If the shooting of fish in barrels offends you, look away. The publication this week of a Nature paper on global warming on Mars offers a fantastic opportunity to kill off one of the silliest climate-sceptic arguments, and I'm more than happy to be pointing the gun at the water.

The sceptical 'argument' — using the word loosely — in question is that global warming on Earth should be seen as a natural, as opposed to anthropogenic, phenomenon because other planets and moons in the Solar System are getting warmer, too (which, indeed, they are). Since what the planets have in common is the Sun, they say, it must thus be the Sun that is driving the warming.

This idea, which I remember first coming across when there were reports of climate change on Mars some years back, regained a certain prominence in February when, a propos nothing much at all, National Geographic 's website dredged up the belief of a Russian astronomer, Habibullo Abdussamatov, that the warming on Mars shows that the Sun is driving climate change on Earth.

This not-really-news (no mention of new observations or publications) was seized on by columnists in a few right-wing papers and bundled together with other reports of climate change throughout the Solar System.

Lorne Gunter, in the National Post, pointed to warmings on Pluto, Neptune's moon Triton, Jupiter and Mars, promising his readers oh-so-wryly that he hadn't left his SUV idling on any of these planets or moons (if you can face it, you can find id=469a4d51-802a-23ad-4e79-696cef1e9d88&Regionid=&Issue_id=">this article archived under 'Fact of the Day' on the web pages of the US Senate's Environment and Public Works committee, thanks to the committee's ranking minority member, James Inhofe (Republican, Oklahoma)).

Former Senator for Tennessee and possible Republican presidential hopeful Fred Thompson expanded on this theme on a radio programme a few days later. In an unexpected demonstration of astronomical acumen he referred to change on "planets, dwarf planets and moons", showing that he — or someone — paid enough attention to such matters to have noticed the recent reclassification of Pluto. Unfortunately, he rather undermined the effect by then comparing, absurdly, the treatment of today's climate sceptics to the inquisition's treatment of Galileo.

Solar spin

Before we take a quick spin around the Solar System looking at these ideas, it is worth noting that the said system contains, in all, ten bodies with atmospheres thick enough to provide something we might call a climate. If these ten climates are all subject to a little natural variation, as the climate on Earth is, then finding that half of them are showing some warming at any given time is hardly surprising.

It is also worth noting that the Sun's radiance is measured from Earth orbit, and these records do not show it increasing over the past few decades, except with the regular rise and fall of the solar cycle. This second fact, you might think, should be enough to scupper the theory about system-wide solar warming on its own; strangely it is notably absent from accounts of the matter.

Moving on to the particulars, in the cases of Pluto and Triton, Neptune's largest moon, the observed warming is due to their current orientation to and distance from the Sun — technically known as summer.

Pluto was closest to the Sun in 1989 and is now moving away, but it is still relatively close. It's not that surprising for the greatest warmth to come a little after the closest approach, any more than it is for afternoons to be warmer than noons. And Triton's orbit is giving its southern hemisphere a particularly hot summer, boiling off frozen material from the southern pole and thickening the atmosphere, keeping in even more heat.

On Jupiter, things are a little different. The patterns of circulation seem to be changing, such that heat at the equator is stuck there, and higher latitudes are getting a little cooler.

On Mars, the warming seems to be down to dust blowing around and uncovering big patches of black basaltic rock that heat up in the day (see 'Mars hots up'). No change in sunshine required.

To take this disparate hodge-podge of phenomena and try to construct a theory of solar influence from it is the sort of foolishness people get driven to when desperate to support a failed theory, or just for a chance to muddy the waters.

The weather elsewhere

What's saddening is that people should miss what these various phenomena really have in common — their explicability. They show that our ideas of atmospheric physics are applicable and useful on bodies that range from the tiny (Pluto, the atmosphere of which is hardly worth mentioning) to the gigantic (Jupiter, the atmosphere of which outweighs a hundred solid Earths).

And computer models based on the ones used to study the climate on Earth provide results even when applied to the hugely different conditions on Mars. That is truly impressive.

So what these disparate observations actually tell us is that the scientific community — the scientific community that enjoys a firm consensus on the causes of Earthly climatic change — has a fairly impressive grasp of the fundamentals of how weather works elsewhere, as well. It's a rather inspiring insight. But it is not the lesson that climate sceptics want their readers to learn.

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