Fact Check: The truth behind global warming

Theresa May's decision not to list climate change among her priorities for the G20 summit in Hamburg has refocused attention on the science and politics of global warming.

The Prime Minister said her principal objectives would be cutting off funding for terrorism, tackling global migration, eradicating modern slavery and "making the global economy work for everyone". But she "will challenge US President Donald Trump over climate change," the BBC reports.

G20 host Chancellor Angela Merkel has said "tackling climate change is to be the key objective of the summit following the US's withdrawal from the Paris climate pact", a decision that prompted protests from many European heads of government, including May.

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Trump, who once said climate change was a Chinese hoax, has appointed several people to his administration who have questioned the science of global warming.

Climate change scepticism - who says what?

Trump's ditching of the Paris accord, which aims to limit the average global temperature increase to below 2C, reflects his administration's scepticism towards the reality and cause of climate change.

Earlier this year, scientists were stunned when the head of the US environmental watchdog questioned the link between carbon dioxide emissions and global warming.

Scott Pruitt, chief of the Environmental Protection Agency (EPA), called for further debate on the issue, saying he was not convinced the gas is a "primary contributor" to rising temperatures.

"I think that measuring with precision human activity on the climate is something very challenging to do and there's tremendous disagreement about the degree of impact," he told CNBC.

As a known climate change sceptic with close ties to the fossil fuel industry, Pruitt was among Trump's most controversial appointments.

His predecessor Gina McCarthy accused him of ignoring established scientific facts: "The world of science is about empirical evidence, not beliefs," she said.

"When it comes to climate change, the evidence is robust and overwhelmingly clear that the cost of inaction is unacceptably high. I cannot imagine what additional information the administrator might want from scientists for him to understand that."

What are the facts?

The link between carbon dioxide and temperature rise, which dates back to the 19th century, is supported by the overwhelming majority of scientists, who all agree that carbon dioxide released by human activities, including the burning of fossil fuels and deforestation, is the main contributor to global warming.

"While climate science is, indeed, a challenging and complex subject, the role of carbon dioxide in warming the planet as well as evidence that recent increases in the gas come from humans is unequivocal," fact-checking website Snopes says.

In a joint report published in January, researchers at Nasa and the National Oceanic and Atmospheric Administration concluded that the planet's average surface temperature has risen by about 1.1 degree since the late 19th century, "a change driven largely by increased carbon dioxide and other human-made emissions into the atmosphere".

As the Washington Post reports, Pruitt's claim is easily refuted by data available on the website of the very agency he runs – EPA's basic information page states that humans are "largely responsible" for recent climate change and warns of the risks of global warming.

Who is right?

While the severity of the impact of climate change is still under debate, the fundamental link between carbon emissions and global warming cannot be disputed. Pruitt's scepticism flies in the face of more than a century of evidence.