Global warming is the process that increased the average temperature of the Earth, near-surface air and oceans. The term global warming is a specific example of the broader term climate change, which can also refer to global cooling. In principle, global warming is neutral as to the period or causes, but in both common and scientific usage the term generally refers to recent warming and implies a human influence. The climate system varies through natural, internal processes and in response to variations in external “forcing” factors including solar activity, volcanic emissions, variations in the earth’s orbit In the past 200 years human industrial activity has injected carbon dioxide and other greenhouse gases into the atmosphere, and recently global average temperatures have been increasing. The predicted effects for the environment and for human life are numerous and varied. The main effect is an increasing global average temperature.
The scientific consensus is that these greenhouse gases have been responsible for most of the present warming trend. That consensus is not unanimous. The earth has experienced natural global warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles with interglacial warming periods much hotter than current temperatures.the time of the last glacial maximum about 20,000 years ago.
Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with some simplifications being necessary because of limitations in computer power. Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. Although global warming and ozone depletion often are linked in the media, the relationship between the two is not strong. Scientists have stated with 66-90% confidence that the effects of volcanic and human-caused aerosols have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for this effect. Many estimates of aggregate net economic costs of damages from climate change across the globe, the social cost of carbon (SCC), expressed in terms of future net benefits and costs that are discounted to the present, are now available.
Peer-reviewed estimates of the SCC for 2005 have an average value of US$43 per tonne. Rising global temperature might cause forest fires to occur on larger scale, and more regularly. This releases more stored carbon into the atmosphere than the carbon cycle can naturally re-absorb, as well as reducing the overall forest area on the planet, creating a positive feedback loop Projected climate changes due to global warming have the potential to lead to future large-scale and possibly irreversible changes in our climate resulting in impacts at continental and global scales. With increasing average global temperature, the water in the oceans expands in volume, and additional water enters them which had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets.
The sea level has risen more than 120 metres since the peak of the last ice age about 18,000 years ago. For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration While local benefits may be felt in some regions (such as Siberia), recent evidence is that global yields will be negatively affected.