It is a paradox of climate economics: How can we know that a few degrees of global warming will be very bad for us, when we do not know how to measure how bad it will be?
The principal benefit of reducing greenhouse gas emissions is that future climate impacts will be less severe. The most serious climate risks will emerge as temperatures climb beyond the range of historical experience, creating conditions qualitatively worse than we have ever encountered. Definitive empirical evidence about the magnitude of future damages will not be available until it is too late to avoid them.
Still, in the prevailing paradigm for climate policy, particularly in the United States, cost-benefit calculations reign supreme and precise numbers are needed. The models used to calculate the Obama administration’s “social cost of carbon” (SCC) estimates all rely on “damage functions” – quantitative estimates of losses that will be incurred as temperatures rise. My own work has criticized the FUND model, which offers the most detailed, and most conservative, damage estimates, often based on dated or limited information (see my 2012 and 2016 articles). The National Academy of Sciences has issued an extensive, damning critique of the analysis behind the SCC estimates.
The point is not to make up [sic] a different damage function. We still have no idea exactly how bad future damages will be. Nonetheless, there are at least two reasons why we know it is urgent to combat climate change. First, our society has emerged and thrived in an era of remarkably stable temperatures. Second, within that era, short-term episodes of global cooling have revealed astonishing, if unpredictable, sensitivity to moderate temperature changes.
One hundred centuries of stability
In One Hundred Years of Solitude, Gabriel Garcia Márquez imagined Macondo, a community that developed its own idiosyncratic and magical life, believing itself to be insulated from outside forces – until suddenly it was not. Our global society has enjoyed one hundred centuries of temperature stability, developing the unmagical realism of agriculture, cities, modern medicine, Facebook and Twitter, and all the rest of modernity, for good and ill. Yet the history of the previous hundred centuries (i.e., 10,000 years) demonstrates that there is no guarantee of such stable temperatures, as shown by the temperature record reconstructed from Greenland ice cores (see graph).
There were three wild swings in temperatures between 10,000 and 20,000 years ago. First came the sudden end of the last ice age, followed by both the beginning and the end of a later cold spell. Known as the “Younger Dryas,” it was named for an alpine wildflower (see picture) that flourished at lower altitudes during the millennium of cold. In all three cases, the temperature changed at by at least 5°C per century, for more than a century. A 5°C change is equivalent to turning Illinois into Arizona, Boston into Raleigh, NC, or London into Barcelona. A change of this magnitude in a matter of decades is much too fast for successful adaptation in society, housing, infrastructure, and particularly agriculture.
Nothing remotely like that has happened in the last 10,000 years, but the Younger Dryas experience shows that disastrously rapid climate change is entirely possible. The cause, in that case, may have been rapidly melting glaciers at the end of the ice age, dumping a huge volume of cold, fresh water into the North Atlantic. This could block the large-scale circulation of ocean currents and shut down the Gulf Stream, which normally warms Europe and North America. Could the same thing happen again as the Greenland ice sheet continues to melt? Or could something else suddenly destabilize temperatures?
This is the scale of risk that matters, not the nuances of measuring incremental damages in a non-crisis environment. The baffling but logical “Dismal Theorem,” proved by economist Martin Weitzman, shows that because we cannot rule out truly catastrophic scenarios with much confidence, the marginal benefit of emission reduction is literally infinite. The timing of crisis is unknown, but action is urgent: as a leading citizen of Macondo realized much too late, “races condemned to one hundred years of solitude did not have a second opportunity on earth.”
The high costs of global cooling
Even within the last 10,000 years of relatively stable temperatures (see graph), there are blips up and down. Some of the strongest evidence on the risks of global temperature changes comes from historical episodes of temporary cooling. Two recent histories of such periods may provide too much information for the average non-historian, but they thoroughly document our sensitive dependence on precise temperature ranges.
In Global Crisis: War, Climate Change and Catastrophe in the Seventeenth Century, Geoffrey Parker describes the bleak political and economic consequences of the global cooling, averaging about 2°C, in the mid-1600s. This “little ice age” was a time of widespread crop failures and extreme weather – and of civil war in England, the bloody conclusion of the Thirty Years War in continental Europe, the overthrow of the Ming Dynasty in China, and on and on around the world. In the seventeenth century, black magic was a popular explanation of mysterious losses; the Salem witch trials followed some years of unusual cold and scarcity. Less magical explanations include the near-absence of sunspots and the high level of volcanic activity at mid-century, both of which reduce the amount of solar radiation reaching the earth.
In The Year Without Summer, William and Nicholas Klingaman chronicle the effects of the immense 1815 volcanic eruption of Mount Tambora, in what is now Indonesia. As epic quantities of dust and debris circulated in the atmosphere, dimming the sunlight, 1816 became the second-coldest year on record in the Northern Hemisphere since at least 1400 (the one colder year, 1601, also followed a massive volcanic eruption). Cold rain, snow, and occasional frosts continued throughout what should have been the growing season in 1816, leading to widespread crop failures, famine, food riots and increased emigration in Europe, North America, parts of China, and elsewhere. It took several years for the dust to finally settle, allowing the climate to return to normal.
In short, our society, and particularly the agriculture that feeds us, is dependent on a narrow range of what we consider normal temperatures, precipitation, and other climate conditions. Human beings can survive wide swings in temperature; the stuff we eat is much fussier about the climate it needs. The rippling social, economic, and political effects of sudden changes in climate are hard to predict in detail, but disastrous in general, even from what may seem like small or temporary changes. A 2°C change in temperature, now taken as the threshold for dangerous climate change, has been enough in the past to cause the collapse of harvests, the overthrow of kings, and wars, poverty and misery on many fronts.
None of that fits in the damage function of an economic model. It is simply immeasurably bad.
You can email Dr. Ackerman at firstname.lastname@example.org. The graph and parts of the text in this article are taken from his recent book, Worst-Case Economics: Extreme Events in Climate and Finance.