from LiveScience Website
Humans must stay within certain
boundaries if they hope to avoid environmental catastrophe, a
leading group of environmental scientists says. Crossing those
limits may not rock the Earth itself, but would lead to harsh
consequences for human existence on the planet as we know it.
Humans have already pushed the planet beyond some of the limits, such as those related to climate change and the nitrogen cycle.
some scientists who responded in the journal Nature questioned the
threshold idea, and others commented that such limits seem
arbitrary. Still, many applauded the idea of limits as benchmarks or
The Ozone Hole
Earth's ozone layer might have eroded to the point where people get sunburned within minutes, if political leaders and scientists had not rallied to regulate the chemicals destroying the ozone, which protects us from solar radiation.
Montreal Protocol banned chlorofluorocarbons (CFCs) in 1989, and
helped banish the specter of a future world with a permanent ozone
hole yawning above Antarctica.
But Molina added the lower
limit on ozone destruction makes sense, given the damage to human
health and the environment beyond ozone loss of 5 percent.
Credit: NASA Earth
Agriculture and industry have long formed the bedrock of human civilization, so that the current crop cover supporting today's population has reached about 12 percent of land.
Now environmental scientists have proposed a 15-percent land
use limit, leaving some wiggle room, but still protecting animals
and plants from losing valuable real estate.
Drinking water represents a basic necessity for life, but humans also use huge amounts for growing crops.
Foley and his colleagues suggested that use of “blue water” sources - evaporation from rivers, lakes, groundwater reservoirs and irrigation - should not go beyond 960 cubic miles (4,000 cubic kilometers) per year, or just a little less than the entire volume of Lake Michigan.
Humans currently use 624
cubic miles (2,600 cubic kilometers) each year.
Still, Molden called the
idea of planetary boundaries an "important warning call" and a
starting point to think about limits.
Higher levels of carbon dioxide can dissolve the minerals necessary for coral reefs and other marine organisms to thrive.
That led environmental scientists to label ocean acidification resulting from the increase in the greenhouse gas carbon dioxide as a tipping point boundary, or one that if crossed could have catastrophic consequences for both marine life and humans who depend on the resources. The boundary definition focuses on aragonite - a mineral building block of coral reefs - so that the aragonite-saturation state should be at least 80 percent of the average global pre-industrial level.
Such a saturation state
reflects the amount of aragonite dissolved within the seawater.
Credit: Jay R. Rooker
Today, species go extinct at a rate ranging from 10 to 100 species per million per year, and many more stand at risk of vanishing from the planet.
scientists say species extinction should not go beyond the threshold
of 10 species per million per year - a boundary that the current
rate of extinction has clearly exceeded.
They could also define species extinction
as a probability based on evolutionary history for different
branches of the tree of life.
Blue Marble: Looking
Back at Earth From Space
Nitrogen represents a crucial element for life, and the amount available decides how much plant life or crops can grow.
Phosphorus is another crucial nutrient for both
plants and animals. Limited amounts of both elements cycle through
the Earth's systems, so that altering the cycles can deplete
available reserves and lead to environmental damage or loss of
species because of the different concentrations.
For phosphorus, they suggested the human
impact should not go beyond 10 times the background weathering that
typically makes phosphorus available.
Waiting to act until humans approach those boundaries simply allows
for bad habits to endure and environmental consequences to
accumulate, he said.
Credit: 20th Century
Many scientists and policymakers have aimed for 350 parts per million (ppm) as the long-term target limit for carbon dioxide concentration in the atmosphere.
The limit was set because beyond that amount, the buildup of the greenhouse gas would push the human contribution to global warming beyond 3.6 degrees Fahrenheit (2 degrees Celsius). Yet that carbon dioxide target largely misses the point, according to Myles Allen, physicist and climatologist at the University of Oxford in England.
that the actions needed to avoid "dangerous
climate change" remain
the same regardless of the long-term concentration target.
That's because scientists know that 15 to 20 percent of CO2 emissions hang around in the atmosphere indefinitely. Releasing a little over 1 trillion tons during the anthropocene era (now) of human-caused global warming would lead to a long-term CO2 concentration of about 350 ppm.
Limiting the excess CO2 emissions to
1 trillion tons would be just about what's needed to keep the
likeliest CO2-related warming peak below 2 degrees C - and humans
are already halfway to that limit.