The Hebrew University debate on Global Warming

By | December 31, 2007

hebrew debate

[Illustrations, footnotes and references available in PDF version]

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On Sunday last week, a global warming debate was held at the Hebrew University, in front of a large public audience. The speakers included myself, and Prof. Nathan Paldor from the HU, on the so called sceptic side, and Prof. Dan Yakir (Weizmann) and Prof. Colin Price (Tel-Aviv Univ.) on the anthropogenic greenhouse gas (AGHG) side.

The panel. From left to right: Prof. Colin Price, Prof. Nathan Paldor, Prof. Dan Yakir, and myself.

You can watch the debate, in Hebrew at the Authority for Community and Youth of the Hebrew University. Since most of the readers are not from Israel (98% of the visitors to, here is a short synopsis. It is followed by a detailed response to the claims raised against the cosmic ray climate link.


Although it was called a debate, it wasn’t really one. It included 4 short presentations (about 12 mins each + 3 min for clarifying questions) and then another 45 mins of questions from the audience.

In my short presentations, I stressed a few major issues. First, there are no fingerprints proving that 20th century warming is necessarily human. Second, once you check the details, you find that there are notable inconsistencies. In particular, the AGHG theory predicts warming over the whole troposphere, while in reality, only the ground appears to have warmed over the past few decades, and that Earth’s climate response to volcanic eruptions is significantly smaller than predicted by computer models. This is because these models tend to have an exaggerated climate sensitivity. Third, the only reason we can attribute the warming to humans, is because allegedly there is nothing else to blame, but there is, the increasing activity of the sun. And then I quickly showed some of the evidence showing that the sun affects climate through the cosmic ray climate link.

The second speaker was Prof. Dan Yakir. He started by saying that there is really no place or need to hold such debates anymore since the vast majority of scientists believe that the warming is anthropogenic. He mentioned Gore’s nobel prize (yes, committees are to decide about scientific truths), Oreskes findings that from a 1000 papers, none contradict anthropogenic global warming, etc. He then attempted to debunk the cosmic ray – climate theory, some of his claims were supposed inconsistencies in the theory, and some were simply non-scientific arguments. Since I was not given a chance to address these claims, the response to each and every point raised can be found below.

The third speaker was Prof. Nathan Paldor. He emphasized the large uncertainties in our current understanding of climate systems. One such example was that of global dimming. Because of these large uncertainties, computer based modeling of 20th century warming or predictions of future climate change is mostly pointless at this time. He mentioned the 70’s during which scientists urged Nixon to prepare the US for the upcoming ice-age, especially considering that the Soviets are better prepared for it!

The fourth speaker was Prof. Price, who emphasized the agreements between computer model predictions of AGHG theory and the observations. He showed, for example, that computer models trying to model 20th warming with only natural radiative forcings cannot explain the observed temperature trend, and models with anthropogenic contributions added, can explain. He then continued by trying to debunk the cosmic-ray climate theory. He mentioned several inconsistencies in the cosmic ray climate link (at least so he supposed). He also showed that for the cosmic ray climate mechanism to work, it rests on many links, some of which he doubted.

Before addressing the critiques, let me add that neither Yakir nor Price brought any evidence proving the standard anthropogenic scenario. Yakir did not attempt to prove anything (there is no need since the majority of scientists anyway support it), and Price did bring supporting evidence, but evidence that in reality does not prove anything about the validity of the AGHG theory.

The main claim raised by Price was that when computer models are used to fit 20th century warming, they do a very lousy job if you include all the natural forcing only, but they do a wonderful job if you include the anthropogenic forcing as well. This supposedly implies that one needs the large anthropogenic contribution to explain the warming. The key point here is that the natural forcings included all the known forcings, and not the unknown forcings, or specifically, the large indirect solar/climate link which these models fail to include, because the modelers bluntly neglect this mechanism. More about it here.

Although Yakir and Price did have a chance to address the critiques I raised about the AGHG (unlike the opposite), they chose not to.

The claims against the cosmic ray climate link, and why they are wrong or irrelevant

Following are the specific claims raised by Profs. Yakir and Price against the cosmic ray climate link and explanations for why the claims are either irrelevant or wrong.

• The Danish group arbitrarily manipulated the cloud data to fit the cosmic ray flux variations: Although many would not like to admit it, there is a clear satellite cross-calibration problem with the ISCCP data, around 1994. This is clearly evident if one looks at the high altitude clouds which exhibit a very unnatural jump around the end of 1994. Although no dramatic climate events took place, there is a jump which is larger than the variations before or after the "event", as can be seen in the figure (from Marsh & Svensmark 2003).

Moreover, if one looks at where the jump actually took place on Earth, it is evidently in the footprints of 3 particular satellites.

In other words, not only is it clear that there was a calibration problem, it is clear which satellites are responsible for it. All that the Danish group did was to correct for this calibration problem. Interestingly, once it is rectified, the correlation continues (Svensmark 2007):

• The cosmic ray climate link is expected to be larger at higher altitudes, where the ionization rate is larger. Under some circumstances, this naive expectation could have been correct, except that the growth of condensation nuclei is more complicated than a simple relation to just the ion density. Near the oceans, ions are not abundant, unlike the aerosol building blocks of the condensation nuclei that are. Under such conditions, ions which where shown to be important in the formation of condensation nuclei become the bottleneck of the process. This of course assumes that other sources of cloud condensation nuclei are absent, which is why the effect is important only over the oceans.

Higher up the atmosphere, there are plenty of ions, but less aerosol building blocks, implying that the ions are not a bottleneck anymore. Thus, changing their abundance at high altitudes will not have a significant effect on the formation of condensation nuclei. This was shown also in a numerical simulation by Yu (2003).

• Prof. Yakir mentioned that if the solar/climate link is correct, there should be climate variations observed in sync with the 11-year solar cycle. In fact, the literature includes many analyses showing that the land and ocean surface temperatures vary by about 0.1°C between solar minimum and solar maximum (e.g., White et al. 1997, Douglass & Clader 2005, Shaviv 2005, and Camp & Tung 2007). This does not sound a lot, but it is about 10 time larger than one would expect from variations in just the solar luminosity.

Two examples showing an 11-year signal. Camp & Tung (2007), on the left, find the 11-year cycle after carrying out an eigenmode decomposition, while Shaviv (2005) finds the 11-year cycle by folding 2 centuries of surface temperature over the 11-year cycle.

• Prof. Yakir said that the CRF/climate theory is not consistent with the observed decrease in the diurnal temperature variations (namely, that the nights warmed more than the days). He expects this result because the warming associated with the CRF/climate link is through a decrease in the amount of clouds. A larger direct insolation during day time would increase the diurnal cycle, and not decrease it. However, Yakir is probably unaware that the CRF/climate link is expected to operate only where there is very little background cloud condensation nuclei, namely, in the marine layer over oceans. This means that direct variation of the cloud cover over land is not part of the link, it will only arise from the feedbacks in the system. Since we expect a warmer climate to have more water vapor in the atmosphere, we will also expect to form more clouds over land, and reduce the IR cooling during the nights, two effects that reduce the diurnal cycle.

• Prof. Price has shown a lack of correlation between cosmic ray and climate over a 20-30 year period (citing the work and showing the graphs of Lockwood and Frölich 2007). In short, L&F assume that there should be a one to one correspondence between cosmic rays and temperature, but they neglect the fact that the climate system is a low pass filter. It introduces delays which increase with the time scale of variations, at least up to about a century. For example, over a day, the maximum radiation is reached around noon while the highest temperature at around 2 pm. On the annual time scale, the maximum radiation to the northern hemisphere reaches around late June, but the warmest period is typically more than a month later.

Similarly, even though there was a reduction in the solar activity (and increase in the average cosmic ray flux) over the last solar cycle, the oceans still emit heat from the large amounts its absorbed over the latter half of the 20th century. Just comparing the average over 2 solar cycles (the 11-year signal was averaged out in the L&F graphs), is like comparing the temperature between noon and 2 pm with the solar insolation, and reaching the conclusion that solar radiation decreases the temperature. More about it here.

Incidentally, I find it to be either hypocritical or out of ignorance that people to call me "disingenuous" for mentioning the same heat capacity used in other aspects of global warming. When it suits their needs, it’s kosher, when it doesn’t, it’s disingenuous. Go figure.

The problematic links within the cosmic-ray climate picture, according to Prof. Price. The black arrows are undisputed links, the red ones are links Price disagrees on, or at least doubts. The added link is one that Prof. Price should have added, this is because forming small condensation nuclei and forming the large cloud condensation nuclei upon which the water vapor condensation takes place, is not the same thing.

• Prof. Price has shown a slide about the different steps of the cosmic ray climate link and where it is wrong or unproven. Before addressing the links doubted by Price, it should be noted that the slide should have included another step, as described in the figure.

Here are my comments about the "red arrowed" links:

1. Formation of condensation nuclei from atmospheric ions: This was shown in several results (Eichkorn et al., Harrison & Aplin 2001, Svensmark et al. 2007). The last is a full fledged cloud chamber mimicking oceanic conditions. It was found that increased ion density increase the formation rate of condensation nuclei. Flat and simple.

2. The formation of cloud condensation nuclei from condensation nuclei, namely, the formation of particles which are perhaps 100 larger (in size), required for the actual water vapor condensation. This step was not shown yet in the lab (e.g., the last experiment described above is not large enough – the particles stick to the walls before they have the time to grow to large particles), but it was shown in a numerical simulation that if the density of background particles is small, then the small condensation nuclei will coalesce to form the large cloud condensation nuclei.

3. Decrease in the cloud cover from a decrease in the cloud condensation nuclei density: The evidence for this effect is ship tracks (besides having ample theoretical support). Namely, in regions where there are more condensation nuclei (e.g., ship exhaust particles) the clouds are whiter.

4. Decrease cloud cover causes a warmer climate: Since the dominant effect is on low clouds (observed empirically, and expected theoretically), and since it is well known from the Earth Radiation Budget Experiment that low clouds have a strong cooling effect (that is, the cooling albedo [reflectivity] effect is much more important that the IR blocking [blanket] effect]), the reduction in low clouds from the decrease in atmospheric ionization will cause warming.

In addition, one should realize that there are many empirical findings showing that it is cosmic rays which are affecting the clouds and climate, and not something else which is more directly related to solar activity. These include my work on cosmic ray flux variations over geological time scales (Shaviv 2003, Shaviv & Veizer 2003), the same latitudinal dependence of the cloud variation as the high energy cosmic rays reaching the lower troposphere (Usoskin et al., 2004), correlations between cloud cover and short term cosmic ray variations (Harrison & Stephenson 2006), etc.

Non-Scientific Arguments:

• Prof. Yakir, and to a small extent Prof. Price used several non-scientific arguments to prove, or at least persuade, that the solar / cosmic-ray / climate link should not be taken seriously. For example, they mentioned that a nobel prize was given to Gore and the IPCC, that I belong to a very small minority, etc. All these kind of arguments are not relevant to any scientific debate.

• Prof. Yakir mentioned that Jan Veizer, a colleague of mine, changed his beliefs, as is evident from his last paper in nature, namely, that the cosmic ray flux climate link has lost its momentum and it is losing its supporters. This is wrong, as can be understood from Veizer’s comments .

• Prof. Yakir said that the cosmic ray climate theory is young and therefore should not be taken on the same par as the standard AGHG theory. This is of course a non-scientific argument. Since when is age a factor in the validity of a scientific theory? Irrespectively, even if the argument had any slight relevance, it is in fact wrong! The idea that cosmic rays could be a link between solar activity and climate was already suggested by Ney (1959). Dickenson (1975) already realized that ions could play a role in the formation of cloud condensation nuclei. Thus, it should have been no surprise that Svensmark and his colleagues discovered the empirical evidence supporting the existence of this link.


•Camp, C. D., and Tung, K. K., Surface warming by the solar cycle as revealed by the composite mean difference projection, Geophys. Res. Lett., 34, L14703, 2007.
•Dickinson, R. E., Solar Variability and the Lower Atmosphere, Bul. Am. Met. Soc., 56, 1240-1248, 1975.
•Douglass, D. H., and B. D. Clader, Climate sensitivity of the Earth to solar irradiance, Geophys. Res. Lett., 29(16), 1786, 2002.
•Eichkorn, S., S. Wilhelm, H. Aufmhoff, K. H. Wohlfrom, and F. Arnold, Cosmic ray-induced aerosol formation: first observational evidence from aircraft based ion mass spectrometer measurements in the upper troposphere, Geophys. Res. Lett., 29, 10.1029/2002GL015,044, 2003.
•Harrison, R. G., and K. L. Aplin, Atmospheric condensation nuclei formation and high-energy radiation, J. Atmos. Terr. Phys., 63, 1811–1819, 2001.
•Harrison, R. G., and Stephenson, D. B. Empirical evidence for a nonlinear effect of galactic cosmic rays on clouds, Prof. R. Soc. A, doi:10.1098/rspa.2005.1628, 2006.
•Lockwood, M., & C. Fröhlich, Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature, Proc. R. Soc. A doi:10.1098/rspa.2007.1880; 2007.
•Marsh, N., and H. Svensmark, Galactic cosmic ray and El Niño–Southern Oscillation trends in International Satellite Cloud Climatology Project D2 low-cloud properties, J. Geophys. Res., 108(D6), 4195, doi:10.1029/2001JD001264, 2003.
•Ney, E. P., Cosmic radiation and weather, Nature, 183, 451, 1959.
•Shaviv, N. J., The spiral structure of the milky way, cosmic rays, and ice age epochs on earth, New Astron., 8, 39–77, 2003
•Shaviv, N. J., On climate response to changes in the cosmic ray flux and radiative budget, J. Geophys. Res, 110, A08105, 2005.
•Shaviv, N. J., and J. Veizer, A celestial driver of phanerozoic climate?, GSA Today, 13, 4–11, 2003.
•Svensmark, H., Cosmoclimatology: A New Theory Emerges, Astron. Geophys., 58, 1.19-1.24., 2007.
•Svensmark, H. et al., Experimental evidence for the role of ions in particle nucleation under atmospheric conditions, Proc. Roy. Soc. A., 463, 385-396, 2007.
•Usoskin, I. G., N. Marsh, G. A. Kovaltsov, K. Mursula and O. G. Gladysheva, Latitudinal dependence of low cloud amount on cosmic ray induced ionization, Geophys. Res. Lett., 31, L16109, doi:10.1029/2004GL019507), 2004.
•White, W. B., J. Lean, D. R. Cayan, and M. D. Dettinger, Response of global upper ocean temperature to changing solar irradiance, J. Geophys. Res., 102, 3255 – 3266, 1997
•Yu, F., Altitude variations of cosmic ray induced production of aerosols: Implications for global cloudiness and climate, J. Geophy. Res., 107(A7), 10.1029/2001JA000248, 2002.

The Author

>This site is maintained by Nir J. Shaviv, who is an associate professor at the Racah Institute of Physics in the Hebrew University of Jerusalem. According to PhysicaPlus: "…his research interests cover a wide range of topics in astrophysics, most are related to the application of fluid dynamics, radiation transfer or high energy physics to a wide range of objects – from stars and compact objects to galaxies and the early universe. His studies on the possible relationships between cosmic rays intensity and the Earth’s climate, and the Milky Way’s Spiral Arms and Ice Age Epochs on Earth were widely echoed in the scientific literature, as well as in the general press."