The Year Without Summer Read Online Free Page B

Association that “the
     cause of this universal fog is not yet ascertained,” he suggested that it may have
     been “the vast quantity of smoke, long continuing, to issue during the summer [from
     Laki] … which smoke might be spread by various winds, over the northern part of the
     world.” And the frigid temperatures, he proposed, probably resulted from this fog
     blocking the rays of the sun, thereby reducing the amount of solar energy that reached
     Earth.
    Throughout the winter of 1815–16, the spreading aerosol cloud from Mount Tambora had
     been doing precisely that: cooling global temperatures by reflecting and scattering
     sunlight. Although the cloud reflected only one half to one percent of the incoming
     energy, it reduced the Northern Hemisphere average temperature in 1816 by about three
     degrees Fahrenheit. This seemingly small cooling had a considerable impact on global
     weather patterns, with devastating consequences for agriculture on both sides of the
     Atlantic. Ironically, however, the effects of Tambora’s aerosol cloud could have been
     far worse if the eruption had been slightly weaker. While immense in size and scope,
     Tambora’s aerosol cloud was not particularly efficient at reflecting sunlight. Stronger
     volcanic eruptions tend to eject more sulfur dioxide into the stratosphere than weaker
     eruptions, which leads to more sulfuric acid droplets within the same volume of atmospheric
     gases. A greater number of droplets increases the chance that droplets will meet and
     collide, forming larger droplets that will be removed more quickly from the stratosphere
     by gravity. A single, larger droplet also has less total surface area than two smaller
     droplets, and so is less effective at scattering sunlight. There is therefore a balance
     to be struck between eruptions that are too weak to penetrate into the stratosphere—and
     so produce small, short-lived cooling—and eruptions that produce large, less effective
     sulfuric acid droplets. By measuring the remnants of Tambora’s aerosol cloud in ice
     cores and lake sediments, modern scientists have determined that the climatic consequences—while
     undoubtedly devastating—could have been far worse if the particles had been roughly
     half their size.
    Unlike the sudden drop in temperatures in the Indonesian archipelago that occurred
     immediately after the eruption of Mount Tambora, the planet-wide cooling was a gradual
     process that took up to a year to be fully realized. While air temperatures can, and
     frequently do, change rapidly in response to variations in solar energy, soil and
     ocean temperatures adjust much more slowly. The land and sea possess considerable
     capacity to store heat, while the atmosphere has practically no storage. When the
     atmosphere is cooler than the land and sea, heat will flow from these reservoirs back
     into the air; but since the air cannot store heat for long, much of this is soon lost
     to space. If, on the other hand, the atmosphere is warmer, some of that excess heat
     will be stored in soil and water until a balance is reached. This process may be seen
     clearly in summer: The warmest weather often occurs not in June, when the sun is strongest,
     but in August, when the ocean and land have warmed.
    As Tambora’s stratospheric aerosol cloud began to cool temperatures by subtly reducing
     the amount of solar energy reaching the earth, the land and oceans would have resisted
     this cooling by transferring stored heat into the atmosphere, and cooling themselves
     as a result. By early 1816, the land, ocean, and atmosphere were shifting toward a
     new balance of energies, largely as a result of the solar-dimming effect of the aerosol
     cloud. The adjustment cooled first air, then land, and finally ocean temperatures
     across the globe. Using information from tree rings—the width of each ring is related
     to the growing conditions (mostly temperature and precipitation) that