Political leaders and climate activists often say that human-caused climate change presents an existential threat to humanity, or could lead to a global catastrophe, but this is rarely defined.

Driving the news: A group of top climate scientists has come forward to argue that more rigorous research is urgently needed into such worst-case scenarios, which they call a "climate endgame."

• In a new perspective piece in the Proceedings of the National Academy of Sciences, 11 researchers from around the world put forward a research agenda into the consequences of global warming that reaches the higher end of plausible scenarios, amounting to 3°C (5.4°F) or greater, by the end of the century.

Why it matters: According to the paper, worst-case scenarios have been understudied by organizations such as the U.N. IPCC due to the focus on the Paris Agreement's temperature goals, as well as the inherent cautiousness of climate scientists whose culture eschews alarmism.

Zoom in: The piece, which is not a study but rather a detailed research proposal, states: "There are ample reasons to suspect that climate change could result in a global catastrophe."

• The authors, who include Hans Joachim Schellnhuber, a physicist who advised German Chancellor Angela Merkel on climate science, and prominent researcher Johan Rockström, see such research as having the potential to motivate society to act with greater urgency to limit warming to the Paris targets.
• The paper notes that if all countries deliver on their non-binding long-term emissions reduction pledges it could limit warming to 2.1°C (3.78°F) above preindustrial levels by 2100.
• The authors warn that even this optimistic scenario would make the planet warmer than any point seen for more than 2.6 million years.

Threat level: The paper notes how the COVID-19 pandemic has revealed the risks of infrequent, high-impact global events that have knock-on effects throughout the global economy.

• The uncertainties involved with catastrophic risks may have policy specific implications, the researchers state, including by boosting the social cost of carbon, which is the price assigned to the consequences of each additional ton of carbon dioxide emissions added to the atmosphere.
• This calculation is used when evaluating a host of government regulations and is heavily contested.

The paper provides a tour of plausible ways in which climate change could tip society into more precarious, if not catastrophic, outcomes, from destabilizing the most fragile countries to causing physical and political "risk cascades" that can ripple around the world.

• The authors propose a rigorous academic research agenda for conducting an “integrated catastrophe assessment” due to climate change, and recommend a special report from the IPCC on this prospect.
• They define such warming as a temperature increase of 3°C or above preindustrial levels, which is well within the current scope of possibility.

Of note: The authors include the "rapid spread of misinformation and disinformation," which has vexed public health officials in responding to COVID-19, as a risk factor that makes the world more vulnerable to the impacts of an extreme climate change scenario.

What they're saying: "We are not saying that we are all doomed," lead author Luke Kemp, of the University of Cambridge, told Axios via email.

• "This isn’t about disaster voyeurism; it is about understanding plausible catastrophic risks so we can prevent them," he said.
• "The greater the understanding of potential catastrophic risks, the better the potential for developing effective risk reduction strategies," study coauthor Kristie Ebi, a professor at the University of Washington, told Axios via email.

The intrigue: Gernot Wagner, a climate economist at Columbia Business School who was not part of the author team, said he agrees that more research is needed into these issues.

• "Even linear changes in underlying climate conditions can lead to socio-economic tipping points," he said, citing the example of the drought in Syria in 2007-2010 that helped spark the civil war there, spurring a refugee crisis that in turn led to a rightward shift in European politics.
• "Cascading risks deserve quite a bit more attention," he said.