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Can we defuse
The Global Warming Time Bomb? 1

JAMES HANSEN 2, 3


NASA Goddard Institute for Space Studies, and Columbia University Earth Institute, 2880 Broadway, New York, NY 10025, USA

Received July 17, 2003; published August 1, 2003

Summary: All glaciers in America's Glacier National Park are retreating inexorably to their final demise. Global warming is real, and the melting ice is a 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.

Introduction

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 (see Box 1). Like a blanket, they absorb infrared (heat) radiation that would otherwise escape from the Earth's surface and atmosphere to space.

In the summer of 1976, Andy Lacis and I, along with other colleagues at the NASA Goddard Institute for Space Studies, 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 (Figure 1).




Figure 1. Wind and tides mix the ocean thereby transporting heat absorbed at the surface to great depths.

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 (1) the sensitivity of the climate system to forcings, (2) the forcings that humans are introducing, and (3) 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.


Box 1: Climate forcings, sensitivity, response time and feedbacks
A climate forcing is an imposed perturbation of the Earth's energy balance. If the sun brightens, that is a positive forcing that warms the Earth. Aerosols (fine particles) blasted by a volcano into the upper atmosphere reflect sunlight to space, causing a negative forcing that cools the Earth's surface. These are natural forcings. Human-made gases and aerosols are also important forcings.

Climate sensitivity is the response to a specified forcing, after climate has had time to reach a new equilibrium, including effects of fast feedbacks. A common measure of climate sensitivity is the global warming caused by a doubling in atmospheric CO2 concentration. Climate models suggest that doubled CO2 would cause 3 °C global warming, with an uncertainty of at least 50%. Doubled CO2 is a forcing of about 4 W/m2, implying that global climate sensitivity is about 3/4 °C per W/m2 of forcing.

Climate response time is the time needed to achieve most of the climate response to an imposed forcing, including the effects of fast feedbacks. The response time of the Earth's climate is long, at least several decades, because of the thermal inertia of the ocean and the rapid mixing of waters within the upper few hundred meters of the ocean.

Climate sensitivity and response time depend upon climate feedbacks, which are changes in the planetary energy balance induced by the climate change that can magnify or diminish climate response. Feedbacks do not occur immediately in response to a climate forcing; rather, they develop as the climate changes.

Fast feedbacks come into play quickly as temperature changes. For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.

Slow feedbacks, such as ice sheet growth and decay, amplify millennial climate changes. Ice sheet changes can be treated as forcings in evaluating climate sensitivity on time scales of decades to centuries.

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1 Edited version of presentation delivered by James Hansen to the Council on Environmental Quality (Washington, DC) on June 12, 2003.
James Hansen2 About the author: Dr. James Hansen heads the Goddard Institute for Space Studies, which is a division of the NASA Goddard Space Flight Center and a unit of the Columbia University Earth Institute located on the Columbia campus in New York City. Dr. Hansen was trained in physics and astronomy in the space science program of Dr. James Van Allen at the University of Iowa. His primary research for the past 25 years has been on studies and computer simulations of the Earth's climate, for the purpose of understanding the human impact on global climate. Dr. Hansen is best known for his testimony on climate change to congressional committees in the 1980s that helped raise broad awareness of the global warming issue. He was elected to the National Academy of Sciences in 1995 and, in 2001, received both the Heinz Award for the environment as well as the American Geophysical Union's Roger Revelle Medal.
3 Affiliations for identification only; interpretations in this paper are the opinion of the author and are not meant to represent the position of any organization.
4 Revised October 28, 2003.

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