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Editor's Summary of Articles
Remembrance of Things Past:
Greenhouse Lessons from the Geologic Record
Questions
How much has the Earth's climate varied in the past? How do we know what
happened, or why, before the time when instruments became available to measure
winds and rain and temperature? How do warmer temperatures of earlier periods
compare with those that are now projected in current models of greenhouse
warming? Were there times when such conditions applied, that might tell us
what to expect in the future? How often have they occurred? What can be
learned from past climates that could reduce the uncertainties in present
climate projections?
An Assessment
Climate has always varied, on all time scales and through wide extremes. Some
of these changes have come about very slowly, through variations in the
location of the continents, the height of mountain ranges, and the shape of the
Earth's orbit. Others have come more rapidly, as from volcanic activity, or
changes in ocean circulation or radiation from the Sun. Once initiated, even
random variations can be amplified by resultant changes, as in the
concentrations of carbon dioxide or other trace gases in the air. Throughout
the last 10,000 years, the mean surface temperature has fluctuated less than
one degree Centigrade. For most of the last two million years the mean surface
temperature was perhaps five degrees lower--in the course of recurring ice
ages--but there is little evidence for global temperatures more than one degree
higher than the present, warm interlude. The possibility of an enhanced
greenhouse effect thus comes at a time of high temperatures that are rare in
the span of human history. Moreover, concentrations of carbon dioxide and
other greenhouse gases are today as high as any that are known in the 200,000
years for which measurements are available, and we now know that they played a
significant role in climatic changes of the past.
Consequences
Were the Earth to warm by the 2°C given in the middle range of the
consensus projections of the Intergovernmental Panel on Climate Change, the
mean surface temperature would climb to higher values than any known in the
last two million years. Were the amount of carbon dioxide in the air to
double, the surface temperatures would rise even higher. We have now consumed
about 5 percent of the total reservoir of fossil fuels, predominantly coal,
that is still in the ground. Were we to burn all or most of what remains, the
carbon dioxide released to the air could drive global surface temperatures to
levels last known about 100 million years ago, at the time of the dinosaurs.
While questions remain regarding possible, ameliorating feedbacks from other
elements of the climate system, all that is known from the record of the past
confirms a direct connection between greenhouse gases and surface
temperature.
Go to: Remembrance of Things Past: Greenhouse Lessons from the Geologic Record
How Bountiful are Ocean Fisheries?
Questions
Can the oceans--that cover so much of the Earth's surface--provide an
ever-increasing supply of fish for our use, or are we now approaching the limit
of what can be taken? Are there prospects that what is caught will keep pace
with the projected needs of an expanding world population? Will aquaculture
solve the problem? How well do we know the number of fish of any kind that
live in the sea, and how well do we manage these natural stocks? How will
possible environmental changes affect the yield of global fisheries?
An Assessment
The total landings of fish taken from the oceans, which had climbed year after
year, reached a peak in 1989 and has since fallen, to a level that is about 7
percent lower. There is no single explanation for the drop, but overfishing is
obviously implicated: since 1970 the number of commercial fishing vessels has
doubled and the potential catch per ship has been vastly increased. As much
involved, or more, are environmental changes that perturb near-shore regions
where many fish spend critical parts of their lives, uncertainties and obvious
inefficiencies in fisheries management, and probable natural cycles of
production. It has been estimated that we now harvest about 80 percent of what
the oceans can naturally provide, but this number is very poorly known.
Various forms of aquaculture add about 8 percent to what is taken in the wild,
but the ocean fish that are raised in this way have been largely limited to
more expensive varieties, such as salmon, shrimp, and oysters. Global warming
could shift the location of traditional fishing grounds, with economic impacts
that would vary regionally. More important, it could alter ocean circulation,
with far greater impacts on aquatic ecosystems. Potential sea-level rise would
particularly affect shellfish that spend all of their lives near the shore, and
the extensive aquaculture facilities that are widely used in their production.
Consequences
In the last seven years, while the world population was climbing by about 10
percent, the total landings of fish from the ocean declined by almost as much,
signaling, perhaps, the end of the long summer of human history when the waters
beneath the ocean surface seemed to hold an unending supply of fish for food.
There is hope that improved knowledge of fish and ocean ecosystems, more
enlightened management, and more prudent allocations can once again increase
the yield. It is certainly true that the science of fisheries has much to
learn, and that fisheries management, as practiced in most seafaring nations,
has more often focused on immediate interests than on what is sustainable in
the long haul. The need to know more and manage better is heightened by the
environmental changes that have altered today's rivers and coastlines, and by
tomorrow's prospect of possible climate change. Technological and economic
advances in aquaculture, worldwide a growing industry, could also help, were
these efforts directed more toward fish as a dietary staple, as opposed to a
luxury.
Go to: How Bountiful are Ocean Fisheries?
The Sun and Climate
Questions
Do possible solar variations challenge the modeled projections of future
climate change? How much does the Sun vary from day to day or year to year?
Do we know why, and can we predict how solar radiation will vary, five or ten
or 100 years in the future? How sensitive is the climate system to
fluctuations in solar energy, and how do these effects compare in magnitude
with the other possible causes of climatic change? How great a role has the
Sun played in climatic changes of the past?
An Assessment
While we count on the Sun as an unfailing source of light and heat, it is not a
constant one. Continuous measurements of solar radiation, made from the
vantage point of space, began in 1979 and documented, for the first time, the
existence of significant changes in the radiation that streams outward from the
solar surface. We now know that what reaches the Earth rises and falls in step
with the eleven-year sunspot cycle, such that today--near the minimum of the
cycle--we receive about 0.1 percent less heat and light than in years when
sunspots are plentiful. The cause is a systematic variation in brightness from
place to place on the surface of the Sun, and the effect on the Earth's
temperature is a change of less than 0.1°Centigrade. What is known
of solar behavior before 1979 suggests that other changes of possibly higher
amplitude also occur. These longer-term solar variations seem to have played
an important part in the climatic changes of the last several hundred years,
and have probably contributed to the documented warming of 0.5°C
since 1850, although if limited to the range of existing measurements, only
about 15 percent of the temperature rise can be attributed to the Sun. Using
the highest estimates of possible solar changes, the Sun could diminish the
2°C mid-range increase in the IPCC projections by about
0.5°C.
Consequences
The Sun is almost certainly implicated in some of the climate changes of the
past, and may today affect, to some degree, the weather from year to year.
Yet, were it the only agent of climatic change, we would live in a world where
the mean global surface temperature varied, in any century, through limits of
at most about 0.5°C . We are not yet able to predict those
changes, beyond what is now known of the Sun's much smaller, eleven-year
variation. Over longer scales of time, possible changes in solar radiation
could modify the warming that has been projected in mid-range, consensus IPCC
models for the end of the next century by at most about 25 percent. Our
understanding of solar variations needs to be extended if we are to refine
these estimates to the level of most of the other climatic variables on which
current projections are based.
Go to: The Sun and Climate
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