Carbon dioxide is a greenhouse gas that leads to global warming. The amount of CO2 that enters the atmosphere each day is around 90 million tons. While water vapour is a more powerful greenhouse gas than CO2, we do not emit vast amounts of water vapour every day; actually, the atmosphere can only hold a certain amount of water vapour at a time, but, by warming the atmosphere, increased CO2 levels enable the retention of greater amounts of water vapour, thus enhancing warming.
Scientists know, from analyzing the isotopes of the carbon in the atmosphere, that the increase in atmospheric CO2 is the result of burning fossil fuels and forests, and not the result of natural processes.
Measured from land or from satellite, global temperatures are increasing. Averaged over all land and ocean surfaces, global mean temperatures have increased by approximately 0.74 degrees Celsius over the past century, but more than half of this warming—about 0.4 °C—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most – about 0.7 °C since 1979 – especially over the Northern Hemisphere. Clearly, the rate of increase is intensifying.
Earth’s climate has undergone numerous significant shifts. It has been at various times, both much warmer and much colder on average than it is now, so how do we know that what is happening now is not one of those natural cycles?
Firstly, none of the natural forces – tilts in the planet’s axis, wobbles in its orbit, or increases solar activity – is a factor. In fact, the sun is going through a slight cooling cycle even as temperatures on Earth are increasing. Secondly, the speed at which the change is occurring indicates that it is by humans-caused.
Data from 73 sites around the world shows that temperatures today are warmer