Reacting to President Donald Trump’s decision to withdraw the United States from the landmark Paris climate agreement, leading scientific organizations say evidence clearly shows the world’s climate is changing and urgent measures must be taken to slow the warming of the planet.

The organizations say the scientific evidence is clear that human activity is behind the changing climate. The Intergovernmental Panel on Climate Change, an independent scientific assessment body, warned that without additional efforts beyond those already in place, warming by the end of the century will lead to very high risk of severe, widespread and irreversible impacts.

IPCC spokesman Jonathan Lynn said the scientific body finds that limiting climate change would require substantial and sustained reduction of greenhouse gas emissions, which together with adaptation can limit climate change risks.

“In its analysis of decision-making to limit climate change and its effects, the IPCC noted that climate change is a problem of the commons, requiring collective action at the global scale,” he said. “Effective mitigation will not be achieved if individual players advance their own interests independently. … It is not clear at this stage how the U.S. withdrawal from the Paris Agreement will affect future emissions.”

Deon Terblanche, head of the Atmospheric Research and Environment department at the World Meteorological Organization, said global warming will continue for as long as the world emits greenhouse gases, especially carbon dioxide, into the atmosphere

“Even a reduction in the emissions will not lead to a reduction in the concentrations of greenhouse gases in the atmosphere because there is a cumulative effect and CO2 remains in the atmosphere for hundreds of years,” said Terblanche. “… The climate will continue to warm in any case.”

In a worst-case scenario, he warned the U.S. withdrawal from the Paris Agreement could result in an additional warming of the atmosphere of 0.3 degrees Celsius above the pre-industrial level.
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