Clouds are just one example. Any new scientific knowledge about the global environment, whether the biochemistry of how plants suck up CO2 and release moisture or the optical properties of sulfate aerosols, is eventually transformed into equations and woven into the computer simulations. “We’re putting more and more elements of the physical system into equations in the model,” Meehl says, “and it hopefully all interacts and produces something that looks like the world we’re living in.”
There are more than a dozen widely used global climate models today, and despite the fact that they are constantly being upgraded, they have already proved successful in predicting seasonal rainfall averages and tracking temperature changes. With temperature, the scientists test their models by replaying history. They can wind the clock back to 1850, plug in the known climate variables at the start, then roll the models forward in time and see whether their forecasts match the historical records. So far, they do. Says Meehl, “It’s kind of a sanity check.”
A major drawback of computer models is that they offer broad, often frightening results without any explanation. Imagine a doctor walking into an exam room, telling you that something terrible will probably happen to your brain in 20 years, then offering nothing but vague shrugs when pressed on the precise what and when, or even whether it might be preventable. All the computers can do is give us a starting point. Then it is up to scientists to work out the specifics (as best they can) of how particular areas will be affected. “We know that these large global mean changes are going to be associated with local and regional changes that are going to cause real problems in some areas,” says Andy Challinor, an expert on climate and agriculture at the University of Leeds in England. “The problem lies in working out those details.”
When scientists try to understand the local impacts of rising global temperatures, for example, they find immense variation. In a world that’s four degrees warmer, many agricultural regions could dry out, become too hot, or both. Overall the planet stands to lose approximately 15 percent of today’s farmland. At the same time, other regions that are too cold today could become arable. One study puts the increase at 20 percent, creating a net increase in productive land. If agricultural production were to shift to currently colder climates, we would still have enough food in the United States; it would just be coming from new locations.
But for sub-Saharan Africa, the changes would be far more calamitous. Most of the people there lack access to a secure, inexpensive global food system. They rely heavily on what is grown and raised locally, so they are far more sensitive to local changes in climate. And farming is a huge part of the economy: 60 percent of the workforce is in agriculture. In a recent paper exploring the consequences of a four-degree world, Challinor and his colleagues, including lead author Philip Thornton of the International Livestock Research Institute in Nairobi, Kenya, found that a four-degree rise in temperatures would decrease the length of the growing season by more than 20 percent. Yields of many key crops in the region, including maize, beans, and a kind of grass used for feeding cattle, would drop significantly in an altered climate, the scientists determined.
At the same time, the population of sub-Saharan Africa is expected to double by 2050, surpassing 1.7 billion people. Growing population and declining food production could have a cascade effect: Food shortages lead to hunger, hunger leads to conflict, conflict leads to mass migration and political instability. (It is worth noting that the current unrest in the Middle East coincides with sharply escalating food prices.) To avoid stumbling into this nightmarish future, Thornton and his coauthors say the region will need extensive investment in agricultural productivity, improved data collection to track the weather, organized testing of how crops and livestock will fare in a hotter, drier climate, and a program to educate local farmers. No farmer, they note, will grow a crop he doesn’t know.
South America should fare better in terms of agriculture, but it faces its own major loss: the Amazon rain forest. Rising ocean surface temperatures in the tropical North Atlantic, combined with warmer ocean surface temperatures in the tropical east Pacific, will affect atmospheric circulation and pressure patterns, reports climate scientist Richard Betts of the Met Office Hadley Centre in England. According to his climate model, precipitation in the region will shift and possibly decline over the Amazon. This is a proven phenomenon: Atlantic Ocean surface temperature is already being used to predict seasonal rainfall in South America.
In the future, Betts says, a consistent decline in annual rainfall could trigger a feedback loop. The rain forest, which covers 2.3 million square miles, needs a certain amount of precipitation to support itself, and if rainfall drops below that level, the forest could start to shrink. Normally trees extract huge amounts of water from the soil and return it to the atmosphere through their leaves. As the forest shrinks, there would be fewer trees to do that. “You get less evaporation, less moisture in the atmosphere, and less rainfall again. The process can feed on itself,” Betts says. “The future of the forest could be threatened.”
The caveat is that other climate models yield different results. Betts traces some of the discrepancy to a disagreement about future ocean temperatures in the tropical North Atlantic. (Scientists have not yet been able to determine why the models disagree.) “One model tells you it’s potentially catastrophic, and others tell you it isn’t,” he says. “You have two physically plausible scenarios. How do you use that in policymaking?”
A four-degree-warmer world will therefore demand a range of responses, from immediate and bold to slow and cautious. Australia, for example, faces both near-term and long-term drought risks due to climate change. In the west, water shortages are a reality now, so the country is considering the construction of three massive desalination plants, at a cost of more than a billion dollars each, to create freshwater. Some experts question the wisdom of planning facilities in parts of Australia that might dry up decades from now, and instead advocate water-conservation efforts. Such adaptation could give the region more time to examine its options and make informed decisions. “We’ve got a lot to do, but we don’t have to do it all at once,” Smith says.