How has the climate changed?
The Earth's climate has varied naturally throughout history, ranging from glacial periods where ice covered a significant proportion of the Earth’s surface, to interglacial periods where ice retreated to the poles or melted entirely. [see reference 1] These long-term patterns in climate are generally cyclical in nature. Evidence comes from various sources including ice cores, deep-sea sediments and continental records. In Antarctica, for example, estimates suggest that regional temperatures have varied by around 10°C over glacial and inter-glacial cycles, which normally last between 20,000 and 100,000 years. [see reference 8] Since the industrial revolution and more visibly, since the mid 20th century, changes in climate have been occurring more rapidly, most noticeably the increase in temperature.
Global warming:
The rise of average global surface and ocean temperatures is known as global warming. On a global scale, temperatures have increased at around 0.2°C per decade over the last 30 years and the global average temperature is now at its warmest level for the last 12,000 years. [see references 9 and 10] The rise in the Earth’s temperature is evident, with eleven of the twelve years between 1995-2006 ranking among the twelve warmest on global surface temperature record since 1850. [see reference 2] The projected scenario is that by 2100, the average global temperature will have increased by 1.4 – 5.8°C. [see reference 2] These changes are expected to be greater in the Northern hemisphere, especially over landmass, and less perceptible over the Southern oceans. [see reference 11] Figure 3 below shows the change in global average surface temperature from 1850 to 2007.
The IPCC report Climate Change 2007: The physical science basis concludes that ‘Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHGs concentrations.' [see reference 2] The report also states that, ‘Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.' [see reference 2]
Figure 3: Observed changes in global average surface temperature
All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties. The left-hand axis is the difference in temperature from 1961-1990, while the right-hand axis is average temperature.
Source: IPCC, (2007): Summary for Policymakers. In: Solomon S, Qin D & Manning M et al 'Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change'.
Observations since 1961 show that the warming of the Earth’s oceans is also occurring, The oceans are absorbing around 80 per cent of the heat added to the climate system, with temperatures rising by 0.10°C from the surface to a depth of 700 m. [see reference 12] This warming of the oceans are leading seawater to expand (thermal expansion) which is one of the factors contributing to the rise in sea level [see reference 12] (see sub-section 'Changes in sea levels').
Melting of snow and ice:
The melting of snow and ice – including the loss of mountain snow, glaciers and ice caps [d] – has increased on average. [see reference 13] The recent increase in melting is correlated with rising surface air temperatures. [see reference 13] In the Northern Hemisphere, there has been a reduction of around 7 per cent in the area covered by seasonally frozen ground since 1900. [see reference 13] The loss of glaciers and ice caps is estimated to be equivalent to a rise of 0.50 ± 0.18 mm yr–1 in sea level between 1961 and 2004. [see reference 13] The losses from the Antarctica and Greenland ice sheets have more than likely contributed to a rise in sea levels between 1993 and 2003. [see reference 13] Projections are for snow cover to continue to contract and for sea ice to continue to shrink. Some projections have Arctic late-summer sea ice disappearing almost entirely towards the end of the 21st century. [see reference 14]
Changes in sea levels:
Sea levels have increased at an average rate of 1.8mm per year between 1961 and 2003, and the total rise in sea level during the 20th century is estimated to have been 0.17m. [see reference 12] This rate of increase was noticeably more rapid between 1993 and 2003, at about 3.1mm per year. [see reference 12] It is unclear, however, if this faster rate is simply reflecting decadal variability or is a direct impact of the recent changes in climate. [see reference 12]
Most of the current and predicted sea level rise will be mainly due to thermal expansion. [see reference 15] As global temperatures increase so does the possibility of the melting of snow and ice which is also likely to contribute to sea level rise. [see reference 7] If the Greenland ice sheet begins to melt irreversibly, or the West Antarctic Ice Sheet collapses, the Earth could witness significant increases in sea level ranging from 5 - 12m over a time period estimated as between centuries to millennia. [see reference 7]
Water vapour content:
Increased water vapour content is associated with an increase in the Earth’s temperature as warmer air can hold extra water vapour. From 1988 to 2004, the average water vapour content has increased over global oceans by 1.2 ± 0.3 per cent per decade. [see reference 11] Water vapour content is an important variable affecting the climate. Along with GHGs, water vapour is responsible for contributing to the greenhouse effect and thus regulating the temperature of the Earth (see section What are the causes of climate change?).
Precipitation patterns and ocean salinity:
Over the last 100 years, long-term trends of increased precipitation have been observed in eastern parts of North and South America, northern Europe and northern and central Asia. [see reference 11] The opposite has been witnessed in areas such as the Mediterranean and southern Africa, where an increase in drying has occurred. [see reference 11]
Precipitation is highly variable both spatially and temporally. It is predicted that there will be increases in precipitation in high latitudes and decreases in subtropical areas. [see reference 2] Changes in precipitation over oceans have also affected salinity, with mid and high latitude waters experiencing increased freshening and low latitude waters experiencing increased salinity. [see reference 12]
Extreme weather patterns:
There is a growing concern that the frequency and intensity of extreme weather events may also be changing due to anthropogenic factors. These changes can be observed in the increased occurrence of more intense and longer droughts, increased frequency of heavy rainfall events, heat waves, and increased occurrence of tropical cyclone activity in the North Atlantic. [see reference 11] It is predicted that these extreme weather patterns will continue to become increasingly frequent. [see reference 14]
Climate change in the UK:
The mean Central England Temperatures (CET) series [e] has shown that temperatures have increased by nearly 2°C, since the coldest period of the ‘Little Ice Age’ which occurred around 1690. [see reference 5] Half of this increase has occurred over the last 40 years. [see reference 5]
The summer of 2003 included record maximum temperatures for both England and Scotland [see reference 5] and 2006 was the warmest year on the CET record. [see reference 16] It is predicted that extreme temperature patterns will become more frequent after 2030 and increase in intensity and duration after 2060. [see reference 5] The duration of annual longest cold spells (a period of days with the lowest daily temperature below 0° C) are expected to decrease; most significantly in Scotland, with a predicted reduction of about 7 days (±1 day). [see reference 5]
UK sea levels rose at a rate of approximately 1mm per year (after adjustment for natural land movements) during the last century. [see reference 15] It is projected that the relative sea level will continue to rise around most of the UK shoreline and the rate of increase will depend on region and circumstances. [see reference 15] In southeast England for example, by the 2080s, sea level may be between 26cm and 86cm above the current level. [see reference 15]
An increase in the contribution to winter rainfall from heavy precipitation events has occurred across all regions of the UK, and in summer there has been a decrease in all regions, except North-East England and Northern Scotland. [see reference 16] While there is uncertainty over future changes in rainfall, it is projected that there will be large regional and seasonal differences in precipitation; [see reference 5 and 15] Northern Ireland for example, is expected to experience a reduction in precipitation, becoming 10 per cent drier by 2050. [see reference 17]
(For consequences from potential climate changes in the UK see the section What are the consequences and implications?, and for health implications see What are the health implications?)
Further sources of information:
- Met Office UK Climate and Weather Statistics www.metoffice.gov.uk/climate/uk/index.html
- Scotland and Northern Ireland Forum For Environmental Research (SNIFFER) www.sniffer.org.uk
- Climate Change 2007: The physical science basis. Contribution of working group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Available at www.ipcc.ch/
- UNFCCC http://unfccc.int/2860.php
- UKCIP www.ukcip.org.uk Includes the following the reports:
- The climate of the UK and recent trends (2008)
- Climate change scenarios for the UK - The UKCIP02 Scientific Report (2002)
[e] The CET series provides time-series of air temperature at the surface for an area covered by the triangle with coordinates Preston, Bristol, and London, with records dating back to 1659.