Chapter 13: The Earth’s Changing Climate Climate change Climate change Possible causes of climatic change Possible causes of climatic change Global warming Global warming 1

The Earth’s Changing Climate The earth’s climate is always changing 18,000 years ago the 18,000 years ago the sea level was so low sea level was so low that the English that the English Channel didn’t Channel didn’t even exist, and Bering land bridge did exist even exist, and Bering land bridge did exist 18,000 years ago 2

Determining Past Climates 1) fossil evidence 2) Ocean floor sediment oxygen isotope ratio (of oxygen 18 over oxygen 16) Isotopes are atoms whose nuclei have the same number of protons but different numbers of neutrons.Isotopes are atoms whose nuclei have the same number of protons but different numbers of neutrons. This ratio is higher during a colder climate (as the oceans contain less water)This ratio is higher during a colder climate (as the oceans contain less water) 3) dendrochronology- tree ring growth depends on temperature and precipitation over certain regions 3) dendrochronology- tree ring growth depends on temperature and precipitation over certain regions Q: Which university established the dendrochronology field first? a) U. of Arizona; b) Harvard; c) Stanford; d) Yale 3

Determining Past Climates sediment cores: ocean, lake sediment cores: ocean, lake ice cores: Antarctic, Greenland ice cores: Antarctic, Greenland borehole temperature profiles borehole temperature profiles Dating calcium carbonate layers of stalactites in caves Dating calcium carbonate layers of stalactites in caves As the global warming debate has intensified in recent years, many methods of reconstructing past climates have undergone close scrutinyAs the global warming debate has intensified in recent years, many methods of reconstructing past climates have undergone close scrutiny Q: What is the main uncertainty of these proxy data in representing temperature? a) relation is not linear b) non-temperature effects are not easy to separate c) correlation is not causality d) all of them 4

Climate During Past 1000 Years and since 1860 “Hockey Stick” graph “Hockey Stick” graph The global warming of the past 100 years has not been constant Q: is the temperature decrease from contradictory with global warming? a) yes; b) no 5 Relative to Relative to

Climate Change and Feedback Mechanisms Q: The water vapor-greenhouse feedback is? a) positive feedback; b) negative feedback c) unknown Q: Why is the snow-albedo feedback positive? A: increasing T decreases snow cover; this decreases surface albedo and hence increases surface solar absorption; leading to further increasing T Q: why is cloud feedback overall negative? A: low clouds are negative by reflecting solar radiation; high clouds are positive just like greenhouse gases; overall low clouds are more dominant, leading to overall negative feedback Feedbacks cause climate changes to be either amplified (positive feedback) or reduced (negative feedback).Feedbacks cause climate changes to be either amplified (positive feedback) or reduced (negative feedback). 6

Possible Causes of Climate Change external: changes in incoming solar radiation external: changes in incoming solar radiation external (e.g., volcano) or human (e.g., CO2): changes in the composition of the atmosphere external (e.g., volcano) or human (e.g., CO2): changes in the composition of the atmosphere external (e.g., mountain uplifting) or human (e.g., land use in urban and agricultural areas): changes in the earth’s surface external (e.g., mountain uplifting) or human (e.g., land use in urban and agricultural areas): changes in the earth’s surface Emissions of CO 2 and other greenhouse gases are by no means the only way to change the climate.Emissions of CO 2 and other greenhouse gases are by no means the only way to change the climate. 7

Climate Change and Atmospheric Particles (decadal-century) Sulfate aerosols cool the climate and therefore counteract global warming to some extent.Sulfate aerosols cool the climate and therefore counteract global warming to some extent. Q: why does volcano eruptions, rich in sulfur, cool the atmosphere in general? A: sulfur aerosols reflect solar radiation; they also modify clouds by increasing the cloud’s reflectivity 8

Climate Change and Variations in Solar Output (decadal-century) Sunspots are huge magnetic storms on the sun that show up as cooler (darker) regions on the sun’s surface Sunspots are huge magnetic storms on the sun that show up as cooler (darker) regions on the sun’s surface Q: why does solar radiation increases with the increase of sunspot numbers? A: sunspots are cooler; but the surrounding margins of sunspots are brighter (i.e., hotter); overall effect is to increase solar radiation 9

Climate Change and Variations in the Earth’s Orbit (10K-100K years) Milankovitch theory: - eccentricity: geometry of orbit (100K yr) - obliquity: tilt angle of axis (41K yr) Milankovitch theory: - eccentricity: geometry of orbit (100K yr) - obliquity: tilt angle of axis (41K yr) - precession: direction of axis (23K yr) - precession: direction of axis (23K yr) Min: 22 o Max: 24.5 o 10

Climate Change, Plate Tectonics, and Mountain-Building (100M yr) theory of plate tectonics theory of plate tectonics Landmasses 150M years ago Today Earth’s land mass in 650 million years: 11

Q: what are the external causes of climate change? a) solar radiation change; b) air composition change; c) earth’s land mass change; d) all of them Q: How do major volcanoes change surface temperature? a) decrease; b) no change; c) increase Q: Since the sunspot number is below average in 1986, the solar radiation that year is a) below average; b) normal; c) above average 12

Rising atmospheric temperature Rising sea level Reductions in NH snow cover And oceans.. And upper atmosphere…. Warming is Unequivocal 13

Human and Natural Drivers of Climate Change CO2 is causing the bulk of the forcing On average, it lives > 100 years in the atmosphere and therefore affects climate over long time scales. 14

Natural versus human-caused temperature increase Climate models are needed: Climate models are needed: aerosols; cloud; precipitation; land processes; …. aerosols; cloud; precipitation; land processes; …. 15

Understanding and Attributing Climate Change Anthropogenic warming is likely discernible on all inhabited continents Observed Expected for all forcings Natural forcing only 16

What’s in the pipeline and what could come Warming will increase if GHG increase. If GHG were kept fixed at current levels, a committed 0.6°C of further warming would be expected by More warming would accompany more emission. 17 Relative to

A1B is a typical “business as usual” ( ) scenario: Global mean warming 2.8 o C; Much of land area warms by ~3.5 o C Arctic warms by ~7 o C; would be less for less emission 18 Relative to Relative to