The main weapon of the greenhouse gassers over the source of ‘recent’ warming in the last 30 years has been the claim that cosmic ray counts appear to be relatively trendless over the last 30 years (apart for solar cycle variability), and therefore cannot responsible for recent warming, supposedly putting the theory prominently associated with Henrik Svensmark under pressure.

For example, the lack of evidence of a downward trend in cosmic rays has been a recurring theme at realclimate, which they regard as one major weakness of Svensmark’s argument that GCR is responsible for recent warming.

I thought I would do a brief review of the evidence and found a number of reliable sources indicating that neutron fluxes have indeed been declining.

1. Long-Term Decline of South Pole Neutron Rates, J. W. Bieber, J. Clem, D. Desilets, P. Evenson, D. Lal, C. Lopate, and R. Pyle, J. Geophys. Res., 112, A12102, doi:10.1029/2006JA011894, 2007.

We suggest that the South Pole monitor, owing to its unique position at both high latitude and high altitude (2820 m), has enhanced sensitivity at 1–3 GV relative to a sea level monitor and may be responding to a change in the intensity of primary cosmic rays in this rigidity region. Measurements of cosmic rays made aboard stratospheric balloons and on the IMP-8 spacecraft support the possibility of a long-term change in cosmic ray intensity.

The paper considers instrument errors as a possibility, but also lists the other independent evidence of long term declines in cosmic rays retrieved from balloons, satellites and radioisotopes.

In the past we presumed this anomalous downtrend at South Pole was artificial and resulted from some unknown instrumental instability perhaps related to the harsh Antarctic environment. Now, however, we are led to reevaluate this presumption on the basis of recent reports of long-term secular changes in the primary cosmic ray flux.

[5] One such report is from cosmic ray measurements
made in the stratosphere with balloon borne instruments [Bazilevskaya and Svirzhevskaya, 1998]. Over a 30-year period the solar minimum value of the stratospheric flux decreases by 2.5% [cf. Stozhkov et al., 2000; Ahluwalia, 2000]. In comparison, our best estimate of the decrease measured at South Pole over the interval 1965–1997 is 8%. (This is somewhat less than that displayed in Figure 1 owing to the renormalization of the IGY neutron monitor rate discussed below.) A long-term decline has also been reported in the >95 MeV proton rate measured aboard the IMP-7 and IMP-8 satellites [Ahluwalia and Lopate, 2001].

Further, these direct measurements of a cosmic ray decrease during the latter half of the Twentieth Century are broadly consistent with long-term trends inferred from measurements of cosmogenic isotopes [Beer, 2000; McCracken and McDonald, 2001; McCracken et al., 2004b].

It appears that the reason for the constant GCR observed in terrestrial neutron detectors is because they can only observe neutrons above a certain energy, due to absorption of lower energy particles in the atmosphere, unless the detector is situated at the poles and altitude, where absorption is much less.

The formation of cosmogenic isotopes would be the most reliable indicator of time integrated cosmic ray flux liable to generate cloud nuclei and affect cloud amounts.

In fact, decline in abundance of the cosmogenic isotope Be10 has been recorded in polar ice over the last century in this paper:

2. A physical reconstruction of cosmic ray intensity since 1610 by Ilya G. Usoskin, Kalevi Mursula, Sami K. Solanki and Manfred Schu¨ssler, and Gennady A. Kovaltsov, presents solid evidence by way of polar Be10 counts, for a strong for a general declining trend in cosmic rays, that could in fact explain increasing recent temperatures.

It looks increasingly like there is evidence that cosmic ray flux has decreased, and what more, the most reliable sources show a decline in Be10 that has continued to track the increase in recent temperatures. Svensmark will be vindicated.

Other interesting links:

http://articles.adsabs.harvard.edu//full/2001ICRC…10.4129M/0004132.000.html

http://solar.physics.montana.edu/SVECSE2008/pdf/steinhilber_svecse.pdf