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Taking the temperature

Updated Monday, 13th February 2006

How do you go about taking the temperature of the entire globe throughout the last 1000 years? Stephen Peake explains the detective work involved

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Melting ice

Global mean surface temperature is a preposterous idea!

‘Global mean surface temperature’. Say it out loud.  Enunciate each word carefully as if it were a line from a poem. Think about the meaning of each word and the overall sense of the phrase.

If you are smiling or giggling even, you may already understand the gist of what I am about to explain. And if you are laughing raucously, you may need to get out more. But if, as I suspect, this isn’t the slightest bit amusing, read on. You’ll find out why the notion of ‘global mean surface temperature’ is as preposterous as it is essential for understanding climate change.

The famous ‘hockey stick’ graphic showing average temperatures in the Northern Hemisphere rising over the last 1000 years is possibly the most critical piece of scientific evidence of climate change we have. This now iconic result has made public understanding of ‘global warming’ synonymous with ‘climate change’. It also has climate sceptics stumbling over their words or silenced altogether.

It’s simple enough to comprehend, but the way the data is collected and the painstaking statistical analysis behind the graphic construction is awesomely complex. It has consumed large parts of many scores of scientists’ careers. This article reveals the science behind such fascinating graphics of rising global temperatures. We begin by unravelling the difference between global warming and climate change.

Climate change and global warming are different but the same
For a pretty innocuous sounding little phrase made up of two relatively common words, ‘climate change’ is in fact a real performance to scientifically measure. To understand why, think about the difference between ‘weather’ and ‘climate’. What’s the weather like outside? Is it cold, warm, hot, dry, humid, wet, windy, sunny, cloudy, snowing? Now if I asked you to describe the climate outside, how would you think to reply? You would, no doubt, think about patterns of weather throughout the year in general. The term ‘climate’ refers to averaged yearly weather conditions for a particular location over a number of years.

Now, when we speak of the climate of a particular region (e.g. Western Europe, North Africa) we are referring to yearly average temperatures, rainfall, wind speeds etc over a large area over a period of several years. When we speak of ‘global climate change’ therefore, we are referring to anomalies in average weather conditions where the yearly average is for all locations over the whole of the earth’s surface area (land, ice and sea) over a number of years. A wry smile now perhaps? Great, you’re starting to think scientifically!

The phrases ‘climate change’ and ‘global warming’ tend to be used interchangeably in the media and in turn by the public. However, these days, it is much more ‘scientific’ to use the term climate change rather than global warming for several good reasons.

Firstly, climate change is not just about changes in global temperature – it is about changes in rainfall, wind speeds, ocean circulation patterns, sea-level, the intensity and frequency of extreme events and many other so-called ‘biogeochemical’ impacts (again you may like to enjoy saying that word out loud, slowly).

Secondly, while it is true that the Earth’s surface is, overall, warming, the trend is not evenly spread. In the Northern Hemisphere recent warming has been more marked at mid to high latitudes. However, some parts of the North Atlantic have actually cooled as a result of changes in ocean circulation patterns!

So global warming and climate change are both scientifically acceptable terms, but are not the same thing. However, rather infuriatingly, when push comes to shove and scientists are asked to show evidence of climate change, they invariably refer to increases in global mean surface temperature, otherwise know as ‘global warming’!

The reason is twofold: temperature is the trigger for other changes in the climate including: sea level, rainfall, sea ice melting, land ice retreat, extreme events, etc and a whole host of other ‘climate change indicators’; secondly while our knowledge of global mean surface temperature increase is uncertain, it is much better than our historical knowledge of other changes in the other climate indicators.

How do we measure changes in global mean surface temperature?
Imagine a web conference with an intelligent life form from another planet in some distant galaxy. ‘What temperature do you live at on the surface of your planet?’ our intergalactic neighbours might ask us. How do we answer and what is the answer?

At any one time, half the Earth is in the cool of night and the other half in the heat of the day. As the Earth journeys round the sun, it might be tending towards summer or winter in the Northern of Southern hemispheres. In turn, at the South Pole the surface temperatures range from -30 to -90°C and at the North Pole from 0 to -30°C. Nearer the equator, temperatures in hot climates reach 40-50°C. The yearly range of temperatures in the Earth’s biosphere is therefore of the order of 70-110°C - greater than the range of temperature between the freezing and boiling point of water.

At any one time, in any one place on the planet, temperatures vary over the course of a day, week, month and by season. In turn, these temperature cycles vary, depending on location. Latitude, longitude, altitude, and local climate factors (valleys, proximity to forests, water, and so on) all play a part.

The best answer we could give our intergalactic neighbours turns out to be around 16°C (or 15.68°C in 1998 to be more precise). But how do scientists come up with this figure and, what’s more, how have they been able to track changes in it as far back as 1000 years?


Melting ice Combining land and sea surface temperatures over the last 140 years
To calculate an average surface temperature for the Earth, we need to measure temperature evenly across its surface. We can, for example, divide it up into grid boxes of 5° latitude by 5° longitude and then average trends in temperatures across all 5184 grid squares.

The raw data for the calculation of mean surface temperature of the earth is measured from two main sources:

  1. monthly readings from a network of over 3000 surface temperature observation stations
  2. sea surface temperatures measurements taken mainly from the fleet of merchant ships, some naval ships and a network of data buoys

Satellite measurements of lower atmosphere temperatures are also available for the period of the last 30 years or so.

While thermometer readings go as far back as 1659 (the earliest are part of what is known as the Central England Temperature series) accurate and geographically diverse coverage on the instrumental record goes back around 140 years to 1860.

Over the years, measurement methods (and therefore accuracy) at both land stations and sea observations have changed. For example, urbanisation around land stations has had an effect on long-term land surface measurements in those locations that were once rural and are now urban.

Methods of collecting sea surface temperatures data have also changed over the years. Sea surface temperature is no longer measured by scooping up a wooden bucket of sea water and sticking a thermometer into it, but is now - believe it or not - usually made by measuring the temperature of cooling water entering merchant ships’ engine systems.

Stripping these and other weird and wonderful effects out of the raw historical data throughout the instrumental record has not been trivial. Methods to do this have themselves been the subject of much scientific debate.

Satellite observations of lower atmosphere temperature
In addition to the land and sea surface temperature series, during the last 30 years or so, we have also been gathering satellite observations of the temperature of  the lowest layer of the atmosphere (the lower troposphere). Meteorological satellites carrying microwave sounding units can remotely observe the average temperature of the lower troposphere.

Currently, scientists agree that the satellite data suggest the lower troposphere has been warming (as we might expect if the surface has been warming), although there is a debate among climatologists about the rate of warming (roughly in the range of 0.1-0.2 °C over 30 years).

Until relatively recently, while the surface temperature data from thermometers showed a significant warming, the satellite data showed a slight cooling. For many years this was a source of great interest for climate change sceptics who smelt a rat. However, the satellite cooling trend turns to a warming trend once scientists took account of subtle changes in the orbits of the satellites making the measurements.

Measuring changes over the last 1000 years
Temperature records before the mid-nineteenth century are rather sparse and inaccurate. If we want to deduce global mean surface temperature in the period before the instrumental record, we have to measure it indirectly using indicators from the paleoclimatic record.

Such ‘paleoclimatic proxies’ come from a variety of sources including:

  • borehole measurements
  • corals
  • ice cores
  • pollen distribution
  • records of lake levels
  • records of glacier advances and retreats
  • tree rings

There are considerable methodological issues in interpreting data obtained from any of these proxies. Tree ring data, for example, are only available on land while coral data relate only to the tropical and sub tropical regions (you won’t be surprised to hear!).

Furthermore, historical documents, for example, tend to be biased towards describing more extreme events. So constructing global mean surface temperature as far back as 1000 years is a question of piecing together a story from lots of different sources of information. It is for this reason that it has been possible to deduce the mean surface temperature for the Northern Hemisphere for the last 1000 years, but not for the Southern Hemisphere.

As we travel back in time, the uncertainties in our measurements of global mean surface temperature cascade. So if you look at the hockey stick graphic from right to left it starts off as a neat, crisp single line which gradually thickens as we look further back in time. Eventually it resembles a sound waveform (like you might see in an audio editing application), a marked change from the crisp line on the right side of the graph. The thickness of our earlier estimates of global mean surface temperature represents the range of confidence we have in the scientific results of using paleoclimatic data.

Who knows, maybe next time you come across a news article on ‘global warming’ or ‘climate change’ it may raise a knowing smile?


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