When you peruse the many sceptic arguments against man-made global warming, you find a tendency to focus on a narrow piece of the puzzle while ignoring the broader picture. This narrow focus serves as a useful distraction from the scientific realities of global warming.

A recent example is the campaign to sow doubts about the US temperature record. To achieve this, an army of volunteers traversed the US photographing weather stations. Pictures were posted on surfacestations.org, showing weather stations positioned near heated buildings, air conditioners and other sources of artificial heat.

Each new photo was greeted with a clucking of tongues and a sense of reaffirmation among sceptics that global warming was largely the product of suspect temperature data. "How do we know if global warming is a problem if we can't trust the temperature record?" asked Anthony Watts who runs the sceptic blog Wattsupwiththat.

Never mind that the Greenland ice sheet is losing ice at an accelerating rate. That Antarctic ice loss is also accelerating, including east Antarctica which until late 2009 was thought too cold and stable to lose ice. Arctic sea ice is melting, sea levels are rising and glaciers are retreating. These and many other physical realities of global warming are well documented in the peer-reviewed literature. However, to some, the accumulated body of empirical data is no match against the persuasive power of a well-framed photograph.

The photos were compiled into a single report by Watts and published by the Heartland Institute, a thinktank that funds climate sceptic activities. For good measure, infrared photos were included to visually drive the point home. Using the National Oceanic and Atmospheric Administration's own classifications, Watts divided the weather stations into five categories. Well-sited stations, positioned well clear of roads, buildings and other heated surfaces, were given a rating one or two. Poorly sited stations, positioned in proximity to warming influences, were ratedthree, four or five. Most weather stations fell into the poorly sited categories. Watts suggested poor siting could contribute a warming of at least 1-5C to individual stations.

The report concludes:

We found stations located next to the exhaust fans of air conditioning units, surrounded by asphalt parking lots and roads, on blistering-hot rooftops, and near sidewalks and buildings that absorb and radiate heat ... The conclusion is inescapable: The US temperature record is unreliable. And since the US record is thought to be "the best in the world," it follows that the global database is likely similarly compromised and unreliable.

The crucial question though is how much extra warming do poorly sited weather stations contribute to the temperature record? Unfortunately, no amount of photos will answer this question. The only solution is data analysis, calculating the temperature trends from poor sites compared with good sites. Curiously, Watt's report contained no such data analysis. While page after page of photos may be effective in sowing doubt about the temperature record, they offer no actual answers on the impact of poor siting.

Finally this month, a peer-reviewed analysis of the temperature data was published in the Journal of Geophysical Research. The paper used Watt's station ratings to split all US weather stations into two categories: good (rating one or two) and bad (ratings three, four or five). The analysis then compared the raw, unadjusted data from the good and bad sites. In typical peer-reviewed understatement, the results were described as "counterintuitive". They were in fact, a great surprise to many. Poorly sited weather stations actually show a cooler trend compared to the good sites.

The cause of this cooling bias appears to have been a change in instruments. In the late 1980s, many sites converted from Cotton Region Shelters (CRS, otherwise known as Stevenson Screens) to electronic Maximum/Minimum Temperature Systems (MMTS). This had two effects. Firstly, MMTS sensors record lower daily maximums compared to their CRS counterparts. So the switch from CRS to MMTS sensors caused a cooling bias in certain stations.

Secondly, the MMTS sensors were attached by cable to an indoor readout device. Limited by cable length, the MMTS weather stations were often located closer to buildings and other artificial sources of heat. This meant most of the stations with the newer MMTS sensors also happened to fall under poorly sited categories. The net result is that poor stations show an overall cooler trend compared with good stations. However, when the change from CRS to MMTS is taken into account in data adjustments, the trend from good sites show close agreement with poor sites.

One might reasonably question whether the goal of surfacestations.org was to lead us into greater scientific truth or merely to sow doubt about the temperature record. Nevertheless, their efforts to rate each individual weather station enabled scientists to identify a cool bias in poor sites and isolate the cause. A net cooling bias was perhaps not the result the surfacestations.org volunteers were hoping for, but improving the quality of the surface temperature record is surely a result we should all appreciate.