Our knowledge of changes in the climate over the last few thousand years was transformed during the 20th century. Before then people didn’t realise that the climate had been any different to the way it is now. This lack of understanding is due in part to an interpretation of classical literature, which appeared to describe the climate in similar terms to current experience. Studies of written works from Classical Greece through to the early years of the 20th century had concluded that there had been no change in the climate since the fifth century BC. These analyses were based on descriptions of the fertility of the country, the nature of streams and rivers, and the dates of sowing and harvests. This view changed gradually during subsequent decades, thanks principally to the painstaking work of several scholars who produced a variety of evidence to show that the climate had indeed changed on almost every timescale and in every part of the world. The image above shows temperature anomalies over the past 1000 years.
The broad picture is that by around 6 000 years ago, the post-glacial warming trend reached a peak. This was mainly a Northern Hemisphere summer phenomenon. On the basis of evidence from tree pollen, the average summer temperature in middle latitudes of the Northern Hemisphere was 2°C to 3°C warmer than at present, largely because of the increased levels of summer sunshine that peaked, as a result of the Milankovitch effect (wobbles in the Earth’s orbit around the sun), about 9000 years ago.
Around 5 500 years ago, in the early stages of the development of the ancient civilizations of Egypt and the Middle East, the climate began to cool gradually and become drier. These changes were small compared with the sudden shifts at the end of, and during, the last Ice Age. Nevertheless, some of the changes were profound. The Sahara dried up. Desert formed where giraffes, elephants and antelope had roamed. More generally, there is evidence of a decline in rainfall in the Middle East and North Africa setting in around 4 000 years ago. At the same time the poleward extent of the treeline across the Canadian Arctic and Siberia started to shift southward. This trend towards cooler, drier conditions continued until near the end of the first millennium AD but was punctuated by warmer periods. There is also considerable evidence of mountain glaciers around the world expanding around 2500 BC, but then receding to high elevations around 2000 BC. Further glacial expansions occurred around 1400 BC to 1200 BC, and between 200 BC and around AD 500–800. In between these colder episodes the climate was warmer with glaciers receding to higher elevations around the world.Although there is no sudden change in the amount of historical evidence on the climate at around AD 1000, much points to the fact that many conditions in northern Europe and around the North Atlantic became warmer during the 9th and 10th centuries. In part, this is inferred from the expansion of economic and agricultural activity throughout that region. Grain was grown farther north in Norway than is now possible. Similarly, crops were grown at levels in northern Britain that have proven uneconomic in recent centuries. The Norse colonization of Iceland in the 9th century and of Greenland at the end of the 10th century is also seen as evidence of a period of more benign climate in this region. There is little doubt the warmer conditions over much of northern Europe extended into the 11th and 12th centuries, but the geographical evidence is complicated.
Much of what we know about climate and how it is varying, is based on hard work and experience. The patient collection of data, day in and day out, forms the basis for many insights on how the climate works as well as how it can change.
Historical documentary sources
In the times where humans lived on Earth, but before instrumental measurements of the climate were made, some of the gaps in our knowledge of the climate can be filled in with documentary records of events that relate to the weather. These include direct references to the weather such as agricultural records, wine harvest dates and phenological records. Phenological records are compilations of the annual events of plants such as leaf opening, flowering, fruiting and leaf-fall together with climatic observations. Phenological calendars were used long ago in both China and the Roman Empire. A lot of work was done on this subject up to the mid-20th century, especially in Europe, but fell out of fashion. Recently there has been a revival of interest in the subject in both Europe and North America as part of efforts to identify the effects of global warming on flora and fauna.
Where we do not have instrumental observations or documentary records, much of our information about the climate has been obtained from what are known as proxy data. These are obtained by analysing a wide variety of materials whose properties are influenced by the surrounding climate. Tree rings, ice cores, ocean sediments, coral growth rings and pollens are the best known examples. Proxy data rarely ever provide a direct measure of a single meteorological parameter. For instance, the width of tree rings is a function of temperature and rainfall over the growing season, and also of ground water levels reflecting rainfall in earlier seasons. Only where the trees are growing near their climatic limit can most of the growth be attributed to a single parameter (e.g. summer temperature). For other records (e.g. analysis of the pollen content in lake sediments, or the creatures deposited in ocean sediments), drawing climatic conclusions depends on knowing the sensitivity of the plants or creatures to the climate and how their distribution might be a measure of the climate at the time.
[More in depth information] on Paleoclimatology can be found in the IPCC Fourth Assessment Report, working group 1, Chapter 6.
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