New statistical projections by researchers at the University of Washington (UW) show a 90 percent chance that temperatures will increase this century by 2.0 to 4.9 degrees Celsius.

Instead of working on scenarios for future carbon emissions, ranging from "business-as-usual" emissions from growing economies to serious worldwide efforts to transition away from fossil fuels, the new study focuses on three quantities that underpin future emissions: total world population, gross domestic product per person and the amount of carbon emitted for each dollar of economic activity, known as carbon intensity.

Using statistical projections for each of these three quantities based on 50 years of past data in countries around the world, the study published this week in Nature Climate Change finds a median value of 3.2 degrees Celsius warming by 2100.

The researchers, including lead author Adrian Raftery, a professor of statistics and sociology, and co-author Dargan Frierson, an associate professor of atmospheric sciences, have recognized that what matters more is the carbon intensity, the amount of carbon emissions produced for each dollar of economic activity. That value has dropped over years as countries boost efficiency and enact standards to reduce carbon emissions.

However, how quickly that value drops in future decades will be crucial for determining future warming. And warming of the planet by 2 degrees Celsius is often seen as a "tipping point" that people should try to avoid by limiting greenhouse gas emissions.

The new findings indicate that there is only a 5 percent chance that Earth will warm 2 degrees or less by the end of this century; and a mere 1 percent chance that warming could be at or below 1.5 degrees, which is the target set by the 2016Paris Agreement on climate change.

In their work, the researchers initially expected to find that higher populations would increase the projections for global warming. Instead, they were surprised to learn that population has a fairly small impact, because most of the population increase will be in Africa, which uses few fossil fuels.