Geological records stretching back millions of years indicate a number of large variations in Earth’s past climate. These have been caused by many natural factors, including changes in the sun, volcanoes, Earth’s orbit and CO2 levels.
However, comprehensive assessment by scientists shows that it is extremely likely that human activity has been the dominant cause of warming since the mid-20th Century.
Greenhouse gas emissions
Evidence that CO2 emissions are the cause of global warming is very robust. Scientists have known since the early 1800s that greenhouse gases in the atmosphere trap heat.
Global CO2 emissions from human activity have increased by over 400% since 1950. As a result, the concentration of CO2 in the air has reached more than 400 parts per million by volume (ppm), compared to about 280ppm in 1750 (around the start of the Industrial Revolution).
The Earth’s natural climate cycle
Over the last 800,000 years, there have been natural cycles in the Earth’s climate, between ice ages and warmer interglacial periods. After the last ice age 20,000 years ago, average global temperature rose by about 3°C to 8°C, over a period of about 10,000 years.
We can link the rises in temperature over the last 200 years to rises in atmospheric CO2 levels. Greenhouse gas levels are now well above the natural cycle of the last 800,000 years.
The sun is the primary source of Earth’s heat, so relatively small changes in solar output can affect our climate.
Satellite observations since the late 1970s have shown a slight decrease in the sun’s total energy output. However, instead of cooling, the Earth has warmed over this period.
Also, warming from the sun would heat all of the atmosphere, including the lowest few kilometres (the troposphere) and the layer above (the stratosphere). Observations show that the stratosphere is in fact cooling while the troposphere warms. This is consistent with greenhouse gas heating and not solar heating.