单选题
The scientific name is the Holocene Age, but climatologists
like to call our current climatic phase the Long Summer. The history of Earth's
climate has rarely been smooth. From the moment life began on the planet
billions of years ago, the climate has swung drastically and often abruptly from
one state to another—from tropical swamp to frozen ice age. Over the past 10,000
years, however, the climate has remained remarkably stable by historical
standards: not too warm and not too cold, or Goldilocks weather. That stability
has allowed Homo sapiens, numbering perhaps just a few million at the dawn of
the Holocene, to thrive; farming has taken hold and civilizations have arisen.
Without the Long Summer, that never would have been possible.
But as human population has exploded over the past few thousand years, the
delicate ecological balance that kept the Long Summer going has become
threatened. The rise of industrialized agriculture has thrown off Earth's
natural nitrogen and phosphorus cycles, leading to pollution on land and water,
while our fossil-fuel addiction has moved billions of tons of carbon from the
land into the atmosphere, heating the climate ever more. Now a
new article in the Sept. 24 issue of Nature says the safe climatic limits in
which humanity has blossomed are more vulnerable than ever and that unless we
recognize our planetary boundaries and stay within them, we risk total
catastrophe. "Human activities have reached a level that could damage the
systems that keep Earth in the desirable Holocene state," writes Johan
Rockstrom, executive director of the Stockholm Environmental Institute and the
author of the article. "The result could be irreversible and, in some cases,
abrupt environmental change, leading to a state less conducive to human
development." Regarding climate change, for instance, Rockstrom
proposes an atmospheric-carbon-concentration limit of no more than 350 parts per
million (p.p.m.)—meaning no more than 350 atoms of carbon for every million
atoms of air. (Before the industrial age, levels were at 280 p.p.m.; currently
they're at 387 p.p.m. and rising.) That, scientists believe, should be enough to
keep global temperatures from rising more than 2℃ above preindustrial levels,
which should be safely below a climatic tipping point that could lead to the
wide-scale melting of polar ice sheets, swamping coastal cities. "Transgressing
these boundaries will increase the risk of irreversible climate change," writes
Rockstrom. That's the impact of breaching only one of nine
planetary boundaries that Rockstrom identifies in the paper. Other boundaries
involve freshwater overuse, the global agricultural cycle and ozone loss. In
each case, he scans the state of science to find ecological limits that we can't
violate, lest we risk passing a tipping point that could throw the planet out of
whack for human beings. It's based on a theory that ecological change occurs not
so much cumulatively, but suddenly, after invisible thresholds have been
reached. Stay within the lines, and we might just be all right.
In three of the nine cases Rockstrom has pointed out, however—climate change,
the nitrogen cycle and species loss—we've already passed his threshold limits.
In the case of global warming, we haven't yet felt the full effects, Rockstrom
says, because carbon acts gradually on the climate—but once warming starts, it
may prove hard to stop unless we reduce emissions sharply. Ditto for the
nitrogen cycle, where industrialized agriculture already has humanity pouting
more chemicals into the land and oceans than the planet can process, and for
wildlife loss, where we risk biological collapse. "We can say with some
confidence that Earth cannot sustain the current rate of loss without
significant erosion of ecosystem resilience," says Rockstrom.
The paper offers a useful way of looking at the environment, especially for
global policy makers. As the world grapples with climate change this week at the
U.N. and G-20 summit, some clearly posted speed limits from scientists could
help politicians craft global deals on carbon and other shared environmental
threats. It's tough for negotiators to hammer out a new climate-change treaty
unless they know just how much carbon needs to be cut to keep people safe.
Rockstrom's work delineates the limits to human growth—economically,
demographically, ecologically—that we transgress at our peril.
The problem is that identifying those limits is a fuzzy science—and even
trickier to translate into policy. Rockstrom's atmospheric-carbon target of 350
p.p.m. has scien-tific support, but the truth is that scientists still aren't
certain as to how sensitive the climate will be to warm over the long-term—it's
possible that the atmosphere will be able to handle more carbon or that
catastrophe could be triggered at lower levels. And by setting a boundary, it
might make policymakers believe that we can pollute up to that limit and still
be safe. That's not the case—pollution causes cumulative damage, even below the
tipping point. By focusing too much on the upper limits, we still risk harming
Earth. "Ongoing changes in global chemistry should alarm us about threats to the
persistence of life on Earth, whether or not we cross a catastrophic threshold
any time soon," writes William Schlesinger, president of the Cary Institute of
Ecosystem Studies, in a commentary accompanying the Nature paper.
But as the world attempts to break the carbon addiction that already has
it well on the way to climate catastrophe, more clearly defined limits will be
useful. But climate diplomats should remember that while they can negotiate with
one another, ultimately, they can't negotiate with the planet. Unless we manage
our presence on Earth better, we may soon be in the last days of our Long
Summer.
单选题
According to the passage, which of the following is NOT the result of
the Long Summer?
A.It is possible to grow crops.
B.Human beings have appeared.
C.Cultures have come into being.
D.It is possible for modern men to increase quickly.