Effects of increased greenhouse gases

2016

The growing concentrations of human-generated GHGs have resulted in an increased absorption, largely in the lower atmosphere, of the heat radiated from Earth’s surface, causing an increase in the global (land and ocean) mean surface temperature of 0.85 ± 0.20 °C from 1880 to 2012 (Stocker et al. 2013a)—a long-term average increase of 0.06–0.07 °C per decade. However, this rise did not occur evenly across the century—for example, average global temperatures did not increase between 1880 and 1910, or between 1940 and 1970.

From 1998 to 2012, global temperatures increased by 0.05 °C per decade, which is similar to the long-term trend, but less than the rate of increase from 1951 to 2012 (0.12 °C per decade). Because of natural variability, trends based on short records are very sensitive to the beginning and end dates, and do not, in general, reflect long-term climate trends (Stocker et al. 2013a). The year 1998 was a strong El Niño year with, at that time, record high global average temperatures (0.1–0.2 °C above 1990–97 temperatures). Thus, decadal or longer temperature trends commencing in 1998 are seemingly suppressed.

The so-called hiatus in global mean surface warming since 1998 was discussed extensively in AR5. Hiatus periods of 10–15 years can arise as a manifestation of internal variability. The relative heat accumulations by the atmosphere and the oceans across short timeframes are likely to be strongly influenced by internal variability of the atmosphere–ocean system. During 1998–2012, the overall climate system, including the ocean below a depth of 700 metres, has continued to accumulate heat and the sea level has continued to rise, consistent with the observed ocean heating and glacial melting.

Each of 2005, 2010, 2013, 2014 and 2015 (a strong El Niño year) set record high annual average global temperatures, showing that the long-term global temperature increase is continuing. The decade 2000 to 2010 has been the warmest decade in the instrumental record.

Box ATM1 Radiative forcing

Radiative forcing is a measure of the balance between the amount of energy (from sunlight) absorbed by Earth and the amount radiated back to space, quantified by the change in energy fluxes at the top of the atmosphere. Positive forcing leads to surface warming; negative forcing leads to surface cooling. Radiative forcing is affected by changes in drivers such as solar radiation, greenhouse gases (GHGs), aerosols and surface albedo (reflectivity). The level and direction of radiative forcing are estimated based on observations of radiation, surface albedo, and the concentrations and radiative properties of GHGs and aerosols.

Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change are shown in Figure ATM3. The net forcing is positive for GHGs as a whole, and negative for aerosols; however, considerable uncertainty exists about the magnitude of forcing from aerosols. The increase in solar radiation has caused a small, positive forcing since the start of the industrial era. Increased surface albedo because of land-use change caused a negative forcing across the same timeframe (Myhre et al. 2013).

Keywood MD, Emmerson KM, Hibberd MF (2016). Climate: Effects of increased greenhouse gases. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/climate/topic/2016/effects-increased-greenhouse-gases, DOI 10.4226/94/58b65c70bc372