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 °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 CO 2

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 ice Warmer, pure ice

“Younger Dryas” cooling event ( ka BP) presentpast Cold reversalLast ice age Warming warm cold

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 …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 salinity Notice 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 water ventilation rate 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 6-8 years long; $1 Billion (1930s dollars) The Dust Bowl ( )

What is Drought?

What caused the ‘30s dust bowl? ObservationsClimate 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)

© 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 Medieval Mega-Droughts Same pattern as Dust Bowl. … except, they lasted years! * *

Lake Powell Today

The Future is now: … “imminent drying of the southwest US” Rainfall projections for the Southwest US. Based on historical climate and future CO 2 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.