Trapped Heat

From: NASANews@hq.nasa.gov <NASANews@hq.nasa.gov
Date: 08. april 1998 21:12
Subject: Increasing Greenhouse Gases May Be Worsening Arctic Ozone Depletion and May Delay Ozone Recovery
David E. Steitz; Headquarters, Washington, DC April 8, 1998; (Phone: 202/358-1730)
Lynn Chandler; Goddard Space Flight Center, Greenbelt, MD; (Phone: 301/286-9016)
Release: 98-58

In late 1997, larger levels of ozone depletion were observed over the Arctic than in any previous year on record. Now, using climate models, a team of scientists reports why this may be related to greenhouse gases, according to a paper published in the April 9 issue of Nature.

The study suggests the increase in greenhouse gas emissions is one possible cause of the observed trends in Arctic ozone losses and that this may delay recovery of the ozone layer. The research team, consisting of researchers from NASA's Goddard Institute for Space Studies (GISS) and Columbia University, New York, investigated the response of ozone to projected future emissions of greenhouse gases and ozone - depleting halogens over time, using the GISS climate model. This is the first time ever that the interaction between ozone chemistry and the gradual buildup of greenhouse gases has been studied in a climate model.

"Buildup of greenhouse gases leads to global warming at the Earth's surface, but cools the stratosphere. Since ozone chemistry is very sensitive to temperature, this cooling results in more ozone depletion in the polar regions," said Dr. Drew Shindell of Columbia University, the lead author of the study. NASA will continue research in this area to determine if these models are accurate.

The "greenhouse effect" is defined as the warming of climate that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere - such as water vapor, carbon dioxide, nitrous oxides and chlorofluorocarbons - act like glass in a greenhouse, allowing sunlight to pass into the "greenhouse," but blocking Earth's heat from escaping into space.

Ozone, a molecule made up of three atoms of oxygen, comprises a thin layer of the upper atmosphere which absorbs harmful ultraviolet radiation from the Sun and protects people, animals and plants from too much ultraviolet sunlight.

Distribution and concentration of stratospheric ozone are influenced in two ways by human-driven activity in addition to natural, seasonal variations. Of first importance is the direct impact of industrially produced chlorofluorocarbons. Although ozone levels around the globe are expected to continue to decline over the next several years, NASA is now detecting decreasing growth rates of ozone-depleting compounds in the upper part of the atmosphere, indicating that international treaties to protect the ozone layer are working. The second influence on stratospheric ozone levels is the indirect impact of "greenhouse gases" on atmospheric temperatures. Ozone destruction is quite sensitive to temperature increases in the atmosphere.

Since upper atmospheric temperatures in the Northern Hemisphere during winter and spring generally are warmer than those in the Southern Hemisphere, ozone depletion over the Arctic has been much smaller than over the Antarctic during the 1980s and early 1990s. The Arctic stratosphere, however, gradually has cooled over the past few decades resulting in very large ozone depletion, especially during 1996-97.

In the simulations performed by Shindell and his team, temperature and wind changes, induced by increasing greenhouse gases, clearly alter the dynamics of the atmosphere. According to this model, as the abundance of greenhouse gases gradually increases, the frequency of Northern Hemisphere sudden stratospheric warming is reduced, leading to significantly colder lower stratospheric temperatures. If proven correct, this dynamic effect would add to the greenhouse cooling of the stratosphere.

"Results suggest that the combination of these two cooling effects causes dramatically increased ozone depletion so that ozone loss in the Arctic by the year 2020 roughly is double what it would be without greenhouse gas increases," said Dr. David Rind of GISS, a co-author of the paper. Increasing greenhouse gases therefore may be at least partially responsible for the very large Arctic ozone losses in recent winters.

The authors caution, however, that though the model predicts a general trend towards increasing ozone depletion, the year-to-year variability is quite large, especially in the Arctic. For example, several years in the late 1990s and early 2000s show very little Arctic ozone depletion, while others show record losses. In fact, the 1997-98 winter that just occurred was characterized by significantly less ozone loss than the preceding six winters. A factor that should be considered, however, is the consistency in model predictions, i.e. whether the same results can be reproduced by other models.

According to this model, the severity and duration of the Antarctic ozone depletion also may increase due to greenhouse gas-induced stratospheric cooling over the coming decades. However, ozone in the Antarctic is already so depleted that any additional losses may be relatively small, Rind added.

The research was conducted by scientists at GISS, The Center for Climate Systems Research, Columbia University, and Science Systems and Applications Inc., New York. The GISS research is part of NASA's Earth Science Enterprise, a long-term coordinated research effort to study the Earth as a global system.