The report by the Intergovernmental Panel on Climate Change (IPCC) concludes that many of the consequences which it forecast in its previous reports are now happening. And it warns that we are perilously close to missing the targets agreed in Paris six years ago to avoid dangerous climate change.
This is the first comprehensive assessment of the state of knowledge about climate change science that the IPCC has issued since 2013. It is nearly 4,000 pages in length, prepared by 234 authors from 66 countries who have spent the last three years reviewing more than 14,000 studies.
The report includes a 41-page ‘Summary for Policymakers’ that was approved line by line over the past two weeks by representatives from almost every government in the world.
It finds that the global average surface temperature has increased by just over one degree Celsius since the second half of the 19th century. The Earth is now warmer than it has been for since 125,000 years ago, when the polar ice caps were smaller and global sea level was about five to ten metres higher than today.
Unlike earlier assessments, this report blames all of the rise in temperature on human activities, particularly the burning of fossil fuels. It notes that the warming has been slightly offset by cooling from fine aerosol particles we have released into the atmosphere, which have been blocking out some of the Sun’s energy.
The consequences of this warming are laid out in stark terms. Glaciers around the world have shrunk, the surface of the vast Greenland ice sheet has started to melt, and all the extra water has led to an accelerating rise in global sea level.
While the IPCC has documented these impacts before, the authors are now confident that our greenhouse gas emissions are making extreme events more frequent and intense. The summary states: “Human-induced climate change is already affecting many weather and climate extremes in every region across the globe.”
It draws attention to stronger evidence for “observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence”.
However, as the report only reviews the evidence that had been documented by scientists before February this year, it excludes the most recent heatwaves, floods and wildfires that have occurred over the past few months.
But it is the forward-looking part of the report that is the most worrying. It lays out five different scenarios for global emissions. The most extreme assumes that the annual amount of global emissions of carbon dioxide triples over the next 60 years, leading to a global temperature by the end of the century that could be almost 6C higher than in the second half of the 19th century.
In its scenario assuming the strongest action against emissions, carbon dioxide output falls rapidly and reaches net zero soon after the middle of the century, before becoming negative. It would mean cutting emissions as much as possible and any residual amounts after about 2050 would need to be outweighed by human activities that remove carbon dioxide from the atmosphere through planting more trees and other vegetation, or by using synthetic means to filter it out and permanently store it, including in disused North Sea oil and gas wells.
In this case, global temperature would probably reach 1.5C above the late 19th century baseline by 2050 before dropping slightly by the end of the century.
In another scenario, carbon dioxide emissions reach zero and become negative by about 2075. Warming would exceed 1.5C but would stay below 2C by the end of the century.
These two scenarios would meet the upper and lower limits of the target set in the Paris Agreement, which commits governments to “holding the increase in the global average temperature to well below 2C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5C”.
But the IPCC report makes clear that even if we succeed in meeting these targets, the climate will continue to change for at least the next 30 years. It warns that there will be “increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, proportion of intense tropical cyclones as well as reductions in Arctic sea ice, snow cover and permafrost”.
In other words, the world will have to become more resilient to these impacts while also making every effort to stop global warming.
The nastiest part of the IPCC assessment lurks towards the end of the summary, in a section that it describes as “low-likelihood outcomes”. These are sometimes described as tipping points or climate thresholds because they result in irreversible, unstoppable or accelerating global or regional impacts. They include destabilisation of the polar ice sheets, leading to rapid or large sea-level rise, and the death of major forests, such as the Amazon.
The summary states: “The probability of low-likelihood, high-impact outcomes increases with higher global warming levels. Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out.”
Given that these could have the most serious consequences for the world, it is surprising that the IPCC report does not give them greater prominence.
Some will regard this IPCC report as another depressing indicator that we are already doomed. But it is important that it leads to a strengthening of resolve by individuals, companies and governments to accelerate and strengthen all our efforts to tackle climate change.
Bob Ward is policy and communications director at the Grantham research institute on Climate Change and the Environment at the London School of Economics and Political Science