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A new climate change study in the journal Nature suggests that “the world has moved decisively away from a no-policy, business-as-usual” pathway on greenhouse gas emissions. And, therefore, a team of scientists conclude, the planet is likely heading toward temperatures by the end of this century that are substantially lower than the 3.9 degrees Celsius (about 7 degrees Fahrenheit) of warming predicted in the absence of climate policy.

Human-caused climate change is just that: human-caused. However, most models of climate change treat the behavior of people—our choices, policies, and perceptions—as a fixed starting point. They develop a range of possible scenarios and policy pathways—and then use those as predetermined and static patterns to run models of the physical system.

“In most models, there’s no feedback between the social and physical systems,” says �պ����� scientist Brian Beckage, a co-author on the new study. “But we know that people respond, and we change our behavior in response to, for example, how climate is changing, what our social networks are doing, and the cost of alternative energy.”

(Graphic: courtesy Brian Beckage et al.)

To better understand how people fit into and will shape the future of climate change, the scientists gathered data sets of public opinion about climate policies, carbon pricing, and greenhouse-gas emissions. Then they created an integrated social-climate-technology model with intersecting feedbacks—and used this public opinion data to constrain several key parameters in the model. Next, they ran the model 100,000 times. This allowed the team to simulate 100,000 possible future policy and emissions trajectories—and fish out relevant variables that could impact climate change in this century.

The results fell into five clusters, with warming in 2100 varying between 1.8 to 3.6 degrees Celsius above the 1880-1910 average, but with a strong probability of warming between 2 and 3 degrees Celsius at the end of the century.

The output of the models suggests that “climate policies will increase in ambition and associated emissions reductions will probably accelerate,” writes the study’s lead author Frances C. Moore, from the University of California, Davis, and co-author Katherine Lacasse at Rhode Island College.

Tipping points

“Small changes in some variables, like the responsiveness of the political system or the level of public support for climate policy, can sometimes trigger a cascade of feedbacks that result in a tipping point and drastically change the emissions trajectory over the century,” said Moore, an assistant professor with the UC Davis Department of Environmental Science and Policy. “We’re trying to understand what it is about these fundamental socio-political-technical systems that determine emissions.”

The study—supported by the and the National Science Foundation—indicates that public perceptions of climate change, the future cost and effectiveness of climate mitigation and technologies, and how political institutions respond to public pressure are all important determinants of the degree to which the climate will change over the 21st century.

“Our research suggests that feedbacks between climate change and the social system will limit the extent of human-caused greenhouse gas emissions and projected climate change—and this is not reflected in the fixed emissions pathways,” says �պ�����’s Beckage, a professor with joint appointments in the Department of Plant Biology and Computer Science, as well as a fellow in the Gund Institute for Environment. “Changing policies, new technologies, direct experience of the weather, shifting opinion, social conformity, and many other factors change our perceptions of climate change and create feedbacks into the system. And these feedbacks reinforce each other and they can lead to rapid changes in behavior that the fixed narratives in other models don’t consider.”