Even if humans could instantly turn off all our emissions of greenhouse gases, the Earth would continue to heat up about two more degrees Fahrenheit by the turn of the century, according to a sophisticated new analysis published in Nature Climate Change. And if current emissions continue for 15 years, odds are good that the planet will see nearly three degrees (1.5 C) of warming by then.

"This 'committed warming' is critical to understand because it can tell us and policy makers how long we have, at current emission rates, before the planet will warm to certain thresholds," said co-author Robert Pincus, a scientist with CIRES at the University of Colorado Boulder and NOAA's Physical Sciences Division. "The window of opportunity on a 1.5-degree [C] target is closing."

During United Nations meetings in Paris last year, 195 countries including the United States signed an agreement to keep global temperature rise less than 3.5 degrees F (2 C) above pre-industrial levels, and pursue efforts that would limit it further, to less than 3 degrees Fahrenheit (1.5 C) by 2100.

The new assessment by Pincus and lead author Thorsten Mauritsen, from the Max Planck Institute for Meteorology is unique in that it does not rely on computer model simulations, but rather on observations of the climate system to calculate Earth's climate commitment. Their work accounts for the capacity of oceans to absorb carbon, detailed data on the planet's energy imbalance, the climate-relevant behavior of fine particles in the atmosphere, and other factors.

Among Pincus' and Mauritsen's findings:

Even if all fossil fuel emissions stopped in 2017, warming by 2100 is very likely to reach about 2.3 F (range: 1.6-4.1) or 1.3 degrees C (range: 0.9-2.3).

Oceans could reduce that figure a bit. Carbon naturally captured and stored in the deep ocean could cut committed warming by 0.4 degrees F (0.2 C).

There is some risk that warming this century cannot be kept to 1.5 degrees C beyond pre-industrial temperatures. In fact, there is a 13 percent chance we are already committed to 1.5-C warming by 2100.

"Our estimates are based on things that have already happened, things we can observe, and they point to the part of future warming that is already committed to by past emissions," said Mauritsen. "Future carbon dioxide emissions will then add extra warming on top of that commitment."

The research was funded by the Max-Planck-Gesellschaft, the U.S. Department of Energy and the National Science Foundation.