The Coupled Climate System

Transcription

1 Introduction to Climatology GEOGRAPHY 300 The Coupled Climate System Tom Giambelluca University of Hawai i at Mānoa Climate Change Defining Climate Change Changes in a system can be ascribed to changes in: Boundary conditions Internal characteristics However a given change in boundary or external conditions will not always produce the same climate response. Climates of the Past Ice Ages Earth's climate history reveals alternating warm and cold periods, with peaks roughly 150 million years apart. Note that the the graph below shows time going back from right to left. 1

2 Climates of the Past Ice Ages Here the graph is oriented more conventionally with time proceeding from left to right. But note that the scale increases as you move closer to the present. Climates of the Past Ice Ages We are currently in one of the cold periods or ice ages, which began with gradual cooling that started about 55 MYA. Climates of the Past Ice Ages We are currently in one of the cold periods or ice ages, which began with gradual cooling that started about 55 MYA. Geological Time Scale Earth is about 4.54 billion years old. 2

3 Climates of the Past Quaternary Period The most recent (current) geological period within the Cenozoic Era, beginning around 2.6 million years ago. Pleistocene: Period of repeated glaciations, 2.6 million to 12,000 years ago. System Quaternary Holocene: Relatively warm period since 12,000 years ago. Subdivisions of the Quaternary Period Series Holocene Pleistocene Ionian (Middle) Stage Tarantian (Upper) Calabrian (Lower) Gelasian (Lower) Age (Ma) Climates of the Past Pleistocene Epoch During the Pleistocene Epoch, periods of cold climate, called glacials, and warm climate, called interglacials, alternated on a roughly 100,000- yr cycle. The most recent Pleistocene glacial peaked around 18,000 years ago. Anthropocene: Some scientists recognize the past years as a new geological epoch. Others suggest that this period started 8,000 years ago with the expansion of agriculture. Paleoclimatology Examples of Proxy Data The study of past climates using non-instrumental records, including: 1. oral and written histories of extreme weather, crop failure, famine, floods, droughts, commodity price fluctuations, etc., 2. biological evidence, including live tree rings, fossil tree rings, fossil pollen, coral layers, marine sediments 3. ice cores, glacial ice deposited as annual layers which trap bubbles of air (atmospheric samples), organisms, and other material; stable water isotopes used to estimate temperature 4. geological evidence, including evidence of glaciation, evidence of inundation, sediments 5. isotopic evidence, radioisotopes used for dating other evidence; ratios of stable water isotopes indicate temperature; other isotope ratios used for a variety of purposes Pollen Tree Rings Ice Cores 3

4 Tree Ring Data Tree Ring Data Using an increment borer to obtain a core for tree ring analysis Bristlecone Pines are the oldest known living trees, up to 5000 years old Extended continuous records can be constructed from overlapping life spans of trees, a process called cross-dating Only certain areas have been sampled for tree ring chrologogies Ice Core Data Causes of Climate Change The climate is highly variable on all space and time scales. But what causes sustained changes over time? Solar Output Sun spots: Ice cores taken during experimental ice core drilling in Greenland, 2005 (Riebeek, 2006) Source: NOAA Arctic Theme Page 4

5 Causes of Climate Change Solar Output Sun spots: Maunder Minimum ( ) coincides with the Little Ice Age Causes of Climate Change Solar Output Faint Early Sun Paradox: Sun's output has increased by about 1/3 since formation of solar system, but Earth surface temperatures were relative high during early times. This paradox was first framed by Carl Sagan and George Mullen in 1972, and focused on the contradiction between low solar radiation and the evidence that liquid water existed on the earth 4 billion years ago. The paradox can be resolved by taking into consideration the changing atmospheric gas composition. The early atmosphere has low in oxygen and rich in methane. As life evolved, oxygen became abundant, which facilitated conversion of method into CO2. Methane is a much more potent greenhouse gas than CO2. Causes of Climate Change Orbital Changes Characteristics: Causes of Climate Change Orbital Changes: Milankovitch Cycles Eccentricity: elliptical shape of earth's orbit Obliquity: tilt of earth's axis Precession: timing of equinox or solstice with respect to aphelion 5

6 Causes of Climate Change Causes of Climate Change Orbital Changes: Milankovitch Cycles Berger's (1991) solution for 65 degrees north latitude from the present to 1 million years ago. In the Northern Hemisphere, peak summer insolation occurred about 9,000 years ago when the last of the large ice sheets melted. Since that time Northern Hemisphere summers have seen less solar radiation. Internal Forcing Factors: Changes in land configuration: continental drift Changes in surface characteristics: deforestation, desertification influences on albedo and ET Changes in atmospheric turbidity: increasing aerosol concentrations; volcanic eruptions Changes in radiatively active gases: early faint sun paradox; recent greenhouse gas increases Causes of Climate Change Possible Feedback Mechanisms: Ice-Albedo Feedback (Positive Feedback) Water Vapor Feedback (Positive Feedback) Cloud-Albedo Feedback (Negative Feedback) Cloud-LW Absorption Feedback (Positive Feedback) Ocean-Atmosphere Interaction Land-Atmosphere Interaction Global Warming Because of increasing concentrations of greenhouse gases in the atmosphere, concerns were raised among scientists that the global average air temperature near the surface would increase. Evidence now strongly points to enhanced greenhouse warming as the cause of the observed increases in global average temperature over the past century and a half. The scientific debate has shifted from the question of whether or not global warming is occuring to what are the consequences of various options to combat (mitigation) and adjust to (adaptation) future climate changes. 6

7 Intergovernmental Panel on Climate Change (IPCC) International scientific body charged with evaluating the risk of climate change caused by human activity. Established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) 1990: First Assessment Report (FAR) 1992: Supplementary Report 1995: Second Assessment Report (SAR) 2001: Third Assessment Report (TAR) 2007: Fourth Assessment Report (AR4) Shared 2007 Nobel Peace Prize with Al Gore Climate Change Deniers Despite scientific consensus, some segments of our society continue to question the validity of contemporary global warming. The group of so-called "global warming skeptics" includes people and organizations, such as: Rush Limbaugh Fox News Wall Street Journal Exxon-Mobile Fred Singer (Univ. of Virginia) Bill Gray (Colorado State Univ.) See Greenpeace-sponsored web site EXXONSECRETS.ORG, which exposes the connections between phony climate science and oil industry funding. Global Warming: The Facts 1. Greenhouse gases, such as CO 2 (carbon dioxide) and CH 4 (methane), cause air temperature near the surface to be higher. Global Warming: The Facts 2. Greenhouse gas concentrations are increasing. MLO: 7

8 Global Warming: The Facts 2. Greenhouse gas concentrations are increasing. Global Warming: The Facts 2. Greenhouse gas concentrations are increasing. CO 2 increases are seen in both hemispheres, compensating decrease in O 2 also observed, global carbon emissions trend matches observed changes in ratio of carbon isotopes, identifying human activities as the source of increasing CO 2 concentrations: Global methane concentration has also been increasing. The rate of increase (bottom panel) has been declining, but large upspike was observed this year (not shown): Global Warming: The Facts 3. Mean global air temperature is increasing. Global Warming: The Facts 3. Mean global air temperature is increasing. The pace of temperature increase is increasing Observed Mean Global Temperature Change Period: Rate ( o C per decade) Past 150 years: Past 100 years: Past 50 years: Past 25 years:

9 Global Warming: The Facts 4. Air temperature increase is global. Both hemispheres: Global Warming: The Facts 5. Global and hemispheric sea surface temperature (SST) is increasing. Global Warming: The Facts 6. Global and hemispheric sea surface temperature (SST) is increasing. Global Warming: The Facts 7. Oceanic dissolved CO 2 is increasing and oceanic ph is decreasing (sea water is acidifying). 9

10 Global Warming: The Facts 8. Mean global sea level is rising. Important Scientific Questions How fast will temperature increase over the coming decades? To what extent will the cooling effects of increased aerosols offset greenhouse warming? How will caused by decreasing ice and snow cover and increasing cloud cover affect the rate of temperature change? How will extreme events (droughts, floods, severe storms) change in terms of frequency or severity? How will regional weather (temperature, precipitation, storm frequency) change? What will be the regional impacts of global warming? Which nations will suffer/benefit the most? Other Important Quesions How can we best respond to global warming? Whom should pay the costs? Modeling Climate Processes The Coupled Climate System Heated by incoming shortwave radiation, strongest at low latitudes Cooled by outgoing longwave radiation, predominant at high latitudes Latitudinal gradient drives large-scale atmospheric and oceanic circulations. Incomplete understanding of feedbacks cause uncertainties about the effects of changes within the system 10

11 Modeling Climate Processes Components of the Climate System Modeling Climate Processes Biogeochemical Cycles atmosphere ocean cryosphere biosphere geosphere nitrogen cycle oxygen cycle carbon cycle phosphorus cycle sulfur cycle water cycle hydrogen cycle Modeling Climate Processes Biogeochemical Cycles Modeling Climate Processes Biogeochemical Cycles nitrogen cycle carbon cycle 11

12 Approaches to predicting changes in the climatic system Palaeo-analogue method Coupled numerical general circulation models GCMs Palaeo-analogue method Approach: Identify periods in the past in which conditions were similar to those for which the prediction is needed. Uses: Estimate global temperature sensitivity to carbon dioxide increases Examine regional climatic patterns associated with higher or lower global average temperatures. Problems: Uncertainty of reconstructions Extending limited data to global scale Interpreting effects of changing orography and equilibrium vs. non-equilibrium conditions Determining the relative influences of various factors that caused past changes in climate Numerical Modeling Approach Equilibrium climate change Time dependent climate change Time Dependent Climate Simulation Strategy: Equilibrium climate: the mean climate after a long period of without change in boundary conditions or internal characteristics; radiative balance. The current climate system is not in equilibrium due to changes in GHG and surface characteristics. So why do equilibrium experiments? Simpler GCMs (requiring less computer time) ignore ocean circulation changes and deep ocean interaction, both of which must be included in time-dependent simulations. Easier to compare equilibrium experiments. Good for doing "sensitivity studies" to see the relative effects of different factors. Can be scaled to approximate time-dependent change. (IPCC, 2007) 12

13 Coupled numerical general circulation models (GCMs) How they work GCMs Grid points, layers, time steps Movement and distribution of mass calculated using differential equations solved numerically Four basic equations: Conservatrion of energy (1st law of thermodynamics) Conservation of momentum (Newton's second law of motion) Conservation of mass Ideal gas law GCMs GCMs Model components and interactions Radiative exchange Atmospheric dynamics Atmospheric Chemistry Carbon Cycle Clouds Ocean Ice Land Surface--Vegetation Issues Sub-grid-scale processes (e.g. convection) Prescribed vs. interactive process Horizontal and vertical resolution Emissions scenarios Flux correction Regional models Data assimilation Reanalysis 13

14 GCM Results CO2 doubling experiments: climate sensitivity 2 o C to 4.5 o C, most likely about 3 o C Time-Dependent Changes To run time-dependent simulations, the timing of future increases in greenhouse gas concentrations needs to be estimated This requires future emissions time series Future emissions cannot be predicted Instead, emissions scenarios are used Emissions Scenarios SRES: Special Report on Emission Scenarios Scenario Families Story lines Emissions Scenarios A1 The A1 scenarios are of a more integrated world. The A1 family of scenarios is characterized by: Rapid economic growth. A global population that reaches 9 billion in 2050 and then gradually declines. The quick spread of new and efficient technologies. A convergent world - income and way of life converge between regions. Extensive social and cultural interactions worldwide. There are subsets to the A1 family based on their technological emphasis: A1FI - An emphasis on fossil-fuels. A1B - A balanced emphasis on all energy sources. A1T - Emphasis on non-fossil energy sources. 14

15 Emissions Scenarios A2 The A2 scenarios are of a more divided world. The A2 family of scenarios is characterized by: A world of independently operating, self-reliant nations. Continuously increasing population. Regionally oriented economic development. Slower and more fragmented technological changes and improvements to per capita income. Emissions Scenarios B1 The B1 scenarios are of a world more integrated, and more ecologically friendly. The B1 scenarios are characterized by: Rapid economic growth as in A1, but with rapid changes towards a service and information economy. Population rising to 9 billion in 2050 and then declining as in A1. Reductions in material intensity and the introduction of clean and resource efficient technologies. An emphasis on global solutions to economic, social and environmental stability. Emissions Scenarios B2 The B2 scenarios are of a world more divided, but more ecologically friendly. The B2 scenarios are characterized by: Continuously increasing population, but at a slower rate than in A2. Emphasis on local rather than global solutions to economic, social and environmental stability. Intermediate levels of economic development. Less rapid and more fragmented technological change than in B1 and A1. Emissions Scenarios Standard Scenarios Used in AR4 Simulations: A2 A1B B2 15

16 Simulated Temperature Change Simulated Temperature Change (IPCC, 2007) (IPCC, 2007) Simulated Hydrological Change Simulated Change in Sea Ice Extent (IPCC, 2007) (IPCC, 2007) 16

17 Simulated Change in Weather Extremes Important Questions about What to Do Response: Mitigation vs. Adaptation Who pays for reducing emissions? (IPCC, 2007) 17