Professor Guy Dagan's computer simulations revealed something that rewrites the textbook on aerosols: these tiny particles warm the planet for the first two days after they increase in the atmosphere, then cool it down. The finding, published in Nature Communications by researchers at Hebrew University of Jerusalem's Fredy and Nadine Herrmann Institute of Earth Sciences, upends decades of climate assumptions and suggests that the atmosphere has a kind of memory we've barely begun to understand.
Aerosols are everywhere—in wildfire smoke, sea spray, dust, and air pollution—and they profoundly shape how clouds form and how much heat Earth retains. Yet for climate scientists, these particles have remained one of the trickiest variables to pin down. Dagan's work explains why: the same aerosols can produce opposite effects depending on how fast they accumulate and how much time the atmosphere has to respond.
Here's what happens in the short term. When aerosol levels spike suddenly, the atmosphere experiences net warming within those crucial first 48 hours. The particles trigger changes in cloud processes that create more high-altitude clouds—the kind that act like a blanket, trapping heat that would otherwise escape into space. It's counterintuitive: the pollution seems to be making the planet warmer, not cooler.
But the story doesn't end there. Over time, the upper atmosphere warms, and cloud development shifts. As conditions adjust, more heat can escape back into space, and the initial warming gives way to a longer-term cooling effect. What determines which effect dominates is not just how many aerosols are present, but the pace of change. Rapid fluctuations favor warming; gradual shifts favor cooling.
The research uncovered something even more intriguing: atmospheric memory. The climate impact of aerosols depends not only on current levels but on whether concentrations are rising or falling. The same amount of particles produces different effects under different circumstances—a nuance that has been largely invisible to scientists relying on single snapshots of the atmosphere.
"Much of what we know about aerosol-cloud interactions comes from observing the atmosphere at a single moment in time," Dagan explained. "Our results show that the atmosphere has a memory. The climatic impact of aerosols depends not only on how many particles are present, but also on how rapidly conditions are changing and how much time the atmosphere has had to respond."
This discovery matters beyond the realm of academic curiosity. Aerosols represent one of the largest sources of uncertainty in climate projections—the models scientists use to predict our future. When you can't accurately account for how aerosols affect warming or cooling, your forecasts become murkier. By recognizing these time-dependent processes, researchers can build better models and produce more reliable predictions of how climate will change.
The implication is clear: climate science needs to think in timescales, not snapshots. Incorporating how atmospheric conditions evolve over days and weeks—not just how they look at any given moment—could measurably improve our ability to understand and predict climate change. For a field wrestling with some of its deepest uncertainties, that's a significant step forward.