Is global warming natural?
While changes in earth's climate are natural, the world's leading authorities on climate have concluded that current warming trends are not natural but are caused by humans.
The planet has undergone multiple changes in its long history and changes in temperature are included.
However, there is overwhelming evidence and no debate among scientists who study climate science that current rates of warming are human-caused. 97% of scientists actively research climate change believe it is caused by human activities (Cook et al., 2013).
Increases in greenhouse gases such as carbon dioxide (CO2) result in the atmosphere retaining more heat. This retention of heat in the atmosphere causes the planet to experience warmer global temperatures on average.
Levels of carbon dioxide in the atmosphere have risen significantly since the industrial revolution. See the image below.
We see CO2 levels in the atmosphere have been relatively stable until the amount emitted by humans increases.
If you're not convinced by the above data and believe that increase in atmospheric CO2 could still be naturally caused, there's even more evidence to the contrary. If we look at the levels of oxygen in the atmosphere, we would expect to see lower levels of oxygen if humans are burning CO2. This is evidence that human activity is directly linked to increase levels of CO2 in the atmosphere. To learn more about this process, click here.
A second piece of indisputable evidence is the ratio of carbon isotopes. Fossil fuels have lower 13C/12C ratios than the atmosphere does. Yet around 1850, when we begin to see a rise in atmospheric CO2 levels, we also see a decrease in the ratio of 13C/12C in the atmosphere, providing further evidence that fossil fuels humans are putting into the environment are causing increases in CO2 levels.
For a more extensive read on the causes of warming temperatures, check out this NASA page on the issue.
J. Cook, et al, "Quantifying the consensus on anthropogenic global warming in the scientific literature," Environmental Research Letters Vol. 8 No. 2, (June 2013); DOI:10.1088/1748-9326/8/2/024024