Both Approaches Supported the Conclusion That Global Warming Has Accelerated

Global warming has accelerated sharply since around 2015, according to a new analysis from the Potsdam Institute for Climate Impact Research (PIK). After filtering out natural influences such as El Niño, volcanic eruptions, and solar cycles, researchers found that the planet has warmed at roughly 0.35°C per decade over the past ten years, compared with just under 0.2°C per decade from 1970 to 2015. The acceleration, which becomes visible around 2013 to 2015, appeared across five major global temperature datasets with more than 98% statistical certainty. "We can now demonstrate a strong and statistically significant acceleration of global warming since around 2015, says Grant Foster, a US statistics expert and co-author of the study, published today in the journal Geophysical Research Letters.

"We filter out known natural influences in the observational data, so that the 'noise' is reduced, making the underlying long-term warming signal more clearly visible. In climate data, "noise" refers to short-term temperature changes that can temporarily hide or exaggerate the underlying trend. El Niño, for example, can push global temperatures higher for a limited period by releasing heat from the tropical Pacific Ocean into the atmosphere.

Volcanic eruptions can have the opposite effect by sending particles into the atmosphere that reflect sunlight, while changes in the Sun's activity can also produce smaller temperature variations. To separate these temporary influences from the broader warming trend, the researchers analyzed measurements from five widely used global temperature datasets: NASA, NOAA, HadCRUT, Berkeley Earth, and ERA5. "The adjusted data show an acceleration of global warming since 2015 with a statistical certainty of over 98 percent, consistent across all data sets examined and independent of the analysis method chosen, explains Stefan Rahmstorf, PIK researcher and lead author of the study. A statistical certainty of more than 98 percent means the researchers found strong evidence that the change in the warming rate is real rather than the result of random variation in the data.

The same overall pattern appeared in all five temperature records, even though the datasets are produced by different scientific organizations and use somewhat different methods. After adjusting for El Niño and the solar maximum, the extreme warmth of 2023 and 2024 was reduced slightly in the analysis, yet both years remained the two warmest since instrumental records began. Across all datasets, the acceleration started to become visible in 2013 or 2014, with the broader shift beginning around 2015. To determine whether the warming rate had changed since the 1970s, the team used two statistical approaches.

The first was a quadratic trend analysis, which tests whether the temperature curve is bending upward over time rather than rising at a constant rate. The second was a piecewise linear model, which divides the temperature record into separate periods and objectively identifies when the rate of warming appears to change. Both approaches supported the conclusion that global warming has accelerated.

The research was designed to detect and measure the statistical acceleration, not to determine exactly why it has occurred. However, the authors noted that climate models can produce periods in which warming speeds up, meaning an increasing rate of warming is consistent with the range of behavior represented in current climate modeling. Climate models are computer simulations that use the laws of physics to estimate how the atmosphere, oceans, ice, and land respond to greenhouse gases and other influences.

They do not predict every short-term fluctuation perfectly, but they help scientists evaluate long-term climate patterns and possible future changes. "If the warming rate of the past 10 years continues, it would lead to a long-term exceedance of the 1.5°C limit of the Paris Agreement before 2030, says Stefan Rahmstorf. "How quickly the Earth continues to warm ultimately depends on how rapidly we reduce global CO2 emissions from fossil fuels to zero."