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December 9, 2025—Bacterial defense systems, ancient Roman cement secrets, and 2025 was HOT. —Andrea Gawrylewski, Chief Newsletter Editor | | The so-called aqueduct de les Ferreres, a Roman arcade that is part of the aqueduct that supplied water to the city of Tarraco (Tarragona). Sergi Reboredo/VW Pics/Universal Images Group via Getty Images | | This year will likely be the second or third hottest on record, coming in slightly cooler than 2024 (the most scorching year in modern history). Each of the world's 10 hottest years have occurred within the last decade. The European Union's Copernicus Climate Change Service (C3S) reported data up to November 2025 that show this year has been 1.48 degrees C above the average from 1850 to 1900.
Why this matters: Ten years ago, under the Paris Climate Agreement, many countries pledged to take action to keep warming from exceeding 1.5 degrees C. Why 1.5 degrees? Most scientists agree that more than 1.5 degrees C of warming will trigger intense and irreversible changes on Earth. The last few years have skirted close to that limit, but it takes several years of data for experts to declare that the planet has officially crossed the 1.5 degree limit.
What this means: The fact that 2025 was so hot worries experts, especially because it was a La Niña year. The opposite of El Niño, La Niña is supposed to cool Pacific Ocean waters and lead to slightly cooler global temperatures. But this year's La Niña was weak and short-lived. The cycle may be weakening due to global warming. | | Amanda Montañez; Source: Copernicus Climate Change Service (data) | | | | |
Bacteria have a sophisticated self-defense system against viruses. To deepen their understanding of the microbes' immunity, researchers infected Streptococcus pyogenes bacteria with phages, viruses that attack these bacteria. Phages either immediately ruptured bacterial cells, or they hid inside the bacterial DNA and went dormant. The scientists collected surviving bacteria cells' DNA and checked their genetic code. Each of these bacteria had more small pieces of the dormant virus's DNA in its own genome, which can help the bacteria recognize the virus next time it appears.
How it works: Bacteria use a CRISPR gene editing system to alter their own genetic code. After being exposed to a virus, a bacterium can use a special enzyme to create openings where it can insert small pieces of the virus's DNA, called spacers, into its own genome, which helps it recognize and fight off the virus next time around.
What the experts say: This new understanding could help researchers design phages that can pierce the defenses of more types of infection-causing bacteria, says North Carolina State University molecular biologist Rodolphe Barrangou, who was not involved in the study. Various bacteria can use any of the more than 150 antiphage defense mechanisms, so new treatments must be tailored to these defenses. —Andrea Tamayo, Newsletter Writer | | - Can you unscramble this jigsaw puzzle of our November 1867 issue cover? The black-and-white image depicts a mosque constructed in Paris to reflect Turkish designs as part of the Exposition Universelle of 1867, a world's fair also known as the 1867 Paris Exposition.
| | Our first news story today reminded me of an amusing social media trend from two years ago. Women asked men in their life: "How often do you think about that Roman empire?" Surprisingly, many men answered that they thought about Rome often, multiple times a month and some even more frequently. Given how much of our world we built atop Roman ideas (sometimes literally, with concrete), maybe those survey answers weren't so strange after all—though still delightfully unscientific. | | How often do you think of the empire? And send other thoughts or feedback to us at: newsletters@sciam.com. —Andrea Gawrylewski, Chief Newsletter Editor
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