Groundbreaking Study Finds 'Fingerprint' of Human-Caused Climate Change in Global Daily Weather

Scientists can now detect the "fingerprint" of human-caused climate change in global daily weather patterns, according to a groundbreaking analysis published Thursday in the journal Nature Climate Change.

"This study provides important new evidence that climate change is influencing the conditions that people and ecosystems are experiencing every day, all around the world."
--Noah Diffenbaugh, Stanford climate scientist

The study, Climate Change Now Detectable From Any Single Day of Weather at Global Scale (pdf), complicates the well-established narrative about the difference between weather and climate. Generally, the former refers to the conditions of the atmosphere in terms of factors like cloudiness, moisture, pressure, temperature, and wind over a short period of time while the latter is average weather in a region over longer periods.

"For generations, climate scientists have educated the public that 'weather is not climate,' and climate change has been framed as the change in the distribution of weather that slowly emerges from large variability over decades," the study says. "However, weather when considered globally is now in uncharted territory."

A research team led by Reto Knutti, a climate professor at ETH Zurich in Switzerland, used statistical learning and climate model simulations to "show that on the basis of a single day of globally observed temperature and moisture, we detect the fingerprint of externally driven climate change, and conclude that Earth as a whole is warming."

According to the study:

The fingerprint of climate change is detected from any single day in the observed global record since early 2012, and since 1999 on the basis of a year of data. Detection is robust even when ignoring the long-term global warming trend. This complements traditional climate change detection, but also opens broader perspectives for the communication of regional weather events, modifying the climate change narrative: while changes in weather locally are emerging over decades, global climate change is now detected instantaneously.

As lead author Sebastian Sippel said in a statement from ETH Zurich, "Uncovering the climate change signal in daily weather conditions calls for a global perspective, not a regional one."

Astrid Caldas, a senior climate scientist at the Union of Concerned Scientists, tweeted that "this study is something we have been expecting to see for a while."

Knutti, in the ETH Zurich statement, suggested that their findings could have broad implications for climate science.

"Weather at the global level carries important information about climate," he said. "This information could, for example, be used for further studies that quantify changes in the probability of extreme weather events, such as regional cold spells. These studies are based on model calculations, and our approach could then provide a global context of the climate change fingerprint in observations made during regional cold spells of this kind. This gives rise to new opportunities for the communication of regional weather events against the backdrop of global warming."

The Washington Postreported that the new study "was in part motivated by [U.S. President Donald] Trump's tweets about how a cold day in one particular location disproves global warming." Since taking office in 2017, Trump repeatedly has turned to Twitter to use cold weather events to cast doubt on the international scientific community's consensus that human activity is driving the climate crisis.

The Post spoke with multiple climate experts who were not involved with the research, including Michael Wehner of Lawrence Berkeley National Laboratory, who described some of the new findings as "profoundly disturbing," and Stanford University climate scientist Noah Diffenbaugh, who highlighted the study's contributions to advancing a global understanding about the impacts of climate change.

"The fact that the influence of global warming can now be seen in the daily weather around the world--which in some ways is the noisiest manifestation--is another clear sign of how strong the signal of climate change has become," Diffenbaugh said. "This study provides important new evidence that climate change is influencing the conditions that people and ecosystems are experiencing every day, all around the world."

There are a few "key caveats" about the new analysis, according to the Post:

The study contains uncertainties, particularly when it comes to the accuracy of computer models in simulating various climate cycles. It also does not tease out the importance of other factors that influence the climate, such as land-use change and human-made and volcanic aerosols.

Knutti notes that the use of machine-learning techniques, which can help tease out patterns in large data sets, can introduce uncertainties, as well, although he's confident those were minimized here.

Sippel said technically the new study does not attribute the climate change trends they found completely to human activities but that there is most likely no other plausible explanation.

Even with the need for further research, reporting on the new findings Thursday has provoked fresh calls for government action to address the climate emergency. Congresswoman Veronica Escobar (D-Texas), in a tweet, tied the research to the failure of President Donald Trump and Senate Republicans to tackle the global crisis, and emphasized this year's upcoming elections:

"This new development means that every day will be marked by the impacts of #climatechange," Rep. Deb Haaland (D-N.M.) tweeted, linking to the Post's report. "We must move forward on bills to reduce our impact on climate. #ClimateActionNow."