Where Clouds and Particles Meet Climate

New research from a team of NOAA-led scientists proposes a totally new approach to understanding how tiny particles in the atmosphere and clouds interact—and that understanding those interactions is critical if you want to know how clouds in turn impact climate.

It's easy to understand why the particle-cloud question is one of the most uncertain pieces in the climate puzzle: there are many unknowns about both atmospheric particles—or aerosols—and clouds, and their interactions. On the aerosol side, there’s the wide variation in the composition of the aerosol, how the particles affect light, how effective they are at initiating the formation of water droplets or ice particles, and their distribution and movement in the global atmosphere. And clouds are the atmosphere’s ultimate shape-shifters: turbulent, ephemeral, and largely unpredictable.

A new paper led by Graham Feingold of the NOAA Earth System Research Laboratories in Boulder, Colorado, takes fresh aim at understanding how the aerosol-cloud interaction affects climate and finds that the key is to consider yet another layer of complexity: that the meteorological conditions that drive cloud formation are changing—or “co-varying”—along with the aerosol. Feingold, who is also a Fellow in the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado-Boulder, along with coauthors at NOAA ESRL, the University of Colorado Boulder, and Leeds University in the UK, describe the new conceptual approach in a paper published this week in the Proceedings of the National Academy of Sciences.

The authors got the first hint that a new approach was needed in cloud-aerosol-climate modeling when they dug into the details of how computer models treat interactions between clouds, aerosols and meteorology. When they provided inputs for aerosol and meteorology that were allowed to vary with respect to each other in one way, and then allowed those inputs to vary in a different way, they discovered that such changes could strongly affect cloud reflectivity and absorption, leading to different implications for climate. “This perplexing outcome motivated us to look further into how to model the complex system of clouds, aerosol, meteorology, and radiation,” said Feingold.

To quantify how cloud-aerosol interactions affect warming vs cooling, the authors find that studies should use inputs for meteorology and aerosols that are allowed to increase and decrease with respect to each other in a natural way, more accurately representing real-world conditions, rather than using the more structured input values typically used in modeling exercises. They call for model simulations in which the initial inputs for aerosol and meteorology are derived from observations—and hence vary simultaneously in space and time. With a stronger use of observations and many model simulations, the authors argue that this approach could advance the study of aerosol-cloud interactions and their implications for the warming or cooling effects of clouds.

The new paper gives a good start on advancing the science on this topic, but it’s far from the end of the story. “To really quantify the warming and cooling effects of clouds, we will have to apply this approach in different cloud regimes in different places on the globe,” said Feingold.