The change in world climate patterns over time. Such change has always occurred, both on a large scale since the formation of the earth and on a smaller scale within the span of human history. For example, there is evidence that the period between 900 and 1300 (the Medieval Warm Period) saw generally high average temperatures and a benign climate, with wine being produced in southern England and the colonization by the Vikings of such northern lands as Greenland; and that this was followed between c.1350 and c.1850 by the Little Ice Age, with generally cooler average temperatures and extreme weather marked by harsh winters, more frequent famines, and advancing glaciers. However, three factors led to climate change becoming an important issue in the late 20th and early 21st centuries: a body of scientific evidence suggesting that the average world temperature was increasing rapidly; a concern that human activity was in part causing this change; and a fear that, if unchecked, it would lead to massive physical, economic, social, and political disruption within a few human lifetimes. The extent to which these fears and concerns are justified is disputed. It is generally agreed that global average temperature increased by about 0.6 °C over the 20th century – a rapid change – with an acceleration after 1975. The majority of scientific opinion holds that this change was too great and too rapid to be natural: human activity has caused this global warming and, unless positive action is taken, it will accelerate with disastrous consequences. These might include: a rise in sea-level, due to thermal expansion of the oceans that will flood coastal areas and perhaps obliterate low-lying countries entirely; altered weather patterns that will affect agriculture and water supply; increasingly extreme weather conditions; and, possibly, an alteration in the ocean currents. The principal human contribution is held to be the emission of large quantities of carbon dioxide and other greenhouse gases, which increase the atmosphere's ability to retain heat and thus upset the balance between the energy received from the Sun and that radiated into space. However, other scientific opinions hold that the threat of global warming either does not exist at all, on the grounds that the observed temperature rise is probably a natural fluctuation that is not caused by human actions and will not continue in the long term, or is not serious enough to warrant urgent countermeasures. This range of scientific opinions is mirrored in the political stances adopted on the issue. For many countries and non-governmental organizations, climate change is an urgent threat that requires actions to restrict carbon emissions, such as those embodied in the Kyoto Protocol. For others, this view is either unnecessarily alarmist or even a political attempt to restrict their economic growth to the benefit of others. The USA, for example, has declined to ratify the Kyoto Protocol, which it considers would penalize US industry excessively compared with that of some developing countries.