Global mean surface temperature trend [IPCC, 2014]

January 2014 temperature anomaly NASA/GISS temperature analysis

Strongest warming in the Arctic [IPCC, 2014]

Trends of multiple indicators of climate change [IPCC, 2014]

EMISSION OF RADIATION Radiation is energy transmitted by electromagnetic waves; all objects emit radiation One can measure the radiation flux spectrum emitted by a unit surface area of object: Here DF is the radiation flux emitted in [l, l+Dl] is the flux distribution function characteristic of the object Total radiation flux emitted by object:

BLACKBODY RADIATION Objects that absorb 100% of incoming radiation are called blackbodies For blackbodies, fl is given by the Planck function: Function of T only! Often denoted B(l,T) F = sT 4 = 2p 5k 4/15c2h3 = 5.67x10-8 W m-2 K-4 is the Stefan-Boltzmann constant lmax = hc/5kT Wien’s law lmax

KIRCHHOFF’S LAW: Emissivity e(l,T) = Absorptivity For any object: …very useful! Illustrative example: Kirchhoff’s law allows determination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 K

TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra for different T Scene over Niger valley, N Africa

RADIATIVE EQUILIBRIUM FOR THE EARTH Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2 Radiative balance c effective temperature of the Earth: = 255 K where A is the albedo (reflectivity) of the Earth

Questions 1. For an object of given volume, which shape emits the least radiation? 2. If the Earth were hollow, would it emit more or less radiation? 3. In our calculation of the effective temperature of the Earth we viewed the Earth as a blackbody.  However, we also accounted for the fact that the Earth absorbs only 72% of solar radiation (albedo = 0.28), so obviously the Earth is not a very good blackbody (which would absorb 100% of all incoming radiation).  Nevertheless, the assumption that the Earth emits as a blackbody is correct to within a few percent.  How can you reconcile these two results?

ABSORPTION OF RADIATION BY GAS MOLECULES …requires quantum transition in internal energy of molecule. THREE TYPES OF TRANSITION Electronic transition: UV radiation (<0.4 mm) Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O3+hn gO2+O) Vibrational transition: near-IR (0.7-20 mm) Increase in vibrational frequency of a given bond requires change in dipole moment of molecule Rotational transition: far-IR (20-100 mm) Increase in angular momentum around rotation axis Gases that absorb radiation near the spectral maximum of terrestrial emission (10 mm) are called greenhouse gases; this requires vibrational or vibrational-rotational transitions

NORMAL VIBRATIONAL MODES OF CO2 forbidden allowed allowed IR spectrum of CO2 asymmetric stretch bend

GREENHOUSE EFFECT: absorption of terrestrial radiation by the atmosphere Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,… Not greenhouse gases: N2, O2, Ar, …

SIMPLE MODEL OF GREENHOUSE EFFECT VISIBLE IR Energy balance equations: Earth system Incoming solar Reflected solar Transmitted surface Atmospheric layer Solution: To=288 K e f=0.77 T1 = 241 K Atmospheric emission Atmospheric layer (T1) abs. eff. 0 for solar (VIS) f for terr. (near-IR) Atmospheric emission Need to supplement with one of those diagrams…indicating that total radiation flux at the ground includes a lot of longwave. Surface emission Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR

THE GRAY ATMOSPHERE MODEL In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection then redistributes heat vertically following the adiabatic lapse rate Integrate over z Absorption ~ ρ(z)dz dz σTo4 surface

The ultimate models for climate research

GENERAL CIRCULATION MODELS (GCMs) Standard research tools for studying the climate of the Earth Solve conservation equations for momentum, heat, and water on global 3-D atmospheric domain Horizontal resolution ~100 km Include coupling to ocean, land, biogeochemistry, atmospheric chemistry to various degrees Solution to equations of motion is chaotic, so that a GCM cannot simulate an observed meterorological year; it can only simulate climate statistics including interannual variability A GCM can be tested by its ability to simulate present-day climate statistics in a repeatable manner when run in radiative equilibrium (equilibrium climate simulation) A radiative imbalance (such as changing concentrations of greenhouse gases) will result in warming or cooling in the GCM

CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS Clouds reflect solar radiation (DA > 0) g cooling; …but also absorb IR radiation (Df > 0) g warming Cloud feedbacks are the greatest source of uncertainty in climate models sTcloud4 < sTo4 sTcloud4≈ sTo4 Tcloud≈ To convection sTo4 sTo4 To LOW CLOUD: COOLING HIGH CLOUD: WARMING

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

TERRESTRIAL RADIATION SPECTRUM FROM SPACE: composite of blackbody radiation spectra emitted from different altitudes at different temperatures Question: How many watts/m2 radiated to space on a clear night in the Niger with the surface still at 320K? (200 cm-1 x 150x10-3 W/m2) x 2pi= 6 x (30+20) = 300 Wm-2. [ + 200x100x10-3 ?] Question: What would be the emission rate if T=280 instead of 320 (a cool clear winter night)? Estimate that it goes at T4: (7/8)4 = .58 => 100 Wm-2. How much does that cool the air? Assume snow cover (perfect insulator), effect through 100 m depth of air. Heat capacity of air = 1005 J/kg/K. In 1 hour each sq. meter loses 36 KJ. 7000m of air = 10,000 kg/m2, so 200m is 300 kg. DelT= 360KJ/300/1.005 = 1.2 K/hr. In a night, 12 hr, 14K; 16 hr, 19K T decline!

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH? Example of a GG absorbing at 11 mm 1. 1. Initial state 2. Add to atmosphere a GG absorbing at 11 mm; emission at 11 mm decreases (we don’t see the surface anymore at that l, but the atmosphere) 2. 3. 3. At new steady state, total emission integrated over all l’s must be conserved e Emission at other l’s must increase e The Earth must heat!

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 mm). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

RADIATIVE FORCING OF CLIMATE CHANGE Fin Fout Reflected solar radiation (surface, air, aerosols, clouds) IR terrestrial radiation ~ T4; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols Incoming solar radiation EARTH SURFACE Stable climate is defined by radiative equilibrium: Fin = Fout Instantaneous perturbation e Radiative forcing DF = Fin – Fout Increasing greenhouse gases g DF > 0 positive forcing At the root of any climate change must be a perturbation of the rad eq of the Earth, a perturbation that we call radiative forcing. The concept of radiative forcing is central to research and policy on climate change, and it is not a difficult concept to understand. The Earth is a thermal engine. A stable climate reflects a close balance between the absorption of solar radiation, indicated here by Fin, and the blackbody emission of IR terrestrial radiation, indicated here by Fout. Aerosols and clouds reflect solar radiation, reducing Fin; greenhouse gases with IR absorption features absorb the terrestrial radiation and reemit it at lower temperatures, decreasing Fout. Perturbations to the levels of aerosols or greenhouse gases thus produces a radiative imbalance which we call radiative forcing. Greenhouse gases, absorbing aerosols result in a positive radiation forcing and the Earth warms; scattering aerosols result in negative radiative forcing and the Earth cools. Eventually, on a time scale of decades limited by the thermal inertia of the ocean, the Earth adjusts to a new radiative equilibrium. For example, the warming resulting from a positive radiative forcing increases the IR terrestrial emission and hence Fout. Many complications and feedbacks are involved in this climate adjustment, involving in particular the effect on the hydrological cycle. Calculations of climate response to a radiative forcing are done by GCMs, which are first-principles physical models for the Earth’s climate. The climate sensitivity factor lambda, defined as the global chance in surface air temperature in response to a unit radiative forcing, varies by a factor of 4 between GCMs, reflecting the uncertainty in climate change calculations. However, for a given GCM, it is found that lambda is relatively insensitive to the type or magnitude of the forcing. Because the radiative forcing can be calculated with much better reliability than the ultimate climate response, it is a widespread metric for use in science and policy. The radiative forcing changes the heat content H of the Earth system: eventually leading to steady state where To is the surface temperature and l is a climate sensitivity parameter IPCC GCMs give l = 0.3-1.4 K m2 W-1, insensitive to nature of forcing; differences between models reflect different treatments of feedbacks

“official chart” IPCC [2007]

CLIMATE MODELS CAN EXPLAIN 20th CENTURY WARMING AS DRIVEN BY ANTHROPOGENIC RADIATIVE FORCING Colored and thin black lines: results from 13 different GCMs Thick black lines: observations Year Year Models including anthropogenic forcing Models not including anthropogenic forcing observed models IPCC [2007]

IPCC PROJECTED WARMING OVER 21st CENTURY for different socioeconomic scenarios (A1, A2, B1, B2) CO2 trend Global temperature Trend (GCM ensemble) IPCC [2001]

Verification of past IPCC projections Surface temperatures IPCC 1995 IPCC 1990 IPCC (2007) actual CO2 emissions

EOCENE (55 to 36 million years ago): The last time in Earth history when atmospheric CO2 was above 500 ppm. The Eocene climate was warm, even at high latitudes: -palm trees flourished in Wyoming -crocodiles lived in the Arctic -Antarctica was a pine forest -deep ocean temperature was 12°C (today it is ~2°C) -sea level was at least 100 meters higher than today Present models cannot reproduce this warm climate – missing processes? Positive feedbacks could cause abrupt climate change but this is not well understood

New IPCC AR5 Scenarios: Representative Concentration Pathways (RCPs) Defined by radiative forcing trajectories rather than socioeconomic storylines Are representative of the Integrated Assessment Model (IAM) literature Provide continuity with older IPCC scenarios: RP8.5 ≈ A2, RP6 ≈ A1B, RP4.5 ≈ B1 Introduce new “peak-and-decline” scenario – aggressive climate policy RCP4.5 to be used for multi-decadal high-resolution simulations RCP8.5 RCP6 RCP4.5 RCP3-PD

ORIGIN OF THE ATMOSPHERIC AEROSOL Aerosol: dispersed condensed matter suspended in a gas Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop) Accumulation mode—happens to be in visible range; also repirable!. Growth rate is proportional to 1/r. Soil dust Sea salt Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…

SCATTERING OF RADIATION BY AEROSOLS: “DIRECT EFFECT” By scattering solar radiation, aerosols increase the Earth’s albedo Scattering efficiency is maximum when particle diameter = l particles in 0.1-1 mm size range are efficient scatterers of solar radiation 2 (diffraction limit) Highest when particle r = wavelength (pi*d)—surface wave, diffraction. Rayleigh=inefficient

AEROSOL OPTICAL DEPTH MODIS satellite data IPCC [2007]

EVIDENCE OF AEROSOL EFFECTS ON CLIMATE: Temperature decrease following large volcanic eruptions Mt. Pinatubo eruption 1991 1992 1993 1994 -0.6 -0.4 -0.2 0 +0.2 Temperature Change (oC) Observations NASA/GISS general circulation model

SCATTERING vs. ABSORBING AEROSOLS Scattering sulfate and organic aerosol over Massachusetts Partly absorbing dust aerosol downwind of Sahara Absorbing (right panel) ; Jfk jr; particle size and composition. Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar radiation

AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES Clouds form by condensation on preexisting aerosol particles (“cloud condensation nuclei”)when RH>100% clean cloud (few particles): large cloud droplets low albedo efficient precipitation polluted cloud (many particles): small cloud droplets high albedo suppressed precipitation

EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS N ~ 100 cm-3 W ~ 0.75 g m-3 re ~ 10.5 µm N ~ 40 cm-3 W ~ 0.30 g m-3 re ~ 11.2 µm from D. Rosenfeld  Particles emitted by ships increase concentration of cloud condensation nuclei (CCN)  Increased CCN increase concentration of cloud droplets and reduce their avg. size  Increased concentration and smaller particles reduce production of drizzle  Liquid water content increases because loss of drizzle particles is suppressed  Clouds are optically thicker and brighter along ship track

SATELLITE IMAGES OF SHIP TRACKS NASA, 2002 Atlantic, France, Spain AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast

OTHER EVIDENCE OF CLOUD FORCING: CONTRAILS AND “AIRCRAFT CIRRUS” Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).

Radiative forcing by aerosols is very inhomogeneous …in contrast to the long-lived greenhouse gases Present-day annual direct radiative forcing from anthopogenic aerosols Leibensperger et al., 2012 global radiative forcing from CO2 Aerosol radiative forcing over polluted continents can more than offset forcing from greenhouse gases The extent to which this regional radiative forcing translates into regional climate response is not understood

Radiative forcing from US anthropogenic aerosol Forcing peaked in 1970-1990

Cooling due to US anthropogenic aerosols From difference of GCM simulations with vs. without US aerosol sources in 1970-1990, including aerosol direct and indirect radiative effects During the period of maximum aerosol pollution (1970-1990), the eastern US cooled by up to 1o C. Leibensperger et al. [2012]

Observed US surface temperature trend o C Contiguous US US has warmed faster than global mean, as expected in general for mid-latitudes land But there has been no warming between 1930 and 1980, followed by sharp warming after 1980 1930-1990 trend “Warming hole” observed in eastern US from 1930 to 1990; US aerosol signature? GISTEMP [2010]

1950-2050 surface temperature trend in eastern US Leibensperger et al. [2012] 1930-1990 trend Observations (GISTEMP) Model (standard) Model without US anthropogenic aerosols US anthropogenic aerosol sources can explain the “warming hole” Rapid warming has taken place since 1990s that we attribute to source reduction Most of the warming from aerosol source reduction has already been realized