By most estimates, global temperatures have been flat for about 15 years, even though many climate models predicted that temperatures should have risen steadily, as they did for most of the 1990s, given the continued increase in CO2 and other greenhouse gases in the atmosphere.

So what is going on? That is a critical question, because depending on the answer, appropriate policies relative to CO2 emissions (and costs to abate them) could be very different.

The amount of CO2 added into the atmosphere between 2000 and 2010 increased by about 100 billion metric tons - some 25% of all the CO2 put there by humanity since 1750.

And yet, James Hansen, the head of NASA's Goddard Institute for Space Studies, told The Economist magazine that "the five-year mean global temperature has been flat for a decade."

Ed Hawkins, a professor at the UK's University of Reading also has noted that surface temperatures since 2005 are already at the low end of the range of projections derived from 20 different climate models. If they remain flat, they will fall outside the models' range within a few years, he says.

So are the past 15 years just an aberration, and temperatures will again resume a rapid climb? Or is it possible that most of the models just have it wrong? Could it be that the 1990s, not the most recent decade, was really the anomaly?

The core of the issue is "carton sensitivity," generally defined as the increase in global temperatures for every doubling of CO2 in the atmosphere.

A doubling of CO2 emissions from the pre-industrial level of about 280 parts per million (ppm) to 560 ppm is estimated by many to warm the Earth by 1 degree Celsius on its own - a relatively modest increase that would be unlikely to cause many problems.

But most climatologists believe that CO2 does not operate on its own. CO2 emissions in turn increases in water vapor and cloud formation that could exacerbate the rise in temperatures. In addition, other pollutants such as soot are also being released in parallel with the CO2 that could also impact global temperature levels.

But how big is the impact from these other factors likely to be? That is where there appears to be a wide range of opinion.

The Intergovernmental Panel on Climate Change (IPCC) in the UK - from which the famous "hockey stick" graph showing a dramatic spike in global temperatures starting in 1900 - estimated in 2007 that the total temperate increase from that doubling of CO2 would be about 3° degrees C, and that it is "very unlikely to be less than 1.5 degrees C. Values higher than 4.5 degrees C cannot be excluded."

A new IPCC's report is due this September, and The Economist says a leaked preview copy showed the same basic range of likely outcome, but now with an upper limit of sensitivity that is a startling 6-7 degrees C.

But are the IPCC's calculations on CO2 sensitivity too aggressive? The Economist cites an as yet unpublished report by the Research Council of Norway that concludes "there is a 90% probability that doubling CO2 emissions will increase temperatures by only 1.2-2.9 degrees C, with the most likely figure being 1.9°C. The top of the study's range is well below the IPCC's upper estimates of likely sensitivity."

These numbers may seem like climate minutia to many, but the reality is that a 3 degree rise would likely cause much more environmental havoc than something closer to a 1.5 degree rise - and also give the world a lot more time to act to lower CO2 emissions.

The Economist says that Nic Lewis, an independent climate scientist, has a paper scheduled for publication is an academic journal that projects an even lower range of sensitivity, just 1.0-3.0 degrees C, with a mean of 1.6 degrees. Lewis' calculations reanalyzed work cited by the IPCC and took account of more recent temperature data. There are several other studies reaching similar conclusions, with most seeing little chance of the sort of cataclysmic increases of over 3 degrees.

That havoc that would ensue from increases over 3 degrees has led many to call for drastic actions to reduce carbon emissions now - likely at big costs to the economy and consumer s, and possibly even to quality of life.

"If, however, temperatures are likely to rise by only 2 degrees in response to a doubling of carbon emissions (and if the likelihood of a 6-degree increase is trivial), the calculation might change," The Economist notes. "Perhaps the world should seek to adjust to (rather than stop) the greenhouse gas splurge."

Researchers are now looking at all kinds of areas, such as whether the impact of factors such as aerosols and clouds may have been miscalculated and thus led to an overestimate of CO2 sensitivity. Or then again, maybe we have other factors that have led to a temporary temperature slowdown of late that will be reverse in due course.

Who is right? No one of course knows. But the world continues to pump more CO2 into the atmosphere every year, especially from developing economies such as China, and we can't see any likelihood of policies that would dramatically change that dynamic any time soon.

So it seems likely to us we'll find out who is right soon enough.

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