Intet nytt under solen

Et av kronargumentene til klimaskeptikere er at Solen indirekte står for hoveddelen av global oppvarming, og at å redusere CO2 -utslipp derfor er meningsløst og uten effekt. Channel4 laget i 2007 en dokumentar ved navn «The Great Global Warming Swindle» som hevdet at kosmisk stråling var hovedårsaken til den globale oppvarmingen:

[…] the amount of low altitude cloud seems to follow changes in the rate at which cosmic rays fall on Earth.[…]Such particles produce ionizing radiation in the atmosphere; under certain conditions water droplets condense on the ions formed […]

[…]

The observation that low cloud cover decreases as the cosmic ray rate drops led to the proposal that ionization from cosmic rays is responsible for the production of clouds i.e. less ionization leads to less cloud. The rate at which cosmic rays fall on the Earth has been observed to decrease over the past century or so.

[…]

So if cosmic rays produce clouds and there are fewer cosmic rays nowadays we will have less cloud cover than we once had, allowing more of the sun’s heat to penetrate to the Earth’s surface and produce the observed global warming.

Og hva har Solen med dette å gjøre?

The sun spews out streams of charged particles, as well as heat and light. The magnetic fields from these charged particles deflect some of the cosmic rays entering the solar system from surrounding space, i.e. galactic cosmic rays. During its periods of high activity the sun emits more such particles. So when the sun is highly active, more galactic cosmic rays are deflected and there is a decrease in the rate of cosmic rays reaching Earth.

Klimaskeptikere jublet og hyllet dokumentaren, mens mange vitenskapsmenn rettet kraftig kritikk mot påstandene filmen serverte.

Solens rolle i global oppvarming har vært studert før uten at den har vist seg å spille noen vesentlig faktor. Men den nye hypotesen om kosmisk stråling har nå blitt studert av et par britiske forskere.

Utgangspunktet er som følger:

The International Panel on Climate Change (IPCC) has concluded that it is 90% probable that global warming is caused by manmade greenhouse gases largely emitted since industrialisation.

[…]

For the IPCC to be wrong, these models must be incorrect, despite the many years of research effort spent developing them. Those of us accustomed to dealing with complicated computer models know that sometimes they can arrive at wrong answers so let’s suppose that the models are indeed wrong. The next question is what can be causing the global warming? That is, we need a new effect which has not yet been discovered to account for the observed rise in temperature. So we need two things to occur before we can declare that the IPCC is wrong. Firstly, the modelling has to be incorrect and secondly, we need an as-yet undiscovered effect to account for the observed global warming.

Forskerne setter så opp hypotesen om at kosmisk stråling har forårsaket den påviste oppvarmingen, og prøver deretter å falsifisere denne hypotesen – hele kjernen i vitenskapelig metode. Etter å ha undersøkt to alternative modeller, kommer de frem til en konklusjon:

To conclude, we could find no evidence that changing cosmic ray rates contribute significantly to changing cloud cover and thereby to global warming. Therefore, cosmic rays are not the new effect that would allow us to declare that the IPCC is wrong. So it is back to cutting our greenhouse gas emissions if we are to avoid the future problems foreseen from climate change.