming of the atmosphere
is expected to increase the VPD. Importantly, VPD increases with warming, even
at constant relative humidity (e.g., Anderson, 1936). Given the fact that the wellmixed
nature of GHGs will drive widespread patterns of global warming, shifts in
evaporative demand are likely to be more spatially homogenous and widespread
than precipitation changes.
The idea that increased evaporative demand in a warmer world will enhance
drought is not new (e.g., Dai, 2011), but it is important to understand where precipitation
or evaporation changes will be dominant individual drivers of drought
and where they will work in concert to intensify drought. To date, however, little
has been done to quantify and explicitly separate the relative contribution
of changes in precipitation versus evaporative demand to the magnitude and extent
of global warming-induced drought. To address this question, we use output
from a suite of 20th and 21st-century GCM simulations, available through the
Coupled Model Intercomparison Project version 5 (CMIP5, Taylor et al, 2012) to
calculate an offline index of soil moisture balance (the Penman-Monteith based
Palmer Drought Severity Index). This index provides an ideal and flexible estimation
of surface moisture conditions, allowing us to vary inputs such as model
precipitation, temperature, and surface energy availability, which in turn allows
us to separate and quantify the influence of specific variables on future drought
projections. Our analysis thus addresses three questions: 1) What are the relative
contributions of changes in precipitation and evaporative demand to global and
regional drying patterns?, 2) Where do the combined effects of changes in precipitation
and evaporative demand enhance drying?, 3) In which regions, if any, are
increases in evaporative demand sufficient to shift the climate towards drought
when precipitation changes would otherwise force wetter conditions?
2 Data and Methods
2.1 CMIP5 Model Output
We use GCM output available from the CMIP5 archive, the suite of model experiments
organized and contributed from various modeling centers in support
of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate
Change (IPCC). Output from the historical and RCP8.5 model scenarios is
used. The historical experiments are run for the years 1850–2005 and are forced
with observations of transient climate forcings over the last 150 years (e.g., solar
variability, land use change, GHG concentrations, etc). These experiments are
initialized in 1850 using output from long, unforced control runs with fixed preindustrial
climate forcings. The RCP8.5 scenario (2006–2099) is one of a suite of
4 Benjamin I Cook et al.
future GHG forcing scenarios; RCP8.5 is designed so that the top of the atmosphere
radiative imbalance will equal approximately +8.5 W m-2 by the end of
the 21st-century, relative to pre-industrial conditions. The RCP8.5 scenario runs
are initialized using the end of the historical runs. Our analysis is restricted to
those models (Table 1) with continuous ensemble members spanning the historical
through RCP8.5 time periods.
2.2 Penman-Monteith Palmer Drought Severity Index
Simulated soil moisture within the GCMs is not easily separated into contributions
from precipitation or PET, making it difficult to identify the extent to which soil
moisture trends in the models are driven by changes in supply and/or demand.
Moreover,