The Sun, still high up in the sky over Long Island on June 7, 2023, taken on my phone without any sort of filters or adjustments.
Over the past month, the northeastern United States has experienced a series of very large haze events due to forest fires in eastern Canada. Since haze is an example of an aerosol, and aerosols have been my primary research focus, I figured that now would be a good time to talk about how aerosols affect the climate picture in general, and how climate change influences these haze events.
The simple definition of an aerosol is that it's anything suspended in the air that is not a water droplet or ice crystal. Aerosols are something of an X-factor in the climate puzzle because their effects on the Earth’s energy balance are difficult to quantify. There are two types of aerosol effects. The direct effect involves the absorption and scattering of incoming solar radiation by the aerosol itself. The indirect effect involves the way the presence of aerosols influences cloud formation and the size of the cloud droplets. The ability of aerosols to absorb and scatter radiation depends on the amount of the aerosol present, the size and shape of the aerosols, and the chemical make-up of the aerosols. For example, sulfate aerosols absorb very little sunlight and produce a strong cooling effect, while black carbon aerosols like the haze in forest fires absorb enough sunlight to produce at least as much of a warming affect as a cooling effect.
Figure 1: Aerosol optical depth data from an AERONET site at Brookhaven National Laboratory for May 2023.
Figure 2: Aerosol optical depth data from an AERONET site at Brookhaven National Laboratory for June 2023.
Scientists monitor the amount and movement of aerosols from satellites in space, and also from a network of stationary devices on the ground. The most established ground-based aerosol monitoring network is called the Aerosol Robotic network, or AERONET. The primary quantity that AERONET measures is called the aerosol optical depth, which is a measure of what fraction of sunlight at a given wavelength is prevented from reaching the ground by the aerosols. There are two AERONET sites near me; one is called LISCO and is located on a small island in the Long Island Sound, and the other is located at the Brookhaven National Laboratory. Figure 1 shows the optical depth measured at the Brookhaven site across the month of May, which was a pretty normal month from an aerosol perspective, and Figure 2 shows the optical depth for the month of June. Typically the aerosol optical depth in this part of Long Island doesn't exceed more than 0.3, though occasionally you get a pollution episode where the optical depth can exceed 1.0. The major issue in these events is not the quantity of pollution, but rather the existence of a temperature inversion in the air that prevents pollution from dispersing and instead causes it to accumulate. That’s not what happened in June. As you can see, on June 6 and 7, the optical depth at the smallest wavelengths exceeded 6.0. (For small particles, the optical depth at low or violet wavelengths is much larger than that at high or red wavelengths.) To put this in perspective, the major Saharan dust storms that produce aerosol clouds which can be easily tracked as they cross the Atlantic Ocean typically have an optical depth of around 2.0.
Why are these fires happening? A number of factors influence wildfires, from natural factors like the dryness of the wood to more artificial factors like human carelessness at campfires. When I’ve looked at articles and posts online concerning the haze and read the comments, forest management has often been presented as the reason that forest fires are raging out of control. (See here, here, and here for examples.) Sometimes forest management is used as a counter-argument to the idea that a warming climate influenced the fires, as though the two ideas are mutually exclusive. There is indeed an element of truth to the notion that forest management can be improved on, but not necessarily in the ways that the people writing and reading these articles may think. Historically, most forest management in the United States and elsewhere has emphasized suppressing all fires, regardless of whether people are being affected by them. The problem with that is that when you don't allow fires to burn in areas that don't directly affect people, the fuel remains for the next fire. You can argue that all it takes to keep fires in line is to clear out the undergrowth and cut enough trees to maintain sufficient space. But even if the labor required to do this over vast stretches of wilderness were not a prohibitive logistical obstacle, the fact remains that the undergrowth is an essential part of the forest ecology — as is the occasional fire. The best strategy to prevent the really big fires is to apply controlled burns in areas that have gone an unnaturally long time between fires. In a controlled burn, fires are set intentionally when the wind is low and the fuel is relatively moist so that the burn can be confined to a specific area. This reduces the amount of fuel to a manageable level in a way that enables the sprouting of certain seeds in the forest and enhances the health of the ecosystem as a whole.
In order to analyze the role that global warming plays in these fires and the associated haze events, you need to connect the dots. Warmer temperatures mean more evaporation, and ultimately more precipitation as well. Most additional evaporation will happen where the most evaporation is already happening, and most additional precipitation will take place where most of the current precipitation is already happening. In other words, wet areas will get wetter and dry areas will get dryer. This logic also applies to areas that experience wet seasons and dry seasons, or periodic wet spells and dry spells. Eastern Canada has had an unusually large dry spell this spring and summer, parching the wood and the undergrowth in the forests. And as I talked about in a previous post about wildfires in Australia in 2020, how a fire spreads does not depend on how it was started. It depends on the quantity and dryness of the fuel.
So the warming climate doesn't create fires where they wouldn't already happen, but it can and does make these fires worse. And you can live pretty far away from the fires and still see the end result.
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