Yesterday afternoon, my six-year-old son practiced swimming with me. Delighted with the water and my attention, Corey stayed in for forty minutes. Despite the water’s chill, I knew if I took his temperature it would be close to 98.6° Fahrenheit. For an hour afterward he ran through the humid July heat, playing tag with his cousins, his hair damp with sweat. Still, a reading would have shown his body to be within a degree or two of 98.6.
This stability is a product of homeostasis. A holy word for biologists, homeostasis refers to an organism’s ability to maintain its ideal temperature and chemistry.
Each species has its own preferred level of warmth, but among most mammals the possible range is surprisingly narrow, generally between 97° and 103°F. Each body’s temperature works to optimize the function of its enzymes, which are critical to its every chemical interaction. Too cold and these catalysts slow down. Too hot and they break down entirely.
Such homeostatic regulation depends on a mechanism known as negative feedback: a response that maintains a system’s balance. Yesterday, for example, when Corey ran in the hot sun, his cheeks got rosy as more blood moved to the surface, maximizing heat loss off his skin. No, his body said to his increasing temperature.
It turns out that our bodies’ homeostasis can provide an analogy with which to understand the complexities of climate change — and the human response to it. Over geological time, the biosphere uses negative feedbacks in a way that maintains a stable global average temperature. When the Earth’s oceans heat up past a certain point, for example, hurricanes (which thrive only on warm water) increase their intensity, leaving a trail of stirred-up nutrients. The food creates a massive bloom of phytoplankton, which suck in enough of the greenhouse gas carbon dioxide to start cooling the global climate.
Conversely, when temperatures fall too low, vast quantities of methane are released into the atmosphere, possibly in part because ice sheets build up, lowering sea levels and exposing coastal methane hydrates. As a greenhouse gas twenty times more powerful than carbon dioxide, the methane warms the biosphere quickly.
Through a large array of negative feedbacks like these, the biosphere has managed to maintain a relatively stable temperature, despite massive volcanic eruptions of greenhouse gasses and orbital and solar irregularities. To Earth-based organisms, the fluctuations may have seemed severe, allowing ice sheets to roll in or crocodiles to paddle across the Arctic. But through its self-corrections, the biosphere has remained habitable.
According to New York University biology professor Tyler Volk, the sun’s temperature has increased 30 percent over the course of Earth’s history, which would have increased temperatures about one hundred degrees were it not for the cooling effect of phytoplankton and other life. And scientists calculate that the Earth would be a frigid fifty to sixty degrees colder without any greenhouse gases. Over the last million years, the biosphere has remained within about eighteen degrees of 59°F.
Humanity’s intemperate carbon emissions, however, have severely tested these negative feedbacks. Since 1900 alone, the Earth has warmed more than a full degree — one-fifth the entire temperature range over the past ten thousand years. The Earth is now within two degrees of its warmest levels in one million years.
The increased heat dries up the soil faster and pumps water vapor into the clouds, exaggerating the severity of drought, then rain. Gentle summer days turn into baking onslaughts, temperate-zone drizzles become tropical monsoons, tropical diseases and pests have whole new latitudes to conquer, and temperate-zone animals and plants are fleeing toward the poles. The increased heat has also warmed the oceans, causing hurricanes to grow in intensity.
And when a situation becomes extreme enough, biological systems can abandon their attempts at moderation. Corey crawled into bed with me in the middle of last night, saying his head hurt. His hands and face felt hot. Touching his ribs I could feel his heart racing. Ill with the flu, his body no longer fought to maintain its normal temperature. Instead it was using positive feedbacks, reactions that amplify a change, to create a fever. Holding him close in bed, I could feel his muscles violently shivering, creating heat and more heat. Within an hour of the first symptoms, his temperature was 102°.
Positive feedbacks can also shift the Earth’s climate quickly, with the kind of results seen in the latest global warming news. As ice sheets melt near the poles, for example, the water slips through the crevasses to the rock below. At some point the water pools up enough to raise the glacier just a fraction, greasing its slide into the ocean. James Hansen of the Goddard Institute, whose science has been uncannily accurate since he alerted Congress in 1988 to signs of human-induced climate change, points out that this phenomenon can cause millennia-old ice sheets to disappear with an explosive splash. At that point, white ice no longer reflects sunlight into space. Instead, dark water draws the sunlight in as heat, accelerating the rise in the Earth’s temperature. Yes, the water says, yes.
The thaw of the Siberian permafrost may provide another example of a positive feedback. Roughly 400,000 megatons of dead plant matter that has never finished decaying because it has been frozen, the permafrost is now thawing more rapidly than expected, according to recent reports. Because all this plant flesh is wet — under snow that’s now melting — the decay is engineered by anaerobic bacteria, which metabolize the plants straight into methane, that muscular greenhouse gas. Once truly underway, this release of methane would dwarf any effect humans have on climate change. There’d be no more discussion of Energy Star appliances or raising the emission standards of cars. The rising temperatures would effectively no longer be powered by us, or subject to our influences. The system would take over.
As medical researchers have discovered in the past decade, our bodies give us fevers for a specific reason: certain antibodies and other infection-fighting agents function optimally at 100°F or higher. When Corey’s body detected the presence of overly aggressive microbes, it turned its thermostat up high. His hypothalamus made his muscles shiver, minimized the blood flow to his skin, conserved heat in his gut. For a little while, his body could withstand a high temperature while the immune system brought out its heavy artillery. Overall this strategy tends to work well: Corey slept straight through the day, and, late in the afternoon, he sat up, skinnier and ferociously thirsty, but healthy again. Research shows, however, that while the feverish response may preserve the human organism as a whole, some of the immune system’s agents have side effects: their activity can kill many “innocent” nonaggressive cells.
We should take note. It may seem a poetic stretch to say the Earth itself ever sickens or has a fever. Seen from a distance, the Earth itself does not become more or less “healthy” — just more or less populated with life. But as seen from the ground, the view is rather different. If our planet’s feedback systems switch over from negative to positive and the biosphere heats up fast, the Earth will certainly seem feverishly ill to a number of species, many of which will not survive.
In terms of our planetary climate, it’s easy to guess which species is playing the role of overly aggressive microbe. But we do have a choice. Some human cultures, through their agriculture and hunting, have respected and adapted to ecological limits. We have the ability to shape our destiny — to be microbial attackers, or humble cells inside a living body.