At the beginning of "To Build a Fire," London's description of the sky foreshadows the grim, depressing conclusion of the story:
Day had broken cold and gray, exceedingly cold and gray... There was no sun nor hint of sun, though there was not a cloud in the sky. It was a clear day, and yet there seemed an intangible pall over the face of things, a subtle gloom that made the day dark, and that was due to the absence of sun.
The lack of sun signals immediately to the reader that it is winter, a time of year when the sun rarely shines in the Yukon. Even before London explicitly describes the temperature, the reader can infer that it is incredibly cold and probably a bad time to be traveling. In addition to this literal meaning, the lack of sun can also be interpreted as an omen presaging the tragic events that follow later in the story—after all, the lack of sunshine creates a dark, moody, and rather ominous setting. Right away, then, readers are plunged into a joyless and unsettling environment, and this ultimately prepares them for the tension and disaster that will eventually befall the man, as well as the story's grim ending.
At the beginning of "To Build a Fire," the omniscient narrator comments on the man's lack of interest in the landscape, thus foreshadowing his fate later in the story:
The trouble with him was that he was without imagination. He was quick and alert in the things of life, but only in the things, and not in the significances. Fifty degrees below zero meant eighty-odd degrees of frost. Such fact impressed him as being cold and uncomfortable, and that was all. It did not lead him to meditate upon his frailty as a creature of temperature, and upon man’s frailty in general.
This passage foreshadows a moment later in the story, when the man fails to recognize the danger of building his fire beneath a tree. This failure to consider the possible outcomes of his actions is a direct result of the man's lack of imagination—he considers the facts of a situation, but not necessarily the somewhat abstract (but, in another sense, very real) implications of those facts. For this reason, he doesn't fully grasp "his frailty as a creature of temperature," which is to say that the temperature only strikes him as impressively cold but doesn't encourage him to consider the way this might adversely affect him as a "creature" who can't survive for very long in such frigid conditions. The fact that he doesn't think about these things foreshadows his ultimate fate of freezing to death.
In the passage below, the story draws attention to the possible dangers of pools of water hidden beneath the ice. This foreshadows the crucial moment later in the story when the man falls into one of these pools.
He had felt the give under his feet and heard the crackle of a snow-hidden ice-skin. And to get his feet wet in such a temperature meant trouble and danger. At the very least it meant delay, for he would be forced to stop and build a fire, and under its protection to bare his feet while he dried his socks and moccasins.
This exact series of events occurs later in the story, when the man falls through the ice, wets his feet, and is forced to stop and build a fire. The specificity of this passage ultimately illustrates just how real the threat of breaking through the ice is—and, for that matter, how likely it is that such a thing will happen to the man as he foolishly travels through the Yukon on his own. And yet, although he seems aware of this danger at the beginning of his journey, he still fails to account for other possible outcomes—like not being able to successfully build a fire—due to his lack of imagination.
At several points throughout the story, London foreshadows the man's eventual fate by commenting on the dog's apprehension at traveling in such cold weather.
The dog was disappointed and yearned back toward the fire. This man did not know cold. Possibly all the generations of his ancestry had been ignorant of cold, of real cold, of cold one hundred and seven degrees below freezing-point. But the dog knew; all its ancestry knew, and it had inherited the knowledge. And it knew that it was not good to walk abroad in such fearful cold.
The dog's instinctual reluctance to leave the fire is validated later in the story when the man fails to build a second fire and freezes to death. If the man had followed similar instincts, he would not have chosen to travel in such cold weather or left the safety of the fire, and he would have survived. This passage signals to the reader that the dog's animal instincts will ultimately prove superior to the man's reason.
A moment later in this passage also foreshadows the very end of the story, when the dog leaves the man's frozen corpse behind and heads toward the campsite.
On the other hand, there was no keen intimacy between the dog and the man. The one was the toil-slave of the other, and the only caresses it had ever received were the caresses of the whip-lash and of harsh and menacing throat-sounds that threatened the whip-lash. So the dog made no effort to communicate its apprehension to the man. It was not concerned in the welfare of the man; it was for its own sake that it yearned back toward the fire.
At the end of the story, once the man has died and no longer has any ability to command the dog, the dog follows its instincts and acts in its own interests:
Later, the dog whined loudly. And still later it crept close to the man and caught the scent of death. This made the animal bristle and back away. A little longer it delayed, howling under the stars that leaped and danced and shone brightly in the cold sky. Then it turned and trotted up the trail in the direction of the camp it knew, where were the other food providers and fire-providers.
While The Call of the Wild, another of London's famous works, emphasizes the special bond between humans and dogs, "To Build a Fire" illustrates that animals are in fact indifferent to their human masters.
At multiple points in the story, the man remembers the advice the old man on Sulphur Creek gave him about traveling in the Yukon. For the most part, the man seems appreciative of this advice, but he grows dismissive of it when he believes that he has successfully built a fire and avoided danger:
He remembered the advice of the old-timer on Sulphur Creek, and smiled. The old-timer had been very serious in laying down the law that no man must travel alone in the Klondike after fifty below. Well, here he was; he had had the accident; he was alone; and he had saved himself. Those old-timers were rather womanish, some of them, he thought. All a man had to do was to keep his head, and he was all right. Any man who was a man could travel alone.
The fact that the old man at Sulphur Creek is mentioned multiple times throughout the story signals to the reader that his advice will eventually become important, and the man's certainty and arrogance in this moment foreshadows the end of the story, when he regrets his earlier hubris and admits that the old man was right all along:
The old-timer on Sulphur Creek was right, he thought in the moment of controlled despair that ensued: after fifty below, a man should travel with a partner.
While a work of Romantic literature might have celebrated the man's self-reliance, London critiques the notion that a person can depend only on themself.
The line "All a man had to do was to keep his head, and he was all right" also directly foreshadows the end of the story, when the man's growing panic causes him to act reckless and irrational.
