NATS 101 Lecture 34 Anthropogenic Climate Change

Anthropogenic Climate Change The data indicate that global-mean land and sea-surface temperatures have warmed about ~0.6 o C during the last century. Is this the start of a man-made global warming? Two main anthropogenic forcing mechanisms: Greenhouse gas concentrations => rising. Aerosol concentrations => also increasing. We will focus attention on CO 2 increases.

CO 2 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

Global Temperatures and CO 2 There is a very strong relationship between CO 2 levels and past global temperatures. CO 2 levels are now higher than during any period of the past 450,000 years. Will global temperatures responding accordingly?

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.

CO 2 makes the biggest contribution to the climate forcing

US (5% of world population) now causes 20-25% of total pollution.

Projections of Global Warming Coupled Atmo-Ocean models are run for hundreds of years to get future climates We assume continued increases in the levels of greenhouse gasses in the atmosphere. The models have sophisticated physics… But they have coarse grid separations! Atmosphere: 250 km horizontal, 30 levels vertical Ocean: km horizontal, m vertical

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

Global Temperature Outlook Assume CO2 levels rise at current rate of 1% per year until 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 2 o C Fig. 9.3 IPCC Report

Global Precipitation Outlook Fig. 9.3 IPCC Report 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

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

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

2x CO 2 Model Runs Details differ; but many areas of general agreement Warming over the entire globe Warming is larger (~10 o C) near poles, especially Arctic Melting of Arctic ice, rise in mean sea level

2x CO 2 Model Runs Less agreement for rain than for temperature More overall rain, but some regions are drier A consistent signal: Wetter high latitudes, suggesting a poleward shift in the primary agriculture zones

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

Good agreement for CO 2 levels of the past 150 yrs Mandatory before use as global warming model

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

CO 2 Increases 1% per Year Surface Temperatures for GFDL Model 2X CO2 Temperature Animation Remote 2X CO2 Temperature Animation Local Disk BIG File (8.2 MB)!!!

4X CO2 Sea Ice Animation 4X CO2 Sea Ice Local Link

July Temperature over Southeastern U.S. Warms 5-9 o C July Heat Index over Southeastern U.S. Rises 7-14 o C due to increase H 2 O vapor Consistent Signal Warmer Southern U.S. CO 2 Increases 1% per Year

Soil Moisture Much Drier over U.S. 60% Soil Moisture Decrease Higher Evapo-Transpiration Altered Balance between Evaporation-Precipitation Agricultural Implications How do we feed ourselves? CO 2 Increases 1% per Year

Arctic Warms the Most Consistent Model Signal

Arctic Sea Ice Melting Consistent Model Signal

(Excludes Melting of Ice) Sea Level Rises: Consistent Model Signal  h=  T  Depth  h=   T  Depth =  1C  4000m =  1C  4000m =0.8m =0.8m

1 meter 4 meters 2 meters 8 meters

Recovery Weaker Gulf Stream No Recovery

Response Sensitivity to Rate of 2xCO 2 Increase

Oceans Get Warmer Absorb More Heat than Land Consistent Model Signal Southern Hemisphere Warms More Than Northern Hemisphere

Hurricane Intensities Increase by 10% Reason: Warmer Sea Surface Temps Increases in the frequency and/or the intensity of extreme weather events is a likely consequence of global warming. GFDL Super TyphoonGFDL Super Typhoon

Key Points: CO 2 Warming CO 2 levels are rising and will likely double by The greenhouse relationship between higher CO 2 levels and warmer temperatures is indisputable. Even with perfect knowledge of future CO 2 levels, there is significant uncertainty about how much warming would occur and how fast it would occur. Model results suggest a 2 o C global warming, with strongest warming in polar regions, and an overall increase in global precipitation. Shifts in precipitation are much more uncertain.

What Might We Do? Common sense precautionary measures suggest that we begin to reduce emissions before enormous changes to the climate and ecosystems could occur. Greenhouse warming is internationally recognized as a serious problem. Kyoto Protocol is a good start, but the Congress and Administrations have refused to ratify it or to support attempts to curb greenhouse emissions. Consider supporting leaders and organizations who provide vision and can make tough decisions.

What Can YOU Do? Nothing, after all you might be gone by 2070! Or… Insist on Energy Efficient Appliances-Merchandize Limit Heating and Air Conditioning Programmable Thermostats Think Alternative Transportation Bicycles, Walking, Public Transportation Practice Smart Use of Personal Automobiles Carpool, Combine Errands, Lighter Loads, Slow Down Consider Fuel Efficient or Hybrid Vehicles

Assignment for Next Lecture NOVA: “What’s Up with the Weather?”