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California's
unprecedented drought, that persisted through three winters
(2011 to 2014), appears to be relenting. More than half of
the state was judged to be in “exceptional drought”
as 2014 ended. The lack of water caused $2.2 billion in damages,
dried up reservoirs, and even forced authorities to close
the taps on all irrigation water for farmers in the prosperous
Central Valley, for all of 2014. A
report1 on
its causes by the US National Oceanic and Atmospheric Administration
(NOAA) immediately generated controversy within climate science.
The drought was caused by natural climate variations, reinforced
by unusually warm water across the North Pacific Ocean, NOAA
affirmed. Warming of the Earth's climate did not bring on
the recent drought; on the contrary, global warming should
have boosted winter rainfall in California, but rainfall dramatically
decreased during the three years mentioned. California has
clearly warmed over the entire 120-year period of weather
observations, just as the USA and the world have, but no clear
trend toward a wetter or drier climate is evident in the state.
Severity and Extent. The
three-year period that just ended was the second driest such
period in California since 1895. The bar graph in Figure 1,
below, shows how unusually dry the recent period has been
for California; only 1974–1977 was a bit drier. The
extent of extreme drought is mapped in Figure 2 (right).
The US Drought Monitor, source of this data, maps severity
of drought in five categories from D0 (abnormally dry) to
D4 (exceptional drought). Most of California was in exceptional
drought on Dec. 2, 2014; while one year earlier, no part of
the state had exceptional drought. Snowpack in the mountains
covered only 24% of its normal extent in early 2014, after
the rather warm and dry winters.
Figure 1: The unusual scarcity of rainfall in California
in the period 2011—2014: the amount that fell in those three
years has been observed only once in the last 120 years, and
only one other 3-year period had even less rainfall. Negative
numbers on the bottom indicate a deficit of rainfall, in millimeters
per day, and the value zero is set to be the average rainfall.
Causes. NOAA asserted that natural
variations of climate, though they can be extreme, were behind
the intense drought. Two natural variations were intertwined:
La Niña predisposed California to dryness, as it generally
does, in the first winter, 2011-2012. By the third winter,
2013-2014, unusually warm water in the Pacific Ocean west
of the US west coast and south of the Gulf of Alaska caused
a strong “resilient ridge” of warm air to form in the atmosphere
above the warm water. This ridge deflected the jet stream
and storms steered by the jet stream far north into Alaska,
and away from California.
A few months before
NOAA proclaimed this, the American
Meteorological Society published reports from three groups
of atmospheric scientists (citations 2, 3,
and 4 below) in its new series of annual reports on extreme
weather of the previous year “from a climate perspective.”
Two groups affirmed the predominant role of natural causes
and discounted global warming as the main cause of the drought;
while the third group concluded that the climate variation
that leads to a “resilient ridge” in the North Pacific is
much more likely in today's climate when the effects of human
greenhouse gas emissions are included.
continues
. . .
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 Figure
2: The intensity and extent of drought in December 2014. More
than one-half of California was in exceptional drought.
Daniel
Swain and his team pointed out2 that the “resilient
ridge” pattern caused dry periods in many California winters,
but this time the ridge was more extensive and resilient than
in previous dry winters. A resilient ridge forces the jet
stream to flow far north over Alaska and the Gulf of Alaska,
and from there, to proceed south toward California. This is
great for transporting tropical moisture into Alaska, but
not for bringing moisture into California. These authors also
compared model simulations of climate in the 20th century
with those in the pre-industrial era before greenhouse gases
were emitted. They found that the likelihood of a strong resilient
ridge was at least four times greater in the 20th century
than in the pre-industrial era. That suggests a strong human
influence.
Funk
and his team4 examined causes for the drought
in two climate simulation models. In the first model, unusually
high sea surface temperatures (SST) were reproduced in the
North Pacific Ocean in the 2012-13 and 2013-14 winters. They
concluded, “Anthropogenic climate change seems likely to have
contributed to these extreme SST and the extreme upper-level
height anomalies” that led to the resilient ridge.
The second
simulation model reproduced high water temperatures in the
North Pacific, but not in the eastern Pacific near California
where the warmest water was actually observed. Funk concluded
that a long-term warming trend of Pacific Ocean water did
not contribute substantially to the recent drought.
Wang
and Schubert3 agreed that the unusually warm
Pacific waters led to a predilection for drought in California,
but that long-term global warming did not increase the risk
of extreme dry years. Two effects of the warming seem to counteract
each other: the first effect is to establish a strong “resilient
ridge” that indeed blocks many Pacific storms from entering
California. The second effect is to boost the humidity in
air over the warmer eastern Pacific; the added moisture is
expected to increase the chances of rainfall over the state.
But the combined effect is, as we have seen, no trend toward
either dryer or wetter winters.
CITATIONS:
Top
1. “Causes
and Predictability of the 2011-2014 California Drought,”
an Assessment Report by Richard Seager, M. Hoerling, S. Schubert,
B. Lyon, A. Kumar, J. Nakamura, N. Henderson (2014). Climate
Program Office, National Oceanic and Atmospheric Administration
(NOAA), Washington DC, 40 p.
2. “The
extraordinary California drought of 2013/2014: Character,
context, and the role of climate change,” by Daniel
Swain and 6 co-authors (2014) [in: Explaining
Extreme Events of 2013 from a Climate Perspective,
a special supplement], Bulletin of the Amer. Meteorological
Society, vol. 95, no. 9, Sept. 2014, p. S3-S7.
3. “Causes
of the Extreme Dry Conditions over California during early
2013,” by H. Wang and S. Schubert (2014), p. S7-S11
(in citation #2 above).
4. “Examining
the contribution of the observed global warming trend to the
California droughts of 2012/13 and 2013/14,” by C. Funk
and 2 co-authors (2014), p. S11-S15 (in citation #2 above).
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