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NATS 101-06 Lecture 20 Anthropogenic Climate Change

NATS 101-06 Lecture 20 Anthropogenic Climate Change. Our changing climate: Key Questions. Climate modelers have predicted the Earth’s surface will warm because of manmade greenhouse gas (GHG) emissions So how much of the warming is manmade? How serious are the problems this is creating?

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NATS 101-06 Lecture 20 Anthropogenic Climate Change

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  1. NATS 101-06Lecture 20 Anthropogenic Climate Change

  2. Our changing climate: Key Questions • Climate modelers have predicted the Earth’s surface will warm because of manmade greenhouse gas (GHG) emissions • So how much of the warming is manmade? • How serious are the problems this is creating? • What, if anything, can and should we do?

  3. What is Climate Change? • Climate change - A significant shift in the mean state and event frequency of the atmosphere. • Climate change has been a normal component of the Earth’s natural variability. • Climate change occurs on all time and space scales. • A plethora of evidence exists that indicates the climate of the Earth has changed. • Humans are radically altering the concentrations of greenhouse gases in the atmosphere which is likely causing a new type of climate change

  4. Climate Change: Moving out of Energy Balance • Changing the • Incoming solar energy • Outgoing energy (IR emitted to space) • Internal transport of energy within the Earth;s atmosphere and oceans will cause a shift in climate

  5. Global Energy Balance In a stable climate, Solar Energy IN = IR Energy OUT IR Out Ahrens, Fig. 2.14 Solar in

  6. Global Energy Imbalance Increasing GHG concentrations decrease the IR Energy out So Energy IN > Energy OUT and the Earth warms IR Out is reduced Ahrens, Fig. 2.14 Solar in Atmosphere

  7. Change in IR Emission to Space • Notice that because of the greenhouse gases in Earth’s atmosphere, 91% (=64/70) of the IR emitted to space comes from the atmosphere and only 9% (=6/70) comes from Earth’s surface • When additional GHG’s are added to the atmosphere, the altitude of IR emission to space rises and less IR from the surface makes it to space • Since air temperature in the troposphere decreases with altitude, the temperature of the emission to space decreases • Therefore Earth’s energy emission to space decreases because the IR energy flux emitted by an object decreases with decreasing temperature • Therefore total Energy flux IN > Energy flux OUT • Earth warms until Energy flux IN = Energy flux OUT

  8. Change in IR Emission to Space BEFORE GHG increase: IN=OUTAFTER GHG increase IR emission to space 3. IR emission to space decreases because of colder emission temperature SH NH Altitude of IR emission to space Ahrens, Fig. 2.21 1. Altitude of IR emission to space rises Altitude Temperature of IR emission to space 2. Temperature of IR emission to space decreases Temperature Temperature

  9. Change in IR Emission to Space (cont’d) AFTER GHG increase IN>OUT Eventual solution IN=OUT 6. IR emission to space increases until it matches the original IR emission before GHG increases 3. IR emission to space decreases because of colder emission temperature SH SH Ahrens, Fig. 2.21 1. Altitude of IR emission to space rises Ahrens, Fig. 2.21 4. Atmosphere warms until… 2. Temperature of IR emission to space decreases 5. Temperature of IR emission to space increases to original temperature Temperature Temperature

  10. Change: Atmo IR Emission to Surface BEFORE GHG increase: AFTER GHG increase Energy DOWN=UP at surface more IR emission into surface Atmospheric IR emission to surface SH NH Ahrens, Fig. 2.21 1. Altitude of IR emission to surface decreases Altitude Altitude of IR emission to surface Temperature Temperature 2. Temperature of IR emission to surface increases Temperature of IR emission to space 3. Atmo IR emission to surface increases because of warmer emission temperature

  11. Out-of-Equilibrium Issues • Land temperature increases faster than Ocean temperature • Atmospheric lifetime of CO2 is ~50 years • System response is NOT instantaneous • System takes a while to heat up in response to increase in CO2 • Initial temperature increase is smaller than final

  12. Anthropogenic Climate Change The data indicate that global-mean land and sea-surface temperatures have warmed about ~0.6oC during the last 35 years. Is this the early stages of a man-made global warming? Two main anthropogenic forcing mechanisms: Greenhouse gas concentrations => rising. Aerosol concentrations => also increasing. We will focus attention on CO2 increases.

  13. Greenhouse Gas Concentrations • The two dominant greenhouse gases are H2O and CO2. • There are several other GHGs with smaller concentrations: O3, CH4, N2O • H2O vapor is categorized separately because it is controlled indirectly by evaporation and condensation and changes in response to other changes in climate (temperature, circulation)

  14. Absorption Visible IR 20% of incident Visible (0.4-0.7 m) is absorbed O2 an O3 absorb UV (shorter than 0.3 m) Infrared (5-20 m) is selectively absorbed H2O & CO2 are strong absorbers and emitters of IR Little absorption of IR around 10 m – atmospheric window Ahrens, Fig. 2.9

  15. Global Warming Potential (GWP) Different gases has different warming potentials which are defined relative to the warming effect of CO2 GasGWP Carbon dioxide (CO2) 1 Methane (CH4) 21 Nitrous oxide (N2O) 310 Hydrofluorocarbons 560-12,100 Perfluorocarbons 6,000-9,200 Sulfur hexafluoride 23,900 Ahrens, Fig 2.10

  16. CO2 makes the biggest contribution to the climate forcing

  17. During last ice age (>18,000 years ago) Temps 6oC colder CO2 levels 30% lower CH4 levels 50% lower H2O levels were lower than current interglacial. 135,000 years ago it was a bit warmer than today 50% increase in CO2 was associated with 6-8oC increase in temperature 6-8oC decrease in temperature produced incredibly different climate: Ice Age Ice Core from Vostok, Antarctica

  18. ChangingCO2 concentrations • CO2 concentrations have varied naturally by ~30-50% over the past few hundred thousand years (ice ages) • Fossil fuel burning since the industrial revolution has created a recent sharp increase in CO2 concentrations • CO2 concentrations are now higher than at any time in past few hundred thousand years • And concentrations are increasing faster with time Last 4 Ice Age cycles: 400,000 years Man made You are here See http://epa.gov/climatechange/science/recentac.html

  19. ChangingCO2 concentrations • CO2 concentrations measured very precisely since 1958 • Over past 45 years they’ve increased by ~21% • Present 0.6%/yr increase You are here Seehttp://www.cmdl.noaa.gov/ccgg/trends/co2_data_mlo.php

  20. US (5% of world population) now causes 24% of total pollution. Seehttp://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions

  21. Currently, 7 gigatons per year of CO2 are injected into the air by burning fossil fuels (80%) and forests (20%). • Half accumulates in atmosphere, where it resides for 50+ years. • If the burning fossil fuels and forests totally ceased, it would still take 50 years for CO2 levels to return to 50% above pre-industrial levels.

  22. Missing Carbon Sink • CO2 is accumulating in the atmosphere more slowly than expected (believe it or not) • Based on our understanding of CO2 emissions and ocean and atmosphere uptake, there is a missing sink/uptake of about 25% NASA OCO mission

  23. Natural Variations in Climate

  24. Milankovitch Theory of Ice Ages • Attempts to explain ice ages by variations in orbital parameters • Three cycles: Eccentricity (100,000 yrs) Tilt (41,000 yrs) Precession (23,000 yrs) • Changes the latitudinal and seasonal distributions of solar radiation.

  25. Milankovitch Theory of Ice Ages • Most recent Ice Ages have occurred for past 2 million years • Ice Ages occur when there is less radiation in summer to melt snow. • Partially agrees with observations, but many questions unanswered. What caused the onset of the first Ice Age?

  26. MilankovitchTheory Change in daily solar radiation at top of atmosphere at June solstice Changes as large as ~15% occur

  27. There is a very strong relationship between CO2 levels and past global temperatures. CO2 levels are now higher than during any period of the past 450,000 years. Will global temperatures responding accordingly? Global Temperatures and CO2

  28. 250 million years ago, the world’s landmasses were joined together and formed a super continent termed Pangea. As today’s continents drifted apart, they moved into different latitude bands. This altered prevailing winds and ocean currents. NA E-A NA E-A India Af SA Aus Af SA India Aus Ant Ant 180 M BP Today Ahrens, Fig 13.6 Long-Term Climate Change

  29. Long-Term Climate Change • Circumpolar ocean current formed around Antarctica 40-55 MY ago once Antarctica and Australia separated. • This prevented warm air from warmer latitudes to penetrate into Antarctica. • Absence of warm air accelerated growth of the Antarctic ice sheet. http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

  30. Circumpolar seaway leads to large latitudinal temperature gradient. Circum-equatorial seaway leads to small latitudinal temperature gradient Long-Term Climate Change http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

  31. Climate System Feedbacks

  32. CO2 and the Greenhouse Effect If the atmosphere were dry, we could predict with high confidence that a doubling of CO2 (likely before 2100) would increase the global mean surface temperature by ~2C. The presence of oceans, ice, water vapor and clouds complicates the analysis significantly. Ahrens, Fig 2.10

  33. Complexity of Climate System The climate system involves numerous, interrelated components.

  34. Closer Look at Climate System

  35. Climate Feedback Mechanisms

  36. Climate System Feedbacks • Feedbacks are what makes the climate problem so tricky • Positive: Increase the impact of the original change • Negative: Reduce the impact of the original change EXAMPLES • Ice albedo (Positive Feedback): • Temperatures increase because of increased CO2 • Ice melts • Reduces Earth’s reflectivity (albedo) • Increases sunlight absorbed by the Earth • Increases warming of the Earth

  37. Positive and Negative Feedbacks • Assume that the Earth is warming. - Warming leads to more evaporation from oceans, which increases water vapor in atmosphere. -More water vapor increases absorption of IR, which strengthens the greenhouse effect. -This raises temperatures further, which leads to more evaporation, more water vapor, warming… “Runaway Greenhouse Effect” Positive Feedback Mechanism

  38. Positive and Negative Feedbacks • Again assume that the Earth is warming. - Suppose as the atmosphere warms and moistens, more low clouds form. - More low clouds reflect more solar radiation, which decreases solar heating at the surface. - This slows the warming, which would counteract a runaway greenhouse effect on Earth. Negative Feedback Mechanism

  39. Cloud Feedback • Clouds affect both solar & IR radiation • Clouds increase solar reflection (albedo) cooling the Earth • Clouds absorb and emit IR radiation like a GHG • Currently their net effect is to cool the Earth • Solar effect > IR effect • Not clear what they will do in the future • If the amount of low clouds increases, the albedo effect wins and they cool the Earth (Negative feedback) • If the amount of high clouds increases, the IR effect wins and the Earth warms (Positive feedback) • There are other subtler cloud feedbacks as well involving aerosols in particular

  40. Positive and Negative Feedbacks • Atmosphere has a numerous checks and balances that counteract climate changes. • All feedback mechanisms operate simultaneously. • All feedback mechanisms work in both directions. • The dominant effect is difficult to predict. • Cause and effect is very difficult to prove at the “beyond a shadow of a doubt” level.

  41. Climate Model Predictions

  42. Projections of Global Warming • Atmosphere and coupled Atmo-Ocean models are run for hundreds of years to simulate future climates • They assume continued increases in the levels of greenhouse gasses in the atmosphere. • The models have sophisticated physics… But they have coarse spatial grid separations! Atmosphere: 250 km horizontal, 30 levels vertical Ocean: 125-250 km horizontal, 200-400 m vertical

  43. Performance Varies by Weather Element In general… Excellent for Surface Temperature Skillful for Sea-Level Pressure (SFC Winds) Marginal Skill for Precipitation How Well Do Models Capture Current Observed Climate? Fig. 8.4 IPCC Report

  44. Assume CO2 levels rise at current rate of 1% per year until 2070. Good agreement for past climate and CO2 levels leads to high confidence. Rather close agreement among models. Consensus of several model runs indicates an average warming of 2oC Global Temperature Outlook Fig. 9.3 IPCC Report

  45. Marginal performance for past climate and CO2 levels means low confidence in outlook. Large differences exist among models. Consensus of several model runs indicates an average increase of 2% in global precipitation Global Precipitation Outlook Fig. 9.3 IPCC Report

  46. Regional Consistency: Warming Fig. 10.1.1 IPCC Report A + or - symbol denotes 7 out of 9 models agree. A2: no sulphate aerosols; B2: has sulphate aerosols

  47. Regional Consistency: Rainfall Fig. 10.1.2 IPCC Report A + or - symbol denotes 7 out of 9 models agree. A2: no sulphate aerosols; B2: has sulphate aerosols

  48. GDFL Model Lets look at some details from a 500-year simulation by a specific climate model. Examine Two Scenarios: 1% CO2 increase per year for 70 years 2X Total Increase in CO2 1% CO2 increase per year for 140 years 4X Total Increase in CO2

  49. Good agreement for CO2 levels of the past 150 yrs Mandatory before use as global warming model www.gfdl.noaa.gov

  50. CO2 Increases 1% per Year Average Global Surface Temperatures… Warm 2oC for 2X CO2 Warm 4o C for 4X CO2 Sea Level Rises… Equilibrium Not Reached until after 500 years North Atlantic Ocean… Circulation Weakens www.gfdl.noaa.gov

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