Anthropogenic forcing has likely contributed to some of the observed changes, including sea level rise, changes in climate extremes (such as the number of warm and cold days), declines in Arctic sea ice extent, glacier retreat, and greening of the Sahara.[134][135]
During the 21st century, glaciers[136] and snow cover[137] are projected to continue their widespread retreat. Projections of declines in Arctic sea ice vary.[138][139] Recent projections suggest that Arctic summers could be ice-free (defined as ice extent less than 1 million square km) as early as 2025-2030.[140]
"Detection" is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. Detection does not imply attribution of the detected change to a particular cause. "Attribution" of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence.[141] Detection and attribution may also be applied to observed changes in physical, ecological and social systems.[142]