You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
a possible influence of solar variability on climate has been discussed for some time .
although the direct solar influence on climate is apparent , variations of the solar irradiance are estimated to be an order of magnitude too small to explain the observed changes in climate ( e.g. , * ? ? ?
* ) . therefore , an indirect mechanism linking solar variability to climate should be involved . according to some modeling studies ,
a response in atmospheric circulation can amplify the terrestrial effect of solar irradiance changes @xcite . on the other hand
, cosmic rays can noticeably affect the earth s climate @xcite . while the energy deposited by cosmic rays into the earth s atmosphere is negligible compared to that from solar irradiance ,
they are the main source of ionization in the troposphere ( see , e.g. * ? ? ?
a possible qualitative link has been proposed that relates cosmic ray induced ionization ( crii ) in the troposphere and cloud properties @xcite .
ions created by cosmic rays rapidly interact with molecules in the atmosphere and are converted to complex cluster ions ( aerosols ) @xcite .
these cluster ions may grow by ion - ion recombination or ion - aerosol attachment and thus affect the number of aerosols acting as cloud condensation nuclei ( ccn ) at typical atmospheric supersaturations of a few percent @xcite .
others have suggested that a crii - cloud link could also arise through changes in the global electric circuit affecting aerosol - cloud interactions at the edges of clouds ( see , e.g. , * ? ? ? * or a review of possible mecahnisms in _ harrison and carslaw _ , 2003 ) .
both mechanisms require that an amplified effect of cosmic rays on climate is realized through the important role that clouds play in the radiation budget of the atmosphere by both trapping outgoing long wave radiation and reflecting incoming solar radiation .
although a detailed physical model quantifying this connection is still missing , correlation studies support its validity .
@xcite found a highly significant correlation between low clouds below @xmath1 3.2 km ( rather than clouds at other altitudes ) and the cosmic ray flux during the period 19831994 .
this basic result has subsequently been confirmed by other independent studies @xcite .
there is also evidence for the reduction of cloud coverage during strong forbush decreases at time scales of a few days @xcite .
this implies that the proposed cloud - cosmic ray relation may also be significant at short - time scales .
more recently @xcite found that the low cloud - cosmic ray correlation can be extended until 2001 but only after the globally averaged cloud data are re - calibrated .
however , the variability in low cloud amount ( lca ) can not be uniquely ascribed to a single mechanism when using globally averaged data since the observed long - term changes in the global lca correlate with different solar - related indices including solar irradiance and cosmic rays .    in this paper
we study the spatial distribution of lca and crii over the period 19842000 . in all previous studies
the count rate of a single neutron monitor was used as a measure of cosmic rays , and assumed to represent the global crii@xmath2 .
although useful for qualitative correlation studies , this approach does not give quantitative estimates since the cosmic ray intensity varies strongly over the globe due to the shielding by the geomagnetic field .
here we study the global distribution of crii and compare that with the measured lca distribution .
following previous studies @xcite , we use the low cloud amount obtained from the isccp - d2 database limited to ir radiances .
isccp provides monthly observations of the global cloud cover based on an intercalibration of up to 5 satellites for the period from july 1983 to september 2001 .
satellites detect a cloud when radiance observations differ significantly from clear sky values .
however , uncertainties can arise if atmospheric transparency is influenced by processes other than clouds , e.g. , aerosol loading from mt .
pinatubo @xcite .
we note that lca as defined from satellite observations is restricted to clouds with their tops below 640 hpa ( 3.2 km ) , which is different from ground - based observations . in the present analysis
annual lca averages are used ( in order to avoid seasonal variations ) on a 5@xmath3x5@xmath3 latitude - longitude grid for the period 19842000 inclusive .
recently , the global distribution of crii has been calculated for the troposphere ( 010 km ) since 1951 @xcite .
first , the electromagnetic - nucleonic cascade initiated by cosmic rays in the atmosphere was simulated for different conditions using the corsika monte - carlo package @xcite .
then the annually averaged ion production rate in the troposphere at a given latitude was calculated using the respective cosmic ray spectra parameterized by the average heliospheric modulation strength @xcite .
finally , the equilibrium ion concentration was calculated at a given location , taking into account processes of recombination and aerosol attachment .
here we use crii values calculated at 3 km altitude which corresponds roughly to the limiting altitude , as defined by isccp - d2 , below which low cloud forms .
.correlation coefficients ( and their significance levels in parentheses ) between lca and crii for the period of 19842000 for different regions : global ( @xmath4 ) , tropics ( @xmath5 ) , and mid - latitudes ( @xmath6 and @xmath7 ) .
[ cols="<,^,^,^",options="header " , ]     using the latitudinal zones with highly significant c.c.s ( see fig .
[ fig:2_zonal ] ) we try to quantify the relation between detrended lca and crii ( fig .
[ fig:5_detrended ] ) . for want of a physical model relating lca to crii , a quantitative phenomenological relation is assumed in the form of a direct proportionality between normalized variations of lca , @xmath8 , and crii , @xmath9 , where @xmath10 is the zonal mean crii value during 19842000 .
the scatter plot of @xmath11 vs. @xmath12 is shown in fig .
[ fig:6_scatter]a . despite the wide scatter of points ,
there is a highly significant correlation between @xmath11 and @xmath12 ( c.c.=0.6 , s.l.@xmath1398% ) , with the corresponding linear relation as follows : @xmath14 the fact that the proportionality coefficient is close to unity implies that inter - annual variations of lca around the long - term trend can be directly ascribed to the variations of crii .
moreover , the amplitude of cyclic relative variations in @xmath11 and @xmath12 shows a similar latitudinal dependence ( fig .
[ fig:6_scatter]b ) .
these results strongly favor the idea that the variations of lca are related to crii rather than other mechanisms , e.g. , solar irradiance , which can not naturally explain such a latitudinal dependence .
while in earlier studies data from a single neutron monitor was used as a proxy of cosmic ray intensity , we have explored the quantitative relationship between temporal and spatial variations of lca and crii over the globe for the period 19842000 .
we suggest that the lca time series can be decomposed into a long - term slow trend and inter - annual variations , the latter depicting a clear 11-year cycle in phase with crii .
the trend whose nature is beyond the scope of the present study , is strong in tropical regions and possibly masks the lca - crii relation .
we then find a highly significant correlation between the detrended inter - annual lca variations and crii over the globe ( polar regions being excluded ) .
a quantitative regression model was obtained ( eq .  [ eq : lin2 ] ) , which implies a one - to - one relation between the relative variations of lca and crii over the latitude range 2055@xmath0s and 1070@xmath0n . the amplitude of relative variations in lca was found to increase polewards , in accordance with the amplitude of crii variations but in contrast to the insolation which decreases polewards .
these results thus support the idea that lca is modulated by crii , rather than by solar irradiance , at inter - annual timescales between 1984 - 2000 .
we thank henrik svensmark for useful discussions .
the financial support by the academy of finland is acknowledged .
ogg and gak were partly supported by the program `` non - stationary processes in astronomy '' .
the cloud data are obtained from isccp d2 web - site ( http://isccp.giss.nasa.gov/products/browsed2.html ) .
gringel w. , j.a .
rosen , and d.j .
hofmann , electrical structure from 0 to 30 kilometers , in studies in geophysics : the earth s electrical environment , p166 - 182 , natl .
acad . press ,
washington d.c . ,
1986 .
hoppel , w.a .
andersen , and j.c .
willet , atmospheric electricity in the planetary boundary layer , in studies in geophysics : the earth s electrical environment , p159 - 165 , natl .
acad . press , washington d.c . , 1986 .
marsh n. , and h. svensmark , galactic cosmic ray and el nio - southern oscillation trends in international satellite cloud climatology project d2 low - cloud properties , _ j. geophys .
_ , * 108*(d6 ) , aac 6 - 1 , doi:10.1029/2001jd001264 , 2003b .            tinsley , b.a . , influence of solar wind on the global electric circuit , and inferred effects on cloud microphysics , temperature , and dynamics in the troposphere , _ space sci .
_ , * 94*(1 - 2 ) , 231 - 258 , 2000 .
a significant correlation between the annual cosmic ray flux and the amount of low clouds has recently been found for the past 20 years . 
 however , of the physical explanations suggested , none has been quantitatively verified in the atmosphere by a combination of modelling and experiment . here 
 we study the relation between the global distributions of the observed low cloud amount and the calculated tropospheric ionization induced by cosmic rays . 
 we find that the time evolution of the low cloud amount can be decomposed into a long - term trend and inter - annual variations , the latter depicting a clear 11-year cycle . 
 we also find that the relative inter - annual variability in low cloud amount increases polewards and exhibits a highly significant one - to - one relation with inter - annual variations in the ionization over the latitude range 2055@xmath0s and 1070@xmath0n . 
 this latitudinal dependence gives strong support for the hypothesis that the cosmic ray induced ionization modulates cloud properties .