5. Climate models cannot simulate the warming observed since the mid-twentieth century without the inclusion of man-made greenhouse gas emissions
- The cause of the warming observed since pre-industrial times has been assessed by considering the likely impacts of all known factors that could have affected climate over this period, including internal variability in the climate system, natural ‘external’ forcing factors such as changes in solar activity and the occurrence of volcanic eruptions, and man-made factors such as greenhouse gas emissions, pollution and land use change.
- Figure 15 (below) shows changes in solar activity (Total Solar Irradiance) since the pre-industrial period. While activity appears to have increased in the first half of the twentieth century, there does not appear to have been an increasing trend in solar output over the past 50 years. A variety of measures of solar-related activity, including cosmic rays, over the past three decades show that trends are in the opposite direction to that required to explain the observed rise in global temperatures . Thus, while it is likely that changes in solar activity made some contribution to the warming observed during the early twentieth century, it is very unlikely that it can explain the warming during the second half.
Figure 15 (above): Annual global temperature change (thin red line) with 11 year moving average (thick red line) (from NASA GISS) and annual Total Solar Irradiance (light blue line) with 11 year moving average (dark blue line) (from Solanki). Note that the solar record is based on satellite records from 1978, and proxy records before that (N.A. Krivova, L. Balmaceda, and S.K. Solanki (2007) Reconstruction of solar total irradiance since 1700 from the surface magnetic flux, Astronomy & Astrophysics, 467, 335–346 and L. Balmaceda , N.A. Krivova, S.K. Solanki (2007) Reconstruction of solar irradiance using the Group sunspot number, Advances in Space Research, 40, 986–989). Note: Fig 15 plots two independent data series alongside each other for ease of presentation purposes (e.g. it should not be inferred that a change in TSI of 1W/m2 equates to a temperature change of 1°C in the earlier part of the record). (Larger version of Figure 15 (GIF, 59 Kb) )
- Explosive volcanic eruptions inject sulphur gases and dust into the upper atmosphere, where they form into aerosols and can cool climate for a few years by scattering solar radiation back into space. Evidence of past eruptions indicates that changes in volcanic activity cannot explain the warming observed during the second half of the twentieth century .
- To explore the underlying causes of recent climate change in a more detailed and systematic way, climate scientists have ‘forced’ climate models with (i) natural factors alone and (ii) natural factors and man-made factors that are known to have operated over the twentieth century (more information on climate models). The results of some of these experiments are summarised in Figure 16 (below). It is clear that while known natural factors (a combination of increasing solar activity, low volcanic activity and natural climate variability) can account for much of the warming in the first half of the twentieth century, they cannot explain the warming that has occurred since the mid-twentieth century. In contrast, both the spatial pattern and trend of the warming observed since the mid-twentieth century are consistent with what the models indicate when man-made factors are included as well as the natural factors. Thus, while it is not possible to rule out the possibility of other unknown causes, the evidence suggests that most of the observed increase in global average temperatures since the mid-20th century is very likely to be due to the man-made increase in greenhouse gas concentrations .
Figure 16 (above): Decadal averages of observed temperature change relative to the corresponding average for 1901 to 1950 (black line). Blue shaded bands show the 5% to 95% range for 19 simulations from 5 climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5% to 95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. Source: IPCC AR4.
The slight cooling observed during the middle of the 20th century (Figure 7 and Figure 16, above) may have been caused in part by an increase in the concentration of sulphate aerosols in the atmosphere due to an increase in industrial activity at the end of the Second World War and a large volcanic eruption in 1963. Man-made emissions of sulphate aerosols – conventional “pollution” – have a dual cooling effect by reflecting more radiation back into space and increasing the reflectivity of clouds .
- It is thought that the cooling effect of sulphate aerosols may have also offset some of the warming due to greenhouse gases during the second half of the twentieth century, although the magnitude of this effect is more uncertain than that of greenhouse gases . Over the last few years, some countries have introduced legislation to restrict the release of pollutants which form these aerosols because they reduce air quality and cause acid rain. This may have reduced their cooling effect, ‘unmasking’ some of the warming caused by greenhouse gases  – an effect that could continue if emissions of these particles continue to decrease.
Black carbon, also known as soot or elemental carbon, has a strong warming effect on the atmosphere but is very short lived, lasting a few days (compared with a few hundred years or more for CO2). Due to its short lifetime, black carbon’s climate impacts are primarily regional and close to sources, in contrast to the long lived and consequently well-mixed global distribution of CO2. The IPCC estimates its contribution to global warming since the pre-industrial era to be about a fifth of that from CO2, though some recent studies have suggested that the forcing could be significantly higher. (See Box 4, top right, for further information).
The hypothesis that greenhouse gases have caused the bulk of the observed warming is supported by direct, empirical evidence: satellite measurements of the infrared radiation emitted by the Earth over the past few decades  show a decrease in the amount of energy escaping into space at the wavelengths that CO2 and other greenhouse gases such as CH4, fluorocarbons and water vapour are known to absorb infrared radiation, while surface measurements suggest  that the amount of infrared radiation directed towards the Earth’s surface (from the emission of infrared radiation by greenhouse gases in the atmosphere) has increased.
- It has sometimes been suggested that the effect of urbanisation and related local warming on temperature measurements can explain the global average temperature rises observed over the twentieth century. It is certainly the case that expansion of towns and cities has caused localised warming: the “urban heat island” effect. But the effect of urbanisation has been carefully analysed and does not explain the changes in global average temperature. Most stations are not affected by urbanisation and there are well-established ways of taking the effect into account, for example by excluding urban stations from analyses. Also, when checks are done on the global temperature databases, no differences are found between temperature trends on still and windy nights, indicating that they are not affected by urbanisation (on a windy night it would be expected that warm urban air would be blown away from the urban measurement sites). It is worth noting too that this effect cannot explain the warming observed over the oceans.
Studies have detected the ‘signal’ of human influence on climate in a number of changes observed over the past few decades, including: warming of the upper several hundred metres of the global ocean ; global average sea level rise ; glacier retreat ; changes in global precipitation patterns ; increasing atmospheric moisture content  retreat of Arctic sea ice extent in every month from May to December ; and increasing salinity of the Atlantic Ocean at low latitudes .
It is important to distinguish the effects of weather, such as we see locally in day-to-day variations, and also of natural inter-annual variability, from the effects of the long term climate change trend. For example, summer sea ice extent in the Arctic has been declining for the past 4 decades and reached a dramatic minimum in September 2007, before recovering somewhat in 2008 and 2009 (Figure 10b). The 2007 minimum cannot be attributed completely to climate change however; a significant proportion was due to an anomalous wind pattern over the Arctic Ocean, which pushed large amounts of ice into one part of the Arctic Ocean. Similarly, sea level fell in the eastern Pacific and the western Indian Ocean between 1993 and 2001 , but this change can be explained by the effect of short-term, natural changes in ocean circulation and wind patterns on regional scales.
- For the same reason, the cold winter in the UK last year (2009-2010) does not indicate that climate change isn’t happening. Rather, albeit relatively severe, it was part of the normal regional variations that take place from year to year (Figure 17, below). In fact, globally, January 2010 was one of the warmest on record. Even in a warmer climate, regionally cold winters will still sometimes occur, although probably with decreasing frequency and decreasing intensity (a signal that already appears to be emerging in Figure 17) – 2009/10 was nowhere near as cold as the winters of 1963 and 1947.
Figure 17 (above): UK monthly temperature anomalies from the 1971-2000 average between 1914 and 2009. Data are provisional from August 2009. (Source: Met Office) (Larger version of Figure 17 (GIF, 39 Kb) )
Some changes that have been observed appear to have been influenced at least in part by other factors. Sea ice extent has increased slightly in some sectors of Antarctica since satellite records began in 1978, for example. This small change is, however, consistent with the combined effects on atmospheric circulation of an increase in greenhouse gases and a decrease in stratospheric ozone (the ozone hole) .
< Previous: 4. Natural factors | Next: 6. Effect of rising greenhouse gas levels >
- 33. Lockwood, M. and C. Frohlich, Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. Proc. R. Soc. A (2007), 463, 2447-2460.
- 34. IPCC AR4 Working Group I (2007) [specifically Sections 2.7.2 and 22.214.171.124]
- 35. IPCC AR4, Working Group I Ch. 9 (2007)
- 36. Aerosols serve as cloud condensation and ice nuclei and can therefore affect the microphysics, radiative properties, and lifetime of clouds (known as ‘indirect effects’). These effects are usually split into two effects: the first indirect effect, whereby an increase in aerosols causes an increase in droplet concentration and a decrease in droplet size for fixed liquid water content, and the second indirect effect, whereby the reduction in cloud droplet size affects the precipitation efficiency, tending to increase the liquid water content, the cloud lifetime, and the cloud thickness (see IPCC TAR, 2001 Ch. 5.3.5).
- 37. Wild, M., Global dimming and brightening : A review. J. Geophys. Res., 114, doi:10.1029/2008JD011470, 2009.
- 38. Harries et al. (2001)
- 39. Philipona et al. (2004)
- 40. Summarised in Stott et al. (2010), which provides original references for the studies concerned.
- 41. Turner et al. (2009)