One of the important storylines emerging from Monday’s synthesis report by the Intergovernmental Panel on Climate Change (IPCC) is the race to avoid overshooting a 1.5°C limit on average global temperature rise—and the added risks that would result from extraordinary efforts to bring warming back below that threshold.
At the COP 21 climate conference in Paris in December, 2015, representatives of 195 nations vowed steps to contain global warming to “well below” 2°C above the average for most of human history, and do so by the century’s end. They didn’t specify a target of no more than 1.5°C, but that was what most everyone understood the decision to mean.
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Almost within months of the Paris Agreement, researchers began warning that the steps announced to reduce emissions would not be enough to meet the 1.5°C target. And they kept on saying so. The global thermometer went on rising, now standing around 1.1°C warmer than the long-term average and still climbing: potentially, to 2.8°C by the century’s end.
“Overshoot entails adverse impacts, some irreversible, and additional risks for human and natural systems,” the IPCC’s Summary for Policymakers warns, “all growing with the magnitude and duration of the overshoot.”
That is, ever-hotter summers would mean more wildfires, more tree deaths, more parched peatland, and more lost permafrost. Not only would these natural carbon sinks fail to absorb atmospheric carbon, they would release ever more greenhouse gases into the atmosphere, to notch up ever-higher heat hazards.
And the worse the overshoot, the tougher life would become for humans, not least in coastal settlements, with wind storms becoming more frequent and powerful and tides creeping ever higher.
The much bigger hazard, the report also suggests, is that some of these impacts would be irreversible: sea levels even at 1.5°C are likely to go on rising for a century or more.
Warming above 2°C heightens the risk that temperatures would reach a tipping point for some ecosystems, beyond which, according to German scientists, irrevocable change could threaten the already-vulnerable ice sheets of Greenland and West Antarctica and raise sea levels dramatically.
Higher temperatures could also change forever the nature of the Amazon rainforest, and perhaps slow or alter the circulation systems of the Atlantic Ocean, with unpredictable consequences.
In February, one research team warned that these things could become inevitable at a temperature of 1.8°C. None of these outcomes is certain at any future temperature, but all become more probable with each tiny increment in the global average.
There is nothing inevitable about exceeding 1.5°C—as Stephan Singer of Climate Action Network-International pointed out for The Energy Mix in January, the barriers to action are political, not technological nor even economic.
But if the world exceeded the 1.5°C target, it would become increasingly risky and difficult to step back. The bigger the overshoot, the greater the need for “more net negative CO2 emissions,” the IPCC says.
“The higher the magnitude and the longer the duration of overshoot, the more ecosystems and societies are exposed to greater and more widespread changes in climatic impact-drivers, increasing risks for many natural and human systems,” the IPCC writes. “Compared to pathways without overshoot, societies would face higher risks to infrastructure, low-lying coastal settlements, and associated livelihoods. Overshooting 1.5°C will result in irreversible adverse impacts on certain ecosystems with low resilience, such as polar, mountain, and coastal ecosystems, impacted by ice sheet, glacier melt, or by accelerating and higher committed sea level rise.”
To put it another way, having accidentally and unthinkingly geoengineered a new and unwelcome global climate, humans would then have to take the calculated decision to apply expensive and energy-greedy technologies to get the carbon dioxide back into the ground—not just by capturing and storing it as it emerges from power stations and oil refineries, which is as daunting enough challenge, but by finding inventive and novel ways to suck it from the air. In the last decade, the world’s laboratories have devised a range of ingenious ways to at least potentially exploit capture and exploit carbon dioxide, and the research goes on. But few of these technologies are anywhere near commercial deployment at even a limited scale, and they require large inputs of energy and other resources.
Conserved natural forests and wetlands, on the other hand, provide the same services for free and offer a huge range of other benefits, as well. Which is why the latest report warns that it might be smarter to head as fast as possible to net-zero, reduce the need for demanding technologies, and at the same time reduce what the IPCC scientists politely called “feasibility and sustainability concerns” and the social and environmental risks that might go with carbon dioxide removal deployment at a global scale.
