Climate changes characterized as global warming are leading to large-scale irreversible effects at continental and global scales. The likelihood, magnitude, and timing is observed to be increasing and accelerating.
Many consequences of Global warming once thought controversial are now being observed. Large reductions in the Greenland and West Antarctic Ice Sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial
biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments are accelerating.
Other expectations of climate changes that might mediate the warming by causing global cooling have not happened.
The Woods Hole proposal that melting ice might bring fresh water to the Gulf Stream bringing significant slowing of the ocean circulation that transports warm water to the North Atlantic isn’t occurring.
The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change. Additionally, the United States National Academy of Sciences has stated, “greenhouse warming and other human alterations of the earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events. Future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected.”
The IPCC reports that the effects of global warming will be mixed across regions. For smaller values of warming (1 to 3 °C), changes are expected to produce net benefits in some regions and for some activities, and net costs for others. Greater warming may produce net costs in all regions. Developing countries are vulnerable to reduced economic growth as a result of warming.
Most of the consequences of global warming would result from physical changes: sea level rise, higher local temperatures, and changes in rainfall patterns, but synergystic affects such as warming causing the release of methane
hydrates or clathrates; oceans forests and species dying off create many unforseen impacts such as a decrease in the amount of oxygen in the earth’s atmosphere. Sea level is generally expected to rise 18 to 59 cm (7.1 to 23.2 inches) by the end of the 21st century.