It appears, once again, that we have nothing to worry about. The Earth isn't going to warm up that much – a mere 1.64C – and even that will be a couple of centuries from now.

Who says so? Well, that's the strange thing. Apparently Nasa says so.

Even a website called Right Wing News was so startled by the news that it could hardly believe its own headline:

Any time an assertion triggers a "wait, what?" kind of response, the chances are it might deserve more than unconditional acceptance. Especially from people who like to describe themselves as "sceptics".

But Right Wing News and scores of other bloggers promoted this assertion without checking Nasa's website. That would have told them straight away there was something not so straightforward about this story.

It started onUK news site The Register, which has been known to turn climate facts upside down. Lewis Page, the author of a piece called New Nasa model: Doubled CO 2 means just 1.64C warming, wrote:

"It now appears, however, that the previous/current state of climate science may simply have been wrong and that there's really no need to get in an immediate flap. If Bounoua [Lahouari Bounoua of Nasa] and her colleagues are right, and CO 2 levels keep on rising the way they have been lately (about 2ppm each year), we can go a couple of centuries without any dangerous warming."

But how does Page know this? Unpicking his calculations gives us an insight into how a myth is put together, and why so many people fall for it.

The Register story starts off, at least, as a straight piece of reporting about a perfectly valid research study.

"A group of top Nasa boffins says that current climate models predicting global warming are far too gloomy, and have failed to properly account for an important cooling factor which will come into play as CO 2 levels rise. According to Lahouari Bounoua of Nasa's Goddard Space Flight Centre, and other scientists from Nasa and the US National Oceanic and Atmospheric Administration, existing models fail to accurately include the effects of rising CO 2 levels on green plants."

What The Register appeared not to know was that the researchers made three climate models. The first was a simple control model, which suggested that a doubling of carbon dioxide would lead to warming of around 1.94C.

In the third model they plugged in evapotranspiration, and found that warming dropped to 1.68C. So their conclusion was that evapotranspiration reduced warming by 0.26C.

But if the researchers worked out a figure of 1.68C, why did The Register tell us the warming would be 1.64C? The difference is small, but it tells a bigger story.

It appears as if Page took his figures from the accompanying Nasa press release, rather than the actual research paper. The problem is, while the Nasa press release had correctly reported the 1.94C of warming shown in the control model, it rounded up the reduction from evapotranspiration from 0.26 to 0.3C. So it seems that Page subtracted 0.3 from 1.94 and got the magic figure of 1.64.

Another mistake was in not understanding that the 1.94C of warming came from a control model. The researchers weren't announcing to the world that they had just discovered the correct warming of the Earth. On the contrary, they stated that their control result was at the low end of a range of other models from 2C to 4.5C. The purpose of the control was to plug in evapotranspiration data and work out the difference. There's nothing to suggest that the control model is any more or less correct than all the other models that have been produced.

So while Page appears to have taken this control figure, subtracted what he thinks is the negative feedback of evapotranspiration, and tells us we no longer have a problem with CO 2 concentrations, the researchers who actually wrote the paper came to a completely different conclusion.

"Bounoua stressed that while the model's results showed a negative feedback, it is not a strong enough response to alter the global warming trend that is expected," read the Nasa press release. It even had a quote from lead author Bounoua: "This feedback slows but does not alleviate the projected warming."

So that explains the spurious temperature projection. But what about the two centuries The Register says it'll take to get there? Page gives no source for this, but once again it's easy to see how he potentially figured it out on the back of an envelope.

The press release says that the researchers based their model on a doubling of CO 2 concentration. So it appears he doubled today's accepted figure of 390ppm [parts per million of CO 2 in the atmosphere], and came up with 780 ppm.

Next he needed a growth rate for CO 2 being pumped into the atmosphere – 2ppm per year. Divide 390 by 2, and you get 195 – which is roughly a "couple of centuries". No wonder people complain about the millions of dollars spent on scientific research, when all you need is five minutes and a pocket calculator.

Unfortunately this calculation is nonsense.

First, Page's assertion that a doubling of CO 2 meant 780ppm was a guess. The paper provides the actual figure that the researchers' model was based on.They state it quite clearly: 700ppm, not 780.

That's another 310ppm more than today, not 390. So at the assumed growth rate of 2ppm per year, it would take 155 years to reach this figure, not 195.

As for the growth rate itself, Page gives us a source, the Mauna Loa monitoring station in Hawaii. But this is only the current growth rate of CO 2 emissions. It assumes the growth rate won't increase and, more importantly, that the land and oceans will continue to absorb carbon dioxide at the same rate. Unfortunately, that absorbtion rate is already declining. (see Canadell et al, 2007)

A 2009 study by the Massachusetts Institute of Technology found that without curbs, CO 2 concentrations are likely to reach between 716 and 1,095ppm by 2095 – just 85 years away, not 155 or 195. (Sokolov et al, 2009)

Hyperbole apart, the Nasa study is important. The idea that evapotranspiration is a significant feedback system is not a new one, but the question is how much of an effect will it have is. Research published in October suggests that evapotranspiration rates are in fact declining, so the negative feedback may turn out to be less than the Nasa study has quantified. (Jung et al, 2010)

Whatever the case, the Register's failure to get figures right, a misunderstanding of what control models are supposed to do, and what appaer to be primary school sums on the back of an envelope combine to create a myth that spreads like a virus through the Internet and is attributed to Nasa.

No doubt those who read it will wonder why the mainstream media is not reporting this stupendous discovery. Must be more of that conspiracy we keep hearing about.