Are Cosmic Rays Changing our Climate? Jose Cardoza University of Utah Atmospheric Science Department Tuesday, February 16, 2010

Transcription

1 Are Cosmic Rays Changing our Climate? Jose Cardoza University of Utah Atmospheric Science Department Tuesday, February 16, 2010

2 OUTLINE Cosmic rays in the atmosphere The supporters The skeptics Summary

3 COSMIC RAY-ATMOSPHERE INTERACTION Cosmic ray (proton,electron,..) Cosmic Rays (CRs): Charged particles from space Mostly protons Extensive Air Shower CRs interact with the nuclei of the atmospheric atoms to produce a variety of secondary particles Primary particle interacts at an altitude of about 30 km

4 EXTENSIVE COSMIC RAY SHOWER Simulated 1TeV proton initiated shower The color shows the density of particles(protons and electrons) ~20,000 per m 3 The flux decreases by about a factor 500 per decade in energy 1000 particles-s -1 m -2 at GeV (10 9 ) energies to 1 particle-century km -2 above 100 EeV. (10 18 )

5 CLOUD COVER AND COSMIC RAYS Low cloud cover 680 hpa Svensmark et al 1997 Svensmark 2000 Cloud fraction follows CR s much more closely than solar flux CR intensity has varied by factor 3 in the past on centennial and millennial times scales 1.7% variation of low clouds = 1 W/m 2 change in the radiation budget Both graphs use ISCCP IR measurements

6 MECHANISMS LINKING CR AND CLOUDS I. Ion-induced nucleation of new aerosols: Ionization from cosmic rays enhance the production rate of new aerosols which then can grow to form more cloud condensation nuclei (CCN). Most favorable for low ionization levels and high trace gas concentrations, such as is found in the lower atmosphere. II. Modulation of global electric current: Cosmic rays alter the ionosphere-earth electric current which, in turn influences cloud properties through charge effects on droplet freezing and other microphysical processes. Both provide an amplification process!

7 Ion-induced nucleation of new aerosols (I) Ways charge increases the number of CCN: Kirby 2008 Stabilizes embryonic cluster, reduces critical size, accelerates early growth process (by factor ~2) increase in droplet number concentration increase cloud reflectivity & suppression of rainfall increase in cloud lifetime net cooling.

8 ION NUCLEATION VS NEUTRAL PARTICLE Sub critical cluster Gibbs Free Energy Ions Stable due to strong electrostatic interactions, growth is faster Small to none Neutral Aerosols Less stable evaporate more quickly Large barrier Critical size Reduced Remains relatively high Net effect: Up to a factor of 2 grow more to particle sizes of 5nm (95% higher chance of becoming CCN than particles 5nm) more low level clouds and a 1 W/m 2 radiative cooling

9 MODULATION OF GLOBAL ELECTRIC CURRENT (II) Attachment of atmospheric ions to cloud droplets decrease in conductivity within clouds by factor of 3-30 w.r.t clear air J z = σe, assuming J z,gradient in E Gauss Law: Stratification of charge density Carslaw et al 2002 Claim: Electrified clouds enhances aerosol efficacy as ice-forming nuclei which grow rapidly and induce rainfall (less cloudiness). Net effect unsure since latent heat release may reverse the sign of the effect.

10 THE SKEPTICS Cloud cover lags and leads CR changes by about half a year. CCN lifetime is a few days formation of clouds should be within a similar time frame IR measurements of low clouds are contaminated by high clouds (thin cirrus) Svensmark 2000 Error in IR-data can be 5 6 % in cloud cover 1.7% variation in low clouds due to CRs

11 CR CORRELATION WITH LOW CLOUDS Significance level of correlations 67% for cosmic rays and low clouds, 98% for solar irradiance and low clouds IR obsevations only. Deviation in total cloud cover is 5% to 6%! Kristjansson et al 2002 Daytime significance level of correlations 30% for cosmic rays and low clouds, 90% for solar irradiance and low clouds Multichannel observations Deviation in total cloud cover by less than 1%

12 CR CORRELATION WITH LOW CLOUDS CALOGOVIC ET AL 2010 Forbush decrease (FD) is a rapid decrease in the observed galactic cosmic ray intensity following a coronal mass ejection (CME). Data of the 6 strongest FD events between with 10-21% CR decrease Correlation histograms for all cloud layers and time lags show no correlation between cloud cover and CR

13 A look at CCN concentrations simulations MODGIL : Aerosol nucleation rate are a function of the ion-pair formation rate, sulfuric acid concentration, temperature, relative humidity and existing aerosol surface area Pierce and Adams 2009 IONLIMIT: Assumes that every ion produced, positive or negative, will nucleate a particle provided that there is enough sulfuric acid to grow the particle to a diameter of 1 nm

14 RESULTS OF MODGIL & IONLIMIT SIMULATIONS Faster nucleation rates each particle grows more slowly. more competition for condensable gases Only a relatively small increase in CCN 0.2%,0.1% between solar maxima and minima as GCRs change by 15%! Net effect of increase in CCN 0.2%,0.1% is W/m 2. Pierce et al 2009

15 SUMMARY The Supporters: 1) Trends in CR fluctuations correlate well with changes in low cloud cover. 2) Two mechanisms: I. Ion-induced nucleation of new aerosols- An increase in the probability of forming more CCNs in the lower troposphere produces more low clouds that have a net cooling effect ~ 1 W/m 2. II. Modulation of Global electric current by which CRs are responsible for Electrified clouds enhances aerosol efficacy as ice-forming nuclei which grow rapidly and induce rainfall (less cloudiness). Net effect uncertain due to latent heat release.

16 SUMMARY The Skeptics: 1) No reliable correlation since cloud cover lags and leads CR changes by about half a year. 2) Correlation is much higher between solar irradiance and low clouds. IR only observations give error in cloud cover (5-6%) that is bigger than claimed by supporters (1.7%). 3) Forbush decrease study for 10-21% CR decrease show s no correlation between cloud cover and CRs. 4) Simulation study of CCN concentrations show that there is only a relatively small increase in CCN 0.2%,0.1% between solar maxima and minima as GCRs change by 15%. This corresponds to an increase of W/m 2.