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Can Global Warming trigger
rapid climate change?
Take Away Concepts
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Rapid climate change mechanisms
The ocean’s role in rapid climate change.
Evidence of past rapid climate change.
The Pentagon Document (2003)
Causes and patterns of drought in the US.
Emissions Projections
2xCO2
450 ppm
IPCC AR4 Warming Projections
2xCO2 = +3°C
2.0 - 4.5 °C range
Modern warming is unusual,
Future warming is “another world”
3°C warming
“Dangerous Climate Change”
A future level of warming sufficiently great to push
climate to a tipping point.
What value is this? Subjective,but can be estimated.
• Level = +2°C above modern temperatures
• Why? Warmer than any time in last three million
years (Pliocene) when world was considerably
warmer than today.
• 450 ppm CO2
Why Dangerous? Feedbacks!
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Ice-albedo feedback
Ice-sea-level feedback
Salinity-Ocean circulation feedback
Glacier-water supply link
Hydrological shifts
“unknowns”
Pentagon Report (2003)
What climate change really looks like
Abrupt Climate Change
warm
Temperature in
Central Greenland
cold
now
then
Younger Dryas interval in the
Greenland Ice Core
Warmer, pure ice
Cold, dusty ice
“Younger Dryas” cooling event (13 - 11.5 ka BP)
warm
cold
present
Warming
Warming
Cold reversal
Last ice age
past
Flowers of “Dryas” plant
Tundra flowering plant
Pollen found in European lake sediments during YD
What happened ?
15°C ∆T within a decade.
Deep ocean circulation
was shut down.
Within years-to-centuries
this signal was
communicated around
the world
What happened?
Glacial
Lake Agassiz
The Global Ocean Conveyor
FRESH
WATER
…shutdown during the Younger Dryas cold period
Past climate has seen MANY of
these abrupt climate changes
WARM
then
now
COLD
Sea-surface salinity
Notice that Atlantic is
saltier than Pacific
Atlantic Thermohaline circulation
AAIW
NADW
AABW
South
North
Density of Seawater
(due to salinity and temperature)
Thermohaline (∆temperature and ∆salinity)
How the ocean can respond
non-linearly to freshwater forcing
+
1
Deep water
ventilation rate
5
2
4
3
-
Fresh water addition to surface ocean
Changes in deep water flow
NADW “On”
NADW “Off”
An overview of the mechanism..
Steady increase in greenhouse gases…
Steady warming of ocean and land surface …
Invigorates hydrological cycle …
High-latitude ice melt and increased river
outflow causes high-latitude freshening …
At some point, the high-latitude oceans become
stratified
--> no convection…
The North Atlantic
is freshening now
Why?
Warmer world speeds up
hydrologic cycle
Dry place become drier
Wet places become wetter
Also, melting ice
contributes fresh water
Red = saltier
Blue = fresher
How ice sheets melt
How likely is this scenario?
• Each step is physically plausible
• Each step is occurring
• BUT - we’re still a long way away from rapid (i.e.
catastrophic) climate change.
• The past is instructive because it shows that climate
changes rapidly.
This “Day” won’t happen
Another face of Climate Change:
Drought
The Dust Bowl (1932-1939)
6-8 years long; $1 Billion (1930s dollars)
What is Drought?
What caused the ‘30s dust bowl?
Observations
Climate Model
Tropical Pacific
Ocean temperatures
“La Niña” conditions
Richard Seager and Ed Cook (Lamont)
US Drought (7 years so far)
Lake Powell levels (1963-Present)
The New York Times
MAY 2, 2004
Drought Settles In, Lake Shrinks and
West's Worries Grow
PAGE, Ariz. - At five years and counting, the drought that
has parched much of the West is getting much harder to
shrug off as a blip. Some of the biggest water worries are
focused here on Lake Powell . . .
© Ron Niebrugge
© Ron Niebrugge
Recall the Medieval
Mega-Droughts
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*
Same pattern as Dust Bowl.
… except, they lasted
20-40 years!
Lake Powell
Today
2002
2003
The Future is now:
… “imminent drying of the southwest US”
American West drought index
Wetter
Rainfall projections for
the Southwest US.
Based on historical
climate and future CO2
18 of 19 climate models
show this trend
Drier
Richard Seager (2007; Lamont)
Summary
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Most warming due to greenhouse gases
Climate sensitive to even weak forcing
Climate changes can happen very quickly.
Not just a “temperature” issue - “water” too.
Bottom line:
Climate is sensitive to “small nudges”
GHG forcing is a “big push”
Surprises are likely.