4. Natural factors affect climate, and have led to periods warmer than today in the past

• Natural processes affect global climate on a wide range of timescales and have led to significant changes in climate in the distant past. On long timescales factors that affect climate include internal variability in the climate system, geological changes including volcanic activity and levels of greenhouse gases in the atmosphere, and changes in ‘external’ forcing factors, such as solar output and the shape of the Earth’s orbit around the Sun.
• Records from ice cores show, for example, that temperatures fluctuated regularly between warm ‘interglacial’ periods and cold ‘glacial’ periods over the last 800,000 years. These changes in temperature were closely correlated with changes in CO₂ concentration (Figure 13, below) [27]. It is believed that these fluctuations occurred because changes in the Earth’s orbit triggered small increases in temperature which resulted in the release of CO₂ (probably by affecting processes in the oceans), and to some extent CH₄, which in turn caused further warming [28]. Without the additional ‘feedback effect’ of CO₂ on climate, the temperature fluctuations over glacial-interglacial cycles would probably have been considerably smaller. This positive feedback can be seen in the relationship between Antarctic temperature and CO₂ records as the Earth’s climate emerged from the last glacial (Figure 14, below). (Further information on CO₂ and temperature phasing).

Figure 13 (above): Records of temperature change in Antarctica and the concentration of CO₂ in the atmosphere over the past 800,000 years from ice cores. The CO₂ data are from Dome C and Vostok ice cores in Antarctica (see Lüthi et al., 2008 for sources). The temperature data are based on deuterium measurements and are 1,000 year averages expressed relative to the average of the last 1,000 years (from Jouzel et al., 2007). (Larger version of Figure 13 (JPG, 36 Kb) )

Figure 14 (above): Deuterium (temperature proxy) and CO₂ from the Dome C (Antarctic) ice core between 22,000 and 10,000 years ago. Data from Monnin et al. (2001) adapted to the age scale of Parrenin et al. (2007). (Larger version of Figure 14 (PDF, 18 Kb) )

• During the last interglacial period, about 125,000 years ago, it is estimated that polar temperatures were to up to 5°C warmer than today. Modelling suggests that a large scale retreat of ice sheets occurred, contributing to global sea levels estimated to be 4-6m higher than in the 20th century, a level that would inundate many of the world's coastal cities today.
• Another period of warming, known as the Palaeocene-Eocene Thermal Maximum (‘PETM’), occurred about 55 million years ago, when global average temperature increased by several degrees Celsius and remained warm for about 100,000 years. The warming appears to have been related to the release of large quantities of CO₂ and / or CH₄ into the atmosphere, perhaps from sediments on the seafloor known as ‘methane hydrates’ or through volcanic activity. The warming was associated with shifts in global precipitation patterns, changes in ecosystems around the world and acidification of the oceans [29].
• Studies of past climate changes such as these confirm that - as expected from basic physics - increases in atmospheric greenhouse gas concentrations tend to increase global average temperatures. They also highlight that warming can lead to significant changes in the environment. It is worth noting, however, that these climate changes exhibit important differences from those that are occurring today. In particular, the concentration of CO₂ in the atmosphere today is already about 30% higher than it has been at any time over the past 800,000 years [30][31], and it is likely that the rate at which CO₂ has increased during the 20th century is unprecedented over at least the past 16,000 years [29].

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Key references 

Footnotes

1. 28. Lüthi, D. et al. (2008)
2. 29. IPCC AR4 Working Group 1 (2007) Ch 6
3. 30. IPCC AR4, Working Group I Ch. 6 (2007)
4. 31. CO₂ data from Lüthi, D. et al. (2008) and NOAA/ESRL
5. 32. Records of the isotopic and chemical composition of fossil organisms and soils and of pores on fossil leaf surfaces have been used to estimate CO₂ concentrations even further back in time. These estimates are much more uncertain than those from ice cores, but they suggest that the level of CO₂ may be higher now than it has been for several million years (see Figure 6.1 of the IPCC AR4, Working Group I, 2007).

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