IPCC's Fourth Assessment Report(AR4) conclusions

Some statistics from the IPCC 2007 report:

The concentration of CO2, the most important green house gas, increased from 280 ppm in the pre industrial era to 379 ppm in 2005, by far exceeding the natural range (180-300ppm) observed in the last 650,000 yrs by samples of ice cores. The growth rate between 1995 and 2005 was 1.9 ppm.

The world avg emission between 2000 and 2005 was 26.4 Gt CO2/yr or approx 7.2 Gt Carbon equivalent per year from fossil fuel burning, as compared to 23.5 Gt CO2/yr in 1990's

The avg methane concentration in the atmosphere increased from the 715 ppb(pre industrial era) to 1774 ppb in 2005. Nitrous Oxide (N2O) increased from 270 ppb in pre-industrial era to 319 ppb in 2005, and this increase has accelerated from 1980's due to the increased agricultural activities.

Between 1906 and 2005, the earth's avg temperature increased by 0.74 degrees C.
The oceans absorb more than 80% of the heat inciding on the earth and their avg temperatures increased in depth of upto 3000 m, leading to a volumetric expansion and to the increase in sea level. The sea level rose 17cm in 2oth century, being 1.8mm a year in 1961-2003 and 3.1 mm a year in 1993 - 2003.

As per the IPCC models, between 1999 and 2099, the avg temperature of the planet will rise by between 0.3 C and 6.4 C; the sea level will rise between 0.18 and 0.59 metres and the ocean pH will be reduced by between 0.14 and 0.35. The models predict that warming will be greater on land compared to oceans which will lead to melting of ice in the northern latitudes, intense cyclones etc. So to stabilize the CO2 concentrations in the atmosphere at 450 ppm and increasing the avg temperation by up to 0.5 C in the 21st century, considerable effort will be necessary to reduce emissions by 2460 Gt CO2 to 1800 Gt CO2. Past and future CO2 emissions caused by human activities will continue to contribute to global warming and an increase in ocean levels for more than a millenium, due to the timescale necessary to remove these gases from the atmosphere.

[ Excepts from "Energy, Environment and Development", 2nd Ed. by Jose Goldemberg and Oswaldo Lucon.]