There is wide agreement among climate researchers – and now also among policy makers – that man-made emissions of greenhouse gases are affecting the Earth's radiation balance.
It will get warmer and more and more knowledge is being acquired on how big the effect will be and where it will be greatest.
The scientific aspects of the climate issue have been described in five major assessment reports from the Intergovernmental Panel on Climate Change (IPCC). The sixth report is planned to be finalised in 2021 and 2022.
In the third report (2001), the panel said it was “likely” that human activities lay behind the trends observed in various parts of the planet. “Likely” in IPCC terminology means between 66 and 90 per cent probability.
In the fourth report (2007) the panel concludes, it is at least 90 per cent certain (“very likely”) that human emissions of greenhouse gases, rather than natural variations, are warming the planet’s surface.
In the fifth report (2013), the panel states that “it is extremely likely that human influence has been the dominant cause of global warming”, which means that they have increased the level of certainty to 95 per cent.
Global surface temperature relative to 1951-1980 average temperatures. The 10 warmest years in the 136-year record all have all occurred since 2000, with the exception of 1998. Source: NASA/GISS
The IPCC has this to say about developments to date:
- The mean global temperature has risen by 0.85°C over the period 1880 to 2012. Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.
- The biggest single impact on the climate that results from human activities comes from emissions of carbon dioxide. This is produced by burning fossil fuels, particularly coal and oil, but also through changes in land use. The level of carbon dioxide in the atmosphere has risen by over 40 per cent since the mid-nineteenth century (from around 280 ppm in 1850 to 391 ppm in 2011).
- Levels of other greenhouse gases (methane and nitrous oxide) have also risen at the same time as a result of human activities, especially agriculture and land use change. The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years.
- Radiative forcing from all anthropogenic emissions of greenhouse gases (CO2, CH4, N2O, and halocarbons) for 2011 relative to 1750 is 3.00 W/m2. The contribution from CO2 alone was 1.82 W/m2.
- There are also human-produced emissions into the atmosphere that lower the surface temperature of the Earth and so counteract some of the warming, such as sulphur particles, which are produced by burning coal and oil. These are estimated to produce an overall cooling effect equivalent to -0.9 W/m2. However they continue to contribute the largest uncertainty to the total radiative forcing estimate.
- The combined effect of the human-induced rise in greenhouse gas and particle levels and change in surface albedo between 1750 and 2011 is an additional net heat input of +2.29 W/m2 (+1.13 to +3.33) at the surface.
- Over the last two decades the Greenland and Antarctic ice sheets have been losing mass, glaciers world-wide have continued to shrink and Arctic sea ice and northern hemisphere spring snow cover have continued to decrease in extent.
- Between 1901 and 2010 global mean sea-levels have risen by 19 centimetres. This rise is partly due to seawater expanding as a result of the warming of the oceans and partly due to the melting of glaciers.
- There is a high degree uncertainty about changes in precipitation due to climate change. Significantly increased precipitation has been observed at mid-latitudes in the northern hemisphere since 1951.
- Some extreme weather events have become more frequent, while others have become less frequent. Examples include a fall in the number of cold winter nights and days with frost over land areas, while at the same time there have been increases in the number of hot summer days and warm summer nights. The frequency and intensity of heavy rains has also increased over most mid-latitude land masses and wet tropical regions.
>> Further reading
Working Group I Report: The Physical Science Basis. Intergovernmental Panel on Climate Change, fifth assessment report, 2013.