Warming of the 20th century explained

In brief

  • Over the last 100 years, global mean temperature has increased by around 0.74 °C. This rapid rate of warming is very unusual in the context of natural climate variability.
  • In the first half of the 20th century, increasing greenhouse gases, increasing radiation (or energy) from the sun and a relative lack of volcanic activity all contributed to a rise in globally averaged temperature.
  • During the 1950s and 1960s, global temperatures levelled off. This is most likely due to an increase in reflective particles in the atmosphere, known as aerosols, from increased industrialisation and the volcanic eruption of Mount Agung in Indonesia in 1963.
  • Since the 1970s, increases in greenhouse gases have been the most dominant factor in influencing temperature, and there has been a period of sustained warming.
  • Scientists have looked very closely at all of the natural factors, such as volcanic eruptions and changes in the sun that have affected climate over the 20th century. Through these studies, they have been able to determine that none of these processes can explain the sustained rise in global temperature that has been observed. Almost all of the climate indicators show that climate change during the late-20th century is consistent with greenhouse gas increases.

In detail

Internal forcing factors

Natural oscillations within the atmosphere and ocean systems affect climate. These natural oscillations are also referred to as internal processes, since they are driven by the internal physics of the climate system itself, rather than external factors. Natural internal processes operate on almost all time scales. For instance the El Niño/La Niña cycle describes periodic warming and cooling of the tropical Pacific on timescales of several years. This see-saw of warm and cool sea surface temperatures is driven by the interactions between surface winds, ocean currents and the physical shape of the Pacific Basin. Similarly, longer, decadal changes to deep ocean currents are also driven by internal processes within the ocean system. Changes in the ocean are typically accompanied by associated changes in the atmosphere.

The magnitude and pattern of natural internal processes can be estimated by studying observed variations in climate and through the use of climate models. Scientists have been unable to find any internal processes that explain 20th century global warming (Foster et al, 2010). However the effects of natural internal forcing can be seen in the instrumental record. For example, an exceptionally warm year occurred in 1998 as a result of a very strong El Niño event (refer to Recent trends in global warming). According to the IPCC (2007, FAQ 9.2) 'Although natural internal climate processes, such as El Niño, can cause variations in global mean temperature for relatively short periods, analysis indicates that a large portion is due to external factors.'

External forcing factors

External forcing factors affect the radiation-energy balance of the planet. They directly influence the amount of solar radiation (energy) reaching the Earth, the amount of radiation reflected back out to space and the amount of radiation absorbed and re-emitted by the atmosphere and the Earth's surface.

Volcanoes

Most volcanic eruptions have little effect on climate, but explosive eruptions can place large amounts of dust and sulphate aerosols into the stratosphere, where they can reflect sunlight back to space. This leads to cooling of the lower atmosphere (troposphere) and warming of the upper atmosphere (stratosphere) that peaks several months after an eruption and lasts a few years (Crowley, 2000; Bertrand et al., 2002; Weber, 2005; Yoshimori et al., 2005; Tett et al., 2007). Volcanic eruptions had a net cooling effect over the last 100 years, because there was stronger volcanic activity towards the end of the 20th century (IPCC, 2007, Ch 9).

Solar radiation

Solar radiation received by the Earth (insolation) varies due to changes in the Earth's orbit around the Sun and changes in actual solar radiation emitted from the Sun (solar irradiance).

Variations in the Earth's orbit occur in cycles of about 20 000 years, 40 000 years and 100 000 years (Bryant, 1997). They have led to major glacial periods (Ice Ages) and warmer inter-glacial periods (refer to Understanding climate change over the long term). Changes in the Earth's orbit have had little impact on annual mean insolation over the past 1000 years (IPCC, 2007, Ch 9). The rate of global warming over the past 50 years (0.13 °C per decade) is about 100 times faster than the 4 °C to 7 °C warming between ice ages and interglacials, which takes about 5000 years (about 0.001 °C per decade).

Solar irradiance is directly affected by sunspot activity. Periods of low sunspot activity correspond with low Earth temperatures, for example the Oort Minimum (1010–1050), Wolf Minimum (1280–1340), Spörer Minimum (1420–1530) and Maunder Minimum (1675–1715) (Bryant, 1997). The timing of past changes in solar irradiance is highly uncertain (Lean et al., 2002; Gray et al., 2005; Foukal et al., 2006). While the magnitude of solar changes is also subject to uncertainty, it has been estimated that solar changes over the 20th century were far too small to explain observed global warming. Climate model simulations show that it is not possible to reproduce the large 20th century warming when using a range of plausible solar forcing scenarios (Bauer et al., 2003; Hegerl et al., 2003, 2007; Lockwood and Fröhlich, 2007). The total solar irradiance, monitored from space for the last three decades, reveals a well-established sunspot cycle of 0.08 per cent (cycle minimum to maximum) (IPCC, 2007, Ch 2). From 1750 to the present, there was a net 0.05 per cent increase in total solar irradiance, which had a much smaller impact on climate than observed increases in anthropogenic greenhouse gases (see below).

About one per cent of the Sun's energy is in the UV part of the spectrum, which creates and modifies the ozone layer. Changes in ozone affect the temperature of both the troposphere and the stratosphere (IPCC, 2007, Ch 2). Various scenarios have been proposed whereby solar-induced cosmic ray fluctuations might influence climate through enhancing or decreasing cloud cover (Gray et al., 2005). While associations between solar-modulated cosmic ray ionization of the atmosphere and globally averaged low-level cloud cover remain ambiguous (IPCC, 2007, Ch 2), it is thought that this forcing mechanism is far too small to explain large changes in climate.

Anthropogenic influences

In general, the contribution of different climate change mechanisms is expressed as a 'radiative forcing' and is measured in units of power (Watts per square metre: Wm-2). Positive radiative forcing acts to warm the Earth's atmosphere, while negative radiative forcing has the opposite effect. Over the period 1750 to 2005, radiative forcing from greenhouse gases was +2.3 Wm-2 (IPCC, 2007, Ch 2), about 20 times greater than the forcing from changes in solar radiation (Figure 1). Other factors over the same period had a cooling effect. The forcing was +0.34 Wm-2 from halocarbons and +0.35 Wm-2 from tropospheric ozone. Aerosols also contributed a total direct forcing of -0.5 Wm-2 and an indirect effect on cloud albedo (reflectivity) estimated at -0.7 Wm-2 but the uncertainty ranges are large. Changes in surface albedo due to land-cover change and deposition of black carbon aerosols on snow had a forcing of -0.2 Wm-2.

Changes in radiative forcing (RF) due to different anthropogenic factors and natural solar irradiance from 1750 to 2005. Volcanic aerosols contribute an additional natural forcing but are not included in the figure due to their episodic nature. The assessed level of scientific understanding (LOSU) is given in the right column. (IPCC, 2007, Fig SPM2).

Figure 1: Changes in radiative forcing (RF) due to different anthropogenic factors and natural solar irradiance from 1750 to 2005. Volcanic aerosols contribute an additional natural forcing but are not included in the figure due to their episodic nature. The assessed level of scientific understanding (LOSU) is given in the right column. (IPCC, 2007, Fig SPM2).

Computer modelling and understanding climate variability

By studying both the observed climate and output from climate models, scientists have been able to estimate the contribution of different forcing factors to 20th century climate variability. Climate models are based on the fundamental physics and chemistry of the atmosphere, ocean and sea-ice system (refer to Climate models). Such modelling allows scientists to simulate so-called control climates (where external forcing factors are held at fixed levels) as well as many different climate change experiments, such as doubling atmospheric carbon dioxide or changing solar radiation. Through comparison with the instrumental record, it is possible to piece together a record of climate forcing over the last 100 years, during which time the global average temperature has risen by 0.74 °C.

In the first half of the 20th century, increasing greenhouse gases, increasing solar radiation and a relative lack of volcanic activity all contributed to a rise in globally averaged temperature. During the 1950s and 1960s, global temperatures levelled off. Climate scientists have shown that this levelling off is most likely explained by an increase in reflective particles in the atmosphere, known as aerosols. More aerosols entered the atmosphere as a result of increased industrialisation and from the eruption of Mount Agung in 1963. Since the 1970s, increases in greenhouse gases have dominated over all other factors, and there has been a period of sustained warming (IPCC, 2007, Ch 9).

Greenhouse gas forcing alone during the past half century would likely have resulted in greater than the observed warming if there had not been an offsetting cooling effect from aerosol and other forcings (IPCC, 2007). The warming took place at a time when natural external forcing factors would likely have produced cooling (IPCC, 2007; Lockwood and Fröhlich, 2007).

In summary, scientists have looked very closely at all of the natural external forcing factors that have affected climate over the 20th century. Through these studies, they have been able to determine that none of these processes can explain the sustained rise in global temperature that has been observed. Rather, changes due to natural forcing have been superimposed on a background warming trend, and it is very likely that most of the observed global warming since the mid-20th century is due to anthropogenic increases in greenhouse gases (IPCC, 2007) (Figure 2). Hence, we can say that greenhouse gases have been the main driver of global warming over recent decades, while a range of natural processes have affected shorter-term temperature variability.

Multiple lines of evidence for attribution of 20th century climate change

As well as global averaged temperature, climate scientists use a range of other climate indicators to monitor and understand climate change and variability. These other indicators form additional lines of evidence that can be used to understand and determine cause and effect in the climate system. Some of the climate indicators have been chosen to explicitly test the presence of an enhanced greenhouse effect. Such indicators are also known as greenhouse 'fingerprints', since detection of these characteristic patterns in the climate system would indicate that increasing greenhouse gases are the likely cause. Comparison of these indicators between the instrumental observations and climate model experiments allows more detailed examination of climate forcing mechanisms.

It is very likely (at least 90 per cent likelihood) that most of the observed global warming since the mid-20th century is due to anthropogenic increases in greenhouse gases (IPCC, 2007). Discernible human influences extend to other aspects of climate including ocean warming, continental-average temperatures, temperature extremes and wind patterns (IPCC, 2007), as well as reductions in Arctic sea ice and changes in the hydrological cycle associated with increasing atmospheric moisture, global and regional patterns of precipitation changes and increases in ocean salinity in the tropical Atlantic (Stott et al., 2010).

Importantly, almost all of the climate indicators show that climate change during the late-20th century is consistent with increases in greenhouse gases. For instance, increases in solar radiation would cause warming in the troposphere and stratosphere. However, cooling in the stratosphere is what is actually observed, which is consistent with greenhouse gas increases (IPCC, 2007, FAQ 9.2). Greater warming over land than the oceans indicates that the cause of the warming is from changes in the atmosphere (a radiative process) rather than changes in the ocean. Similarly, warming of the upper oceans shows that the warming is not being driven by long timescale, internal ocean processes.

Comparison between global mean surface temperature anomalies (°C) from observations (black) and model simulations forced with (a) both anthropogenic and natural forcings and (b) natural forcings only. All data are shown as global mean temperature anomalies relative to the period 1901 to 1950, as observed (black, Hadley Centre/Climatic Research Unit gridded surface temperature data set (HadCRUT3)) and, in (a) as obtained from 58 simulations produced by 14 models with both anthropogenic and natural forcings. The multi-model ensemble mean is shown as a thick red curve and individual simulations are shown as thin yellow curves. Vertical grey lines indicate the timing of major volcanic events. Those simulations that ended before 2005 were extended to 2005 by using the first few years of the IPCC Special Report on Emission Scenarios (SRES) A1B scenario simulations that continued from the respective 20th century simulations, where available. The simulated global mean temperature anomalies in (b) are from 19 simulations produced by five models with natural forcings only. The multi-model ensemble mean is shown as a thick blue curve and individual simulations are shown as thin blue curves. (IPCC, 2007, Figure 9.5).

Figure 2: Comparison between global mean surface temperature anomalies (°C) from observations (black) and model simulations forced with (a) both anthropogenic and natural forcings and (b) natural forcings only. All data are shown as global mean temperature anomalies relative to the period 1901 to 1950, as observed (black, Hadley Centre/Climatic Research Unit gridded surface temperature data set (HadCRUT3)) and, in (a) as obtained from 58 simulations produced by 14 models with both anthropogenic and natural forcings. The multi-model ensemble mean is shown as a thick red curve and individual simulations are shown as thin yellow curves. Vertical grey lines indicate the timing of major volcanic events. Those simulations that ended before 2005 were extended to 2005 by using the first few years of the IPCC Special Report on Emission Scenarios (SRES) A1B scenario simulations that continued from the respective 20th century simulations, where available. The simulated global mean temperature anomalies in (b) are from 19 simulations produced by five models with natural forcings only. The multi-model ensemble mean is shown as a thick blue curve and individual simulations are shown as thin blue curves. IPCC, 2007, Figure 9.5).

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