can global warming trigger rapid climate change?
TRANSCRIPT
Can Global Warming trigger rapid climate change?
Take Away Concepts
1. Rapid climate change mechanisms
2. The ocean’s role in rapid climate change.
3. Evidence of past rapid climate change.
4. The Pentagon Document (2003)
5. 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!
• 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
now then
Temperature in Central Greenland
warm
cold
Younger Dryas interval in the Greenland Ice Core
Cold, dusty iceWarmer, pure ice
“Younger Dryas” cooling event (13 - 11.5 ka BP)
present past
Cold reversal Last ice ageWarmingWarming
warm
cold
Flowers of “Dryas” plantTundra 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
…shutdown during the Younger Dryas cold period
FRESH WATER
Past climate has seen MANY of these abrupt climate changes
then now
WARM
COLD
Sea-surface salinityNotice that Atlantic is
saltier than Pacific
NADW
AABW
AAIW
Atlantic Thermohaline circulation
NorthSouth
Density of Seawater(due to salinity and temperature)
Isopycnals
(Equal density lines)
Thermohaline (∆temperature and ∆salinity)
How the ocean can respond non-linearly to freshwater forcing
Deep waterventilation rate
Fresh water addition to surface ocean
1
2
34
5
+
-
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 drierWet places become wetter
Also, melting ice contributes fresh water
Red = saltierBlue = 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
6-8 years long; $1 Billion (1930s dollars)
The Dust Bowl (1932-1939)
What is Drought?
What caused the ‘30s dust bowl? Observations Climate Model
Tropical PacificOcean temperatures“La Niña” conditions
Richard Seager and Ed Cook (Lamont)
US Drought (7 years so far)
Lake Powell levels (1963-Present)
© Ron Niebrugge
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
Recall the MedievalMega-Droughts
Same pattern as Dust Bowl.
… except, they lasted 20-40 years!
**
Lake PowellToday
2002
2003
The Future is now:… “imminent drying of the southwest US”
Rainfall projections for the Southwest US.
Based on historical climate and future CO2
18 of 19 climate models show this trend
Drier
Wetter
American West drought index
Richard Seager (2007; Lamont)
Summary
1. Most warming due to greenhouse gases
2. Climate sensitive to even weak forcing
3. Climate changes can happen very quickly.
4. Not just a “temperature” issue - “water” too.
Bottom line:
Climate is sensitive to “small nudges”
GHG forcing is a “big push”
Surprises are likely.