Papers on the non-significant role of cosmic rays in climate

This list contains papers which show that cosmic rays don’t have significant role in recent climate change, so this list doesn’t contain the papers from Svensmark et al. or other papers symphatetic to the strong role for cosmic rays, but such papers and issues are discussed in papers below (see also Anti-AGW papers debunked section for some Svensmark et al. papers). The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

LATEST UPDATE (December 25, 2012): Laken et al. (2012) added.

A cosmic ray-climate link and cloud observations – Laken et al. (2012) “Despite over 35 years of constant satellite-based measurements of cloud, reliable evidence of a long-hypothesized link between changes in solar activity and Earth’s cloud cover remains elusive. This work examines evidence of a cosmic ray cloud link from a range of sources, including satellite-based cloud measurements and long-term ground-based climatological measurements. The satellite-based studies can be divided into two categories: (1) monthly to decadal timescale analysis and (2) daily timescale epoch-superpositional (composite) analysis. The latter analyses frequently focus on sudden high-magnitude reductions in the cosmic ray flux known as Forbush decrease events. At present, two long-term independent global satellite cloud datasets are available (ISCCP and MODIS). Although the differences between them are considerable, neither shows evidence of a solar-cloud link at either long or short timescales. Furthermore, reports of observed correlations between solar activity and cloud over the 1983–1995 period are attributed to the chance agreement between solar changes and artificially induced cloud trends. It is possible that the satellite cloud datasets and analysis methods may simply be too insensitive to detect a small solar signal. Evidence from ground-based studies suggests that some weak but statistically significant cosmic ray-cloud relationships may exist at regional scales, involving mechanisms related to the global electric circuit. However, a poor understanding of these mechanisms and their effects on cloud makes the net impacts of such links uncertain. Regardless of this, it is clear that there is no robust evidence of a widespread link between the cosmic ray flux and clouds.” Benjamin A. Laken, Enric Pallé, Jaša Čalogović and Eimear M. Dunne, J. Space Weather Space Clim. 2 (2012) A18, DOI: http://dx.doi.org/10.1051/swsc/2012018. [http://www.swsc-journal.org/articles/swsc/pdf/2012/01/swsc120049.pdf”>Full text]

Solar irradiance, cosmic rays and cloudiness over daily timescales – Laken & Čalogović (2011) “Although over centennial and greater timescales solar variability may be one of the most influential climate forcing agents, the extent to which solar activity influences climate over shorter time periods is poorly understood. If a link exists between solar activity and climate, it is likely via a mechanism connected to one (or a combination) of the following parameters: total solar irradiance (TSI), ultraviolet (UV) spectral irradiance, or the galactic cosmic ray (GCR) flux. We present an analysis based around a superposed epoch (composite) approach focusing on the largest TSI increases and decreases (the latter occurring in both the presence and absence of appreciable GCR reductions) over daily timescales. Using these composites we test for the presence of a robust link between solar activity and cloud cover over large areas of the globe using rigorous statistical techniques. We find no evidence that widespread variations in cloud cover at any tropospheric level are significantly associated with changes in the TSI, GCR or UV flux, and further conclude that TSI or UV changes occurring during reductions in the GCR flux are not masking a solar-cloud response. However, we note the detectability of any potential links is strongly constrained by cloud variability.” Laken, B. A. and J. Čalogović(2011), Geophys. Res. Lett., 38, L24811, doi:10.1029/2011GL049764. [Full text]

Relationship of Lower Troposphere Cloud Cover and Cosmic Rays: An Updated Perspective – Agee et al. (2011) “An updated assessment has been made of the proposed hypothesis that “galactic cosmic rays (GCRs) are positively correlated with lower troposphere global cloudiness.” A brief review of the many conflicting studies that attempt to prove or disprove this hypothesis is also presented. It has been determined in this assessment that the recent extended quiet period (QP) between solar cycles 23–24 has led to a record high level of GCRs, which in turn has been accompanied by a record low level of lower troposphere global cloudiness. This represents a possible observational disconnect, and the update presented here continues to support the need for further research on the GCR-Cloud hypothesis and its possible role in the science of climate change.” Ernest M. Agee, Kandace Kiefer and Emily Cornett, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00169.1.

The contribution of cosmic rays to global warming – Sloan & Wolfendale (2011) “A search has been made for a contribution of the changing cosmic ray intensity to the global warming observed in the last century. The cosmic ray intensity shows a strong 11 year cycle due to solar modulation and the overall rate has decreased since 1900. These changes in cosmic ray intensity are compared to those of the mean global surface temperature to attempt to quantify any link between the two. It is shown that, if such a link exists, the changing cosmic ray intensity contributes less than 8% to the increase in the mean global surface temperature observed since 1900.” T. Sloan and A.W. Wolfendale, Journal of Atmospheric and Solar-Terrestrial Physics, doi:10.1016/j.jastp.2011.07.013. [Full text]

Cosmic ray effects on cloud cover and their relevance to climate change – Erlykin et al. (2011) “A survey is made of the evidence for and against the hypothesis that cosmic rays influence cloud cover. The analysis is made principally for the troposphere. It is concluded that for the troposphere there is only a very small overall value for the fraction of cloud attributable to cosmic rays (CR); if there is linearity between CR change and cloud change, the value is probably ~1% for clouds below ~6.5km, but less overall. The apparently higher value for low cloud is an artifact. The contribution of CR to ’climate change’ is quite negligible.” A.D. Erlykin, B.A. Laken and A.W. Wolfendale, Journal of Atmospheric and Solar-Terrestrial Physics, doi:10.1016/j.jastp.2011.03.001.

Cosmic rays and global warming – Erlykin et al. (2010) A brief review article. “Is global warming man made or is it caused by the effects of solar activity on cosmic rays as claimed by some? Here we describe our search for evidence to distinguish between these claims. … In our view the jury is back and the verdict is that cosmic rays and solar irradiance are not guilty for most of the Global Warming. Nevertheless, they could be responsible for a contribution and we look forward to future experiments such as CLOUD at CERN which should be able to quantify to what extent ionization plays a part in the production of aerosols, the precursors of cloud formation.” [Full text]

Sudden Cosmic Ray Decreases: No Change of Global Cloud Cover – Calogovic et al. (2010) “Here we report on an alternative and stringent test of the CRC-hypothesis by searching for a possible influence of sudden GCR decreases (so-called Forbush decreases) on clouds. We find no response of global cloud cover to Forbush decreases at any altitude and latitude.” [Full text]

Cosmic ray decreases and changes in the liquid water cloud fraction over the oceans – Laken et al. (2009) “Svensmark et al. (2009) have recently claimed that strong galactic cosmic ray (GCR) decreases during ‘Forbush Decrease (FD) events’ are followed by decreases in both the global liquid water cloud fraction (LCF) and other closely correlated atmospheric parameters. To test the validity of these findings we have concentrated on just one property, the MODIS LCF and examined two aspects: 1) The statistical chance that the decrease observed in the LCF is abnormal. 2) The likelihood of the observed delay (∼5 to 9 days) being physically connected to the FD events. On both counts we conclude that LCF variations are unrelated to FD events: Both the pattern and timing of observed LCF changes are irreconcilable with current theoretical pathways. Additionally, a zonal analysis of LCF variations also offers no support to the claimed relationship, as the observed anomaly is not found to vary latitudinally in conjunction with cosmic ray intensity.” [Full text]

Results from the CERN pilot CLOUD experiment – Duplissy et al. (2009) “During a 4-week run in October–November 2006, a pilot experiment was performed at the CERN Proton Synchrotron in preparation for the CLOUD1 experiment, whose aim is to study the possible influence of cosmic rays on clouds. … Overall, the exploratory measurements provide suggestive evidence for ion-induced nucleation or ion-ion recombination as sources of aerosol particles. … In conclusion, therefore, the experimental variables were not well enough controlled to exclude the presence of ion-induced nucleation on the basis of Fig. 7; it merely does not support the presence of strong contributions from this source.” [Full text]

On the correlation between cosmic ray intensity and cloud cover – Erlykin et al. (2009) “Various aspects of the connection between cloud cover (CC) and cosmic rays (CR) are analyzed. Most features of this connection viz. an altitude dependence of the absolute values of CC and CR intensity, no evidence for the correlation between the ionization of the atmosphere and cloudiness, the absence of correlations in short-term low cloud cover (LCC) and CR variations indicate that there is no direct causal connection between LCC and CR in spite of the evident long-term correlation between them. … The most significant argument against causal connection of CR and LCC is the anticorrelation between LCC and the medium cloud cover (MCC).” [Full text]

Atmospheric data over a solar cycle: no connection between galactic cosmic rays and new particle formation – Kulmala et al. (2009) “More than a decade ago, variations in galactic cosmic rays were suggested to closely correlate with variations in atmospheric cloud cover and therefore constitute a driving force behind aerosol-cloud-climate interactions. Later, the enhancement of atmospheric aerosol particle formation by ions generated from cosmic rays was proposed as a physical mechanism explaining this correlation. Here, we report unique observations on atmospheric aerosol formation based on measurements at the SMEAR II station, Finland, over a solar cycle (years 1996–2008) that shed new light on these presumed relationships. Our analysis shows that none of the quantities related to aerosol formation correlates with the cosmic ray-induced ionisation intensity (CRII). We also examined the contribution of ions to new particle formation on the basis of novel ground-based and airborne observations. A consistent result is that ion-induced formation contributes typically less than 10% to the number of new particles, which would explain the missing correlation between CRII and aerosol formation.” [Full text]

Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? – Pierce & Adams (2009) “In this paper, we present the first calculations of the magnitude of the ion-aerosol clear-air mechanism using a general circulation model with online aerosol microphysics. In our simulations, changes in CCN from changes in cosmic rays during a solar cycle are two orders of magnitude too small to account for the observed changes in cloud properties; consequently, we conclude that the hypothesized effect is too small to play a significant role in current climate change.”

On the possible connection between cosmic rays and clouds – Erlykin et al. (2009) “Various aspects of the connection between cloud cover (CC) and cosmic rays (CR) are analysed. We argue that the anticorrelation between the temporal behaviour of low (LCC) and middle (MCC) clouds evidences against causal connection between them and CR. Nevertheless, if a part of low clouds (LCC) is connected and varies with CR, then its most likely value averaged over the Globe should not exceed 20% at the two standard deviation level.” [Full text]

Solar activity and the mean global temperature – Erlykin et al. (2009) This study finds that the changes in the cosmic ray rate lags the changes in temperature. “The cyclic variation in the cosmic ray rate is observed to be delayed by 2–4 years relative to the temperature, the solar irradiance and daily sun spot variations suggesting that the origin of the correlation is more likely to be direct solar activity than cosmic rays. Assuming that the correlation is caused by such solar activity, we deduce that the maximum recent increase in the mean surface temperature of the Earth which can be ascribed to this activity is ~<14% of the observed global warming.” [Full text]

Cosmic rays, cloud condensation nuclei and clouds – a reassessment using MODIS data – Kristjánsson et al. (2008) “Averaging the results from the 22 Forbush decrease events that were considered, no statistically significant correlations were found between any of the four cloud parameters and GCR, when autocorrelations were taken into account.” [Full text]

Testing the proposed causal link between cosmic rays and cloud cover – Sloan & Wolfendale (2008) “A decrease in the globally averaged low level cloud cover, deduced from the ISCCP infrared data, as the cosmic ray intensity decreased during the solar cycle 22 was observed by two groups. The groups went on to hypothesize that the decrease in ionization due to cosmic rays causes the decrease in cloud cover, thereby explaining a large part of the currently observed global warming. We have examined this hypothesis to look for evidence to corroborate it. None has been found and so our conclusions are to doubt it. From the absence of corroborative evidence, we estimate that less than 23%, at the 95% confidence level, of the 11 year cycle change in the globally averaged cloud cover observed in solar cycle 22 is due to the change in the rate of ionization from the solar modulation of cosmic rays. “ [Full text]

Cosmic Rays and The Climate – Sloan (2008) Summarizes different views on the issue. “A number of papers and posters were presented at the ECRS on the subject of the relationship between cosmic rays (CR) and both the climate and the weather. I was asked by the organisers to attempt to summarise them.” [Full text]

Cosmic Rays and Global Warming – Sloan & Wolfendale (2007) “It has been claimed by others that observed temporal correlations of terrestrial cloud cover with `the cosmic ray intensity’ are causal. The possibility arises, therefore, of a connection between cosmic rays and Global Warming. If true, the implications would be very great. We have examined this claim to look for evidence to corroborate it. So far we have not found any and so our tentative conclusions are to doubt it. Such correlations as appear are more likely to be due to the small variations in solar irradiance, which, of course, correlate with cosmic rays. We estimate that less than 15% of the 11-year cycle warming variations are due to cosmic rays and less than 2% of the warming over the last 35 years is due to this cause.” [Full text]

Solar activity, cosmic rays, clouds and climate – an update – Kristjánsson et al. (2004) “Eighteen years of monthly averaged low cloud cover data from the International Satellite Cloud Climatology Project are correlated with both total solar irradiance and galactic cosmic ray flux from neutron monitors. When globally averaged low cloud cover is considered, consistently higher correlations (but with opposite sign) are found between low cloud variations and solar irradiance variations than between variations in cosmic ray flux and low cloud cover.” [Full text]

Pattern of Strange Errors Plagues Solar Activity and Terrestrial Climate Data – Damon & Laut (2004) “Links have been made between cosmic rays and cloud cover, first total cloud cover and then only low clouds, and between solar cycle lengths and northern hemisphere land temperatures. … Analysis of a number of published graphs that have played a major role in these debates and that have been claimed to support solar hypotheses shows that the apparent strong correlations displayed on these graphs have been obtained by incorrect handling of the physical data.” [Full text]

Solar activity and terrestrial climate: an analysis of some purported correlations – Laut (2003) “The last decade has seen a revival of various hypotheses claiming a strong correlation between solar activity and a number of terrestrial climate parameters: Links between cosmic rays and cloud cover, first total cloud cover and then only low clouds, and between solar cycle lengths and Northern Hemisphere land temperatures. These hypotheses play an important role in the scientific as well as in the public debate about the possibility or reality of a man-made global climate change. I have analyzed a number of published graphs which have played a major role in these debates and which have been claimed to support solar hypotheses. My analyses show that the apparent strong correlations displayed on these graphs have been obtained by an incorrect handling of the physical data.” [Full text]

Cosmic Rays, Clouds, and Climate – Carslaw et al. (2002) A review paper. “It has been proposed that Earth’s climate could be affected by changes in cloudiness caused by variations in the intensity of galactic cosmic rays in the atmosphere. This proposal stems from an observed correlation between cosmic ray intensity and Earth’s average cloud cover over the course of one solar cycle. Some scientists question the reliability of the observations, whereas others, who accept them as reliable, suggest that the correlation may be caused by other physical phenomena with decadal periods or by a response to volcanic activity or El Niño.” [Full text]

A new look at possible connections between solar activity, clouds and climate – Kristjánsson et al. (2002) “We present a re-evaluation of the hypothesis of a coupling between galactic cosmic rays, clouds and climate. We have used two independent estimates of low cloud cover from the International Satellite Cloud Climatology Project, covering 16.5 years of data. The cloud cover data are used in conjunction with estimates of galactic cosmic ray flux and measurements of solar irradiance. It is found that solar irradiance correlates better and more consistently with low cloud cover than cosmic ray flux does. The correlations are considerably lower when multichannel retrievals during daytime are used than retrievals using IR-channels only.” [Full text]

Some results relevant to the discussion of a possible link between cosmic rays and the Earth’s climate – Wagner et al. (2001) “However, the smoothed combined flux of ¹⁰Be and ³⁶Cl at Summit, Greenland, from 20–60 kyr B.P. (proportional to the geomagnetically modulated cosmic ray flux) is unrelated to the corresponding δ¹⁸O and CH₄ data (interpreted as supraregional climate proxies). (3) Furthermore, although a comparison of the incoming neutron flux with cloud cover in Switzerland over the last 5 decades shows a significant correlation at times during the 1980s and 1990s, this does not occur during the rest of the period.” [Full text]

Sunshine records from Ireland: cloud factors and possible links to solar activity and cosmic rays – Pallé & Butler (2001) “The importance of cosmic rays as a link between solar activity and climate was assessed from a study of the ISCCP-D2 satellite cloud factors and Irish sunshine data. Whilst these results confirmed the strong correlation between total cloud factor and cosmic rays over non-tropical oceans between 1984 and 1991 previously reported, it was found that this correlation did not hold in the subsequent period 1991-1994. Other work has established a link through specifically low cloud. Indirect evidence of cloud formation by cosmic rays from a variation in the sunshine factor following Forbush decreases, and over the sunspot cycle, was mostly negative. Although a dip at seven years past sunspot minimum is evident in the sunshine factor for all four sites and in most seasons, it is of marginal statistical significance.” [Full text]

Cloud cover variations over the United States: An influence of cosmic rays or solar variability? – Udelhofen & Cess (2001) “To investigate whether galactic cosmic rays (GCR) may influence cloud cover variations, we analyze cloud cover anomalies from 1900–1987 over the United States. … The cloud cover variations are in phase with the solar cycle and not the GCR.”

Is there a cosmic ray signal in recent variations in global cloudiness and cloud radiative forcing? – Kristjánsson et al. (2000) “In order to evaluate a recent hypothesis of a coupling between galactic cosmic rays, clouds, and climate we have investigated temporal variations in global cloudiness and radiative fluxes at the top of the atmosphere. … When the results are related to temporal variations in cosmic ray activity, we do not find support for a coupling between cosmic rays, total cloudiness, and radiative forcing of climate. … The net radiative effect of clouds during the period 1985–1989 shows an enhanced cooling effect despite a reduction in both total and low cloud cover. This contradicts the simple relationship between cloud cover and radiation assumed in the cosmic-ray-cloud-climate hypothesis.”

Are Cosmic Rays Influencing Oceanic Cloud Coverage – Or Is It Only El Niño? – Farrar (2000) “The monthly average (C2) cloud coverage data produced by the International Satellite Cloud Climatology Project (ISCCP) for the period of July 1986–June 1991 show strong global and regional cloud coverage variations associated with the El Niño of 1986–1987. The Pacific Ocean, in particular, shows strong regional variations in cloud coverage. These agree well with contemporaneous satellite observations of broadband shortwave infrared cloud forcing measured by the Earth Radiation Budget Experiment. Svensmark and Friis-Christensen (1997) noted a similarity between the shape of the timeseries curve of average cloud coverage fraction for mid- to low-latitude ocean-areas and the time series curve of cosmic ray flux intensity. They proposed a causal relationship – a `missing link’ for solar cycle influence on Earth climate. Further spatial and temporal analysis of the same ISCCP C2 data in this paper indicates that the cloud coverage variation patterns are those to be expected for the atmospheric circulation changes characteristic of El Niño, weakening the case for cosmic rays as a climatic forcing factor.”

Closely related

Testing the link between terrestrial climate change and Galactic spiral arm transit – Overholt et al. (2009) Tests the correlation of climate changes and Earth’s passage through spiral arms of the Milky Way. Possible climate effects largely relate to cosmic rays. “We re-examine past suggestions of a close link between terrestrial climate change and the Sun’s transit of spiral arms in its path through the Milky Way galaxy. These links produced concrete fits, deriving the unknown spiral pattern speed from terrestrial climate correlations. We test these fits against new data on spiral structure based on CO data that do not make simplifying assumptions about symmetry and circular rotation. If we compare the times of these transits with changes in the climate of Earth, the claimed correlations not only disappear, but we also find that they cannot be resurrected for any reasonable pattern speed.” [Full text]

Toward Direct Measurement of Atmospheric Nucleation – Kulmala et al. (2007) A paper on the results of SMEAR project which (among other activities) provides direct measurements of atmospheric nucleation. They find that ion-induced nucleation is not very important (ion-induced nucleation fraction is only 10 % of total nucleation at best). “We introduce an instrumental setup to measure atmospheric concentrations of both neutral and charged nanometer-sized clusters. By applying the instruments in the field, we come to three important conclusions: … (iii) neutral nucleation dominates over the ion-induced mechanism, at least in boreal forest conditions.”

For those interested in the SMEAR project results, see the presentation of Markku Kulmala in “Climate Change – Man Made?” seminar in Stockholm (2009) (click the “cosmic rays and climate change”, Kulmala’s presentation starts after Svensmark’s, about at 00:34:15).

There are plenty of papers which deal with this cosmic ray issue while concentrating solar forcing as a whole (for example a string of papers from Lockwood & Fröhlich). Many of those papers would belong to the list above, but I shall make a separate entry on them, and add link to that post here when I have made it (separate post is already on the works). UPDATE (September 3, 2009): Here is the link to the post about the Sun’s role.

Original claims of Svensmark et al. were based on the apparent correlation between the cosmic rays and an observed decreasing trend in ISCCP cloud cover data, but it has been found out that ISCCP trend was an artifact of satellite viewing geometry, so it seems that there is no observational basis for the original claim. [UPDATE (March 17, 2010): I have recently discussed about this here.] There are few papers discussing this, and I will give one of them below.

Arguments against a physical long-term trend in global ISCCP cloud amounts – Evan et al. (2007) “Here we show that trends observed in the ISCCP data are satellite viewing geometry artifacts and are not related to physical changes in the atmosphere. Our results suggest that in its current form, the ISCCP data may not be appropriate for certain long-term global studies, especially those focused on trends.” [Full text]

UPDATE (September 10, 2009): As it has been suggested that cosmic rays affect by changing the cloud cover, this is relevant here:
Papers on global cloud cover trends

Update history

UPDATE (January 2, 2012): Laken & Čalogović (2012) added and my opinion statement removed.
UPDATE (September 26, 2011): Agee et al. (2011) added.
UPDATE (August 16, 2011): Sloan & Wolfendale (2011) added.
UPDATE (April 27, 2011): Erlykin et al. (2011) added.
UPDATE (April 4, 2010): Farrar (2000) added, thanks to Paul Farrar for pointing it out, see the comment section below.
UPDATE (March 23, 2010): Erlykin et al. (2010) added, thanks to Pekka Pirilä for pointing it out (elsewhere).
UPDATE (February 23, 2010): Pallé & Butler (2001) added, thanks to Barry for pointing it out, see the comment section below.
UPDATE (February 8, 2010): Wagner et al. (2001) added.
UPDATE (January 8, 2010): Laken et al. (2009) added. Calogovic et al. (2010) added, thanks to PeterPan for pointing it out, see the comment section below. Kristjánsson et al. (2000), Udelhofen & Cess et al. (2001), Kristjánsson et al. (2002), Damon & Laut (2004), and Kristjánsson et al. (2004) added.
UPDATE (November 29, 2009): Duplissy et al. (2009) added.
UPDATE (October 26, 2009): Erlykin et al. (2009) added. Thanks to PeterPan for pointing it out, see the discussion section below. Update history section also added.
UPDATE (October 23, 2009): Overholt et al. (2009) and Pierce & Adams (2009) added. Thanks to PeterPan for pointing these out, see the discussion section below.
UPDATE (October 13, 2009): Kulmala et al. (2009) added.
UPDATE (October 8, 2009): I modified the text relating to the SMEAR project; I had misunderstood the bit about 1 % effect, so I left that part out and everyone can just check out Kulmala’s presentation and see what he says about that (thanks to Theo Kurtén for pointing this out).
UPDATE (October 7, 2009): Kulmala et al. (2007) and some more information on SMEAR project added (thanks to Theo Kurtén and Tuomas Helin for the information on this).