Global warming is happening, but not statistically ‘surging,’ new study finds

The planet is getting warmer - but at a statistically steady rate, a new study reveals.

Clemson University's Colin Gallagher joined an international team of researchers from the U.S. and U.K. in investigating if the rate of global warming has increased significantly, or "surged," over the last half century at statistically detectable rates.

The new study, published in the journal Nature Communications Earth & Environment, confirms the broad consensus that the planet is getting warmer but at a statistically steady rate - not at a sufficiently accelerated rate that could be statistically defined as a "surge."

Recent years have seen record-breaking temperatures and heat waves globally. Data show 2023 was the warmest year since global records began in 1850 - by a wide margin - and that the 10 warmest years in the historical record have all occurred in the past decade (2014-2023).

These alarming records have spurred discussion and debate about whether the rate of global warming has increased, with some arguing that global warming has accelerated over the past 15 years. The team's findings demonstrate a lack of statistical evidence for an increased warming rate that could be defined as a "surge."

Yearly surface temperature from 1880-2023 compared to the 20th-century average (1901-2000). Blue bars indicate cooler-than-average years; red bars show warmer-than-average years. NOAA Climate.gov graph, based on data from the National Centers for Environmental Information.

"Using careful analyses of the global temperature anomaly series, we detect the well documented temperature increase that started in the 1970s but find no significant changes in the rate of global warming after that," said Colin Gallagher, Emily Peek Wallace '72 Director of the School of Mathematical and Statistical Sciences at Clemson University. "The recent record-breaking temperatures are alarming, but not yet inconsistent with steady global warming."

"Of course, it is still possible that an acceleration in global warming is occurring," said lead author Claudie Beaulieu, professor of ocean sciences at University of California Santa Cruz. "But we found that the magnitude of the acceleration is either statistically too small, or there isn't enough data yet to robustly detect it." 

The research team performed a rigorous analysis of sets of global surface-temperature averages from the four main agencies that track the average temperature of Earth's surface, including NASA and the National Oceanic and Atmospheric Association (NOAA), dating back to 1850. Since that year, Earth's temperature has risen by 0.11 degrees Fahrenheit per decade, according to NOAA.

Specifically, the study analyzed the "global mean surface temperature" (GMST), which is widely studied to monitor climate change but also presented some challenges. GMST tends to rise over time due to human-caused pressures, and it fluctuates around that long-term trend because of natural phenomena, like major volcanic eruptions and the El Niño Southern Oscillation, that affect global temperatures. Thus, distinguishing between that natural variability and true underlying changes in the pace of warming is a statistical challenge, the team acknowledged.

The team's analysis deemed an increased warming surge to be statistically detectable if it exceeded and sustained a level above those temporary fluctuations over a long period of time. Imagine temperature records plotted on a graph. A small change in the slope would require more time to detect it as significant, whereas a large change would be evident quicker. The team applied that threshold in their statistical analysis of more recent records dating back to the 1970s to see if temperature trends crossed that threshold and found that none did.

"Our concern with the current discussion around the presence of a 'surge' is that there was no rigorous statistical treatment or evidence," said study co-author Rebecca Killick, a statistics professor at Lancaster University in Lancaster, England. "We decided to address this head on, using all commonly used statistical approaches and comparing their results."

After accounting for short-term average temperature fluctuations, which can mask long-term trends and create the appearance of slowdowns and surges in warming, and using a range of statistical methods, the team determined the level of increased warming that would be needed for a given number of years in order to detect surges.

"A warming surge would be indicated by sustained observed temperatures well above those expected under the current warming scenario, and we wanted to give climatologists an idea of how many years it might take to statistically detect a surge or hiatus relative to the current estimated rate," Gallagher said.

For example, they identified that, for the year 2012, the rate of warming would have needed to increase by at least 55% before its trajectory could be statistically detectable in 2024 - and therefore be called a "surge." In another example, they show that a change in the warming rate of around 35% in 2010 would become statistically detectable by around 2035.

They applied that threshold in their statistical analysis of more recent records dating back to the 1970s to see if temperature trends crossed that threshold and found that none did.

The team also provide the minimum percentages for statistical detectability going into the future up to 2040.

"Alongside our results, we give a benchmark to scientists - a minimum threshold that must be exceeded before a change may be detectable. We hope this helps add rigor to future discussions on potential surges or hiatus," Killick said.

Beaulieu emphasized that although the findings show there is currently no statistical evidence for a warming surge, this should not be taken to read that climate change is not happening.

"Earth is the warmest it has ever been since the start of the instrumental record because of human activities - and to be clear, our analysis demonstrates the ongoing warming," Beaulieu said. "However, if there's an acceleration in global warming, we can't statistically detect it yet."

Other co-authors on the paper, "A Recent Surge in Global Warming is not Detectable Yet," are Robert Lund, professor and department chair of statistics at the UC Santa Cruz; and Xueheng Shi, assistant professor of statistics at the University of Nebraska-Lincoln.

Adapted from a media release by Lancaster University and UC Santa Cruz.

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