Effects of Galactic Cosmic Rays on the Atmosphere and Climatecondensable material, which slows the...

Effects of Galactic Cosmic Rays on the Atmosphere and Climate

Jón Egill Kristjánsson, Univ. Oslo

Overview of talk

• Hypotheses for Coupling between Galactic Cosmic Rays and Climate

• Observational studies

• Recent Results from Lab Studies (CERN)

• Recent Results from Modeling Studies

• Physical Mechanisms

• Unexplored issues

Galactic Cosmic Rays and the Atmosphere

• GCR are the dominant source of penetrating ionizing particle radiation

• GCR provide the sole source of ions away from terrestrial sources of radioisotopes

• GCR directly influence the global atmospheric electrical circuit

Carslaw et al. (2002: Science)

GCR Ion Production vs. Height

Solar Min.

Solar Max.

Neher (1971: JGR)

Max. at 10-15 km (upper troposphere / lower stratosphere)

Clouds and Climate

Thin, high ice clouds: Warming Dense, low liquid clouds: Cooling

Trapping of LW > Reflection of SW Trapping of LW << Reflection of SW

Observational Studies

Correlations between Galactic Cosmic Rays and Low Clouds

(ISCCP IR data)

Marsh and Svensmark (2000: Phys.Rev.Lett.)

GCR Low Clouds Climate

• Galactic Cosmic Ray Flux is Declining (3.5% during 1964-1995)

• Fewer Low Clouds (6.1% during 1983-1997)

• Less Reflected Solar Radiation (assumed)

• Warmer Climate

Marsh and Svensmark (2000: Space Sci.Rev.)

Cosmic Ray Flux 1964 - 2012

Kilde: http://cosmicrays.oulu.fi

Low Cloud Amount (MODIS)

Laken et al. (2012: J. Climate)

GCR Low Clouds Climate

• No trend in Galactic Cosmic Ray Flux (during 1964-2012)

• More Low Clouds (MODIS: 0.9% during 2000-2011) / Fewer Low Clouds (ISCCP)

• No Trend in Reflected Solar Radiation (2000-2005)

• Warmer Climate

Forbush Decrease Events

~ Same amplitude as Solar cycle variation (Min minus Max)

5 Strongest FD events of 2000-2005

Kristjánsson et al. (2008: Atmos.Chem.Phys.)

5 Strongest FD events of 2000-2005

Globally averaged data: Signals in particle size, liquid cloud cover and cloud liquid water content 5-10 days after FD onset

Svensmark et al. (2009: Geophys.Res.Lett.)

5 strongest FD events of 2001-2005: TOA Fluxes from CERES

a) b)

In addition to the GCR signals, non-cosmic ray signals of similar amplitude as the cosmic ray signals appear

Physical Mechanisms

Near-cloud mechanism: GCR ionization creates electrical charges on clouds

Carslaw et al. (2002: Science)

Power Spectra: High-Pass Filtered GCR vs Cloud Base Height (Stratus)

1983-1991: 1.68 yr GCR signal 1993-2001: GCR ~ clouds

Harrison et al. (2011: PRSLA)

GCR GCR

Stratus

Stratus

Clear-air mechanism: GCR ionization aids particle formation

Carslaw et al. (2002: Science)

A typical particle formation event

Kulmala et al. (2010: ACP)

5 July 2006 at Hyytiälä, Finland

Mechanisms for aerosol nucleation

Measurement sites Ion contrib. vs. Nucleation rate

Kulmala et al. (2010: ACP)

Ion contribution large only when nucleation rates are small => Overall ion contribution < 10%

Kirkby et al. (2011: Nature)

NEUTRAL GCR PBL GCR UPPER TROP.

Kirkby et al. (2011: Nature)

Kirkby et al. (2011: Nature)

292 K

278 K

248 K

Dots: PBL measurements

Lines: Lab measurements at room temp.

NH3 < 35 pptv

NH3 < 50 pptv

NH3 = 150 pptv

NH3 = 190 pptv

- Ion-induced binary nucleation at a significant rate in the free troposphere at atmospheric [H2SO4] - Binary nucleation within the PBL is negligible - In the PBL, ion-induced nucleation can not explain observed nucleation rates

Kirkby et al. (2011: Nature)

Recent modeling studies

Solar Min minus Solar Max

• Global Aerosol-Climate Model: ECHAM5-HAM

• Ionization by GCR based on the analytical theory of O’Brien [2005]

• The GCR produced ions drive aerosol formation from the gas phase via charged nucleation of sulfuric acid (H2SO4) and water (H2O) in the model

Kazil et al. (2012: GRL)

TSI signal by itself: - 0.24 W m-2

Negligible Impact on Top of Atmosphere Short Wave Radiation!

Forbush Decrease events

Snow-Kropla et al. (2011: ACP)

• Global CTM GEOS-Chem model extended with the TOMAS aerosol microphysics model

• 40 size bins from 1 nm to 10 µm diameter

• Sulfate, sea-salt, organic carbon, black carbon, mineral dust

• Ion-mediated nucleation (IMN); Yu (2010: JGR)

The fate of the nucleated aerosols

Growth

• Nucleated aerosols (~2 nm) may grow by condensation and coagulation to CCN size (~100 nm)

Limitations

• Re-evaporation

• Competition for condensable vapors => Slower growth

• Scavenging by coagulation

Summary

• Do GCR influence Earth’s climate via clouds? • Mechanisms involving charges on clouds – some support from

observations; poorly understood • Mechanisms involving particle formation; some support from

observational and modeling studies • Aerosol nucleation important for CCN • GCR stimulates aerosol nucleation • But, globally, GCR variations do not seem to have a large influence

on [CCN] clouds climate • Growth of aerosols to CCN size still highly uncertain • Other mechanisms?? E.g., involving ice clouds??

http://folk.uio.no/jegill

Thank you!

Photo: Michael Gauss

TROPICS SUBTROPICS

H2O, SO2

Cosmic Rays

Aerosol nucleation

Sulfate aerosols, growing in size by coagulation, spreading horizontally

STORM TRACKS

Solar Min minus Solar Max: Simulated Changes in Aerosol Number Conc.

Solar signal weak in all cases! Snow-Kropla et al. (2011: ACP)

Simulated Forbush Decrease Events

Snow-Kropla et al. (2011: ACP)

Quite strong signals also outside FD time window

Cosmic Ray signals with a few days’ delay

Aerosol nucleation vs [H2SO4]

• GCR ionization enhances nucleation rate twofold at 292 K and more than tenfold at 278 K and 248 K

• Binary nucleation extremely slow in the PBL, compared to observations

• Unrealistically high [H2SO4] applied

292 K

278 K

248 K

Kirkby et al. (2011: Nature)

6 Largest FD events of 1989-2001

Calogovic et al. (2010: Geophys.Res.Lett.)

No relation found between GCR signal and cloud amount

• Ion production rate calculated as a function of latitude, longitude and height

• 3-hourly ISCCP D1 cloud data

• Gridded data at 5°x 5° resolution

Sensitivity Experiments

• Results robust to uncertainties in assumptions concerning background aerosols

• Cosmic Ray signal real, but small

Snow-Kropla et al. (2011: ACP)

Simulated GCR Ionization Rate

Kazil et al. (2012: GRL)

Snow-Kropla et al. (2011: ACP)

What are galactic cosmic rays?

• Galactic cosmic rays are high-energy charged particles that enter the solar system from far away in the galaxy. They are composed of protons, electrons, and fully ionized nuclei of light elements.

Source: Wikipedia

Signals in Aerosols and Clouds

Kazil et al. (2012: GRL)

Reflected SW radiation at TOA: Sensitivity to selected number of FD events

a) b) c)

As more and weaker events are added, signals become weaker, less significant

TOA Radiation (CERES data) vs. Cosmic Rays

a) b)

Signal a few days after strong FD events in mainly SW radiation, followed by an opposite signal a few days later in mainly LW radiation

Kristjánsson et al. (2008: ACP)

No signals in cloud droplet size, cloud cover or cloud water path associated with FD events

Focus on areas of high cloud susceptibility

Findings from CERN so far

• For typical boundary-layer NH3 mixing ratios (< 1 pptv), ternary nucleation NH3-H2SO4-H2O, with or without ions, is unable to explain atmospheric observations

• The fraction of the freshly nucleated particles that grow to sufficient sizes to seed cloud droplets remains an open question experimentally

• The same goes for the role of organic vapours in the nucleation

Kirkby et al. (2011: Nature)

Exploring Model Uncertainties

Snow-Kropla et al. (2011: ACP)

Modeling of aerosol nucleation CCN

• We estimate that 45% of global low-level cloud CCN at 0.2% supersaturation are secondary aerosol derived from nucleation (ranging between 31–49% taking into account uncertainties in primary emissions and nucleation rates), with the remainder from primary emissions.

• The model suggests that 35% of CCN (0.2%) in global low-level clouds were created in the free and upper troposphere.

• In the marine boundary layer 55% of CCN (0.2%) are from nucleation, with 45% entrained from the free troposphere and 10% nucleated directly in the boundary layer.

Merikanto et al. (2009: Atm.Chem.Phys.)

Particle Formation over Europe

Kulmala et al. (2010: ACP)

Annual data

Monthly data

No link found between: - variations in aerosol nucleation events - variations in GCR flux

Data Sources

• MODIS = MODerate resolution Imaging Spectro-radiometer

• Measures cloud and aerosol properties using 36 bands in VIS, NIR and IR

• Time resolution: Daily

• Spatial resolution: 1 km 1 degree

• CERES = Clouds and the Earth’s Radiant Energy System

• Measures broadband SW and LW energy fluxes from 0.3 – 200 µm

• Time resolution: Hourly Every 3 hours Daily

• Spatial resolution: 20 km 1 degree

AERONET data

• AERONET = AErosol RObotic NETwork

• Surface-based measurements using sun photometer instruments

• Measure aerosol properties

• Ångström Exponent is a Measure of Particle Size:

• Purely Gaseous Atmosphere: AE ~ 4

• Small Particles: AE ~ 1

• Large Particles: AE ~ 0

340/440log

/log 340440340440

AE

The GCR clear-air mechanism

• With increasing nucleation rates, there is increased competition between the new particles for condensable material, which slows the growth rates of these new particles. Slower growth rates increase the probability of scavenging by coagulation

• Therefore, we would expect a smaller fractional change in CCN than the fractional change in the cosmic-ray flux

• Away from sources of precursor gases it may take on the order of a week or more for nucleated aerosols to grow to CCN sizes

Carslaw et al. (2002: Science)

Snow-Kropla et al. (2011: ACP)

Aerosol Sources and Sinks

Seinfeld and Pandis (1998)

Accumulation mode: Most CCN

Aerosol Nucleation due to Cosmic Rays

• Simulated Ion-Mediated Nucleation

Maxima:

• Upper tropical troposphere

• Lower mid-latitude troposphere

Yu et al. (2010: JGR)

Global Cloud Cover

Mid-latitude Storm tracks => Mid-level Clouds (mixed-phase)

ITCZ: Deep Convection => High Clouds (ice)

Subtropical subsidence inversion => Low Clouds (liquid)

Pierce & Adams (2009: Geophys.Res.Lett.)

CCN at SolarMin – SolarMax

• Standard assumption on the influence of ionization

• Extreme assumption on the influence of ionization

Pierce & Adams (2009: Geophys.Res.Lett.)

The simulated signal in Cloud Condensation Nuclei

(CCN) from changes in cosmic rays over a solar cycle

is very weak

The resulting radiative forcing is only ~0.01 W m-2

GLOMAP results vs. obs.

Spracklen et al. (2010: Atmos.Chem.Phys.)

Validation of IMN nucleation scheme

Observed IMN scheme

Yu et al. (2010: JGR) Good agreement between model results and aircraft campaign observations

Validation of IMN nucleation scheme

Snow-Kropla et al. (2011: ACP)

Yu et al. (2010: JGR)

Solar Min minus Solar Max: Simulated Changes in Aerosol Properties

Snow-Kropla et al. (2011: ACP) Solar signal weak in all cases!

Ionization Chamber as a proxy for the Earth’s atmosphere

Kirkby et al. (2011: Nature)

Pathways that need to be explored

• Near-Cloud Mechanism (G. Harrison) largely unexplored

• Favorable Conditions - Pristine Areas?

• Mechanisms involving Ice Clouds?

• Relevance for Paleoclimate?

292 K

[H2SO4] = 4.5∙108 cm-3

278 K

[H2SO4] = 1.5∙108 cm-3

- Nucleation rate roughly proportional to negative ion concentration -Binary nucleation extremely slow in the PBL, even with ion enhancement

Kirkby et al. (2011: Nature)

248 K

278 K

292 K

NH3 < 35 pptv

NH3 < 35 pptv

NH3 = 230 pptv 292 K

NH3 < 35 pptv - A clear progression from almost binary nucleation at 248 K to pure ternary nucleation at 292 K; both contributing at 278 K - At 292 K clusters grew by a striking stepwise accretion of NH3 molecules, each stabilizing a distinct additional number of acid molecules

Kirkby et al. (2011: Nature)

292 K

278 K

[H2SO4] = 6.3∙107 cm-3

[H2SO4] = 1.5∙108 cm-3

[H2SO4] = 4.3∙107 cm-3

- Nucleation rate highly sensitive to small additions of ammonia up to ~100 pptv - Saturation at higher NH3 mix. ratios

Kirkby et al. (2011: Nature)

Approach

• Study recent Forbush Decrease events (time scale ~2 weeks)

• Search for signals of cosmic rays / solar activity in observational data (CERES) of TOA radiative fluxes

• Compare to signals of cloud and aerosol properties from MODIS data

• CERES and MODIS are instruments on the NASA Aqua and Terra satellites – we use Terra data at 1°x 1° resolution

CERES data (annual average) Reflected SW Emitted LW

-150 -100 -50 0 50 100 150

-50

0

50

SWtotal

0

50

100

150

200

-150 -100 -50 0 50 100 150

-50

0

50

LWtotal

0

100

200

300

400

Signals dominated by latitude and clouds Low clouds: Large SW effect High clouds: Large LW effect

Summary

• We have investigated possible links between cosmic rays, clouds and climate in Forbush Decrease events of ~2 week duration

• Signals for strong events only (< 1 such event per year) • At +5-7 days, a signal is found in Ångström exponent at

the surface, TOA SW fluxes, LWP and mid-level cloud amount

• At +12-13 days an opposite signal is found in TOA LW fluxes, IWP and high clouds – not understood!

• Applying a longer time window, signals of similar amplitude, unrelated to cosmic ray events, are found

• Our results indicate that the GCR – cloud link is real, but weak – Consistent with recent model studies

Cloud Properties vs. Cosmic Rays

a) b)

c) d)

Signals a few days after strong FD events in several cloud parameters

Cloud Amount (blue) vs. TOA Radiation (red, green)

Liquid Clouds

Ice Clouds

Mid-Level Clouds

SW signal PRECEDES cloud signal by 2 days

LW signal correlates well with cloud signal (negative correlation)

SW signal correlates well with cloud signal

Signal at +5 days correlates well with mid-level clouds Signal at +12 days correlates well with ice clouds

Cloud Properties vs. TOA Radiation

a) b)

c) d)

Cloud Optical Depth ~ SW

Ice Water Path ~ LW, SW Liquid Water Path ~ SW

Cloud Droplet Size does not correlate with TOA SW or LW radiation!

Spatial Signatures - Chaotic

The mean of (days 5 to 8) minus the mean of (days -15 to -1 plus days 10 to 20)

a) b)

c) d)

% %

W m-2 W m-2

Ångström Exponent

Why 5 episodes? How about 4, 6 or 7?

Does not seem to make much difference

Removing the Jan05 event

-20 -10 0 10 2028

28.5

29

29.5

30

30.5

31

Days (0 at FD minimum)

CALIQ, mean of strong FD

jan05 removed

all 5 FD

-20 -10 0 10 2095

95.5

96

96.5

97

97.5

98

98.5

99

99.5

100

SWtotal, mean of strong FD

Days (0 at FD minimum)

jan05 removed

all 5 FD

Statistical procedure

• Arbitrary 5 * 36 (or 130) day periods, averaged

• Repeated 15 times

• Calculate 2 standard deviations around the mean

Residual from vertical partitioning of the cloud cover

IR only

IR+VIS+NIR

IRLOW, IRMID, IRHIGH do not add up to ISCCP total cloud cover!

Simulated Cosmic Ray Ion-Pair Formation Rate

Snow-Kropla et al. (2011: ACP)

Simulated Rate of Aerosol Nucleation

Snow-Kropla et al. (2011: ACP)

Negative values in some areas due to more competition and larger coagulation sink

ISCCP Low Clouds MODIS Liquid

The high correlation disappears after 1998

r = - 0.56

Gray et al. (2010: Rev. Geophys.)