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Analysis: Why scientists think 100% of global warming is due to humans

The extent of the human contribution to modern global warming is a hotly debated topic in political circles, particularly in the US.

During a recent congressional hearing, Rick Perry, the US energy secretary, remarked that “to stand up and say that 100% of global warming is because of human activity, I think on its face, is just indefensible”.

However, the science on the human contribution to modern warming is quite clear. Humans emissions and activities have caused around 100% of the warming observed since 1950, according to the Intergovernmental Panel on Climate Change’s (IPCC) fifth assessment report.

Here Carbon Brief examines how each of the major factors affecting the Earth’s climate would influence temperatures in isolation – and how their combined effects almost perfectly predict long-term changes in the global temperature.

Carbon Brief’s analysis finds that:

  • Since 1850, almost all the long-term warming can be explained by greenhouse gas emissions and other human activities.
  • If greenhouse gas emissions alone were warming the planet, we would expect to see about a third more warming than has actually occurred. They are offset by cooling from human-produced atmospheric aerosols.
  • Aerosols are projected to decline significantly by 2100, bringing total warming from all factors closer to warming from greenhouse gases alone.
  • Natural variability in the Earth’s climate is unlikely to play a major role in long-term warming.

Animation by Rosamund Pearce for Carbon Brief. Images via Alamy Stock Photo.

How much warming is caused by humans?

In its 2013 fifth assessment report, the IPCC stated in its summary for policymakers that it is “extremely likely that more than half of the observed increase in global average surface temperature” from 1951 to 2010 was caused by human activity. By “extremely likely”, it meant that there was between a 95% and 100% probability that more than half of modern warming was due to humans.

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