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| What do we really know about the Sun-climate connection? |
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| Written by Eigil Friis-Christensen and Henrik Svensmark | ||||
| Thursday, 19 July 2007 | ||||
Page 2 of 2 COSMIC RAY FLUX AND CLOUDNESS One of the major uncertainties in climate models is the role of cloud effects (Houghton et al., 1992;1995). In particular there are large difficulties associated with the parameterization of these effects in general circulation models. Recent studies (Tinsley, 1994) indicate that cloud formation may be influenced by galactic cosmic rays through ionization changes that cause microphysical changes in the atmosphere. Hereby nucleation and growth of ice particles may be affected. Along these lines Pudovkin and Veretenenko (1995;1996) found local decreases in the amount of cloud cover related to short term changes in the cosmic rays due to increased solar activity (Forbush decreases). The effect, however, seemed to disappear at latitudes lower than 55°.
A
change in cloud cover would indeed be a very effective amplifying mechanism for
climate forcing because the energy necessary to condense water vapour is small
compared to the resulting changes in energy of solar radiation received at the
Earth’s surface. Svensmark and Friis-Christensen (1997) examined the compiled
International Satellite Cloud Climatology Project (ISCCP) data. In Figure 4
from Svensmark and Friis-Christensen (1997) is shown the 12 months running mean
of the total cloud cover (thick line) together with the 12 months running mean
values of cosmic ray intensity measured at the Climax Neutron Monitor station,
Colorado. The correlation between the cosmic ray flux and the global cloud
cover is 0.93 for the 12 months running means. The effect is larger at higher
latitudes in agreement with the increased shielding effect of the Earth’s
magnetic field at low latitudes. For latitudes excluding the tropics the
correlation increases to even 0.97. Svensmark and Friis-Christensen (1997)
emphasize that a large seasonal variation exists in the unfiltered monthly
average cloud data probably due to the North-South asymmetry in ocean coverage.
This effect could account for an apparent slight but not significant time lag
of the cosmic ray data relative to the cloud data.
Fig.4 Variation of total cloud cover and cosmic ray flux observed at Climax. (Svensmark and Friis-Christensen, 1997). The amount of the decrease of cloud cover is considerable. The satellite data documented a decrease of 3% in global cloud cover from the solar minimum around 1987 to the solar maximum around 1990. The effect of a decrease of cloud cover would be dependent on the type of clouds that are affected. A decrease of high clouds would result in lower temperatures while a decrease in the low-altitude clouds would mean an increase in temperature (Manabe and Wetherald, 1967). On the average a decrease in the various cloud types will mean a warming. The effect of a 3% decrease in cloud cover is believed to represent a global warming corresponding to 1-1.5 Wm-2 (Rossow and Cairns, 1995). Compared to the 0.1% change in solar irradiance during the same interval which corresponds to 0.25 Wm-2 when taking into account the effect of the albedo of the Earth, the mechanism is therefore stronger by a factor of possibly 6. With this amount the solar forcing of long-term variations in global temperature seems plausible and a number of the reported correlations between solar activity variations and climate may then be immediately explainable. DISCUSSION Scientific discussions about the possible role of solar activity variations on climate have suffered from the lack of a precise physical mechanism that could account for the vast number of reported correlations. In particular it has not been possible to identify unambiguously neither the important solar activity parameter nor the primary climatic parameter in such relationships.
Solar
activity variations have traditionally been associated with the sunspot number
although it is well known that solar activity may not be described by a single
number. In particular it has been difficult to find a good representation of
the long-term variations of solar activity. That solar cycle relationships
cannot just be extrapolated to represent long-term behaviour is demonstrated by
the relative variations of the sunspot number and the geomagnetic activity
index, aa. Although aa is an index specifically associated with the
fluctuations in the terrestrial magnetic field, it does represent the result of
the continuous interaction between the geomagnetic field and the solar wind,
and hence some form of solar activity. In Figure 5 is shown a comparison between
the aa-index and the sunspot number R since 1868 when systematic geomagnetic
recordings allowed the derivation of the aa-index. The aa-index does show the
11-year cycle but two main differences between R and aa are clearly noticeable.
Firstly, the individual cycles are very different with the aa record normally
displaying several maxima whereas the sunspot record has only one dominant
maximum in each cycle. The second fundamental difference is the different
long-term variation seen in aa and R, in particular in the level at solar
activity minima. This clearly demonstrates that some long-term change in the
solar wind has taken place during this century which is not reflected in the
sunspot number at solar minima.
Fig. 5 Yearly values of the sunspot number R and the geomagnetic activity index aa. The formation and radiative effect of clouds is one of the major uncertainties in climate modeling (Houghton et al., 1995). Due to the large radiative effect of clouds, any insufficiency in the parameterization of clouds will introduce major uncertainties in the results of the climate models. Recent results have indicated strong correlations between the total cloud cover and the cosmic ray flux, indicating that this could be the missing link between solar activity variations and climate changes. If this relationship can be confirmed and understood, a major obstacle in our understanding of natural climate variations may be removed and our chances of a credible estimate of the effects of manmade greenhouse gases could be significantly improved. REFERENCES Dickinson, R., Solar variability and the lower atmosphere, Bull. Am. Met. Soc., 56(12), 1240-1248 (1975). Eddy, J.A, The Maunder minimum, Science, 192, 1189-1202 (1976). Friis-Christensen, E. and K. Lassen, Length of the solar cycle: an indicator of solar activity closely associated with climate, Science, 192, 1189-1202 (1991). Haigh, J.D. , The role of stratospheric ozone in modulating the solar radiative forcing of climate, Nature, 370, 544-546, (1994). Houghton, J.T., B.A. Callander., and S.K. Varney (eds.). Climate Change 1992: The Supplementary Report to the IPCC 1991 Scientific Assessment, Cambridge University Press, Cambridge, UK. (1992). Houghton, J.T., L.G. Meira Filho, B.A. Callander., N. Harris, A. Kattenberg and K. Maskell (eds.)., The science of climate change. Cambridge University Press, Cambridge, UK. (1995). Kelly, P.M. and T. M. L. Wigley, Solar cycle length, greenhouse forcing and global climate, Nature 360: 328-330 (1991). Labitzke, K. and H. van Loon, Some recent studies of probable connections between solar and atmospheric variability, Ann. Geophysicae, 11: 1084-1094 (1993). Lassen, K. and E. Friis-Christensen, Variability of the solar cycle length during the past five centuries and the apparent association with terrestrial climate, J. Atmos. Terr. Phys., 57(8), 835-845 (1995). Lean, J., A. Skumanich, and O. White, Estimating the Sun's radiative output during the Maunder minimum, Geophys. Res. Lett., 19, 1591-1594 (1992). Lean, J., J. Beer, and R. Bradley, Reconstruction of solar irradiance since 1610: Implications for climate change, Geophys. Res. Lett., 22, 3195-3198 (1995). Manabe, S., and R.T. Wetherald, Thermal equilibrium of the atmosphere with a given distribution of relative humidity, J. Atmos. Sci., 24, 241-259 (1967). Ney E.R, Cosmic radiation and the weather, Nature, 183, 451-452 (1959). Pudovkin, M. and S. Veretenenko, Cloudiness decreases associated with Forbush-decreases of galactic cosmic rays, J. Atm. Terr. Phys., 57, 1349-1355 (1995). Pudovkin, M. and S. Veretenenko, Variations of the cosmic rays as one of the possible links between the solar activity and the lower atmosphere. Adv. Space Res., vol. 17, No. 11, (11)161-(11)164 (1996). Reid, G.C, Influence of solar variability on global sea surface temperatures, Nature, 329, 142-143 (1987). Rossow, W.B., and B. Cairns, Monitoring changes of clouds, J. Climate, 31. 305-347 (1995). Schlesinger, M.E., and N. Ramankutty, Implications for global warming of intercycle solar irradiance variations, Nature, 360: 330-333 (1992). Svensmark, H. and E. Friis-Christensen, Variation of cosmic ray flux and global cloud coverage - a missing link in solar-climate relationships, J. atmos. sol.-terr. Phys., 59, 1225-1232, (1997).
Tinsley,
B. A. ,Solar wind mechanism suggested for weather and climate change, EOS,
75(32), 369 (1994).
Robert Ferguson, President This e-mail address is being protected from spam bots, you need JavaScript enabled to view it 209 Pennsylvania Ave., SE Suite 299 Washington, D.C 20003 www.scienceandpublicpolicy.org (202) 288-5699 *The views expressed here are those of the author and not necessarily those of SPPI |
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