WMOD00435

• Title: Limits on Global Warming
• Date: 5/9/1991
• Weather Modification - Doc Type: Report

LIMITS ON GLOBAL W ARMINGl

A. S. Dennis

U.S. Bureau of Reclamation

Denver, CO 80225

Abstract. Consideration of the earth's carbon reservoirs and carbon exchanges among them
indicates that the concentration of carbDn dioxide (C00 in the atmosphere will increase
markedly over the next 100 years and mDre. A doubling of atmospheric CO2, along with
anticipated feedback effects on atmospheric water vapor, will be roughly equivalent in
radiative terms to an increase of 5 percent in the intensity of solar radiation at the top of the
atmosphere. The laws of physics predict a resultant global warming, which likely will
average around 3 to 4 DC, unless it is modified by other feedback effects. Concern about
global warming is heightened by evidence that the average temperature at the earth's surface
has risen about 0.5 oC over the last century. Global warming will be limited in intensity as
the atmosphere approaches total opacity in the CO2 radiation bands and will be ended by
advection of excess CO2 into the ocean depths, a process which will take several centuries.
Possible measures to offset global warming include restrictions on fossil fuel consumption
and active measures, such as the introduction of artificial dust clouds into the stratosphere to
scatter solar radiation.

1. INTRODUCTION

The temperature at the surface of the earth is a
function of the atmospheric concentrations of greenhouse
gases, particularly water vapor and CO2, as well as several
other variables. As early as the 1860s, some scientists
speculated that changes in the composition of the
atmosphere due to widespread use of coal to fuel the
Industrial Revolution eventually could affect the weather.
Arrhenius (1896) calculated that doubling the concentration
of carbon dioxide (COz) in the atmosphere would raise
surface temperatures by about 5 to 6 DC depending on
latitude.

Global warming, if it proves real, will surpass in
importance all other cases of inadvertent weather and
climate modification to date. Interest in it and other
aspects of global climate change has been increasing in the
scientific community over the past 25 years. However,
most Americans paid little attention until 1988. That year
brought to much of North America a searing drought,
which climaxed a series of hot summers in the 1980s. It
also brought TV newscasts combining pictures of
drought-stricken fields with ominous speculations that the
drought was merely a taste of an imminent climate change.
Almost immediately "the greenhouse effect" and "global
warming" became household words.

Among the hundreds of articles and reports that
have appeared in the last few years dealing with various
aspects of climate change, I have not found one that brings
together in a few pages all of the factors that are likely to

determine the intensity and duration of global warming due
to an enhanced greenhouse effect. This paper does that. It
describes no original research and it references, for the
most part, reviews rather than original sources. It is
intended to provide a perspective on the anticipated global
warming and some ability to judge the merits of various
methods that have been proposed to offset its impacts. No
attempt is made to assess the potential impacts of global
climate change on agriculture, commerce, or any other
human activity.

2. CARBON RESERVOIRS AND CARBON CYCLES

2.1 An Inventory of Carbon Reservoirs

Any assessment of the possibility of global warming
due to changes in the composition of the atmosphere must
consider present and future trends of C~ concentrations.
Carbon is stored in the earth's crust, ocean and
atmosphere, and moves freely among the three reservoirs.
Because carbon is a constituent of many inorganic
compounds and is also a key element in the composition of
all organisms, tracking it involves consideration of
geological, physical, chemical, and biologiGal processes.

While the quantities of carbon in its various locations are
subject to considerable uncertainty, various authors have
produced useful inventories; an example is that of Berner
and Lasaga (1989), which is the basis of table 1. Nearly all
the earth's carbon is tied up in sedimentary rocks, many of
which were laid down on the seafloor as shells ages ago
and subsequently lifted above sea level by geologic
processes.

IBased on a presentation at the Fortieth Anniversary Meeting of the Weather Modification
Association, Ontario, California, May 9, 1991.

L-,__L

106