Chapter 8 Paleoclimate This chapter discusses 1 Proxy

Chapter 8: Paleoclimate This chapter discusses: 1. Proxy data 2. Climate change at different time scales

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

Climate Through Time Late 1990’s 1976 Variations of the Earth’s surface temperature for the past 140 years Temperatures over last 400, 000 years Quelcaya Ice Cap, Andes Mountains, Peru Temperatures 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

1. The last time atmospheric CO 2 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 6 5 million years 10 million PRESENT 55 million years 3. 5 Million years 18, 000 years 1 Million years 230, 000 years 1, 000 years 100, 000 years

The only mammals living at this time were small rodents.

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

Climate Data Tools for studying climate and climate change Data Instrumental measurements (direct) Historical documents Natural recorders of climate or proxy data Climate models Understand climatic cause and effect External factors climate system Feedbacks Test hypothesis Quantitative (put numbers on ideas) and Predict the future

Instrumental Measurements (Direct) Weather Stations • • ~40 years old Stevenson Screen Temperature ~140 years old Automatic Weather Station Satellite

Historical documents The Hunters in the Snow by Pieter Brueghel the Elder (Kunshistorisches Museum, Vienna)

Proxy Records of Climate • Uses of proxy records of climate depend on both - time span of record - resolution of record

Proxy Records of Climate • 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

Tree Rings • Lighter, thicker wood tissue formed by rapid growth in spring and much thinner, darker layers marking cessation of growth in autumn and winter • Limited to land areas outside of tropics • Variations of tree ring width and density act as recorders of year to year changes in temperature and rainfall

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.

Corals multi-celled organisms that build reefs in tropical oceans • Texture of calcite (Ca. CO 3) incorporated in skeletons varies; lighter parts during periods of rapid growth in summer and darker layers during winter • Measurements of oxygen-18 isotope concentration records sea surface temperature and salinity (precipitation and runoff) variations • Limited to tropical oceans

Ice Cores • Limited to polar latitudes and mountain glaciers • Darker and lighter layers are more or less dust blown in seasonally • Measurements provide information on temperature, snowfall, atmospheric composition (gases, dust, volcanic aerosols), sunspots, …

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 groundwater Uranium 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

Marine Sediments • Isotopes in shells of foraminifera can reveal temperature, salinity, and ice volume • Granular debris from land can indicate icebergs breaking off of continental ice sheets, suggesting cold climates

Proxy Records of Climate • Recent times: instrumental • More recent times: historical, tree rings, ice cores • 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

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

Proxy Records of Climate • 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

The Tropical Cooling Debate (21 k yrs ago) • How cold were the glacial tropics? • Evidence for a small tropical cooling • Evidence for a large tropical cooling • Was the actual tropical cooling medium-small? • Relevance of global tropical temperatures to future climate

CLIMAP (Climate Mapping and Prediction) Project Began in 1970 s, published its first map in 1976 and then 1981 LGM August SST Reconstructing the Last Glacial Maximum Mainly based on ocean sediments 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 Ice sheets were too distant. Greenhouse gases must have been a major factor.

Ocean-Based Evidence for a Small Tropical Cooling CLIMAP Distribution of plankton species depends on ocean water temperature. During LGM, high-latitude coldadapted species moved to the tropics a large cooling in the tropics. But … Tropical cooling: 1. 5°C Biochemical Composition Relative abundance of alkenone molecules is sensitive to ocean water temperature. Tropical cooling: less than 2°C Oxygen isotope measurements Difference in δ 18 O values (LGM and today) = difference by ice sheets + difference by ocean temperatures Tropical cooling: 2 -3°C

Land-Based Evidence for a Large Tropical Cooling Drop of the ice line Descent of the lower limit of mountain glaciers by 600 -1000 meters in the tropics. Lapse-rate cooling: 6. 5°C/1000 meters Tropical cooling: 4 -6°C Descent of the upper limit of forests Tropical cooling: 5°C Temperature-sensitive noble gases (xenon, krypton, argon, neon) in groundwater SW USA and SE Brazil cooling: 5°C

Was the Actual Tropical Cooling Medium-Small? Ocean-based evidence: small cooling; Land-based evidence: large cooling Critics of small cooling Plankton relatively insensitive to temperatures at low latitudes Food more important than temperature for survival The Pacific is a difficult region to apply CLIMAP Seafloor sediments poorly preserved (altered by dissolving) Critics of large cooling Drier glacial tropical climate increases lapse cooling rate 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 CO 2 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? CO 2 = 190 ppmv (LGM) 280 ppmv (preindustrial, 47% higher) 381 ppmv (in 2006) ? (by 2100) CH 4 = 350 ppbv (LGM) 700 ppbv (preindustrial 100% higher) 1751 ppbv (in 2006) ? (by 2100) Tropical cooling between 1. 5°C (CLIMAP) and 5°C (land evidence) Greenhouse gases by 2100: doubling of the preindustrial values? This range matches the range of uncertainty about Earth’s CO 2 sensitivity simulated by GCMs

Orbital-Scale Changes in CO 2 Vostok Ice in Antarctica Four 100, 000 -year cycles 23, 000 -year cycle not prominent Maxima: 280 -300 ppm Minima: 180 -190 ppm Major CO 2 cycles match marine δ 18 O (ice volume) cycles in an overall sense Which is driving which? Difficulties: Low accuracy in dating in Antarctica Dust reacts with CO 2 bubbles in Greenland

Climate Change in the Last 7, 000 Years • The strength of tropical monsoons • The warmth of northern high-latitude summers • Vegetation responses • Bedrock rebounding and sea level fall

Causes of Climate Change Since Deglaciation Climate controls: 21 k yrs ago Large ice sheets Low CO 2 21 -6 k yrs ago Increasing summer insolation Increasing CO 2 6 -0 k yrs ago Decreasing summer insolation High CO 2

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 Years Red: recent estimates; Global Warming Blue: earlier estimates Data from tree rings, corals, ice cores, and historical records are shown in various colors. Thermometers data in black. About 1000 y. a. , Medieval Warm Period. Certain regions were warmer than others. Warm and dry summers in England (1000 -1300): vineyards flourished and wine was produced. Vikings colonized Iceland Greenland. http: //upload. wikimedia. org/wikipedia/commons/b/bb/1000_Year_Temperature_Comparison. png

Yearly Temperature Change Since 1850 1998 Data from thermometers http: //commons. wikimedia. org/wiki/Image: Instrumental_Temperature_Record. png

The Earth’s Climate History 1. 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.