In the Biblical story of Joseph, following seven years of abundance, the Kingdom of Egypt was confronted by seven years of drought and famine. In the parlance of modern climate science, intervals with several consecutive extremely dry years are described as ‘“megadroughts”. In this short talk, I’ll describe how climate scientists combine clues from natural weather archives (including corals, tree rings, lake sediments, and many other sources) to reveal the history of ancient megadroughts across our planet. And I’ll highlight new research that combines these surrogate drought records with simulations from state-of-the-art climate models to help us better anticipate the risks of unusually persistent droughts during the coming century.

1. Network Meeting of the Alexander von Humboldt Foundation | Bielefeld, Germany | October 18-20, 2017
LONG DROUGHTS
USING NATURAL CLIMATE ARCHIVES
TO GAGE THE RISKS OF FUTURE “MEGADROUGHTS”

3. EXPERIENCE

5. EXPERIMENTAL LAKES AREA, CANADA

7. THE HILLS LOOK LIKE SAWDUST,
REALLY, THAT COLOUR.
I’VE NEVER SEEN IT
WHERE THE GRASS DIDN’T
TURN GREEN IN THE SPRING BEFORE.
“ ”
Jerry Murphy, Elnora Alberta (Canada)
Globe and Mail, 1 July 2009

8. LONG SHORT

9. BY LOOKING FARTHER
INTO THE PAST,WE ARE ABLE TO UNDERSTAND
HOW DYNAMIC OUR EARTH REALLY IS.

12. Source: Mina Guli
VERTICAL COLUMNS HAVE BEEN USED TO MEASURE
FLOODS ON THE NILE SINCE THE EARLY 8th CENTURY.

14. DENDROCHRONOLOGY

15. ALEXANDER VON HUMBOLDT

16. SEVERAL EXAMPLES… SEEM TO CONFIRM
THE IDEA THAT THERE ARE STILL
TREES ON THE GLOBE
OF A TREMENDOUS ANTIQUITY
AND PERHAPS WITNESSES
TO ITS MOST RECENT PHYSICAL REVOLUTIONS.
“ ”
Alexander von Humboldt
Views of Nature

17. QUERCUS DOUGLASII
Source: Dan Griffen

18. PINUS LONGAEVA

19. MOST TREE-RING RECORDS FROM NORTH AMERICA REFLECT THE EFFECTS
OF WINTER (GREEN DOTS) OR SUMMER PRECIPITATION (BLUE DOTS).
Source: St> George and Ault, Quaternary Science Reviews, 2014

20. ?WHAT HAVE WE LEARNED
ABOUT PAST (AND FUTURE) DROUGHTS
BY READING THE RINGS OF LONG-LIVED TREES?

21. 1Tree rings (and other natural climate archives) provide clear evidence
that droughts have been longer lasting during the ‘recent’ past.

22. 7 7PLENTY FAMINE

24. THESE STUMPS TOOK ROOT IN A DRY MEDIEVAL PERIOD BEFORE BEING
KILLED BY INCREASED PRECIPITATION AND SUBSEQUENT FLOODING.
Source: Bryce Gray

25. 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 .

26. 2The Earth’s climate does not require any unusual conditions to
produce megadroughts. It only needs time and a bit of (bad) luck.

27. 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

29. EL NIÑOSOUTHERN OSCILLATION

30. +TIME

31. MORE THAN ONE OUT OF THREE SIMULATIONS HAD (AT LEAST) ONE EVENT
AS SEVERE AS THE BIGGEST 11th CENTURY MEGADROUGHT .

32. 3In the U.S. Southwest, future warming could make megadroughts
almost certain. But mitigation would cut their risks in half.

33. SOUTHWESTERN UNITED STATES

35. Source: Ault et al. Science Advances, 2016

37. EXPERIENCE