Paris climate agreement goal still within reach, new study suggests

The Paris Climate Agreement’s goal of limiting global warming this century to 3.6 degrees Fahrenheit (2 degrees Celsius) above pre-industrial temperatures is still within reach, while the most doomsday scenarios unfavorable are no longer plausible, suggests a new analysis by CU Boulder.

released today in Environmental Research Letters, the new study finds that a subset of climate scenarios from the Intergovernmental Panel on Climate Change (IPCC) most consistent with recent data and predictions from the International Energy Agency (IEA) for 2050 project between 3.6 and 5.4 degrees F (2 and 3 C) of warming by 2100, with a median of 3.96 F (2.2 C) degrees. By comparison, some implausible worst-case scenarios have projected as much as 7.2 or 9 degrees F (4 or 5 C) of warming by the end of the century.

“This is good news that is cautiously optimistic about where the world is today, about where we thought we were,” said lead author Roger Pielke Jr., professor of environmental studies. “The goal of two degrees from Paris remains within reach.”

In order to explore and plan for possible futures, the climate research community uses scenarios: predictions of how the future might develop based on factors such as projected greenhouse gas emissions and different possible climate policies.

The most commonly used scenarios, called the Representative Concentration Pathways (RCPs), were developed by the IPCC beginning in 2005. The Shared Socioeconomic Pathways (SSPs) that followed, beginning in 2010, were intended as an update . Together, the two sets of scenarios inform the IPCC’s fifth and forthcoming sixth assessment reports.

For their study, Pielke Jr. and his co-authors started with a total of 1,311 climate scenarios from which the climate research community selected the 11 RCPs and SSPs. Pielke and his colleagues compared the scenarios to projected growth rates of fossil fuels and industry carbon dioxide emissions for 2005-2050 most consistent with actual observations from 2005-2020 and IEA projections through in 2050. The number of scenarios that most closely matched data from the past 15 years and subsequent emissions projections ranged from less than 100 to nearly 500, depending on the method applied. These scenarios represent plausible futures if current trends continue and countries adopt the climate policies they have already announced to reduce carbon emissions.

Additional, more optimistic or pessimistic futures could also exist, the authors said.

“Because we haven’t updated our [IPCC] scenarios [for many years]there are also futures that are plausible but have not yet been considered,” said Pielke Jr.

Tracks and plausibility

The analysis joins a growing consensus of independent groups around the world whose work shows that the most extreme climate scenarios are unlikely to occur this century, and that mid-range scenarios are more likely. A report from the IPCC’s Sixth Assessment Report (AR6) published in 2021 also notes that the likelihood of high emissions scenarios is considered low.

Why are these worst-case scenarios now less plausible? Generally, they were all developed over ten years ago, and a lot has happened since then.

For example, renewable energy has become more affordable and, therefore, more mainstream faster than expected, said Matthew Burgess, co-author and researcher at the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.

These rapid changes are captured in scenarios developed by the IEA, an intergovernmental organization based in Paris, which provides updates every year.

Climate scenarios also tend to overestimate economic growth, especially in poorer countries, according to Burgess, assistant professor of environmental studies.

Moreover, while the 2010 scenarios were meant to serve as an update of the socio-economic assumptions of the initial RCPs, the RCPs have continued to be widely used by scientists. And the commonly used “worst case” scenario, RCP8.5 (named after 8.5 watts per square meter, a measure of solar irradiance) projects an increase of 7.2 to 9 F (4 to 5 C) by 2100.

“It’s hard to overstate how [climate] the research focused on the four- and five-degree scenarios, RCP 8.5 being one of them. And those seem less and less plausible every year,” Burgess said.

Relying not only on outdated scenarios, but also on scenarios that are no longer plausible, for research and policy has big implications for how we think, act and spend money on issues of change. climate, the authors said.

“There is a need for these scenarios to be updated more frequently. Researchers may be using a 2005 scenario, but we need a 2022 outlook,” Pielke Jr. said. “You’ll have better policies if you have a more accurate understanding of the problem, whatever the political implications on one side or the other.”

The authors point out that 3.6 degrees F (2 C) of warming will still have a dramatic impact on the planet, and now is not the time to be complacent.

“We’re getting closer to our two degree goal, but we certainly still have a lot of work to do if we’re going to hit 1.5,” Burgess said.

Justin Ritchie of the Institute for Resources, Environment and Sustainability at the University of British Columbia is co-author of this publication.