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 Variability Ch. 16 Global Warming and 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 evidence Measured temperatures at weather stations. Spotty coverage beginning ~1850 AD Moderate 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 trends 2nd order trends 3rd order trends

How do we look at trends? 1st order trends 2nd order trends 3rd order trends

How do we look at trends? 1st order trends 2nd order trends 3rd order trends

How do we look at trends? 1st order trends 2nd order trends 3rd order trends: probably ENSO 1983

Unit VIII. Global Warming A. 20th Century Warming 1. Direct evidence: Measured temperatures General warming for the past century…..but not steady…. initial warming from ~1880 to 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 evidence 2. Indirect evidence a. climate proxies Climate proxy: “something preserved that represents a particular aspect of climate.” pollen tree ring widths ice-rafted stones in ocean sediment hippos in England

Unit VIII. Global Warming A. 20th Century Warming 1. Direct evidence 2. Indirect evidence a. climate proxies b. Indirect evidence of 20th century warming. Glaciers are retreating Sea level rise

Global Sea Level Change

Sea Level Rise How 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 Rise Why 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 Rise Why 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 evidence 2. 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 century warming. 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 evidence 2. 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 Shelf Antarctica: 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 evidence 2. 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 century warming. 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 CO 2 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 ago 2. 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 Warming b. 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 Warming b. Medieval Warm Period ~800 to 1300 AD c. 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 Warming b. Medieval Warm Period c. The Little Ice Age By 1400 AD Greenland Colonies lost. Glaciers grew Sea ice more extensive (Iceland / Inuit) Lakes froze earlier Rivers/canals froze in NW Europe Crops in Europe were less productive.

Unit VIII. Global Warming b. Medieval Warm Period c. The Little Ice Age d. 20th century warming

Unit VIII. Global Warming A. 20th Century Warming 1. Direct evidence 2. Indirect evidence B. 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 energy b. Medieval Warm Period c. The Little Ice Age d. 20th century warming