Unit VII. Global Warming Is the planet warming? How do we know? How confident are we? If it is warming, how long has it been warming? How unusual is the.

Unit VII. Global Warming • Is the planet warming? • How do we know? How confident are we? • If it is warming, how long has it been

warming? • How unusual is the 20th century? • Why?

Natural variability vs human impacts? • Can we predict the future?

Unit VIII. Global Warming

Readings: Ch. 15 Short-term Climate VariabilityCh. 16 Global Warmingand pages 3-7

We will cover much of what is in these chapters that covers the last 2000 years, and a bit of the older records back to the last ice age.

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidenceMeasured temperatures at weather stations.

Spotty coverage beginning ~1850 ADModerate coverage since ~1880 AD

Problems with the direct measurements:• Developing countries shorter records• Oceans cover 70% of Earth• Heat Island Effect

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence: Measured temperatures a. Spatial distribution biased and of

unequal duration. b. Heat Island: Cities are hotter than

surroundings. • Albedo• Extra local heat, often trapped• Inversions

3 °C (6 °F) warmer than surroundings not uncommon.

How do we look at trends?1st order trends2nd order trends3rd order trends

How do we look at trends?1st order trends2nd order trends3rd order trends

How do we look at trends?1st order trends2nd order trends3rd order trends

How do we look at trends?1st order trends2nd order trends3rd order trends: probably ENSO

1983

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence: Measured temperatures

General warming for the past century…..butnot steady…. initial warming from ~1880 to 1940. Little change (maybe even cooling) 1940 to mid 1960s. Steady, rapid warming since the early 1970s.

Is the warming uniform across the planet?

Why does the land show twice the temperature change as the ocean?

Land

Ocean

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence: Measured temperatures General warming for the past century

What about precipitation?

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence2. Indirect evidence a. climate proxies

Climate proxy: “something preserved that represents a particular aspect of climate.”

pollentree ring widthsice-rafted stones in ocean sedimenthippos in England

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence2. Indirect evidence a. climate proxies b. Indirect evidence of 20th century

warming.• Glaciers are retreating• Sea level rise•••

Global Sea Level Change

• Sea Level RiseHow do we know?

• Tide gauge records. 30 cm/century• Satellites. Also 30 cm/century

Why is sea level rising?• Glaciers melting• Thermal expansion of the ocean• Spatial differences mostly related to last

ice age and plate tectonics

• Sea Level RiseWhy is sea level rising?

• Glaciers and ice sheets melting: 50%• Thermal expansion

Rule of thumb: 20 cm of sea level rise for every 1 °C rise in temperature of the top 100 m

Global temp has increased 0.7 °C,so ocean expansion could be estimated to be 0.7 x 20 cm = ~ 14 cm, or ~ half the observed sea level rise.

• Sea Level RiseWhy is sea level rising?

• Glaciers and ice sheets melting: 50%• Thermal expansion: 50%

What about the BIG ice sheets, Greenland and Antarctica?

Greenland seems to be melting more and more

But Antarctica is either not changing, or perhaps growing a little.

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence2. Indirect evidence a. climate proxies b. Indirect evidence of 20th century

warming.• Glaciers are retreating• Sea level rise• Sea ice melting ••

Unit VIII. Global Warming A. 20th Century Warming

b. Indirect evidence of 20th centurywarming.

• Glaciers are retreating• Sea level rise• Sea ice melting: How does loss of sea ice impact sea level?What other impacts might sea ice reduction

have? Ice Albedo Feedback: Summer and Winter implications.

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence2. Indirect evidence a. climate proxies b. Indirect evidence of 20th century

warming.• Glaciers are retreating• Sea level rise• Sea ice melting • Ice shelves disappearing•

Unit VIII. Global Warming A. 20th Century Warming

2. Indirect evidence• Ice Shelves Arctic Canada: Ward Hunt Ice ShelfAntarctica: losing ice shelves

How does the loss of ice shelves impact global sea level?

Other impacts?

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence2. Indirect evidence a. climate proxies b. Indirect evidence of 20th century

warming.• Glaciers are retreating• Sea level rise• Sea ice melting • Ice shelves disappearing• Permafrost melting in the Arctic

Unit VIII. Global Warming A. 20th Century Warming

• Permafrost melting in the Arctic

Permafrost: Permanently frozen ground (ground temperatures never rise above 0 °C during the year).

Is there much permafrost?

Unit VIII. Global Warming A. 20th Century Warming

b. Indirect evidence of 20th centurywarming.

• Permafrost melting in the Arctic: Some on land, some under the sea.

Arctic warming has resulted in permafrost warming, and reduction in the area of permafrost.Impacts? Positive feedbacks as warming releases trapped CO2 and gas hydrates.

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence of 20th century warming.2. Indirect evidence of warming

• Glaciers are retreating• Sea level rise• Sea ice melting • Ice shelves disappearing• Permafrost melting in the Arctic• Melting glaciers reveal humans, tools The Iceman, Swiss Alps Melting Alaska glaciers/snowbanks

Unit VIII. Global Warming A. 20th Century Warming

1. Direct evidence of 20th century warming.2. Indirect evidence of warming

B. A perspective on 20th century warming. The last ice age and the present interglacial.

The last ice age.Throughout the past 2 million years, Earth

has been mostly in a “glacial cycle”, with brief (10,000 year) “interglacials” and long (100,000 year “glacials”.

The last ice age.Throughout the past 2 million years, Earth

has been mostly in a “glacial cycle”, with brief (10,000 year) “interglacials”.

The most recent glacial period period peaked 25,000 to 15,000 years ago and ended about 10,000 years ago, and we have been in the current interglacial (The Holocene) since then.

Primary explanation of ice age cycles

Solar insolation forcing:changes in solar energy received by Earth due to irregularities in Earth’s orbit.

Precession of the equinoxes.

The Holocene

Within the Holocene, solar insolation in the Northern Hemisphere was at a maximum at the beginning of the Holocene, and has been decreasing ever since.

We expect the present to be the coolest summers of the Holocene.

Unit VIII. Global Warming A. 20th Century Warming B. A perspective on 20th century warming.

1. The last ice age: peak 20,000 yr ago2. The Holocene 10,000 to present a. In N Hemisphere, decreasing solar

energy in summer throughout the Holocene. b. Medieval Warm Period c. The Little Ice Age.

Unit VIII. Global Warmingb. Medieval Warm Period

• Vikings establish colonies on Greenland• Inuit from Alaska hunting bowhead

whales colonize eastern Canadian Arctic.• Wine grown in England.• Farming at higher altitudes and farther

north through out Scandinavia

Viking exploration: 800 to 1200 AD

Unit VIII. Global Warmingb. Medieval Warm Period ~800 to 1300 ADc. The Little Ice Age

Beginning in 1300 AD, sea ice in the North Atlantic increased, and travel to Greenland Colonies became difficult.

By 1400 AD sea ice so heavy, ships unable to bring supplies and Greenland Colonies lost.

Unit VIII. Global Warmingb. Medieval Warm Periodc. The Little Ice Age

By 1400 AD Greenland Colonies lost.Glaciers grewSea ice more extensive (Iceland / Inuit)Lakes froze earlierRivers/canals froze in NW EuropeCrops in Europe were less productive.

Unit VIII. Global Warmingb. Medieval Warm Periodc. The Little Ice Aged. 20th century warming

Unit VIII. Global WarmingA. 20th Century Warming 1. Direct evidence 2. Indirect evidenceB. A perspective on 20th century warming. 1.The last ice age and the present interglacial. 2. The Holocene 10,000 to present

a. N Hemisphere decreasing solar energyb. Medieval Warm Periodc. The Little Ice Aged. 20th century warming