Scientists all over the world are busy trying to work out a benchmark for the amount of CO2e (Carbon-dioxide or equivalent green house gases) presence so that atmosphere's mean temperature doesn't rise beyond 2 degree centigrade as compared to Pre-Industrial Revolution.
Scientific studies in last 50 years point to a fact that temperatures have been rising. There is a broad consensus among the scientific community that the rising level of CO2 gases in the atmosphere is responsible for the increase. What is still debatable is how much of it is created by humans? What would be its impact on earth? How much decrease in CO2e can bring down the projected temperature to stabilize earth's climate so that we don't suffer catastrophic losses?
According to various studies right now human beings are emitting an estimated 26 billion tons of CO2e every year. If we have to contain temperature rise within 2 degree centigrade then we will have to limit carbon emission to a trillion tons within 40 years. Since Industrial Revolution in mid 1800s the world has consumed half a trillion tons of carbon and will have to restrict its emission to half a trillion tons in the next 40 years.
Pre- Industrial Revolution carbon emission was at 280 ppm. Since then it has risen at a rate of 2ppm per year. According to studies conducted 1955 the CO2 level in the atmosphere was 310 ppm (Parts Per Million molecules of atmospheric gases). By 1990 it had risen to 350 ppm. In the year 2009 it is 385.99 ppm.
International Protocol on Climate Change (IPCC) the United Nation's designated body to deal with and track the climate change developments has suggested that all the nations should agree to limit the CO2 level at 450ppm. According to IPCC if the CO2 levels increase beyond this point it would lead to long-term changes in climate. These changes would be reflected in melting glaciers, decreased capacity of oceans to absorb carbon and irregular weather patterns.
The IPCC estimates suggest that to reach or cap carbon emission at 450 ppm emissions will have to reach a maximum by 2020 and from then on reduce at a rate of 3 percent annually for the next 30 years.
But there are NGOs, experts and activists insisting that the emission level benchmark should not be fixed at 450ppm. Instead they say it should be fixed at 1990 level of 350 ppm. Their argument is that the Earth Summit in Rio (1992) had suggested that the emissions should be cut by 5 percent from 1990 level. If we aim for the 1990 carbon level of 350 ppm we may be able to arrest the climate change.