Part One
TWO
Were it not for the human hand, the climate would likely still be in the sweet spot (at least for humans) that it has been in for most of the past 11,500 years. It’s not a coincidence that almost all of human civilization developed during this hiatus from the ice ages, nor is it coincidence that our numbers exploded during this period, rising from about 5 million to more than 7.8 billion as of this writing.
This sweet spot is a geophysical reality. Innumerable factors create a given climate, but dominant among them is where earth is positioned in space relative to the sun and how it is tilted relative to our star. Earth’s orbit around the sun changes in a 100,000-year pulse. We’re presently in the rounder part of this pulse, which means that earth is now at the beginning of a 100,000-year ice age cycle (note that any return to ice age conditions could be thousands of years in the future). The home planet’s spin axis is now tilted about 23.5 degrees. This should accentuate the difference of the seasons, but countering that effect is where we are in the precession of that spin axis. Precession is easiest to visualize if you imagine a rod driven through earth’s poles with a pen on each end. Precession would be the circle that pen draws as the spin axis shifts back and forth on a regular basis. Right now (and for the next many thousands of years, because precession also has a 100,000-year cycle), the Northern Hemisphere is tilted away from the sun when the earth is closest to the sun, and toward the sun when it is farthest. The net effect is to reduce summer-winter differences in the Northern Hemisphere, where most of the world’s food is grown. That’s a good thing.
Humanity has been the beneficiary of other orbital dynamics. There’s the 100,000-year oscillation of earth’s orbit from rounder to flatter, the 41,000-year cycle that characterizes changes in the tilt of earth’s axis, and the circle described every 26,000 years by the precession of that axis. Geophysicist Richard Alley notes that we would have to go back 115,000 years to find an equally warm, human-friendly set of orbital dynamics as we have enjoyed over the past ten thousand years.
There are also climate cycles related to natural events here on earth, cycles with periods ranging from millions of years to the quarterly changes of the seasons. Roughly 2.7 million years ago, for instance, the Panama land bridge rose, separating the Atlantic and Pacific oceans and diverting equatorial currents. Along with some other events, this set in motion changes in how heat was distributed around the world, and, voilà, the ice ages began.fn1
Other, shorter cycles also derived from events here on the planet. During glacial periods, so-called Heinrich events drove down temperatures every 10,000 years or so (Heinrich events involve mass discharges of icebergs into the North Atlantic, whose melting then shuts down the currents that deliver significant amounts of heat to the northern latitudes). Another regular cycle of abrupt warming and then cooling recurred every 6,100 years (also described by Heinrich, this cycle involves a lagging response of the ice sheets to changes in solar radiation). There are many other cycles related to sunspots or to the interactions between the oceans and the atmosphere, including the now familiar El Niño/La Niña cycle that has a period of just a few years.
There have been blips over the past 11,500 years, and those blips give a foretaste of what happens when climate goes haywire. The most recent blip was the Little Ice Age. It began around AD 1300 and was at its most intense between 1645 and 1715. The Little Ice Age ended in the mid-nineteenth century. During the Medieval Warm Period that preceded the Little Ice Age, populations exploded, with England’s population tripling in the 1200s. Life spans lengthened as well.
Then around 1300, climate began to whipsaw, with deep freezes alternating with hot years, droughts with floods, and Europe was battered by epic storms. Waterlogged fields became incubators for molds, pests, and disease. One of these, St. Anthony’s fire, emerged from a blight that blackened kernels of rye. As described by pioneering climate historian H. H. Lamb, whole villages would succumb to convulsions, hallucinations, and gangrene. Bad as it was, St. Anthony’s fire was just a warm-up act for the Black Death, which blitzed through the compromised immune systems of people weakened by famines and prior disease. The Little Ice Age stalled population growth in Europe for four hundred years, and by its deepest part in 1715, it had shortened average life spans and even shortened the people.
After that cold spell, population growth resumed its upward march. From about 1.25 billion people in 1860, population has increased more than sixfold. The spread of sanitation, the discovery of antibiotics, and huge advances in plant breeding helped foster that rise. A largely unacknowledged factor, however, was that during most of that period, at least until the 1980s, the weather offered a clement context for human advance. The lesson of history is that climate is consequential.
Apart from these regular cycles, unpredictable external shocks such as asteroid strikes and volcanic eruptions have had outsized impacts on the climate in the past. In April 1815, for instance, Mount Tambora in Indonesia exploded in one of the most violent eruptions in recorded history. As its ash and gases circled the globe, it cooled the climate in what came to be regarded as the “year without a summer.” No external event in the past 150 years has had any lasting impact on climate. The eruption of Mount Pinatubo in 1991 gave the United States a cool summer in 1992, but its effects faded within a year.
After about 150 years of stability, nested in 11,500 years of stability, climate is now changing. Over the past three decades, the changes have been coming faster, and the amplitude of the changes is more extreme. What’s changing climate today could be called an internal shock or, to be more apt, a self-inflicted wound. Until the Industrial Revolution, changes in the carbon balance of the atmosphere were reactions to changes in climate. For instance, as an ice age ended, vegetation would flourish, and decay would put more CO2 in the atmosphere, which, in turn, would further enhance the warming. With our billions of tons of emissions, humanity has turned this pattern on its head, with greenhouse gases driving climate change rather than the reverse.
We began our planetary-scale science experiment with our own climate in the nineteenth century, when the coal-fired “dark satanic mills” of the Industrial Revolution began a steep ramp-up of the amount of greenhouse gases we poured into the atmosphere each year.
What scientists knew about climate change and when they knew it will be discussed in a separate section. What became evident in the 1980s was that, after about a century of increasing fossil fuel use and increasing human numbers, climate began warming noticeably. Other changes began to become manifest. A “five-hundred-year” flood hit the U.S. Midwest. In 1994, northern India suffered a heat wave of the century with ninety consecutive days of temperatures above 100 degrees Fahrenheit. Subsequently, India has been hit by many worse heat waves, including one in 2019 that led to speculation that parts of the country were becoming too hot for human habitation.
Then Antarctica started changing, first on its outermost fringes. The Larsen Ice Shelf, the continent’s northernmost and thereby most sensitive to warming, began shedding huge portions of its ice. In 1995, the Larsen A Ice Shelf disintegrated, shrinking by 2,000 square kilometers (followed by the Larsen B in 2002 and the Wilkins in 2008). In the United States, floods of the century were becoming an annual occurrence, another trend that has accelerated in the new millennium.
In April and May 2011, the Mississippi River received so much water from snowmelt and a series of four storms in the Midwest that the federal government was forced to open the Morganza Spillway for the first time in thirty-seven years, deliberately flooding parts of Louisiana in order to save Baton Rouge and New Orleans. At the same time, farther west, Texas was suffering one of the most intense droughts in its history. This led to a bizarre situation in which some landowners were suffering drought on the western part of their property and floods toward the east.
The new millennium added wildfires to the growing mix of ugly surprises and extreme events. As the climate warmed, the normal winter rain bands shifted north in the American West, leading to a succession of droughts, turning large parts of the West into kindling. The warmer weather led to an explosion of bark beetles and other pests that caused mass die-offs in forests, adding more fuel waiting for a spark.
The sparks were not long in coming. Devastating wildfires swept through western states in 2012, 2015, 2016, 2017, 2018, 2019, 2020, and 2021. Amid severe drought and the hottest year in Australia’s history, the worst wildfires on the planet hit the continent’s southeast in 2019, burning more than seven times the area of the California fires of 2018 and killing an estimated half-billion animals. On the other side of the globe, another enormous fire raged out of control for weeks in Siberia. Then, in 2020, the 4.4 million acres burned in California amounted to double the record set just two years earlier.
While many different factors govern earth’s climate, the one that matters to us right now is our own behavior as a species. While humans are powerless to change the big cycles that affect climate, these man-made impacts remain entirely within our control. How long that remains the case is currently unknown.
Reality is the clock against which all the other clocks will be measured, the Greenwich Mean Time of climate change. What has happened in reality, however, is only meaningful to the public and financial community after the changes have been interpreted by the scientific community. Are the changes naturally occurring or a response to human actions? Is climate likely to continue to change, and if so, how fast? What are the likely impacts? These and myriad other questions flow from the events.
Because the scientific community is the intermediary between changing climate and the broader public, what various scientific disciplines understood about climate, when they came to that understanding, and their confidence in their assertions were critical to the response of the public and financial communities to the changes we began seeing in the 1980s.
As the changes began to become evident, people wanted answers as to what they meant. Scientists, however, found themselves in the impossible situation of having to invent means to document and interpret the changes in climate even as the changes were accelerating. Because researchers had to design experiments and studies, collect data, analyze data, and then publish it, the state of scientific knowledge always lagged what was actually happening by a couple of years. To put this another way, to some degree climate science is condemned to live in the past, even as the present is changing rapidly. That is the nature of science. It is to the story of the science that we now turn.