Climate change and California drought in the 21st century

Abstract

Climate science has advanced over decades
from an initial focus on the development and
use of numerical models of Earth’s climate
and compilation of rich networks of observational
data, to now being in a position to
“detect” and “attribute” specific impacts and
events to anthropogenic climate change.
Recent analyses have thus established the
“fingerprint” of anthropogenic climate change
in an increasingly diverse array of meteorological
and hydrological phenomena around
the world, from heat waves to coastal damages
during extreme tides and storms, flooding
from more intense precipitation events,
and severe drought.

Full text

COMMENTARY
Climate change and California drought in
the 21st century
Michael E. Mann
a,1
and Peter H. Gleick
b
a
Department of Meteorology, Pennsylvania State University, University Park, PA 16802; and
b
Pacific Institute for Studies in Development, Environment, and Security, Oakland, CA 94612
Climate science has advanced over decades
from an initial focus on the development and
use of numerical models of Earth’sclimate
and compilation of rich networks of obser-
vational data, to now being in a position to
“detect” and “attribute” specific impacts and
events to anthropogenic climate change.
Recent analyses have thus established the
“fingerprint” of anthropogenic climate change
in an increasingly diverse array of meteoro-
logical and hydrological phenomena around
the world, from heat waves to coastal dam-
ages during extreme tides and storms, flood-
ing from more intense precipitation events,
and severe drought (1). In a new study pub-
lished in PNAS, Diffenbaugh et al. now add
weight to the accumulating evidence that
anthropogenic climatic changes are already
influencing the frequency, magnitude, and
duration of drought in California (2). The
authors show that the increasing co-occur-
rence of dry years with warm years raises
the risk of drought despite limited evidence
of a trend in precipitation itself, highlighting
the critical role of elevated temperatures
in altering water availability and increasing
overall drought intensity and impact.
Golden State Goes Brown
California’s nickname is the Golden State,
a name that owes as much to the golden hue
of its landscapes during the dry summer
season as to the 19th century Gold Rush or
the fields of golden poppies. The grasses
green up again in late fall when the mid-
latitude storms and rainfall return. What
happens if those rains come late, come little,
or in some cases don’tcomeatall?Suchhas
been the case in recent years.
California is experiencing extreme drought.
Measured both by precipitation and by run-
off in the Sacramento and San Joaquin river
basins, 10 of the past 14 y have been below
normal,andthepast3yhavebeenthedriest
and hottest in the full instrumental record. A
plot of temperature and precipitation anom-
alies over the full instrumental record from
1895 through November 2014 shows that
the 3-y period ending in 2014 was by far
the hottest and driest on record (Fig. 1). As
of the publication of this commentary, the
state appears headed into a fourth consec-
utive year of water shortfall, leading to
massive groundwater overdraft, cutbacks to
farmers, reductions in hydroelectricity gen-
eration, and a range of voluntary and man-
datory urban water restrictions.
As drought has taken hold, the Golden
State is slowly becoming a more arid, brown
state, where constraints on water availability
threaten a large and growing population (up
nearly 80% since the severe drought of
1976–77), unique ecological resources, a major
source of agricultural produce, and one of
the largest economies in the world. What
a sadly ironic destiny that would be for the
state currently led by one Governor Brown:
The growing threat of climate change to
California is one of the drivers for exten-
sive efforts under the Brown (and prior
Schwarzenegger) administration to understand
the risks to the state and develop strategies to
reduce greenhouse gas emissions and imple-
ment adaptation strategies (3). Of course,
we’re not just talking about whether or not
the grass is green. There are growing con-
cerns about whether California can continue
tomeetitstremendousdemandforwaterfor
industrial use, growing food, sustaining eco-
systems, and expanding cities in the face of
drought (4).
As part of the effort to understand the
influence of climate change on extreme re-
gional events, there has been a robust scien-
tific debate over the role of climate change on
California’s current drought. Some studies
(5–7) have argued that we cannot yet discern
a link between storm tracks (and the Pacific
Ocean surface temperature patterns that in-
fluence their behavior) and drought in the
western United States. Others (8) do, how-
ever, see a relationship in the form of ob-
servational data, physical analysis of possible
Fig. 1. California temperature (°F) and precipitation (inches) anomalies from January 1895 to November 2014,
plotted as 3-y anomalies relative to 1901–2000 mean. Data from the National Climatic Data Center nClimDiv dataset.
Author contributions: M.E.M. and P.H.G. wrote the paper.
The authors declare no conflict of interest.
See companion article 10.1073/pnas.1422385112.
1
To whom correspondence should be addressed. Email: mann@
psu.edu.
www.pnas.org/cgi/doi/10.1073/pnas.1503667112 PNAS Early Edition
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COMMENTARY

mechanisms, and model results that human-
caused climate change is strengthening
atmospheric circulation patterns in a way
that “implies that the periodic and in-
evitable droughts California will experience
will exhibit more severity.” Seeming to
weigh in favor of a climate change con-
nection is the fact that by several measures
the current drought appears to be unprece-
dented in at least 1,200 y (9).
Don’t Blame It on the Rain
Part of the challenge is that the term
“drought” can be defined in different ways:
for example, meteorological, hydrological,
agricultural, and socioeconomic drought
(10).Drought,mostsimplydefined,is
the mismatch between the amo unts of
water nature provides and the amounts of
water that humans and the environment
demand. The National Drought Mitigation
Center notes (11):
In the most general sense, drought originates
from a deficiency of precipitation over an ex-
tended period of time—usually a season or
more—resulting in a water shortage for some
activity, group, or environmental sector. Its
impacts result from the interplay between the
natural event (less precipitation than expected)
and the demand people place on water supply,
and human activities can exacerbate the impacts
of drought. Because drought cannot be viewed
solely as a physical phenomenon, it is usually
defined both conceptually and operationally.
Commonly used indicators (such as the
Palmer Drought Severity Index and the
Standard Precipitation Index) evaluate
the balance between incoming (through
precipitation + snow/ice melt runoff) and
outgoing (through evaporation, transpira-
tion, and groundwater recharge) moisture
(12, 13). Previous studies dismissing any
link between anthropogenic climate change
and the current California drought (5–7)
have focused exclusively on only one part
of this balance, the “incoming” part. These
studies argue that climate change cannot be
tied to the low levels of precipitation that
have accompanied the drought. Another
study (8) argues instead that the unusually
strong atmospheric ridge in the west that has
been associated with the drought (what has
been termed the “ridiculously resilient ridge”)
was made more likely by global warming.
How can one reconcile these divergent
findings? Diffenbaugh et al. (2) seem to solve
that mystery in their latest assessment. As
noted earlier in Fig. 1, recen t years haven’t
just been hot and they haven’tjustbeendry:
they’ve been very hot and very dry at the
same time. Climate change appears to be
Diffenbaugh et al.
now add weight to the
accumulating evidence
that anthropogenic
climatic changes are
already influencing the
frequency, magnitude,
and duration of drought
in California.
increasing the likelihood of a large-scale atmo-
spheric pattern that yields warm, dry weather
in California. That’s a double whammy when
it comes to the hydrological balance that
governs drought: less precipitation and more
evaporation and transpiration, at the same
time. Combined with the role that tempera-
ture plays in increasing the loss of water from
agriculture, soils, surface water bodies, and
snowpack, the authors note that 100% of the
moderately dry years between 1995 and 2014
co-occurred with a positive temperature
anomaly. Diffenbaugh et al. (2) note:
efforts to understand drought without exam-
ining the role of temperature m iss a critical
contributor to drought risk. Indeed, our results
show that even in the absence of trends in mean
precipitation—or trends in the occurrence of
extremely low-precipitation events—the risk of
severe drought in California has already increased
due to extremely warm conditions induced by
anthropogenic global warming.
In addition, Diffenbaugh et al. (2) highlight
model results that suggest the emergence of
a climatic regime in which all future dry years
coincide with warmer conditions. As they
note, the region is moving toward “atransi-
tion to a permanent condition of ∼100% risk
that any negative—or extremely negative—
12-mo precipitation anomaly is also ex-
tremely warm” (2).
The conclusions of Diffenbaugh et al. (2)
do not stand in isolation. Indeed, they re-
inforce the results of another new study an-
alyzing future climate model projections (14).
That study similarly concludes that there
is growing risk of unprecedented drought
in the western Un ited States driven pri-
marily by rising temperatures, regardless
ofwhetherornotthereisacleartrendin
precipitation.
That might sound like bad news, and
certainly the trends are moving rapidly in the
wrong direction. The good news, however, is
that this is only one possible future. If society
works to limit global warming to under 2 °C,
which is still possible (1), then we can likely
avoid committing to a brown C alifornia.
California still has a chance to remain the
Golden State.
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California drought? Geophys Res Lett 41(24):9017–9023.
10 Wilhite DA, Glantz MH (1985) Understanding the drought
phenomenon: The role of definitions. Water Int 10(3):111–120.
11 National Drought Mitigation Center (2015) What is drought?
Available at drought.unl.edu/DroughtBasics/WhatisDrought.aspx.
Accessed March 4, 2015.
12 Choi M, Jacobs JM, Anderson MC, Bosch DD (2013) Evaluation
of drought indices via remotely sensed data with hydrological
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