What is meant by climate change?
Has climate changed in the past?
Has the climate changed recently?
Should we trust the evidence?
What does the data from instruments show?
What is causing global warming?
Is climate change necessarily a bad thing?
Can anything be done about climate change?
Further information
   

What is meant by climate change?

Climate

Before we answer this question, we should understand something about climate. We first need to realise that there is a difference between weather and climate. Weather consists of those meteorological events, such as rain, wind and sunshine, that can change day by day, even hour by hour. Climate is the average of all these events over a period of time.

The climate varies over different parts of the world. The climate of the Tropics is hot and often humid, whereas around the Poles it is cold and often very dry. In between these extremes, there are other different climates that have elements of either the Tropics or the Poles, sometimes of both. They are also dependent on local factors, such as whether they are near the sea or inland, low lying or in mountainous regions.

Another factor to consider is the time of year. In the tropics, the temperature does not change much throughout the year and seasons are called 'wet' or 'dry'. There are the 'long' and 'short rains' in Africa, the monsoons in south-east Asia, the 'Wet' in northern Australia. Away from the tropics, seasons become more recognisable. Summer is when the pole is in permanent daylight, winter when it is in permanent night-time, with Spring and Autumn as the seasons in between.

The region around the North Pole is surrounded by land, but is situated mostly over water. The South Pole is on a large, elevated land mass surrounded by ocean. This means that their climates are different from each other. The inland Antarctic is very cold and dry in winter, changing to less cold and dry in summer. In the Arctic, the change from winter to summer is often quite sudden, with little in the way of spring or autumn. In summer, it stays cold where ice lingers, but it can become warm inland.

Changes to climate

The seasons can differ from year to year. We sometimes talk of, say, spring being colder/warmer and/or wetter/drier than usual, or compared with last year. This is just a natural year by year variation that is part of climate itself. Sometimes two or three years may appear very different from what is normally expected, but over time, the situation balances out.

The climate is usually defined as the average of the weather over a 30-year period. It is when that average changes significantly that we start thinking the climate might be changing. However, we need to look at other sources of evidence before we can really believe this.

Has climate changed in the past?

The study of climate is not only undertaken by climatologists. Many other scientists, such as oceanographers, ecologists and geologists are also involved. Geology has shown that, millions of years ago, the land masses were in different positions from where they are now. Fossils, or the lack of them, provide a lot of useful information for this period of time.

Over the period of a few thousand years, climatic evidence comes from deep in the ice covering Greenland and the Antarctic, or from ocean sediments. This period also covers the existence of human beings, whose ways of life have partly been influenced by climate. Tree rings, river sediments and changes in glaciers provide evidence of changes in climate over hundreds of years. It is only for the last 150 years or so that instrumental records are widely available and these are only over certain parts of the world.

There is little doubt, from the evidence so far, that there have been enormous changes in climate. These ranged from a complete absence of ice over the Poles to ice sheets extending across much of Europe, Asia and North America. The last major extension of polar ice retreated only 10,000 years ago. Since then, the climate has sometimes been warmer and sometimes cooler than it is now.

Has the climate changed recently?

Natural sources, such as tree rings and glaciers, as well as human records, show that climate has changed significantly over the past few hundred years. There was a relatively warm period in Europe during the 14th century, followed by a quite sudden change to cooler conditions in the 15th century. This extended into the Little Ice Age of the 17th and 18th centuries, followed by a warming trend that has recently accelerated. The evidence for this recent warming comes largely from direct measurements of temperature. Fig 1 below shows temperatures over the past 2000 years determined from proxy data.

Fig 1: Temperatures over the past 2,000 years determined from proxy data

 

Should we trust the evidence?

Proxy data

	Fig 2: An example of tree rings

There is a lot of information available in historical written records, such as diaries, ships' logs, personal letters. Agricultural events, such as the dates of seed-time and harvest give a good idea of weather conditions during that time. The periods of exceptionally good and, more especially, disastrous harvests are usually known. These provide time frames, but such information is qualitative and, by themselves, do not give actual figures.

Tree-ring data is even more difficult to assess, since the variations in tree rings are due as much to changes in rainfall and the immediate surroundings as to changes in temperature. Figure 2 shows a good example of tree rings.

Pictures showing how far glaciers have retreated over the past 50-100 years can be very impelling, see figures 3a and 3b. It seems clear there must have been a rise in temperature at their level, but is that the only reason for the retreat? The snows of Kilimanjaro are disappearing rapidly, but that could be due to a lack of snowfall rather than rising temperatures.

Fig 3a and 3b: The retreat of the South Cascade Glacier, Washington State between 1928 and 2000. Images courtesy of USGS

Nevertheless, when these types of information are gathered together and related with each other, a better idea of the past climate can be made. As an example, the study of historical records and volcanology has shown that there was a particularly violent eruption in Iceland during 1783. This released dust and poisonous gases into the atmosphere that not only caused a lot of deaths in Iceland itself, but can also be linked to poor harvests in Europe for some years afterwards. Measurements of temperature were also being made in a few places and these show that summer temperatures were lower than in previous years. Hence, there are interconnections between volcanic activity, temperature and harvests.

Incidentally, there was particularly severe deprivation in France at this time, largely due to bad harvests. This is widely believed to have been a trigger for the French Revolution.

Instrumental data

There are records from various types of instruments that go back over hundreds of years. Until about 300 years ago, these records tended to be short and incomplete, partly because few people had the time or inclination to keep writing them down and some will have been lost or destroyed. Also, the instruments were probably too expensive for most people, were cumbersome and not particularly accurate. All the same, such records have been useful as a form of calibration for proxy data that can be dated to the same time.

As scientific enquiry grew, observatories were set up that were intended mainly to observe the heavens, but included an increasing number of instruments for measuring and studying the weather. Reasonably accurate measurements of temperature, rainfall and atmospheric pressure have been possible for a long time. A variety of different scales have been used for each, but they are relatively easy to convert to standard units.

In recent years, techniques for looking at meteorological information have been developed that make it possible to study changes on a global scale. It is believed that this can reasonably be done with data back to the middle of the 19th century, since, by then, there was a sufficiently large number of observing stations available.

What does the data from instruments show?

The instrumental record shows an uneven trend towards higher temperatures that seems to have accelerated during the past 20 years. Figure 4 shows temperature anomalies as measured by thermometers. It might be thought that, since this information comes from scientific sources, there should be little argument about the results. However, a number of questions should be asked before we decide whether this "global warming" is real.

1. Where are the observations made?
2. How are the observations made?
3. When are the observations made?
4. What instruments are being used?
5. How are the global averages calculated?

• The environment around a site is likely to change. The most obvious is housing development, but there can be more subtle changes, such as the growth or removal of vegetation. Another change may be the site itself, with small changes in the position of the instruments.
• Over time, observers will change and some will be better than others. The exposure of the instruments may change; either different screens are used or are not properly maintained. An increasing number of sites are becoming fully automated.
• The times at which observations are made may change and, over time, the climate averages for a station may be calculated differently.
• Liquid-in-glass thermometers are being replaced by platinum resistance thermometers, but these may have different characteristics.
• Attempts are made to correct observations or make them more compatible with each other. The methods used involve complex statistics. Also, designated climate stations may close and/or be replaced by others.

All these factors can pose significant questions about the figures.

Fig 4: Temperature anomalies as measured by thermometers. Data from Met Office

This is not the only manifestation of global warming and it is difficult to deny that the international expansion and greater rigour of scientific enquiry over recent years means that there is less reason for doubt. Apart from the retreat of glaciers, Arctic ice has become thinner over the last 30 years, now being barely half its earlier thickness. Its expansion southwards during the winter months is also much less. Figure 5 shows variations from average of the extent of both Arctic and Antarctic sea ice. Land in the far north is deep frozen throughout the year, known as permafrost, but soil temperatures have risen sufficiently to allow this to begin to thaw in places.

Fig 5: Variations from average of the extent of both Arctic and Antarctic sea ice Source Met Office Hadley Centre.

In the more temperate northern latitudes, winters are less severe than 30 years ago, with cold snaps generally being short-lived. This has been especially noticeable in places with a more maritime climate. The signs of Spring (bulbs, flowers, birds nesting) appear earlier, there are more frequent hot spells during summer and the first frosts of autumn tend to be later.

What is causing global warming?

Put simply, the 'Greenhouse Effect'. When the sun shines on a greenhouse (a conservatory, a porch) the temperature inside begins to rise above the outside temperature, due to the heat becoming 'trapped'. The reason for this is that glass allows through the radiation from the sun, but blocks the radiation from the surfaces inside.

In the atmosphere, there are a number of gases that act like the glass in the greenhouse by reducing the amount of heat radiated back to space. The two most effective are methane and carbon dioxide. Measurements taken over the past 50 years show a steady increase in the concentration of carbon dioxide and this has the effect of allowing global temperatures to rise. Figure 6 shows the concentration of carbon dioxide (CO2) measured at Mauna Loa Observatory in Hawaii by monitoring the absorption of infrared radiation. The sawtooth shape of the graph is due to the changes caused by take-up of CO2 by plants during the growing season and release of CO2 during autumn and winter.

Fig 6: the concentration of carbon dioxide measured at Mauna Loa Observatory in Hawaii

 

Is climate change necessarily a bad thing?

In some ways, no. Warmer weather would allow a longer growing season in temperate latitudes, reduce the need for heating and lead to warmer seaside holidays. However, reduced rainfall in tropical regions can lead to the expansion of deserts and rises in sea level would threaten low-lying coasts and islands as well as a number of major cities with inundation.

Can anything be done about climate change?

On present evidence, 'global warming' could be slowed if emissions of methane and carbon dioxide were reduced. The main artificial sources of these gases are (a) for methane - agriculture, emissions from landfill sites and natural gas and (b) for carbon dioxide - the burning of fossil fuels, cutting down and burning trees. This may seem to be something that only governments or large organisations can tackle, but the individual can also contribute significantly by, for example, not using a car unnecessarily and recycling.

Fig 7: Traffic pollution	Fig 8: Recycling

 

Further information

Met Office Hadley Centre
Life in a changing climate