The report, approved by 195 governments and based on more than 14,000 studies, is the most comprehensive summary to date of the physical science of climate change. It will be a focal point when diplomats gather in November at a U.N. summit in Glasgow to discuss how to step up their efforts to reduce emissions.
A growing number of world leaders, including President Biden, have endorsed the goal of limiting global warming to 1.5 degrees Celsius, though current policies in the major polluting countries are still far off-track from achieving that target. The 10 biggest emitters of greenhouse gases are China, the United States, the European Union, India, Russia, Japan, Brazil, Indonesia, Iran and Canada.
The new report leaves no doubt that humans are responsible for global warming, concluding that essentially all of the rise in global average temperatures since the 19th century has been driven by nations burning fossil fuels, clearing forests and loading the atmosphere with greenhouse gases like carbon dioxide and methane that trap heat.
The changes in climate to date have little parallel in human history, the report said. The last decade is quite likely the hottest the planet has been in 125,000 years. The world’s glaciers are melting and receding at a rate “unprecedented in at least the last 2,000 years.” Atmospheric levels of carbon dioxide have not been this high in at least 2 million years.
Ocean levels have risen 8 inches on average over the past century, and the rate of increase has doubled since 2006. Heat waves have become significantly hotter since 1950 and last longer in much of the world. Wildfire weather has worsened across large swaths of the globe. Bursts of extreme heat in the ocean — which can kill fish, seabirds and coral reefs — have doubled in frequency since the 1980s.
In recent years, scientists have also been able to draw clear links between global warming and specific severe weather events. Many of the deadly new temperature extremes the world has seen — like the record-shattering heat wave that scorched the Pacific Northwest in June — “would have been extremely unlikely to occur without human influence on the climate system,” the report says. Greenhouse gas emissions are noticeably making some droughts, downpours and floods worse.