Ten years ago, close to 200 nations signed the Paris Agreement, an international treaty designed to cut greenhouse gas emissions and curtail global warming. Under the treaty, most nations made a 15-year promise to reduce emissions. Now, armed with a decade of data, a new University of Washington-led study shows global progress, but not enough to compensate for the environmental cost of economic growth.
With its adoption in 2015, the Paris Agreement kicked off a concerted effort to reduce emissions by prioritizing renewable energy, reducing waste and tracking impact. The treaty aimed to keep the planet from warming more than 2 degrees Celsius by 2100, with a target of 1.5 C. A new statistical analysis of the data shows that the agreement has helped some nations cut emissions, but the net impact is still too high to curb warming.
"The efforts made in response to the Paris Agreement did change the course of things, but the effects were knocked out by an increase in gross domestic product," said Adrian Raftery, a UW professor emeritus of statistics and sociology who led the study.
The results were published Oct. 17 in Communications Earth and Environment.

Carbon dioxide emissions (top) and carbon intensity (bottom) for the world, China, the U.S. and Germany. Black shows observed trends, green represents each country meeting its NDC and red is a median projection. Carbon intensity is trending down but emissions in China, and the world, are increasing.Jiang et al./Communications Earth and Environment
This study is the third in a series of papers by Raftery and colleagues. The first, published in 2017, provided a probability-based assessment of what would happen if every country satisfied its Nationally Determined Contribution, as defined by the Paris Agreement. The researchers' predictions were based on three components: total world population, GDP and carbon intensity, a measure of carbon emissions per dollar.
"One of the key findings from that work was that basically, it's not going to be enough," Raftery said. "Even if every country met their goal, there was just a 5% chance that we would stay below the 2-degree mark."
Meeting the collective goal would require everyone to do more, but just how much more was unclear.
Four years later, in 2021, the authors came back to say that emissions goals needed to be about 80% more ambitious to keep warming below 2 C. If each country increased its emissions reduction goal by 1.8% annually, and continued doing so after the Paris Agreement lapses in 2030, temperatures might stay just under the threshold.
This new study updates the statistical methods developed in 2021 and applies them to data collected in the past decade. It reveals that carbon intensity is trending downward by 3.1% each year, compared to 1.1% before the Paris Agreement was signed in 2015.
"It's an improvement," Raftery said, but the net result is not positive because world economies grew faster than expected. Even though less carbon was released to produce each economic unit, global GDP increased enough to drive total emissions up.
"Reducing economic growth is unpopular," Raftery said, and the world population is growing. "So realistically, carbon intensity is the only factor under some kind of policy control."

Emissions projections have shifted in the past decade, likely due to the Paris Agreement and reductions in carbon intensity, a measure of CO2 per dollar.Jiang et al./Communications Earth and Environment
Still, the data does contain promising trends. The chance of "the most catastrophic climate change," where temperature increases by 3 or more degrees, has gone down since 2015, from 26% to 9%. The likelihood of keeping warming below 2 degrees has also increased from 5% in 2015 to 17%.
All countries can and should continue looking for ways to contribute but some have more power than others, according to Raftery. The bigger the economy, the greater its impact on carbon intensity.
China, which is responsible for nearly one-third of total global carbon emissions, saw dramatic economic growth over the past decade. Although the country managed to reduce its carbon intensity 36% by 2024, its emissions shot up as GDP rose.
Both India and Russia followed similar trends, with observed emissions climbing far above their projected goals.
"China and the U.S. have the biggest economies and are among the most wasteful countries," Raftery said. The carbon intensity for China was three times that for Germany, which is the largest economy in the European Union. Carbon intensity for the U.S. was 50% higher than that for Germany.

Average global temperature projections to 2100 show how the Paris Agreement has impacted trends. Blue dashes represent each country meeting their NDC, purple shows continued improvement after the Paris Agreement lapses and orange reflects U.S. exclusion.Jiang et al./Communications Earth and Environment
When President Donald Trump took office earlier this year, he announced plans to withdraw the U.S. from the Paris Agreement for a second time. After he withdrew during his first term, former President Joe Biden rejoined and pledged that the U.S. would reduce its emissions 60% from peak levels by 2035. To evaluate how the climate will change if the U.S. makes no contribution, the researchers ran the numbers again without it.
If all the other participating countries meet their goals, the projected temperature increase goes up by 0.1% and the chance of staying below 2 C decreases from 34% to 27%. The researchers note that this is an optimistic projection. It assumes that the U.S. will stop making cuts, but does not account for the possibility that it will reverse the trend by increasing carbon intensity.
"The fact that the Paris Agreement did work in reducing at least carbon intensity is good news," Raftery said. "There need to be bigger efforts made to offset economic growth, but there is reason for hope."
Coauthors include Jitong Jiang and Skylar Shi, both former UW graduate students of statistics. This research was funded by the National Institutes of Health.