Climate - Temperatures

From Worldtraveller

One of the pitfalls for a layman studying the climate is to overestimate the importance of the present day. To me, the increase in temperatures over the last twenty years now seem very noticeable and very significant. Over the last few years, heatwaves have become more common here in the UK, and winters have been amazingly mild. This is enough to convince many people that global warming really is happening, but is it really that hot in comparison to long-term historical records?

1 Direct measurements
2 Reconstructions
3 Urban heat islands
4 Useful reading
- 4.1 Direct measurements

Direct measurements

In some parts of the world there are good temperature records stretching back to the 1600s; in others, only a few decades. And over most of the world it's only since the advent of satellites that temperature measurements have been possible. Truly global temperature measurements are only available for the last thirty years or so. There are a number of issues with calibrating satellite measurements and for a time it appeared that there was a conflict between ground-based measurements showing a much greater rise in temperatures than the satellite measurements. However, the resolution of some calibration issues has brought the satellite measurements into good agreement with the ground-based measurements, and it is now clear that the temperature record from direct measurements shows a rise in global temperatures of about 0.7°C over the 20th century.

Reconstructions

Is such a rise unusual? How can we tell what the climate was doing before global direct temperature measurements were available? For this, you have to rely on proxies - things which can be measured in ice cores or boreholes or similar, and which vary with temperature. One of the most familiar examples is tree rings - the warmer the summer, the more a tree will grow, and the thicker the visible ring will be when the tree's trunk is later analysed. Another example is based on the fact that just as carbon has several isotopes, so does oxygen. Water can be made of of two hydrogen molecules and one 'normal' oxygen molecule, or two hydrogen molecules and one of a less common, heavier isotope of oxygen. Heavier water molecules don't evaporate as easily as lighter ones, so evaporation and condensation of water changes its isotopic ratios. From ice cores, you can measure the isotopic content of water stretching back hundreds of thousands of years and use this to estimate temperatures over this period.

What the reconstructions show is that the temperature today is about 0.5°C higher than it was at any point between 1000AD and 1900AD. The last time temperatures were significantly higher than today was at least 2000 years ago.

Urban heat islands

Cities are hotter than surrounding rural areas. This is because buildings and roads trap heat very effectively. Therefore, any temperature measuring station in a city will generally record a higher temperature than one in a nearby rural area. Many urban areas have dramatically increased in size over the 20th century and many formerly rural areas are now urban. Could this have distorted the temperature record and given rise to a spurious warming trend?

It's quite easy to show that this can't be so. First, you can correct for the effect by comparing urban measurements with rural ones. Second, you can compare temperature trends based only on windy nights with those based on calm night measurements; the urban heat island effect is significantly reduced by wind. Studies show no difference between the two trends. And third, you can look at the direct observational evidence of warming, which finds that the greatest warming trends are seen not in heavily urbanised countries but in thinly populated Arctic regions, and the equally thinly-populated Antarctic Peninsula. Sea surface temperatures measurements also show a strong rise and cannot be affected by urban heat islands.

Useful reading

Direct measurements

The world's longest continuous direct measurement of surface temperature is the Central England Temperature. The first link below is a public web page and the second is a scientific paper describing the measurement and accuracy of the CET.

There are several teams of people who determine global temperatures from surface records. The best known are those from NASA/GISS (GISTEMP) and the UK's Hadley Centre/Climate Research Unit (HadCRUT). Links below are to their web pages and a couple of useful scientific publications.