Well may you ask. Or even “What are they?” And we answer that h/t JoNova we learn of a pair of new papers in Nature (here and here) that point to trees as massive sources of global isoprene emissions. Boo trees! Well, not exactly. In fact all those vapours mean greenhouse gasses might have far less climate impacts than models are programmed to show. Isoprene is a very common organic vapour which has long been known to be emitted by forests, especially tropical rainforests. But since the sources were near the ground and isoprene molecules are chemically unstable they break up quickly and were assumed to have no general effect. However the new studies, one based on aircraft measurements in the tropics and the other on precise experiments at the CERN particle accelerator, suggest that isoprene molecules are carried by nighttime air currents over vast distances where they play key roles in cloud formation. So declining forest cover potentially plays a bigger role in climate change than thought, one that, naturally, climate models ignore. These discoveries may force a re-think of conventional assumptions about climate sensitivity to greenhouse gasses. Or a determined effort to avoid such a re-think lest data undermine dogma.

The story concerns aerosols, tiny particles in the air that form nuclei around which water vapour condenses, forming the droplets that make up visible clouds. Where do cloud condensation nuclei come from? That’s the question. The answer is aerosols, and they matter for understanding the climate because clouds have an overall cooling effect. If over time there is a long slow trend towards more or less cloud cover that will in turn cause a long term trend towards, respectively, cooler or warmer conditions.

When climate scientists look at historical data to attempt to determine how much warming greenhouse gasses cause, they first need to figure out how much cooling aerosols caused, remove that from the historical record, and then assign the blame for what remains to greenhouse gasses. Since there hasn’t been all that much warming over the past 150 years, if aerosols caused very little cooling then GHGs didn’t cause much warming. But if aerosols caused a lot of cooling, then GHGs must have caused a lot of warming to make up for it. Naturally the IPCC favours theories that aerosols caused a lot of cooling, which means greenhouse gasses have a powerful warming effect and future warming will be rapid.

To get a story that aerosols caused a lot of cooling over the last hundred years you need to assume there weren’t many natural aerosol sources in preindustrial times, but once industries got going in the middle of the last century they emitted copious amounts of smoke around the world and that made the climate cloudier and cooler. But what if there were a lot of natural aerosols in the preindustrial period?

If so, there would necessarily be little or no change in aerosol cooling over the 20th century, meaning less warming for greenhouse gasses to explain. We already reported on a Harvard study in 2021 that found evidence in ice cores that preindustrial wildfire activity yielded a lot more aerosols than were previously supposed. The new isoprene study does the same thing.

The authors found evidence that, as Joanne Nova explains, “there is still quite a lot of isoprene left in a rainforest at night, and tropical storms suck it up ‘like a vacuum cleaner’ and pump it up and spray it out some 8 to 15 kilometers above the trees. Then powerful winds can take these molecules thousands of kilometers away.” As the sun comes up, high up in the atmosphere the isoprene reacts with minute amounts of nitrous oxides (from lightning), hydroxyl radicals, sulphur dioxide and other trace gasses to form enormous quantities of cloud-condensation nuclei.

The effects were first measured by aircraft survey then reproduced in the lab at the CERN particle accelerator. And as Nova points out, this phenomenon constitutes a natural warming driver climate scientists haven’t known about before:

“[If] forests of broadleaf trees turn out to be seriously helpful at seeding clouds, presumably that means the last few centuries of deforestation might have reduced cloud cover on Earth, which would have allowed much more sunlight in to heat the planet. If that’s true, it’s just one more climate forcing the modelers didn’t know about. It’s one more thing that warmed the planet which we blamed on carbon dioxide, but were wrong about. And it’s yet another feedback. More CO2 makes more forest grow, which may seed more clouds.”

Since it’s one of those cases where the alarmist press can’t spin the outcome as “worse than we expected” you shouldn’t expect to hear about these studies on the TV news, in the usual newspapers or in Scientific American. In fact they’ll be the last to find out, right after the IPCC, who are probably already working out ways to keep these findings out of their summaries, if not the reports themselves lest they should seem obtuse or disingenuous.