Climate science’s worst case scenario isn’t just an awful warning. It describes what is already happening right now.
LONDON, 10 August, 2020 – A trio of US researchers has grim news for people worried about climate science’s worst case outcome. Forget about the other options. The worst case is already happening.
Christopher Schwalm and colleagues at the Woods Hole Research Center in Massachusetts report in the Proceedings of the National Academy of Sciences that they took a closer look at the evidence for climate change in terms of carbon dioxide emissions and climate models.
This is the kind of research that assesses the future under a number of possible scenarios. These scenarios are based on mathematical models and global assumptions about economic growth, carbon budgets and land use changes, and they are couched in language arcane enough to make even committed followers of climate science reach for the aspirin.
The most optimistic of these is one in which the world makes a determined, drastic and concerted effort to contain global heating to well below 2°C above the average for most of human history. At the other end of the scale is one notoriously called “business as usual”, in which the nations of the world carry on burning ever more fossil fuels, while sea levels rise ever higher, and the thermometer readings get ever higher. It has been intended from the start as an awful warning rather than as a guide to what is most likely to happen.
“RCP8.5 has continued utility … if RCP8.5 did not exist, we’d have to create it”
Since 195 nations met in Paris in 2015 and vowed to take action to keep global heating if possible to well below 2°C, and ideally no higher than 1.5°C, there has been an assumption that the “worst case”, or “business as usual” scenario – known in climate science shorthand as Representative Concentration Pathway 8.5, or RCP8.5 – was no more than that: the worst case.
Under the terms of the Paris Agreement, nations accepted commitments to plans to reduce emissions. Researchers have repeatedly warned that such plans as have been announced were not ambitious enough, and not being implemented fast enough.
Even so, after Paris, it became clear there would surely be change. The world had been alerted, the worst could indeed be averted. The RCP8.5 scenario was, some said, of no great help. It has even been described as “extreme, alarmist and ‘misleading’.”
Implications for 2100
Sadly, it may not be. Dr Schwalm and his colleagues looked at cumulative greenhouse gas emissions since 2005. By 2020, the emissions matched the “business as usual” or RCP8.5 predictions very closely.
They then extended the trends to 2030, and to 2050, with the same outcome. That means that – by the end of the century – the planet could be 3.3°C to 5.4°C warmer than it was at the launch of the Industrial Revolution and the worldwide switch to fossil fuels. In which case, the worst-case scenario would remain on the table as a useful risk assessment tool.
“The implied probability of occurrence similar to RCP8.5 even at the end of the century is large enough to merit its continued use,” the scientists write.
“RCP8.5 has continued utility, both as an instrument to explore mean outcomes as well as risk. Indeed, if RCP8.5 did not exist, we’d have to create it.” – Climate News Network