Volcanic cooling, El Niño, and regional climate feedback

A volcano erupts, and the next summer is unusually cold. Did the volcano cause that?

The opening gives us a plausible reason it might have. Dust and sulfuric acid droplets can remain in the atmosphere long after the lava cools. That veil could block sunlight and cool the climate. The eruption of Tambora in 1815 is often blamed for the following year's snowstorms in June and frosts in August in parts of the northeastern U.S. and southeastern Canada.

But one event following another isn't enough to establish the cause. Maybe it would have been an unusually cold year anyway. Maybe another weather phenomenon contributed. We should want more than the sequence of an eruption and a cold summer, even if the proposed sunlight-blocking mechanism sounds sensible.

Paragraph 2 introduces scientists Clifford Mass and David Portman, who think the connection is weaker than previously believed. Notice the degree of that claim. They aren't saying volcanoes cannot affect the climate. They are questioning how much of an effect we should attribute to them.

Their study compares temperature data before and after nine eruptions. To identify the volcanic contribution, they try to remove the effects of El Niño, which warms the equatorial Pacific's surface and, through it, the atmosphere.

Why does that matter? Because temperature can change for more than one reason. An eruption might cool things while El Niño warms them. Looking only at the resulting temperature could make the volcanic cooling look smaller than it really is: the warming partly hides it.

The timing can also mislead us in the other direction. Suppose an eruption happens just as an El Niño warm period is fading. Temperatures would fall as that warmth subsides. If we credited the whole drop to the eruption, we would exaggerate the volcano's effect. El Niño can both mask real volcanic cooling and create cooling that gets mistaken for a volcanic effect.

That makes filtering it out useful. It doesn't establish that El Niño is the only other influence on climate, or prove what happened in every particular year. It helps the researchers separate two influences that would otherwise get mixed together.

What do they find? Minor eruptions have “no discernible effect on temperature.” Major eruptions do cool things, but less than expected: half a degree centigrade or less in the average temperature of the eruption's hemisphere, with an even smaller drop in the opposite hemisphere. Small isn't the same as nonexistent, and an average over a hemisphere isn't a temperature reading for one particular region.

Now the final paragraph asks us to consider what a small initial change could set off.

Imagine early spring is just cold enough that snow takes longer to melt. The remaining snow keeps reflecting sunlight away from the surface, which adds to the cooling. That additional cooling helps keep the snow around. The original change starts a process that strengthens it.

The regional cold could also affect the jet stream, which follows the boundary between cold northern air and warmer southern air. If it moves farther south, more polar air could enter behind it and deepen the cold spell. We don't need to become weather experts to follow the structure: a small cooling starts changes that produce more cooling.

So the modest broad effect doesn't settle how severe the effect could become in one region. Feedback could amplify it into something like a year without a summer.

Could is important here. The writer presents a way that a small volcanic effect might produce a much larger regional effect, not proof that this exact sequence caused the weather in 1816. Our understanding should include both parts: the research reduces the estimated broad cooling, while the final response explains how substantial regional cooling could still occur.