7 Tipping points
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
There are two kinds of boundaries, the researchers proposed in
October 2009. "One represents a tipping point — you cross that and
irreversible, catastrophic bad stuff happens," said Jonathan Foley, an
ecologist at the University of Minnesota. "The other would involve more
gradual changes, but still well outside the range of anything we've seen
in human history."
Humans have already pushed the planet beyond some of the limits, such
as those related to climate change and the nitrogen cycle. But 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 starting points.
Here are the seven planetary boundaries that have been put on the
table for discussion.
Earth's ozone layer might have eroded to the point where people get <a href="http://www.livescience.com/environment/090320-ozone-simulation.html">sunburned
within minutes</a>, if political leaders and scientists had not rallied
to regulate the chemicals destroying the ozone, which protects us from
solar radiation. The Montreal Protocol banned chlorofluorocarbons (CFCs)
in 1989, and helped banish the specter of a future world with a
permanent ozone hole yawning above Antarctica.
Environmental scientists have proposed a tipping point of a 5-percent
decrease in ozone in the stratosphere (an upper layer of the
atmosphere), based on ozone levels from 1964-1980.
A more realistic tipping point for stratospheric ozone might be
higher, said Mario Molina, a physical chemist who heads the Center for
Strategic Studies in Energy and the Environment in Mexico City. Truly
catastrophic ozone depletion all across the globe would be something
like a 60-percent decrease. 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.
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.
The limit is a "sound idea" but also premature, according to Steve
Bass, a senior fellow at the International Institute for Environment and
Development in London. Bass pointed out that the arbitrary limit might
leave policymakers unconvinced. After all, converting land to farming
and industry has delivered huge benefits for human populations.
A better boundary of environmental health might be a limit on soil
degradation or soil loss, Bass said. That could gauge the environmental
impact of different types of land use, such as intensively farmed
cropland versus more sustainable agriculture. Poor land-use practices
have historically led to loss of soil and have also created terrible
dust storms, whether in the <a href="http://www.livescience.com/environment/080505-dust-bowl.html">1930s
Dust Bowl</a> or in <a href="http://www.livescience.com/environment/china-sandstorm-space-100323.html">modern-day
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.
But that global limit on freshwater might be too high, said David
Molden, deputy director general for research at the International Water
Management Institute in Sri Lanka. Molden contends the global view
overlooks local conditions that limit how easily people can access
freshwater, whether it's lack of infrastructure or lack of money, as
well as the proportion of its water each region uses.
Intense agriculture might use up most of the freshwater in one
region, not to mention a growing demand for biofuel crops that <a href="http://www.livescience.com/environment/090410-bioethanol-water.html">stresses
water supplies</a>. Another part of the world with plenty of freshwater
might not use much for farming at all. So water limits might have to be
customized for the region. Still, Molden called the idea of planetary
boundaries an "important warning call" and a starting point to think
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 <a href="http://www.livescience.com/environment/071213-coral-acid.html">ocean
acidification</a> 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.
The boundary is based on lab experiments that have shown less
aragonite leads to slower coral reef growth, said Peter Brewer, an ocean
chemist at the Monterey Bay Aquarium Research Institute in Moss
Landing, Calif. Some marine life can apparently survive low aragonite
levels in the ocean, but growing ocean acidification would likely kill
off many species that live around reefs.
One problem with the tipping point: Brewer doesn’t know if anyone has
a serious plan to convince humans to stay within the environmental
Today, species go extinct at a rate ranging from 10 to 100 species
per million per year, and many more <a href="http://www.livescience.com/animals/rarest-rare-endangered-species-100407.html">stand
at risk</a> of vanishing from the planet. Now environmental 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.
The complexity of just keeping track of all species presents a
problem for using extinction rate as a boundary, said Cristian Samper,
director of the Smithsonian National Museum of Natural History in
Washington D.C. Scientists don't even <a href="http://www.livescience.com/environment/top-10-new-species-1.html">discover
all existing species</a> before they go extinct, with rates of
extinction especially unknown for insects or most marine invertebrates.
There's also the fact that past <a href="http://www.livescience.com/strangenews/070808_GM_mass_extinctions.html">mass
extinctions</a>, such as the Permian-Triassic event, have also exceeded
the proposed extinction rate boundary. And some species have a higher
natural rate of extinction compared with others.
Instead, Samper suggested two alternatives to extinction rates.
Instead of focusing on a single extinction rate, scientists could focus
on how population size, distribution and threat levels change for each
group of species. They could also define species extinction as a
probability based on evolutionary history for different branches of the
tree of life.
Nitrogen and Phosphorus cycles
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.
Environmental scientists have suggested that humans shouldn’t add
more than 25 percent to the nitrogen that gets transferred from the
atmosphere to land surface. For phosphorus, they suggested the human
impact should not go beyond 10 times the background weathering that
typically makes phosphorus available.
Those limits seemed too arbitrary for William Schlesinger, president
of the Cary Institute of Ecosystem Studies in Millbrook, N.Y.
Schlesinger noted that soil bacteria and ecosystem management can change
nitrogen levels, so that the nitrogen cycle might tolerate greater
human impact with the right counterbalances. By contrast, he said the
proposed phosphorus cycle boundary was unsustainable and would allow for
the depletion of phosphorus reserves within 200 years.
Schlesinger's biggest problem was with the idea of thresholds.
Waiting to act until humans approach those boundaries simply allows for
bad habits to endure and environmental consequences to accumulate, he
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. He argued that the actions needed to avoid "dangerous climate
change" remain the same regardless of the long-term concentration
Modern humans can't possibly claim to have control over whether
carbon dioxide concentrations are 350 ppm or any other specific level in
the future, Allen said. He also criticized the proposed boundary based
on its high estimate of climate sensitivity, or the long-term warming
response to the doubling of atmospheric carbon dioxide.
But Allen admitted that the 350 ppm concentration might still serve
as a useful 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 <a href="http://www.livescience.com/environment/080127-new-epoch.html">anthropocene
era</a> (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.