As world leaders prepare for the COP26 climate summit on October 31, AFP Fact Check investigates some common claims that call into question the existence of human-caused global warming.

Some claim that the crisis is a hoax perpetrated by scientists to justify research grants, or that it is a government conspiracy to control people. If that is the case, it will have to be one of extraordinary complexity, coordinated by successive governments in dozens of countries with a large number of scientists.

Tens of thousands of peer-reviewed studies in the public domain have resulted in a scientific consensus that human-caused climate change is real. The Intergovernmental Panel on Climate Change is the most comprehensive source (IPCC). Far from being a clandestine operation, its evidence and methods are available on the internet at www.ipcc.ch.

Its most recent report, 3,500 pages long, was approved by delegates from 195 states this year. Contributors include 234 authors from 66 countries.

The panel was established by a UN resolution, which fuels conspiracy theories while providing proof of its legitimacy to others.

Scientists know that the Earth has long alternated between ice ages and warming periods, with roughly one ice age every 100,000 years over the last million years. Is the current heating just a part of this cycle? No, it’s different this time because of the speed, relative abruptness, and global extent of the warming over the last 50 years.

“Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2,000 years,” the IPCC claims, accompanied by graphs.

This is based on several types of data, including paleontological analysis of sediment, ice, and tree rings from before the Industrial Revolution, as well as temperature records dating back to 1850.

As evidence of unusual warming has become unmistakable, some skeptics admit it is occurring but deny it is caused by carbon emissions from humans burning fossil fuels. The IPCC created a climate model that assesses the impact of various factors. It computes the extent of heating with and without human activity.

“It is unequivocal that human influence has warmed the atmosphere, ocean, and land,” the IPCC concluded in its annual report this year.

“Massive amounts of snow and near-record-breaking cold have blanketed large parts of the country…. It wouldn’t hurt to have some good old-fashioned Global Warming right now!”

Donald Trump’s tweet on January 20 combined a common climate myth, that cold weather is evidence against climate warming, with the assumption that even if warming occurs, it isn’t all bad. Climate is a metric that measures average weather variations over time. One day or one week of snow does not, therefore, suffice to demonstrate that average temperatures are not rising over decades.

Could “a little global warming” be a good thing? Parts of Siberia may become arable, increasing food resources, but melting permafrost in the same region threatens to exacerbate the situation.

A two-degree rise may sound pleasant, but the IPCC calculates that it is enough to raise sea levels by half a meter or more, drowning coastal cities.

Specialists frequently express their skepticism, signing joint statements and editorials. However, an examination of their credentials in a number of cases has revealed that they are almost never climate scientists.

One of the most important criteria for measuring the legitimacy of claims used by scientists is consensus – and the consensus on climate change is now overwhelming.

A recent Cornell University survey of thousands of peer-reviewed studies on climate change discovered that the authors agreed in more than 99 percent of them that climate change was caused by humans.