We know from tree rings and other natural drought records that the western United States has been affected by several 'megadroughts' during the past millennium. But are these exceptionally long-lasting droughts due to unusual external forcings, or are they inevitable given a sufficiently long period of time? Here we present a statistical model that combines sea surface temperature records and drought severity statistics from the western USA, and use that tool to set out an expectation for megadrought, given no other changes in the climate system. Even though this model was trained using only modern climate data (and incorporates no information from tree rings or other proxies), it still produced megadroughts. Moreover, those simulated megadroughts were just as long-lasting, covered as large an area, and were just as severe as real megadroughts estimated from tree rings. That result means that megadroughts can occur in the western United States even if nothing else changes in the climate -- they really are just a matter of time. On the other hand, the only aspect of real-world megadroughts that the model cannot duplicate was the high number of these events during the so-called Medieval Climate Anomaly (800 to 1300 CE). So that cluster of megadroughts may have been caused by some sort of unusual climate circumstances that have not been observed by us during the past few decades. The proxy record tells us that many different kinds of exceptional or unusual climate events happened in the past. But it is often difficult to determine what caused those exceptional events because even, within a period of a thousand years, we still have very few cases. So besides being an aid to understand the causes of past megadroughts, we hope this approach can be applied to other paleoclimate records to distinguish between real interrelations between separate components of the climate system and simple coincidences.

1. TRACE 2018 – Tree Rings in Archaeology, Climatology and Ecology | Greifswald, Germany | April 26, 2018
SCOTT ST. GEORGE TOBY AULT JASON SMERDON SLOAN COATS CARLOS CARRILLO BEN COOK SAMANTHA STEVENSON
A NEW FRAMEWORK TO TEST
THE ORIGINS
OF WESTERN AMERICAN MEGADROUGHT

2. LONG-DURATION

3. Megadroughts are rare but not unlikely.
Clusters of them truly are unusual and
require out-of-sample climate states.

5. Source: Bryce Gray
Based on a survey of relict tree stumps in the Sierra Nevada,
Scott Stine identified two centennial-scale dry periods in
California between 862–1074 CE and 1122–1299 CE.

6. NADA(North American Drought Atlas)

7. During the 11th century, more than 40% of the western United States
experienced drought, even when averaged over several decades.
Sources: Cook et al. (2004). Long-term aridity changes in the western United States. Science 306:1015–1018; Cook et al., (2016),
North American megadroughts in the Common Era: reconstructions and simulations. WIREs Climate Change, 7:411–432 .

8. WHAT ARE THE RISKS POSED
BY FUTURE MEGADROUGHTS?

9. WHAT ARE THE CAUSAL MECHANISMS
RESPONSIBLE FOR MEGADROUGHT
IN THE WESTERN UNITED STATES?

11. In order to get prolonged megadroughts in the western
United States, the climate system needs to be “pushed’
by exotic external forcings that have not occurred during
the more recent instrumental period.
HYPOTHESIS

13. When considered over a sufficiently long interval,
megadroughts are inevitable and occur purely as a
consequence of internal climate variability.
HYPOTHESIS

14. ?
IS IT POSSIBLE THAT
(WITH ENOUGH TIME)
INTERANNUAL VARIABILITY
PLUS SPATIOTEMPORAL STRUCTURE
IN GLOBAL SSTs AND WESTERN US HYDROCLIMATE
CAN PRODUCE MEGADROUGHT?

15. GCMs are expensive and are known to exhibit systematic biases
in mean precipitation and tropical Pacific variability.

16. MEGADROUGHTS
— BY THEIR
NATURE — ARE
RARE EVENTS.

17. Toby Ault Cornell University

18. LIM(Linear Inverse Model)

22. 1000 SIMULATIONS
EACH 1000 YEARS LONG

23. NULL HYPOTHESIS
The characteristics of megadrought in the western US (as reconstructed
by tree rings) are consistent with those arising from a stochastically
forced, linearly damped system with stationary statistics.

24. REAL(TREE-RING ESTIMATES OF
SUMMER DROUGHT SEVERITY)

25. We use the NADA, which was recently updated to a spatial resolution
of one-half degree (Cook et al., 2014), as our ‘gold standard’
to test how well the LIM reproduces reality.
Source: Cook et al., 2014: Pan-continental droughts in North America over the last millennium. Journal of Climate, 27, 383–397.

26. FAKE(SYNTHETIC PDSI AND SST DATA
GENERATED BY THE LIM)

27. Source: Ault et al., Journal of Climate (2018)
Even though it’s conditioned on seasonal climate data from the late
20th century (only), the LIM still produces synthetic megadroughts
(several in this single run) if it’s allowed to run for a millennium.
Drought Area Index (%)

28. Can a simple statistical emulator actually reproduce
the characteristics of real (reconstructed) megadroughts?

32. IMPLICATION
1000800 CE 1200 1400 1600 1800 2000
The cluster of megadroughts reconstructed
between 800 and 1300 CE is unlikely to be generated
by stationary statistics and stochastic forcing.

34. “ ”GIVEN THE OFTEN DRAMATIC CONSEQUENCES
OF EVEN SINGLE-YEAR DROUGHT EVENTS, AND
THE UBIQUITY
OF MULTIDECADAL MEGADROUGHTS
IN THE PALEOCLIMATE RECORD
IT IS PERHAPS FORTUITOUS
THAT NORTH AMERICA
HAS NOT EXPERIENCED A MEGADROUGHT
IN THE LAST SEVERAL HUNDRED YEARS.
Cook et al., 2016
WIREs Climate Change

35. Megadroughts are rare but not unlikely.
Clusters of them truly are unusual and
require out-of-sample climate states.

36. WHAT ARE THE RISKS POSED
BY FUTURE MEGADROUGHTS?

37. H0

38. Adapting the LIM to high-resolution proxies could help determine
the importance of internal variability and forced climate change
across a broad range of phenomena.

39. @scottstgeorge
github.com/cornell-eas/LIM
Source: Ault et al., Journal of Climate (2018)