Chapter 8: PaleoclimateChapter 8: Paleoclimate

This chapter discusses:This chapter discusses:

1.1. Proxy dataProxy data

2.2. Climate change at different time scalesClimate change at different time scales

Time Scales of Climate ChangeTime Scales of Climate Change

Earth’s climate changes all the time, e.g., last 300 Earth’s climate changes all the time, e.g., last 300 Myr, last 3 Myr, last 50,000 yr, and last 1000 yr.Myr, last 3 Myr, last 50,000 yr, and last 1000 yr.

Climate Through Time

Quelcaya Ice Cap, Andes Mountains, Peru

1976

Late 1990’s

Variations of the Earth’s surface temperature for the past 140 years

Temperatures over last 400,000 yearsTemperatures over last 570 million years

Climate is not constant

Change occurs over a variety of time scales

Most Earth history is warmer and wetter than at present

100,000 years

18,000 years

230,000 years

1 Million years

3.5 Million years

10 million years6 5 million years

10,000 years

1,000 years

55 million years

PR

ES

EN

T1. The last time atmospheric CO2 concentrations and temperatures were much higher than today was in the age of dinosaurs.2. Agriculture revolution began 10,000 years ago.3. Human population explosion in the past 100 years. Today: 6 billion

The only mammals living at this time were small rodents.

Sea levels were much higher than today, and Texas was mostly under water

Climate DataClimate DataTools for studying climate and climate change

Data

Climate models

Natural recorders of climate or proxy data

Instrumental measurements (direct)

Historical documents

Understand climatic cause and effect

Test hypothesisQuantitative (put numbers on ideas) andPredict the future

External factors climate systemFeedbacks

Instrumental Measurements (Direct)Instrumental Measurements (Direct)

Weather Stations• Stevenson Screen

• Temperature

Automatic Weather Station

Satellite

~140 years old

~40 years old

The Hunters in the Snow by Pieter Brueghel the Elder (Kunshistorisches Museum,

Vienna)

Historical documentsHistorical documents

Proxy Records of Climate

• Uses of proxy records of climate depend on both

- time span of record - resolution of record

• Proxies that record annual growth patterns can indicate year to year variations in climate

-tree rings -ice cores -deep lake sediments -coral reefs

• Limited to last 500-1000 years except ice cores

Proxy Records of Climate

• Variations of tree ring width and density act as recorders of year to year changes in temperature and rainfall

• Lighter, thicker wood tissue formed by rapid growth in spring and much thinner, darker layers marking cessation of growth in autumn and winter

Tree Rings

• Limited to land areas outside of tropics

Varved Lake Sediments• Complement tree ring records; most

common in cold-temperature environments

• Occur in deeper parts of lakes that do not support bottom-dwelling organisms that would obliterate annual layers with their activity

• Layers usually result from seasonal alternation between light, mineral-rich debris and dark, organic rich material brought in by runoff – act as proxy of precipitation amount

Varves: sediments deposited annually on the bottoms of lakes that freeze in winter and thaw in summer. Winter varve: fine sediments; summer varve: coarse sediments. Varve thickness – length of freeze-free period – summer

temperature.

• Measurements of oxygen-18 isotope concentration records sea surface temperature and salinity (precipitation and runoff) variations

• Texture of calcite (CaCO3) incorporated in skeletons varies; lighter parts during periods of rapid growth in summer and darker layers during winter

Corals

• Limited to tropical oceans

multi-celled organisms that build reefs in tropical oceans

• Measurements provide information on temperature, snowfall, atmospheric composition (gases, dust, volcanic aerosols), sunspots, …

• Darker and lighter layers are more or less dust blown in seasonally

Ice Cores

• Limited to polar latitudes and mountain glaciers

Speleothems (cave deposits)Mineral formations occurring in limestone caves (most commonly

stalagmites & stalactites, or slab-like deposits known as flowstones)

Primarily calcium carbonate, precipitated from groundwaterUranium can be used to determine the age

Fossils of Past Vegetation• Climate can be inferred from distinctive vegetation types

• Palm-tree like fossil in Wyoming 45 Myrs ago indicating the Cretaceous warm climate

• Climate can be inferred from leaf size and shape.

• Climate can be inferred from pollen in sediments: All flowering plants produce pollen grains with distinctive shapes.

Marine Sediments

• Long cores drilled by specially equipped ships

• Dating only accurate to about 40,000 years ago and can resolve climate changes that occur on century scale or longer

• Granular debris from land can indicate icebergs breaking off of continental ice sheets, suggesting cold climates

• Isotopes in shells of foraminifera can reveal temperature, salinity, and ice volume

Marine Sediments

Proxy Records of Climate

• Proxies for more ancient climates are found in sediments or inferred from fossils and land forms• Can generally only resolve changes that occur over 100 years or greater

• Recent times: instrumental

• More recent times: historical, tree rings, ice cores

Why bother studying ancient climate? Who cares what happened a long time ago?

1. Past variability can show climatic extremes that have not been experienced during recorded history

2. In order to understand the effects of human activity on climate, we must establish what the planet, the atmosphere, and climate change was like before human perturbations

3. Constructing and interpreting long-term records of climate are the only means to determine how periodic climate change is (All in all, we are just a blip)

4. Past is prologue

“The farther backward you can look, the farther forward you are likely to see.” - Winston Churchill

• Proxies that record annual growth patterns can indicate year to year variations in climate

-tree rings -ice cores -deep lake sediments -coral reefs

• Limited to last 500-1000 years except ice cores

Proxy Records of Climate

(21 k yrs ago)

• How cold were the glacial tropics?

• Evidence for a small tropical cooling

The Tropical Cooling Debate

• Evidence for a large tropical cooling

• Was the actual tropical cooling medium-small?

• Relevance of global tropical temperatures to future climate

Reconstructing the Last Glacial Maximum

CLIMAP (Climate Mapping and Prediction) Project

Mainly based on ocean sediments

Began in 1970s, published its first map in 1976 and then 1981

LGM August SST

Difference between LGM and Today

Overall: 4°C cooler than today

N Atlantic: 8°C cooler

N Pacific: 2-4°C cooler

Tropical oceans: 1-2°C cooler

What Caused The Tropical Cooling?

Insolation was close to today

Greenhouse gases must have been a major factor.

Ice sheets were too distant.

Ocean-Based Evidence for a Small Tropical CoolingCLIMAP

Distribution of plankton species depends on ocean water temperature.

During LGM, high-latitude cold-adapted species moved to the tropics a large cooling in the tropics. But …

Biochemical Composition

Relative abundance of alkenone molecules is sensitive to ocean water temperature.

Oxygen isotope measurements

Difference in δ18O values (LGM and today) = difference by ice sheets + difference by ocean temperatures

Tropical cooling: 1.5°C

Tropical cooling: less than 2°C

Tropical cooling: 2-3°C

Land-Based Evidence for a Large Tropical Cooling

Drop of the ice lineDescent of the lower limit of mountain glaciers by 600-1000 meters in the tropics.

Tropical cooling: 4-6°CLapse-rate cooling: 6.5°C/1000 meters

Descent of the upper limit of forestsTropical cooling: 5°C

Temperature-sensitive noble gases (xenon, krypton, argon, neon) in groundwater

SW USA and SE Brazil cooling: 5°C

Ocean-based evidence: small cooling; Land-based evidence: large cooling

Was the Actual Tropical Cooling Medium-Small?

Plankton relatively insensitive to temperatures at low latitudes

Critics of small cooling

Food more important than temperature for survival

The Pacific is a difficult region to apply CLIMAPSeafloor sediments poorly preserved

(altered by dissolving)

Drier glacial tropical climate increases lapse cooling rate

Critics of large cooling

from present-day 6.5°C/km toward 9.8°C/km of dry air

Mountain glaciers poorly dated Descent of vegetation due to lower CO2

Where is the truth? Somewhere between 1.5°C (CLIMAP) and 5°C (land evidence)

Relevance of Glacial Tropical Temperatures to Future Climate

Lower values of greenhouse gases caused glacial tropical cooling (1.5 to 5ºC); how large the future warming will be in response to large increases in greenhouse gases?

This range matches the range of uncertainty about Earth’s CO2 sensitivity simulated by GCMs

Tropical cooling between 1.5°C (CLIMAP) and 5°C (land evidence)

CO2 = 190 ppmv (LGM) 280 ppmv (preindustrial, 47% higher) 381 ppmv (in 2006) ? (by 2100)CH4 = 350 ppbv (LGM) 700 ppbv (preindustrial 100% higher) 1751 ppbv (in 2006) ? (by 2100)

Greenhouse gases by 2100: doubling of the preindustrial values?

Orbital-Scale Changes in CO2

Maxima: 280-300 ppm

Minima: 180-190 ppm

Vostok Ice in Antarctica

Four 100,000-year cycles23,000-year cycle not prominent

Major CO2 cycles match marine δ18O (ice volume) cycles in an overall sense

Which is driving which?

Low accuracy in dating in Antarctica Difficulties:

Dust reacts with CO2 bubbles in Greenland

• The strength of tropical monsoons

• The warmth of northern high-latitude summers

Climate Change in the Last 7,000 Years

• Vegetation responses

• Bedrock rebounding and sea level fall

Causes of Climate Change Since Deglaciation

21k yrs agoClimate controls:

Low CO2Large ice sheets

21-6k yrs ago Increasing CO2Increasing summer insolation

6-0k yrs ago Decreasing summer insolation High CO2

Stronger, Then Weaker Monsoons

High lake levels in the north tropics 9000 years ago

Warmer, Then Cooler North Polar Summers

Pollen in lake sediments indicates northward large-scale shifts in spruce and oak.

Yearly Temperature Change for the Last 2000 YearsYearly Temperature Change for the Last 2000 Years

Data from Data from tree rings, corals, ice cores, and historical recordstree rings, corals, ice cores, and historical records are are shown in shown in various colorsvarious colors. . ThermometersThermometers data in data in blackblack. .

Red: recent Red: recent estimates;estimates;

Blue: Blue: earlier earlier estimatesestimates

About 1000 y.a., Medieval Warm Period. About 1000 y.a., Medieval Warm Period. Certain regions were Certain regions were warmer than others.warmer than others. Warm and dry summers in England (1000-1300): vineyards flourished and wine was produced. Vikings colonized vineyards flourished and wine was produced. Vikings colonized Iceland and Greenland. Iceland and Greenland.

Global Global WarmingWarming

http://upload.wikimedia.org/wikipedia/commons/b/bb/1000_Year_Temperature_Comparison.png

Yearly Temperature Change Since 1850Yearly Temperature Change Since 1850

Data from Data from thermometersthermometers

19981998

http://commons.wikimedia.org/wiki/Image:Instrumental_Temperature_Record.png

The Earth’s Climate History1. Over the last century, the earth’s surface temperature has increased by

about 0.75°C (about 1.35°F).

2. Little Ice Age = Cooling during 1,400 A.D. – 1,900 A.D. (N.H. temperature was lower by 0.5°C, alpine glaciers increased; few sunspots, low solar output)

3. Medieval Climate Optimum (Warm Period) = Warming during 1,000 A.D. – 1,300 A.D. in Europe and the high-latitudes of North Atlantic (N.H. warm and dry, Nordic people or Vikings colonized Iceland & Greenland)

4. Holocene Maximum = 5,000-6,000 ybp (1°C warmer than now, warmest of the current interglacial period)

5. Younger-Dryas Event = 12,000 ybp (sudden drop in temperature and portions of N.H. reverted back to glacial conditions)

6. Last Glacial Maximum = 21,000 ybp (maximum North American continental glaciers, lower sea level exposed Bering land bridge allowing human migration from Asia to North America)

7. We are presently living in a long-term Icehouse climate period, which is comprised of shorter-term glacial (e.g., 21,000 ybp) and interglacial (e.g., today) periods. There were four periods of Icehouse prior to the current one.

8. For most of the earth’s history, the climate was much warmer than today.