Thursday, July 19, 2007

The Global Warming Primer for the Reasoning Layperson (but not for pure dummies),Part 1

Note -for a context of how this primer came to be, the motives behind it, terminology and formatting issues, please see my following post -Why the heck am I wasting time trying to make sense of the Global Warming debate when I can just lie on the couch and watch Ben-Hur for the 21st time?

The Global Warming Primer for the Reasoning Layperson (but not for pure dummies)

Part 1 - Gathering data, measuring temperatures

This part is divided in following sections-

1) The theory and practice of finding average global temperature (surface record).
2) Who is doing the data collection?
3) Why 1961-1990 as the base?
4) Are there other methods of measuring temperatures?
5) Since when do the records exist?
6) Is this all relevant at all to the climate change debate? Do I have to know any of this?

(1.1) 1)The theory and practice of finding average global temperature (surface record).

(1.1.1) How do the experts arrive at their value of average global temperature of the Earth?

Well, how would you go about measuring the 'average' temperature of your house? An intuitive method would be to place a room thermometer in each room(including the kitchen, all the bathrooms, closets, attics etc.).For simplicity sake, let there be five rooms in the house. You place a thermometer in each of these rooms.Then you take several readings from those thermometers each day.The average of those readings for a particular room is the average temperature of that room for that day.And an average of the average temperatures of all the rooms is the average or the mean temperature of your house for that day.For example, suppose the average temperatures of the rooms for a certain day are- 10, 15, 25(kitchen?),15 and 10°C.Then the average temperature of the house for that day is 10+15+25+15+10 divided by 5 = 15°C.
Similarly, by taking the mean temperature of the house over a desired period of time, you can find out the mean temperature of house over a week, a month, a year or ten years if you so wish.

So do we now have a perfectly fine method to find out the average global temperature?
Well, not yet. Consider this :
the temperature readings taken in your house will be scientifically incorrect (or 'biased') if -

a) you had placed your thermometers carelessly, e.g. close to an air-conditioner(which would 'bias' the readings towards lower values), or close to a stove(which would give an upward 'bias' to the readings).
b) you did not take the readings at a fixed time. You cannot take readings at 4 am, 11 am, 3 pm and 7 pm on one day and at 11 am, 1 pm, 7 pm and 11 pm on the next and still get 'scientific' results.
c) the rooms are not of same size. We implicitly assumed the size of all the rooms to be equal. But of course, that is unlikely to be true in a real life example. So the readings must be adjusted or 'equalized' (or 'weighted') for size. To take a simple example, suppose that in a house of two rooms of sizes 10 and 20 square meters, the average temperatures of the rooms for a certain day are 30°C and 40°C respectively.The average temperature of the house is not given by 30+40 divided by 2=35°C. Look at it this way- you may imagine that the larger room of 20 square meters to be made up of two rooms of 10 square meters, each having a mean temperature of 40°C. So you can now imagine the house to be made up of three rooms, all of 10 square meters with mean temperatures of 30, 40 and 40°C. So the average temperature of the house is 30+40+40 divided by 3= 36.7°C.

So are we there yet? Yes, there are other quibbles but this just about is the basic theory.
The average global temperature on any given day is the average of the temperature readings all over the globe, weighted for relative area coverage. The mean global annual surface temperature is the daily figures averaged over a year. Simple.


(1.1.2) How are these temperatures taken? Where are the thermometers or sensors placed? What about----? Questions, questions!

Imagine the whole of the earth divided into invisible 'rooms' in each of which one might place a temperature sensor. Deciding on the 'rooms' is easy. When you look at a map, you see a grid of parallel lines- verticals and horizontals which divide the map into squares. These, of course, are the longitudes and the latitudes. These give us the ready made 'rooms' to place thermometers in. The climatologists take one ' room' to be a 'square' of size 5°X5°(latitude by longitude). But if you look at a globe(not a map), you will see that 'squares', i.e. our imaginary 'rooms' are not equal in size, since the Earth is not flat like a map but a sphere. So the readings from these 'rooms' have to adjusted for size, as explained above(1.1.1). Fortunately, for people as brainy as those whose job is to do these calculations, it is a piece of cake. These brainy folks also call our invisible 'rooms' grid-boxes.
The `surface record' comprises the combined average of thousands of thermometers world-wide in every country, recording temperatures in standard white louvred boxes called Stevenson Screens, usually mounted one metre above the ground. The boxes are mostly placed where there are suitable people to read and maintain them, such as at post offices in town/city centres, airports, pilot stations, lighthouses, radio/tv stations, farms, and cattle stations. By far the majority are located in towns and cities.

Stevenson Screen

(Image Source)

(Image Source)

Marine temperatures are determined from ship data, usually measuring the temperature of the marine atmosphere from stevenson screens mounted near a ship's bridge, and sea surface temperature from intake pipes in the ship's hulls.

The resulting data is statistically collated by two leading institutions, the NASA Goddard Institute for Space Studies (GISS) in the U.S. and the Climatic Research Unit (CRU) at the University of East Anglia in Britain [11]. The process they follow is -
1) Select the stations to be used in the global database
2) Apply corrections for urbanisation to data originating from urban areas.
3) Divide the globe into 5°x5° latitude/longitude boxes
4) Determine the temperature `anomalies' for each box based on available data.
5) Combine the trends from all the boxes to arrive at an overall `global mean temperature'.
(boxes which have no data are left blank. They are not estimated from neighbouring boxes).

The final two steps are achieved by calculating a weighted average of the monthly mean temperatures of the chosen stations within the grid-box . This average is then compared against a 1961-1990 reference period, the final figure obtained being the temperature anomaly for that grid-box for any particular month. The weighted hemispheric and global annual average anomaly is then determined from that monthly data.


(1.1.4) In fact, there is also another, newer sensor known as MMTC.
Another and newer type of thermometer is the Maximum Minimum Temperature System or MMTS. An MMTS is an electronic thermometer not too different from the type you buy at the local electronics store. The MMTS is a thermistor housed in a shelter which looks similar to a bee hive.


(1.1.5) What else?
Over land regions of the world over 3000 monthly station temperature time series are used. Coverage is denser over the more populated parts of the world, particularly, the United States, southern Canada, Europe and Japan. Coverage is sparsest over the interior of the South American and African continents and over the Antarctic. The number of available stations was small during the 1850s, but increases to over 3000 stations during the 1951-90 period. For marine regions sea surface temperature (SST) measurements taken on board merchant and some naval vessels are used. As the majority come from the voluntary observing fleet, coverage is reduced away from the main shipping lanes and is minimal over the Southern Oceans.

This is the 'surface record' of temperatures, taken over land and sea.

(1.2) 2) Who is doing the data collection?

(1.2.1) Who is doing the hard work of collecting all this data and doing the calculations?

Currently, the Hadley Centre maintains the HADCRUT3, a global surface temperature dataset [2], NASA maintains GISTEMP, which provides a measure of the changing global surface temperature with monthly resolution for the period since 1880[3], and the NOAA maintains the Global Historical Climatology Network (GHCN-Monthly) data base contains historical temperature, precipitation, and pressure data for thousands of land stations worldwide [4].


Note- The Hadley Centre and CRU(Climate Research Unit) (referenced earlier, see 1.1.3) maintain the dataset of surface temperatures known as HADCRUT3.( More on this in Part 2 of this primer). NASA GISTEMPS stands for 'GISS Surface Temperature Analysis.' GISS has also been referenced earlier(1.1.3) in this primer.

(1.3) 3) Why 1961-1990 as the base?

(1.3.1) What is this 1961-1990 thing, as mentioned in (1.1.3)?
It a method of interpreting data collected over 150 years. More on this in Part 2 of the primer -Making sense of the data(or Has Global Warming been happening?)

(1.4) 4) Are there other methods of measuring temperatures?

(1.4.1) Yes, there are two other methods- satellite observations and radiosonde observations(from instruments sent up in what we know as weather balloons). Satellites (indirectly) measure the temperatures in the lower part of the Earth's atmosphere(Troposphere).
Since 1979, Microwave Sounding Units (MSUs) on NOAA polar orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen. The intensity is proportional to the temperature of broad vertical layers of the atmosphere, as demonstrated by theory and direct comparisons with atmospheric temperatures from radiosonde (balloon) profiles.
Satellites do not measure "temperature" as such. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.


(1.5) 5)Since when do the records exist?

(1.5.1) For the surface record-
The time period for which reasonably reliable near-surface temperature records exist from actual observations from thermometers with quasi-global coverage is generally considered to start in about 1850 - earlier records exist, but coverage and instrument standardization are less. The instrumental temperature record is viewed with considerable skepticism for the early years.


For the satellite observations-
Since 1979 (see 1.4.1 above)

For the radiosondes-
The radiosonde data set becomes usably global in about 1958. Changes in balloon instrumentation and data processing over the years have been pervasive, however, resulting in discontinuities in these temperature records.


(1.6) 6)Is this all relevant at all to the climate change debate? Do I have to know any of this?

(1.6.1) No, you don't have to know any of it. But it would help if you wish to have a better understanding of the bitter disputes and controversies, where experts and non-experts, scientists and activists have a go at their 'rivals'(i.e.,those who do not agree with them) with the ferocity of attack dogs. As a bewildered layperson who would like to know what the mauling and snarling is about, at least some basic knowledge of the fundamentals would be necessary.

Part 2 of the primer(this is where the fun begins and the firecrackers of controversy start to burst) -
Making sense of the data(or Has Global Warming been happening?)
Coming soon.