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.