The effects of global warming on the environment and human life are numerous and varied.

Scenarios studied by the Intergovernmental Panel on Climate Change (IPCC) predict that global warming will continue and get worse much faster than was expected even in their last report. Research by NOAA indicate that the effects of global warming are already irreversible. The IPCC reports attribute many specific natural phenomena to human causes. The expected long range effects of recent climate change may already be observed. Rising sea levels, glacier retreat, Arctic shrinkage, and altered patterns of agriculture are cited as direct consequences of human activities. Predictions for secondary and regional effects include extreme weather events, an expansion of tropical diseases, changes in the timing of seasonal patterns in ecosystems, and drastic economic impact. Concerns have led to political activism advocating proposals to mitigate, or adapt to it. Geoengineering is a further potential response, which could potentially reverse some effects that may otherwise be irreversible.

It is notable that many of the effects of global warming are non-linear in nature, with potential for dramatic positive feedback effects. This means that the climate may enter a critical state where small changes can trigger runaway or abrupt climate change. Geoengineering has been suggested as a means of interrupting or reversing these effects. A notable example of a 'runaway' effect is the release of methane from clathrates (known as the clathrate gun effect).

The 2007 Fourth Assessment Report by the IPCC includes a summary of the expected effects.

Climate changes characterized as global warming are leading to large-scale irreversible effects at continental and global scales. The likelihood and magnitude of the effects are observed and predicted to be increasing and accelerating.

Many consequences of global warming once controversial or thought to be unlikely are now being observed. Arctic shrinkage and Arctic methane release, alongside 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, leading to expectations of runaway climate change.

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 (of up to 3°C, or about 5°F), 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 (or to reduce the benefits from smaller warming) 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. Sea level is expected to rise 18 to 59 cm (7.1 to 23.2 inches) by the end of the 21st century, not including the unknown contribution from non-linear changes to large ice sheets.

It has also been proposed that the melting in the Arctic may bring fresh water to the North Atlantic to disrupt the Gulf Stream, which may cause a destabilisation or shutdown of the Thermohaline circulation.