Cloudy, with a chance of malaria.

Every day, perhaps 50 million insects, spiders, and other tiny arthropods drift out of the sky and onto the volcanic rocks of the little Indonesian island of Krakatau. These immigrants come mainly from neighboring islands, where the winds have lifted them from their native foliage. We know about them thanks to the painstaking efforts of scientists who counted every bug landing within a set of marked-out squares, and then extrapolated a rate of arrival for the island as a whole. Such studies reveal a little-noticed atmospheric fact: a rain of arthropods is a normal part of our planer's weather.

It's easy to get into the habit of thinking about climate change as an abstract social liability - rather like an economic down-turn. No doubt, you might say, some people on the "front lines" of change could get hurt badly - drought-stricken farmers, for example, or people facing the rising seas. But for most of us, the threat might appear to be only a set of diffuse and incremental costs.

In fact, however, the front lines are everywhere. Think about the insects that are raining down, invisibly, all around you. Under more-or-less constant climate conditions, your local insect rain is not a threat, because any species that can drift into your area and survive is probably already living there. As for the other drifters - your local climate is probably "immune" to them. If it's too wet for a particular forest pest, say, or too cool for a disease-carrying mosquito, those species won't survive no matter how often the winds bring them in. But as the climate changes, the arthropod rain may begin to leave some very nasty "puddles," and it's going to be difficult to avoid stepping in them. Is anyone in your family infected with malaria?

These puddles would be bad enough, but the global economy is generating a far larger "rain" of invading organisms, which I describe in a new book, Life Out of Bounds. All over the world, people are releasing organisms of just about every description into just about every kind of ecosystem - sometimes by accident (weeds, for example, or wildlife pathogens) and sometimes on purpose (plantation trees or aquaculture fish). To most of us, this biotic mixing is nearly as invisible as the insect rain, even though the cumulative result is already massive ecological disruption and billions of dollars in annual damage to our natural resources, public health, and infrastructure.

The current rates of invasion would be unsustainable even without climate change. But shifts in moisture and temperature patterns, in seasonal rhythms, and perhaps even in ocean currents will force those rates even higher. Native species, adapted to previous conditions, will tend to die off and aggressive invaders will gain a better foothold.

In a sense, we've "booby-trapped" the landscapes in which we live by releasing so many invaders into them. Now, by pumping so much carbon into the atmosphere, we're about to set those booby traps off. So if you think climate change just means ominous weather, wait until you see what it could mean on the ground.