Emissions of greenhouse gases and aerosols due to human activity continue to alter the atmosphere in ways that are expected to affect the climate. Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activity. There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. There are preliminary indications that some human systems have been affected by recent increases in floods and droughts. Natural systems are vulnerable to climate change, and some will be irreversibly damaged.
Many human systems are sensitive to climate change, and some are vulnerable.
Projected changes in climate extremes could have major consequences. The potential for large-scale and possibly irreversible impacts poses risks that have yet to be reliable quantified. Adaptation is a necessary strategy at all scales to complement climate change mitigation efforts. Those with the least resources have the least capacity to adapt and are the most vulnerable. Lower emissions scenarios require different patterns of energy resource development.