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It's the sun
Climate's changed before
There is no consensus
Surface temp is unreliable
Models are unreliable
It's cooling
Ice age predicted in the 70's
Al Gore got it wrong
We're heading into an ice age
CO2 lags temperature
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It's cosmic rays

The skeptic argument...

Fig1When solar activity is high, galactic cosmic rays are reduced due to increased magnetic shielding by the Sun. Cosmic rays may influence Earth's climate through formation of low lying clouds.
They ionize the atmosphere and an experiment found production of aerosols depends on the amount of ionization. As aerosols are precursors for formation of cloud droplets, this indicates cosmic rays may influence cloud formation. (Source: Danish National Space Center).

What the science says...

While the link between cosmic rays and cloud cover is yet to be confirmed, more importantly, there has been no correlation between cosmic rays and global temperatures over the last 30 years of global warming.

The fatal flaw in the theory that cosmic rays (or lack thereof) are driving global warming is that cosmic radiation has shown no correlation with global temperatures since 1970.

This led the Max Planck Institute (Krivova 2003) to conclude that "between 1970 and 1985 the cosmic ray flux, although still behaving similarly to the temperature, in fact lags it and cannot be the cause of its rise. Thus changes in the cosmic ray flux cannot be responsible for more than 15% of the temperature increase."

Similarly, Lockwood 2007 compared neutron monitor measurements, Beryllium 10 and Carbon 14 isotopes (both proxies for cosmic radiation) as well as several other measures of solar activity and concluded "the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanism is invoked and no matter how much the solar variation is amplified."

Other difficulties with cosmic ray theory

There are other problems proving the causality link between cosmic rays and cloud formation. This is not to disprove the theory but just to show there are enough question marks to show some healthy skepticism about cosmic ray theory.

Breakdown in the correlation between low clouds and cosmic rays
One of the key proofs of Svensmark's cosmic ray theory is the high correlation between low cloud cover and cosmic rays. However, the correlation broke down in 1991 (Laut 2003). At that point, cloud cover began to lags cosmic ray trends by over 6 months while cloud formation should occur within several days (Yu 2000). The correlation completely breaks down in 1994.


Graph courtesy of Laut 2003. Low cloud cover (blue line) versus cosmic ray intensity (red line)

Svensmark explained the 6 month lag as data uncertainty (Svensmark 2003). He also claims the loss of correlation after 1994 is due to long term calibration drift with the ISCCP satellites (Marsh & Svensmark 2001). The ISCCP disagree.

Critique by Sloan and Wolfendale
A recent paper (Sloan 2007) scrutinizes the link between cosmic rays and cloud cover and finds several discrepancies. As cosmic radiation shows greater variation in high latitudes, one would expect larger changes in cloud cover in polar regions. This is not observed. Examining the nuclear reactor accident at Chernobyl, ionization from the radioactivity would be expected to produce an increase in cloud cover. There is no evident increase in cloud cover following the accident.

The bottom line is even if these difficulties can be resolved and the causality link between cosmic rays and cloud formation is proven, all they'll find is the cloud formation 50 years ago is similar to now and has had little to no impact on the last 30 years of long term global warming.

Further reading

A team of scientists from 17 countries have found the most likely origin of galactic cosmic rays - the centres of distant galaxies (Active Galactic Nuclei) powered by supermassive black holes. This discovery is not particularly pertinent to the global warming debate but it is cool :-)

Comments

  1. I always thought this hypothesis seemed unlikely, but...

    What about the last 10 years of global not warming? Why does CO2 get an 800 year wrong direction lag and the sun has to be exactly in step to be a viable hypothesis?
    [ Response: The point is that because the sun has correlated so closely with temperature in the past, when the correlation ended in the 1970's, it's reasonable to conclude some other forcing imposed itself on the climate. Re the CO2 lag, the ice core records actually confirm the amplifying effect of atmospheric CO2. As for the last 10 years of global not warming, you'll find the warming rate is the same over the last 10 years as over the last 30 years. ]
  2. Den siste mohikanen at 02:10 AM on 3 January, 2008
    First, the Harrison paper of 2006 state

    "...Furthermore, during sudden transient reductions in cosmic rays (e.g. Forbush events), simultaneous decreases occur in the diffuse fraction, showing that the diffuse radiation changes are unambiguously due to cosmic rays."

    http://www.cosis.net/abstracts/EGU06/07661/EGU06-J-07661.pdf

    Hence, it is pointless to argue that cosmic rays do not affect cloudiness. The correlation is there on a timescale of hours to decades, and as others have shown, on centennial to billion of years. The mechanism might not be what Svensmark has proposed but it doesn't change the basic fact if he is wrong about that.

    Secondly, your argument is nonsensical, as what Svensmark and other argue is not that cosmic rays can account for all climate influences. But if he is right - and evidence is piling up that he is - two things follows.

    1) the parametrisation in current GCMs are wrong as they fit past temperatures without taking this effect in consideration. Hence they are not reliable as tools for forecasts.

    2) climate sensitivity is overestimated by earlier attempts such as Hansens, as one major forcing was not considered when calculating those sensitivity values.

    What "sceptics" such as me claim is that there is precious little evidence to support the higher estimates on future temperatures as presented by UN (IPCC). And quite a bit of evidence against it. Emission scenarios is, well, rather extravagant, as they include projections of emissions many times higher than todays in year 2100 in spite of our likelhood to develop good alternatives to the ever more pricier fossile fuel (current trends are cutting the cost of renewables at half each decade). Climate scenarios based on these extravagant emission scenarios is then calculated with GCMs that are likely overestimating the response to a particular forcing. In general I would say sceptics accept that the climate warms when we add CO2 to the atmosphere, but we believe its effect will be muted by the climate systems rather than enhanced.

    I also want to add that I recognise all other environmental (and geopolitical) problems associated with burning fossile fuel and find that a compelling reason to put higher efforts in developing alternatives.
  3. Den siste mohikanen at 08:03 AM on 14 March, 2008
    Sir, you state that
    "The point is that because the sun has correlated so closely with temperature in the past, when the correlation ended in the 1970's, it's reasonable to conclude some other forcing imposed itself on the climate. Re the CO2 lag, the ice core records actually confirm the amplifying effect of atmospheric CO2. As for the last 10 years of global not warming, you'll find the warming rate is the same over the last 10 years as over the last 30 years."

    For sure, neither the sun nor CO2 nor the two together make up for the only climate forcing. So your argument is a bit weak by itself, but even if we assume that all the unexplained difference is due to CO2, that doesn't give as much room for IPCCs +6°C forecast that you seem to imply.

    If all the difference from Krivova et al graph is attributed to CO2 that means that burning half of our known reserves of oil and gas has yielded us less than 0,3°C in temperature change. Some may be in the pipeline but several degrees? The climatic response time would need to be many hundreds of years for that to be possible, which clearly is not the case with TSI & cosmic ray forcings.

    I would also think that the discussion would be less confusing if we started to define what kind of cosmic rays we are discussing. If I remember correctly Laut is using low energy CR but what Svensmark and others claim is that it is cosmic rays of a certain energy (around 10 GeV) that makes the difference: only these energies create the secondary particles needed for the ionisation over low altitudes far from land. Sometimes the CR of high and low energies follow eachother, other times they won't; hence we should clearly state what kind of cosmic rays we are refering to.
  4. Here are a few new studies in favor of the climate-GCR link:

    http://aps.arxiv.org/ftp/arxiv/papers/0803/0803.2765.pdf

    http://aps.arxiv.org/ftp/arxiv/papers/0803/0803.2766.pdf

    GCRs are certainly a fascinating aspect of climate.

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