Radiative Forcing Definition: A change in the net radiation at the top of the atmosphere due to some external factor.

Net Radiation Net radiation = Incoming - Outgoing Positive net radiation Incoming > Outgoing Incoming > Outgoing Negative net radiation Outgoing > Incoming Outgoing > Incoming

Positive & Negative Forcing Positive forcing  warming Negative forcing  cooling

Forcing and Feedbacks “ Forcing” is produced by an external process, e.g. Changes in solar flux Changes in solar flux Volcanic eruptions Volcanic eruptions Human actions Human actions A feedback is a response to temperature changes Example: Increased water vapor due to warming Anthropogenic increases in greenhouse gases are considered forcings Increases in greenhouse gases that are caused by temperature changes are feedbacks

The same gas can be involved in forcings and feedbacks, e.g., CO 2 Forcing: CO 2 increase from burning of fossil fuels CO 2 increase from burning of fossil fuels Largest – by far: increased greenhouse gases Largest – by far: increased greenhouse gases Increase is almost entirely anthropogenic Increase is almost entirely anthropogenicFeedback  temp   decay   CO 2  temp   decay   CO 2

Initial Equilibrium Absorbed Shortwave OLR Top of atmosphere Now, add greenhouse gas Keep temperatures fixed

Reduced Upward Flux Absorbed Shortwave OLR Top of atmosphere

Net Downward Flux Net Flux Top of atmosphere Result: A positive radiative forcing

Negative Radiative Forcings Largest: Increase in sulfate aerosols Mostly anthropogenic Mostly anthropogenic

Effect of Anthropogenic Sulfate Aerosols on Temperature Direct effect The aerosols themselves reflect sunlight The aerosols themselves reflect sunlight This is similar to the effect of volcanic aerosols This is similar to the effect of volcanic aerosols Indirect effect Sulfate aerosols act as condensation nuclei Sulfate aerosols act as condensation nucleicondensation nucleicondensation nuclei This increases the droplet concentration in clouds This increases the droplet concentration in clouds Result: Increased cloud albedo Result: Increased cloud albedo Both effects tend to increase the Earth’s albedo

Solar Irradiance Some evidence suggests solar irradiance may have increased lately Current estimate of forcing: very small Note: Evidence is very weak!

Climate Sensitivity The change in equilibrium temperature per unit of radiative forcing

Temperatur e Time Start in equilibrium Apply radiative forcing Temp. rises Change in equilibri um temp New Equilibrium Temp

Example Suppose Sensitivity = 2  C per unit of forcing (1 Wm -2 ) Radiative forcing = 3 Wm -2 Radiative forcing = 3 Wm -2 Then, eventual warming = 2 x 3 = 6  C

Differing Sensitivities Same radiative forcing applied at t= 0 System 2 is twice as sensitive 1C1C 2C2C

Comparing Models Double CO 2 content of model atmosphere Radiative forcing ~ 4 W/m 2 Radiative forcing ~ 4 W/m 2 IPCC has compared many climate models Results used to estimate actual climate sensitivity of Earth

Sensitivity Estimates Sensitivity Estimates Model sensitivities have a range of 2  C to 4.5  C for a doubling of CO 2 Model sensitivities have a range of 2  C to 4.5  C for a doubling of CO 2 (A technical point – don’t memorize.) (A technical point – don’t memorize.)

The Role of Feedbacks Model sensitivity is determined by the strength of the feedbacks in the model Positive feedbacks increase sensitivity Negative feedbacks decrease sensitivity

Differences in Model Sensitivity Main Cause of Variation: Cloud Feedbacks In most models, cloud feedback is positive However, magnitude varies a lot from one model to another However, magnitude varies a lot from one model to another

Thermal Inertia Determines rate of temperature change

Rate of Warming Thermal inertia: resistance of system to temp. change Measured by heat capacity Measured by heat capacity Higher heat capacity  slower warming

System 1: 70% of warming has occurred at t = 1.2 Time Temperature Change (  C) System 2: 70% of warming has occurred at t = 2.4

Earth-Atmosphere System Most of the heat capacity is in oceans Presence of oceans slows down warming

Comparison Look at two systems with same radiative forcing and sensitivity, but different heat capacities

Summary Positive (negative) radiative forcing causes warming (cooling) System warms (cools) until equilibrium is restored Amount of eventual warming (cooling) depends on radiative forcing and sensitivity Eventual warming (cooling) = sensitivity x rad. forcing Eventual warming (cooling) = sensitivity x rad. forcing Rate of warming is inversely proportional to heat capacity

More Realistic Situation Previous examples assumed radiative forcing applied instantaneously i.e., all GHG & aerosols added instantaneously i.e., all GHG & aerosols added instantaneously Real life: GHG & aerosols added gradually

27 21 st Century Climate Change Dominant influence likely to be increase in greenhouse gases (anthropogenic) Projections of temperature change are made using climate models

28 Climate Models – 3 Climate Model Anthropogenic forcing Climate change Input Output

29 Calculation of Future CO 2 Concentrations -- Method Carbon Cycle Model Anthropogenic Emissions CO 2 Concentration increase Model Input Model output

30 Climate Models -- 5 Complication: Models have differing sensitivities  models produce different results for same emission scenarios  models produce different results for same emission scenarios

31 Differing Response of Models for Same Scenario Time Global Mean Temperature High Sensitivity Low Sensitivity

32 Summary: Two Causes for Large Range in Projections 1.Wide range in emission scenarios 2.Wide range in model sensitivities #1 due to uncertainty in future human actions (i.e., it is not a fault of the models) #2 is due to our imperfect understanding of the climate system (i.e., it is a fault of the models)

33 Impacts Arctic: large reduction in summer sea ice Arctic could be ice-free in summer by end of century Arctic could be ice-free in summer by end of century Permafrost = soil that remains frozen throughout the year Warming  softening of permafrost Warming  softening of permafrost

34 Impacts Glaciers and Ice Sheets Glaciers and Ice Sheets Mountain glaciers will continue to shrink Greenland ice sheet will very probably lose mass Antarctica (?)

35 Impacts: Sea Level Melting glacial ice and thermal expansion will cause sea level to rise Melting glacial ice and thermal expansion will cause sea level to rise Estimated rise Low-emission scenario: 18 – 38 cm Low-emission scenario: 18 – 38 cm High-emission scenario: 26 – 59 cm High-emission scenario: 26 – 59 cm Estimates are probably too low Contribution from ice sheets was not taken into account! Contribution from ice sheets was not taken into account!

36 Impact of Rising Sea Level Greatest in countries with heavily populated coastal regions, e.g. Bangladesh and in small-island nations

37 Fresh Water Supplies Warming  shrinking glaciers, reduced snowfall in mountains Problem: 1/6 of world population depends on glacial & snow melt for drinking water

38 Precipitation Models project increases in precipitation in some regions, decreases in others Regions of decrease include: Southwestern U. S., Mexico, Central America, Caribbean Southwestern U. S., Mexico, Central America, Caribbean Mediterranean Mediterranean Regions of increase include: Canada, most of Asia Canada, most of Asia

39 Soil Moisture, Runoff  precip.   soil moisture and runoff But, can have  soil moisture even with  precip.

40 Effect on California Warming  less snowfall in mountains Warming  less snowfall in mountains  less summer runoff  less summer runoff  less water in summer for  less water in summer for irrigation irrigation hydroelectric power hydroelectric power drinking water drinking water Loss of salmon habitat

41 More about precipitation Models project increased variability  increased flooding and increased droughts!  increased flooding and increased droughts! Another problem: increased demand for water.

42 Agriculture Reductions in soil moisture  reduced crop yields However, areas with increased soil moisture could benefit However, areas with increased soil moisture could benefit (If warming isn’t too large.) (If warming isn’t too large.)

43 Other Potential Agricultural Benefits of Warming Increased growing season in higher latitudes Could benefit Canada, Russia Could benefit Canada, Russia Beneficial effects of increased CO 2 could offset damaging effects of reduced soil moisture Called “CO 2 fertilization” Called “CO 2 fertilization” Only works if warming is relatively small Only works if warming is relatively small

44 Ecosystems In past, ecosystems have been able to adapt, but … In past, ecosystems have been able to adapt, but … “ resilience of many ecosystems is likely to be exceeded by 2100” Effects of climate change aggravated by increased human demands increased human demands fragmentation of habitats fragmentation of habitats

45 Ecosystems, continued Up to 30% of species at “increasingly high risk of extinction” if average global temp increase above 2 -3  C Oceans becoming more acidic Will hurt organisms that make shells Will hurt organisms that make shells

46 Carbon Cycle Now, biosphere is a net “sink” of carbon i.e., carbon uptake > carbon released i.e., carbon uptake > carbon released By mid-century, biosphere likely to become a net source of carbon i.e., carbon release > carbon uptake i.e., carbon release > carbon uptake (mainly due to increased rate of decay) (mainly due to increased rate of decay) Ocean carbon uptake will diminish Result: Faster rise of CO 2

47 Impacts on U. S. Forests Each tree species requires a specific environment for optimum growth Climate change will cause a shift in tree habitats Projections of habitat changes Projections of habitat changes _atlas.html# _atlas.html# _atlas.html# _atlas.html#

48 Tropical Cyclones – basic info Called hurricanes in Atlantic, eastern Pacific Called hurricanes in Atlantic, eastern Pacific Called typhoons in western Pacific (north of equator) Energy source: heat stored in oceans Energy source: heat stored in oceans Theory: warmer oceans  stronger storms Theory: warmer oceans  stronger storms (There is evidence this already happening) (There is evidence this already happening)

49 Forest Fires In western U. S., warming  more forest fires

50 Human Health More deaths from heatwaves Like 1995 Chicago heat wave Like 1995 Chicago heat wave Increases in some tropical diseases

Adaptation Strategies for coping with climate change Strategies for coping with climate changeMitigation Strategies for reducing the rate of climate change Strategies for reducing the rate of climate change

Why Adaptation? Some warming is inevitable because of thermal inertia

Adaptation Examples Sea-level rise Build coastal defenses Build coastal defenses (e.g., dikes, as in the Netherlands) (e.g., dikes, as in the Netherlands)Expensive! Relocation of affected populations Relocation of affected populations (Millions of refugees possible.)

More Examples Agriculture Increased use of irrigation Increased use of irrigation Development of heat- and drought- resistance crops Development of heat- and drought- resistance crops (Genetic engineering?) Human health and comfort Increased use of air conditioning Increased use of air conditioning (Would require increased electricity)

Mitigation Reduction of greenhouse-gas emissions Probably the most effective way Probably the most effective way Carbon capture and storage Send emissions into ground or deep ocean Send emissions into ground or deep ocean Geoengineering (more later) Increase albedo (e.g., sulfur in stratosphere) Increase albedo (e.g., sulfur in stratosphere) Remove CO 2 (e.g., artificial trees) Remove CO 2 (e.g., artificial trees)

Reducing Global Emissions This is a massive problem Very large reductions are required to make much difference International cooperation is required

Some Methods to Reduce Emissions Using alternative energy sources, e.g., solar, wind, nuclear (France: 80% of power generation is nuclear) (France: 80% of power generation is nuclear) Increased efficiency Improved gas mileage in cars Improved gas mileage in cars Compact fluorescents instead of incandescent light bulbs Compact fluorescents instead of incandescent light bulbs (Problem: CFs contain mercury)

Geoengineering Schemes Disposal of CO2 in the Deep Ocean Carbon Capture and Storage Aerosol Injection into the Stratosphere Injection of other small reflectors into Stratosphere Shielding the Earth from Afar Placing reflective plastic over deserts Modify ocean reflectivity Damming the Bering Strait