Changes to the climate system have been observed at a, in the measurements of temperature at the surface, and the middle atmosphere, increased sea level, increased sea surface temperature, increased ocean heat content, and increased water vapour in the atmosphere.
Decreased polar ice sheets, decreased global sea ice extent (with regional variation but overall net loss), and net decrease in glacier volumes (with regional variation) have also been observed (see figure).
Drivers of thesehave been identified as a positive radiative forcing of the atmosphere, leading to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.
The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. Carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.
Observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing. In this regard, human influence on the climate system is clear.
Future global and regional climate change indicate that continued emissions of greenhouse gases will cause further warming and changes in all components of the. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.