ATTRIBUTED AND EXPECTED EFFECTS

Global warming may be detected in natural, ecological or social systems as a change having statistical significance. Attribution of these changes e.g., to natural or human activities, is the next step following detection.

NATURAL SYSTEMS

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming. Most of the increase in global average temperature since the mid-20^th century is, with high probability, attributable to human-induced changes in greenhouse gas concentrations.

Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades. Over the course of the 21^st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20^th century.

In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21^st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes, heat waves, and heavy precipitation will increase.

ECOLOGICAL SYSTEMS

In terrestrial ecosystems, the earlier timing of spring events, and pole ward and upward shifts in plant and animal ranges, has been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs. It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.

SOCIAL SYSTEMS

There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere. Future climate change is expected to particularly affect some sectors and systems related to human activities. Water resources may be stressed in some dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt. Reduced water availability may affect agriculture in low latitudes. Low-lying coastal systems are vulnerable to sea level rise and storm surge. It is expected that some regions will be particularly affected by climate change, including the Arctic, Africa, small islands, and Asian and African mega deltas. Some people, such as the poor, young children, and the elderly, are particularly at risk, even in high-income areas.