Global warming is the increase in the average temperature of the Earth 's near-surface air and oceans since the mid- twentieth century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 ° C (1.33 ± 0.32 ° F ) during the 100 years ending in 2005.   The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the temperature increase since the mid-twentieth century is "very likely" due to the increase in anthropogenic greenhouse gas concentrations,   and natural phenomena such as solar variation and volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.   These basic conclusions have been endorsed by 30 scientific societies and academies of science ,  including all of the national academies of science of the major industrialized countries .
Global Warming Temperatures
Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes , including glacial retreat , Arctic shrinkage , and worldwide sea level rise . Changes in the amount and pattern of precipitation may result in flooding and drought . There may also be changes in the frequency and intensity of extreme weather events. These changes are not likely to be reversible on timescales shorter than a thousand years.  Other effects may include changes in agricultural yields, addition of new trade routes,  reduced summer streamflows , species extinctions ,  and increases in the range of disease vectors .
Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far yielded no conclusive findings; in a survey of 100 estimates, the values ran from US$ -10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$ 350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide). 
Global Mean Temperature
One alternative hypothesis to the consensus view that anthropogenic forcing has caused most of the recent temperature increase is that recent warming may be the result of variations in solar activity.   
A paper by Peter Stott and colleagues suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.  They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases. Another paper suggests that the Sun may have contributed about 45–50 percent of the increase in the average global surface temperature over the period 1900–2000, and about 25–35 percent between 1980 and 2000. 
Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4 percent per decade in the total volume of ozone in Earth's stratosphere (ozone layer) since the late 1970s, and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole . In addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone depletion events , which occur near the surface in polar regions during spring.
The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine.  The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons , and of bromofluorocarbon compounds known as halons . These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased.
Cause Of Ozone Hole
Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from man-made compounds such as CFCs are now accepted as the main cause of this depletion.
It was first suggested by Drs. M. Molina and S. Rowland in 1974 that a man-made group of compounds known as the chlorofluorocarbons (CFCs) were likely to be the main source of ozone depletion. However, this idea was not taken seriously until the discovery of the ozone hole over Antarctica in 1985 by the British Antarctic Survey .
Chlorofluorocarbons are not "washed" back to Earth by rain or destroyed in reactions with other chemicals. They simply do not break down in the lower atmosphere and they can remain in the atmosphere from 20 to 120 years or more. As a consequence of their relative stability, CFCs are instead transported into the stratosphere where they are eventually broken down by ultraviolet (UV) rays from the Sun , releasing free chlorine. The chlorine becomes actively involved in the process of destruction of ozone . The net result is that two molecules of ozone are replaced by three of molecular oxygen, leaving the chlorine free to repeat the process:
Ozone is converted to oxygen, leaving the chlorine atom free to repeat the process up to 100,000 times, resulting in a reduced level of ozone. Bromine compounds, or halons, can also destroy stratospheric ozone. Compounds containing chlorine and bromine from man-made compounds are known as industrial halocarbons .
We Should Not
Deforestation is the logging or burning of trees in forested areas. There are several reasons for doing so: trees or derived charcoal can be sold as a commodity and are used by humans while cleared land is used as pasture , planations of commodities and human settlement. The removal of trees without sufficient reforestation, has resulted in damage to habitat , biodiversity loss and aridity . Also deforestated regions often degrade into wasteland.
Disregard or unawareness of intrinsic value, and lack of ascribed value, lax forest management and environmental law allow deforestation to occur on such a large scale. In many countries, deforestation is an ongoing issue which is causing extinction , changes to climatic conditions, desertification and displacement of indigenous people.
Submitted By:- Submitted to:- Harshit Agarwal Mr. Ashok Sir X-G 14