Two centuries ago — only 10 years after a hungry, angry populace had ushered in the French Revolution — the dour Englishman predicted that exponential population growth would condemn humanity to the edge of subsistence.
“The power of population is so superior to the power in the earth to produce subsistence for man, that premature death must in some shape or other visit the human race,” he wrote with alarm.
This was, we now know, wrong. The gloomy forecast was soon buried under an avalanche of progress that spread from England around the world. Between 1820 and the year 2000 the world’s population grew sixfold. Economic output multiplied by more than 50.
Nonetheless, Malthus’s prediction was based on an eminently sensible premise: that the earth’s carrying capacity has a limit. On Monday, the United Nations Intergovernmental Panel on Climate Change provided a sharp-edged warning about how fast we are approaching this constraint.
“In many cases, we are not prepared for the climate-related risks that we already face,” Vicente Barros, co-chairman of the panel and professor emeritus of climatology at the University of Buenos Aires, said.
The list of present damages outlined by the United Nations panel — melting ice caps and rising sea levels, stressed water supplies, heat waves and heavy rains — underscored the risk if humanity does not figure out how to curb the use of fossil fuels that have provided the lifeblood for economic development since the time of Malthus.
But what most stood out in the report from the panel, which gathers every few years to produce a synthesis of mainstream science’s take on climate change, was that it rolled straight into Malthus’s territory, providing its starkest warning yet about the challenge imposed by global warming on the world’s food supply.
The panel’s past report in 2007 had concluded: “Globally, the potential for food production is projected to increase with increases in local average temperature over a range of one to three degrees Celsius.”
But the new report is much more pessimistic about the prospect of extra grain production in the globe’s temperate zones, where more carbon dioxide in the atmosphere would increase the rate of photosynthesis, raising yields, and warmer weather would lengthen the growing season.
Faster photosynthesis will help weeds more than cereal crops, while the accumulation of ozone and high temperatures would reduce yields of all the major grains, according to the report.
This would be bad enough if demand for food were to remain constant. It won’t. Studies suggest that feeding more than nine billion people in 2050 will require 70 percent more calories than the world’s population consumes today, according to Craig Hanson, director of food, forests and water programs at the World Resources Institute.
Indeed, the panel calculates that food demand is rising at a pace of 14 percent per decade. But it estimates that climate change is already reducing wheat yields by 2 percent each decade — compared with where they would be in the absence of climate change — and corn yields by 1 percent.
“This is a wake-up call for the agriculture sector,” Mr. Hanson said. “Climate change is a food security issue. It’s not just an environmental issue.”
The climate panel’s findings do not quite endorse the Malthusian idea that famine will spread practically everywhere. But a world with a more unstable food supply is likely to be a more volatile place. And those most exposed, of course, will be the world’s poor.
Recent experience suggests that the productivity of farmland won’t decline gradually as the world grows warmer. World food prices stopped their long secular decline around 2007 and have been on a roller-coaster ride since. More volatile weather patterns promise to bring sharp disruptions to agricultural production that can cause spikes in food prices.
“There is a rigorous correlation between food price spikes and urban unrest,” said Andrew Holland, who studies climate change at the American Security Project, a research group in Washington. “There was a food price spike in 2008, and you can see unrest spread throughout Africa. And there’s a relatively clear line that leads from the food price spike in 2010 to unrest in the Middle East and the Arab Spring.”
Instability spreads easily. When rice prices jumped in 2007, big producers like India and Vietnam banned exports to protect their domestic markets, while importers like Bangladesh, Nigeria and Iran went out on the market to hoard as much grain as they could. The combination wreaked havoc in commodity markets.
Since then big food importers, like China, Saudi Arabia and South Korea, have tried to insulate themselves from future food shortages by buying or leasing agricultural land in places like Sudan, Madagascar and Uzbekistan. The strategy is still to be tested in a situation in which Africa or Central Asia were to suffer itself shortages of grain.
“I have run some war game scenarios,” Mr. Holland said. “The tendency becomes very quickly for a country to look after its own interests.”
Still, there are good reasons to take prophesies of doom with more than a pinch of salt. Ecological Cassandras have consistently underestimated humanity’s capacity to invent ways around constraints, using resources more efficiently and switching from scarcer commodities to more abundant ones.
In “The Population Bomb,” published in 1968, the noted Stanford ecologist Paul R. Ehrlich wrote “in the 1970s the world will undergo famines — hundreds of millions of people are going to starve to death.” In “The End of Affluence,” written six years later, he forecast “a genuine age of scarcity” by 1985.
Today, Professor Ehrlich is perhaps best known for his bet with the economist Julian L Simon — a committed believer in the power of human ingenuity — who in 1980 challenged Mr. Ehrlich to choose any five commodities and accurately predicted that Mr. Ehrlich’s basket would be cheaper 10 years later, not scarcer and more expensive.
Indeed, the climate panel suggests a variety of ways in which countries could adapt to a changing climate. Farmers could breed new species to better resist heat and drought. Water harvesting techniques could be used to delay evaporation. Rotation of crops could help improve yields.
The United Nations panel reported that a survey of various studies concluded that adapting crop management could raise yields of wheat, rice and maize from 15 to 18 percent compared with doing nothing.
Changes in demand and logistics could also help cope with scarcer food. Mr. Hanson pointed out that fully one-quarter of the food produced in the world today is wasted — by either poor storage and transport infrastructure in developing countries or wasteful consumers in the rich world.
But for all the evidence of humankind’s ability to adapt to its environmental constraints, it would be reckless to assume that ingenuity will arrive just in time to pull us from the brink.
The Competitive Enterprise Institute, a libertarian think tank that is skeptical about global warming, 13 years ago created the Julian L. Simon Memorial Award to celebrate his “vision of man as the ultimate resource.” But Mr. Simon got lucky, too. Had the bet extended for 30 years rather than 10, it would have gone to Mr. Ehrlich.
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