Causes of past and recent global climate changes
In climate science, ‘attribution’ describes accounting for the causes of observed changes in the climate system.
Researchers typically use a combination of climate modelling, instrumental observations, studies of feedback processes and sometimes palaeoclimate reconstructions to investigate cause and effect (see Technical Report Section 3.4).
Climate models can characterise both natural climate variability and changes to the climate system that are driven by factors such as increases in greenhouse gases, variations in solar radiation and emissions of volcanic aerosols. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions.
Evidence of human influence on the climate system has strengthened over the past decades. Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes.
The Fifth IPCC Assessment Report concluded that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.
Global mean temperature has risen by around 0.85 °C from 1880 to 2012, at a rate of around 0.12 °C per decade since 1951. Increasing greenhouse gases were likely to have been responsible for between 0.5 °C and 1.3 °C of warming from 1951-2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of −0.6°C to 0.1°C. The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from natural internal variability is likely to be in the range of −0.1°C to 0.1°C. Together these assessed contributions are consistent with the observed warming of approximately 0.6°C to 0.7°C over this period.
The IPCC also concluded:
- It is very likely that anthropogenic influence, particularly greenhouse gases and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961.
- It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0–700 m) observed since the 1970s.
- It is likely that anthropogenic influences have affected the global water cycle since 1960. Anthropogenic influences have contributed to observed increases in atmospheric moisture content in the atmosphere (medium confidence), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence), and to changes in surface and sub-surface ocean salinity (very likely).
- It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heat waves in some locations.
- Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979.
- Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface mass loss of the Greenland ice sheet since 1993.
- It is likely that there has been an anthropogenic contribution to observed reductions in Northern Hemisphere spring snow cover since 1970.
- It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s.
- There is high confidence that changes in total solar irradiance have not contributed to the increase in global mean surface temperature over the period 1986 to 2008, based on direct satellite measurements of total solar irradiance. There is medium confidence that the 11-year cycle of solar variability influences decadal climate fluctuations in some regions. No robust association between changes in cosmic rays and cloudiness has been identified.
Further information (external links)Fifth Assessment Report > Intergovernmental Panel on Climate Change (IPCC)