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, with best estimates indicating the humans have contributed 100% of the observed warming between today’s climate conditions and those of the 1850-1900 period, an approximation of pre-industrial levels.
The bulk of emissions derive from our demand for energy. The largest contributor is carbon dioxide (CO2), emitted when fossil fuels are burnt to meet those demands. There are also other emissions attached to industrial processes and agriculture.
Greenhouse gas emissions
Scientists have known since the early 1800s that greenhouse gases (such as water vapour, carbon dioxide and methane) trap heat in the atmosphere. The concentration of CO2 in the air has reached more than 410 parts per million by volume (ppm), compared to about 280ppm in 1750 (around the start of the Industrial Revolution), increasing the amount of energy that is being trapped in the climate system and causing the surface temperature of the planet to rise. This increase in atmospheric concentrations is largely the result of burning fossil fuel for energy resulting in CO2 accumulating in the atmosphere.
The warming from increased levels of greenhouse gases in the atmosphere, and a smaller cooling effect of other human emissions such as aerosol particles that are often emitted alongside CO2, where assessed by the IPCC to explain 100% of the global average warming observed between the pre-industrial period and the 2006-2015 decade.
Natural influences on the climate
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.