The storms, floods, and other weather calamities of recent years are just a start.
Consider the "500 year" floods in the Midwestern U.S. that caused $27 billion of damage in 1993. Decades of development had channeled and otherwise altered the Mississippi and other great rivers of the Midwest, reducing their access to floodplains that had absorbed and moderated the effects of extreme rainfall. Without those buffers, the rivers in 1993 rose higher than they might have in years past. When they breached dikes and other barriers, they spilled into the old floodplains, now largely occupied with farms and homes, amplifying the damage. We saw this pattern repeated in New Orleans after Hurricane Katrina and in the Christmas tsunami of 2004. While the tsunami killed more than 280,000 people and destroyed settlements over a swath of several thousand miles, a series of powerful tsunamis in that part of the world during the 19th century passed with far less damage and loss of life. They took place before protective buffers of mangroves were destroyed, before hundreds of millions of people moved into the potential path of the waves, and before cars, trucks, and other contrivances proliferated only to become projectiles when the 2004 tsunami swept them up.
Around the world, humanity has reduced nature's capacity to dampen extremes to an astonishing degree: more than 59% of the world's accessible land degraded by improper agriculture, deforestation, and development; half the world's available fresh water now co-opted for human use at the expense of other species and ecosystems; more than half the world's mangroves destroyed; half the world's wetlands drained or ruined; 1/5 of the world's coral reefs (including crucial barrier reefs) destroyed and 1/2 damaged--the list goes on and on.
Nature does not alert us to all her tripwires. Perhaps that's why, in recent years, the unprecedented has become increasingly ordinary. When pushed past a certain magnitude, the damage of natural events increases exponentially, and that threshold falls as natural buffers are eliminated. Research led by MIT climatologist Kerry Emmanuel suggests that hurricanes have doubled in intensity during the past 30 years, as the oceans have warmed. Hurricane Katrina surged to its immense power when the storm passed over a deep layer of 90°F water in the Gulf of Mexico. Hurricane Rita transfixed meteorologists, when it strengthened from Category 2 to 5 in less than 24 hours, while moving over those same hot seas. And in October, Wilma bested that by strengthening from tropical storm to Category 5 hurricane in a single day.
Since we are dismantling natural buffers just at the point when we really need them, it's tempting simply to conclude that humanity has a self-destructive streak. The explanation, of course, is not masochism but a collective failure of imagination--compounded by the fact that we are only now learning to weigh the threat. There are no models to estimate the economic impact of rapid changes in temperatures, storm tracks, precipitation, and so on. In a 2001 report entitled "Abrupt Climate Change: Inevitable Surprises," the National Research Council, the principal operating unit of the National Academy of Sciences, noted that most modeling of impacts has been confined to cases in which changes are gradual and moderate. Modeling the effects of abrupt change is a lot harder, but the study makes a couple of important points.
First, economies can minimize the effects of a gradually changing climate if people recognize the threat and respond. With abrupt climate change, however, things happen so rapidly that neither markets nor ecosystems have time to adapt. Moreover, a dynamic market economy with capacity to respond to intermittent crises, by spreading risk and reallocating assets, may be unable to respond when crisis is ubiquitous and risks loom everywhere.
Second, even gradual climate change would pose immense challenges. Tim Barnett, an oceanographer at Scripps Oceanographic Institution, took part in a study of the likely effects of climate change on the Los Angeles area. Surprisingly, he says, even modest decreases in rainfall during what he called a "best-case scenario for future climate change" (a gradual and small change, decades in the future) could reduce available water for the area by 50% by 2050. The region has limited storage capacity for water. It relies on the winter snowpack that builds up in the Sierra Nevada and the Rockies for water during the dry summer months. Under even modest climate-change scenarios, however, the snowpack would be smaller and would melt earlier. The region would dry up before its driest months.
Angelinos wouldn't necessarily go thirsty. California has plenty of agricultural water that could be diverted to human needs. The ancillary effects would be harder to manage. Farm output would be reduced, and water shortages could idle hydroelectric plants. Drought also makes trees more vulnerable to pests, such as the pandora moth that afflicts ponderosa pine. Dead trees are tinder for wildfires, like the ones that destroyed hundreds of homes in Southern California in 2003. Such impacts would roil the economy. Consider how increased fire risk and other effects of acute water scarcity might affect housing prices or the job market.
Keep in mind that the 50% reduction of available water was a best-case scenario. While the richest state in the world's richest nation has some ability to weather a drought, such shifts would not be occurring in isolation. The changing climate that brought drought to Southern California would also be affecting weather throughout the American West and beyond--damaging property, disrupting agriculture, and spurring migrations.