The thermometer records

The warming signal we are looking for is small compared with the variations in temperature from day to day, season to season. We want to know whether the planet surface as a whole is getting warmer, so we will have to average out all the weather. It could be treacherously easy for some bias in the raw data to be undetected or otherwise not corrected for. A computed average temperature for some particular place has to be balanced between daytime and nighttime temperature measurements, for example, and summer versus winter. If we want to compare the average temperature between one year and another, we also have to worry about the possibility that the way temperature is measured might have changed with time.

On land there is a potential source of bias known as the urban heat island effect. Sunlight hitting pavement is converted into warmer temperatures, what climatologists call sensible heat, "sensible" because you can measure it with a thermometer. Some of the sunlight hitting vegetation drives water to evaporate and is carried away as latent heat (Chapter 5), which you can't feel or measure with a thermometer until the water condenses. A vegetated landscape doesn't warm up as much as a paved landscape because some of the heat is escaping invisibly, or insensibly I suppose you would say, as water vapor. This is an observed phenomenon; cities can be 5°C warmer than their surrounding countryside. Nighttimes in particular are warmer in cities. What if we established a weather station in some idyllic rural area, only to have the landscape urbanize over the decades? The temperature record would show a warming, which would be real for that location, but will confuse you if you try to infer the global average temperature trend from it. The warming in the city is real and should be counted in the global average, but if the data were biased toward cities, the computed average temperature could be higher than the real average temperature of the Earth.

Four independent studies of the land temperature data made an attempt to throw out data from locations that might have seen urbanization (Fig. 11.1). The studies all agree that removing urban locations has only a small impact on the global average temperature trend that you calculate from the data. They find that it doesn't matter how you deal with the urban data, you get pretty much the same answer.

The sea surface temperature, known to seagoing types as SST, used to be routinely measured by heaving a bucket into the ocean to collect surface water and sticking a

Year

Fig. 11.1 Instrumental land temperature reconstruction from four independent studies. Replottedfrom IPCC (2001).

Year

Fig. 11.1 Instrumental land temperature reconstruction from four independent studies. Replottedfrom IPCC (2001).

thermometer into the bucket on deck. Some of the buckets were made of wood, others of canvas. Evaporation of water stole some heat from the surface of the bucket, cooling the water somewhat. Canvas buckets cooled more than wooden ones because wet canvas is a poor insulator. After 1942, mariners began measuring the temperature of surface water as it was pumped into the ship for engine cooling water. This is a more accurate measurement but there is still the possibility of bias. The waters in the surface ocean sometimes get cooler with depth. A large ship might sample subsurface waters, which would be systematically cooler than the real SST. These effects must be corrected for, if we are to calculate a global average value of SST through time.

The land and sea records are consistent with each other, in that the SST record can be used to drive an atmospheric climate model, and the temperature history of the atmospheric model on land matches the reconstructed land surface temperature record (Fig. 11.2). You can see that the largest correction that had to be applied to the temperature records was the bucket correction for the SSTs. The abrupt change in SST in 1942 resulted from a switch between bucket and engine room temperature measurement.

Michael Crichton in his fiction book State of Fear shows temperature records from selected locations around the world, where temperatures have dropped over the past few decades. Climate has regional variations, this is true. The global average temperature rise incorporates these regions of cooling, but still shows warming on average. There are other places that Crichton does not show, the Arctic for example, where temperature has risen far more than the global average. The global average temperature record is based on thousands of individual locations around the world. The existence of a few where climate is cooling does not disprove the warming trend in the global average.

Measured land o

Measured land

1870

1910

1950

1990

Year

1870

1910

1950

1990

Year

Fig. 11.2 Land temperatures reconstructed from land thermometers and predicted using SST reconstructions, corrected and uncorrected for the bucket effect. Replotted from IPCC (2001).

Was this article helpful?

0 0
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Responses

  • Goytiom
    What is a thermometer that records sensible temperatures?
    3 years ago

Post a comment