'If RCP8.5 Did Not Exist, We'd Have to Create It' for Climate Change by 2050

The "worst-case scenario" for climate change set out by the Intergovernmental Panel on Climate Change (IPCC) is our best assessment for the risk posed to the planet over the next 30 years, scientists have found. If the RCP8.5 projection—which assumes greenhouse gas emissions will continue to rise throughout the 21st century—did not exist, "we'd have to create it," researchers writing in the journal PNAS said.

The team, led by Christopher Schwalm from the Woods Hole Research Center in Massachusetts, published a short report in the journal outlining why RPC8.5 is an important way of tracking emissions, especially over the shorter-term.

The IPCC sets out several different climate change scenarios based on different levels of emissions and measures taken to limit them. Known as the Representative Concentration Pathway (RCP), the most optimistic scenario—RCP1.9—represents the pathway needed to keep global warming to below 1.5 C by 2100 compared with the start of the Industrial Revolution. RCP8.5 is the worst-case scenario and would see temperatures exceed 2 degrees Celsius by 2100, with a predicted range between 2.6 and 4.8 C.

The researchers say there has been a move away from RCP8.5 recently, with some within the scientific community calling the scenario is"alarmist" and "misleading." A comment piece published in Nature in January said it should be seen as "unlikely worst cases rather than as business as usual." Zeke Hausfather, director of climate and energy at the Breakthrough Institute, California, told the BBC at the time that the scenario was created in 2005, and that there have been huge changes since then. "What originally was a sort of worst-case with less than 10 percent chance of happening is today, exceedingly unlikely."

Schwalm and colleagues say dismissing the scenario "is not only regrettable, it is skewed." In the report, they say RCP8.5 is in closest agreement with the total cumulative CO2 emissions we have, coming within one percent of actual emissions. They also said it is the best scenario for assessing the risk posed by climate change by 2050, as it is the "best match" to what is going on with current and stated policies.

The researchers say RCP8.5 is important for assessing climate over the shorter-term, and should not be dismissed as an unlikely event. Schwalm said commentary that focuses on the climate by the end of the century is largely unknowable. The Nature commentary, he said, looks at how a scenario is constructed. "We are concerned with physical climate risk in the here and now," he said.

Many climate change models look at what will be happening in 2100. Schwalm and colleagues say that while there is reason to believe we will not be facing RCP8.5 by the end of the century, focusing this far ahead is not necessarily helpful. In order to make societal decisions, shorter-term approaches that convey climate change are useful.

"2100 is 80 years from now," he said. "Think back to young adults trying to imagine 2020 in 1940...The next 30 years is more human relatable.

Schwalm said he is "cautiously optimistic" we will not be in a RCP8.5 scenario by the end of the century. However, the team says dismissing RCP8.5 would be a mistake. "Given the agreement of 2005 to 2020 historical and RCP8.5 total CO2 emissions and the congruence between current policies and RCP8.5 emission levels to midcentury, 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."

global warming
Stock image representing climate change. Researchers say the IPCCs worst case scenario for future warming is the best model for assessing risk for 2050. iStock