The Global Warming Time Bomb?

All glaciers in Glacier National Park are retreating inexorably to their final demise. Global warming is real, and the melting ice is an apt portent of potentially disastrous consequences. Yet most gloom-and-doom climate scenarios exaggerate trends of the agents that drive global warming. Study of these forcing agents shows that global warming can be slowed, and stopped, with practical actions that yield a cleaner, healthier atmosphere.

A paradox in the notion of human-made global warming became strikingly apparent to me one summer afternoon in 1976 on Jones Beach, Long Island. Arriving at midday, my wife, son and I found a spot near the water to avoid the scorching hot sand. As the sun sank in the late afternoon,a brisk wind from the ocean whipped up whitecaps. My son and I had goose bumps as we ran along the foamy shoreline and watched the churning waves.

It was well known by then that human-made "greenhouse gases," especially carbon dioxide (CO2) and chlorofluorocarbons (CFCs), were accumulating in the atmosphere. These gases are a climate "forcing," because they alter the energy budget of the planet. Like a blanket, they absorb infrared (heat) radiation that would otherwise escape from the Earth's surface and atmosphere to space.

That same summer, Andy Lacis and I, along with other colleagues at the NASA Goddard Institute for Space Studies, had calculated that these human-made gases were heating the Earth's surface at a rate of almost 2 W/m2. A miniature Christmas tree bulb dissipates about 1 W,mostly in the form of heat. So it was as if humans had placed two of these tiny bulbs over every square meter of the Earth's surface, burning night and day.

The paradox that this result presented was the contrast between the awesome forces of nature and the tiny light bulbs. Surely their feeble heating could not command the wind and waves or smooth our goose bumps. Even their imperceptible heating of the ocean surface must be quickly dissipated to great depths, so it must take many years, perhaps centuries, for the ultimate surface warming to be achieved.

This seeming paradox in the notion of human-made global warming has now been largely resolved through study of the history of the Earth's climate, which reveals that small forces, maintained long enough, can cause large climate change. And, consistent with the historical evidence, the Earth has begun to warm in recent decades, at a rate predicted by climate models that take account of the atmospheric accumulation of human-made greenhouse gases. The warming is having noticeable impacts as glaciers are retreating worldwide, Arctic sea ice has thinned, and spring, defined by the cyclical behavior of organisms, the average temperature and the breakup of winter ice, comes about one week earlier than when I grew up in the 1950s.

Yet many issues remain unresolved. How much will climate change in coming decades? What will be the practical consequences? What, if anything, should we do about it? The debate over these questions is highly charged because of the economic stakes inherent in any attempts to slow the warming.

Objective analysis of global warming requires quantitative knowledge of
· The sensitivity of the climate system to forcings,
· The forcings that humans are introducing, and
· The time required for climate to respond.

All of these issues can be studied with global climate models, which are numerical simulations on computers. But our most accurate knowledge about climate sensitivity, at least so far, is based on empirical data from the Earth's history.