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Q. Press releases and reports on climate change, including the Met Office website, make statements such as "The climate IS changing: globally, the average temperature has risen by more than 0.7 °C over the last 100 years." Over 100 years, techniques for measurement and recording of data have changed beyond the imagination of scientists even just a few decades ago. May I ask how the question of measurement uncertainties has been addressed?
Even today, information on measurement uncertainties must vary from nation to nation, and even from station to station within the UK. For example, I have three external thermometers at my house, and they never read the same! Uncertainties of measurement from up to 100 years ago, from all over the globe, must be difficult to estimate. 0.7°C is a very small rise over 100 years: may I please ask what is the level of uncertainty associated with this value?
Another point: in order to support the global warming theory, has the Met Office changed its temperature predictions in its forecasts to quote the temperature expected in urban areas? The reason I ask is that the daytime maximum temperatures here at my home, which is in a rural location about 3km from the southwest coast of Cornwall, have rarely risen to the levels forecasted locally, and on many occasions have been several degrees lower. For example, the maximum temperature recorded here for the whole of April 2007 was no higher than 16°C. Finally, I must congratulate the Met Office on its superb website! (John Holden)
A. I'm pleased to hear you think our website, www.metoffice.gov.uk, is superb!! Measurement uncertainty is obviously a crucial issue and I can reassure you that a huge amount of work goes into making sure it is addressed. Meteorological measurements (both past and present) from the global network of weather stations undergo rigorous quality control – high-quality measurements are absolutely key to getting a reliable weather forecast. Observation stations are carefully set up to minimise factors which might bias the measurements: thermometers are placed in Stevensons Screens (http://en.wikipedia.org/wiki/Stevenson_screen), which shield them from direct sunlight and rainfall, and these screens are placed in an open area away from buildings, trees, etc, to minimise their effects.
Are your three external thermometers all in Stevenson's Screens or are they (for example) attached to the side of the house? If they are not in Stevenson's screens, then they will be measuring very localised temperatures which will depend on (for example) whether they are in direct sunlight or are near a south-facing wall which will generate its own warmer microclimate. As you say, measurement techniques have changed over time, so it is important to take this into account when looking for trends in the data. Only data from well-established sources (in which the measurement technique is well known) are included in the datasets we use for studying past climate change. The effects of changes in measurement technique are studied and this can be accounted for by including a correction factor in the data coming from some sources, and estimating the uncertainty.
To double-check, the corrected dataset is cross-checked for consistency with other data from sources which did not need correcting. The greatest challenge is not so much in minimising the errors in the measurements by individual weather stations, but in putting together all the measurements to give an estimate of global average temperature. This is a challenge because the global coverage of weather stations has changed over the last 150 years, and if this wasn't accounted for then you would think you were seeing a change in the global temperature when really you are seeing the effects of adding in more stations from places with different climates. Again, this is accounted for extremely rigorously. The error in the estimated global warming is estimated to be 0.2°C, which is smaller than the warming, which is more than 0.7°C.
I can categorically state that the Met Office does NOT "change its forecasts to support global warming theory". Our weather forecasts do take account of urban heat island effects, but that is in order to get a reliable forecast for people in cities and not to fiddle anything! When you see differences between the forecast temperatures and those you measure yourself, you should remember that the forecasts are for a general area and the temperatures can vary from place to place, according to local micro-climates, for the reasons I gave earlier. The average temperature measured in your garden could well be generally lower than somewhere else in the vicinity, especially if (for example) your thermometer is exposed to rain and wind.
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