A new study suggest that the cheapest way to limit global warming is to use every available means to reduce emissions. Photo courtesy of U.S. Fish & Wildlife Service |
New
computer modeling work shows that by 2100, if society wants to limit
carbon dioxide in the atmosphere to less than 40 percent higher than it
is today, the lowest cost option is to use every available means of
reducing emissions. This includes more nuclear and renewable energy,
choosing electricity over fossil fuels, reducing emissions through
technologies that capture and store carbon dioxide, and even using
forests to store carbon.
Researchers
from the Joint Global Change Research Institute introduced the work,
called the RCP 4.5 scenario, in a special July 29 online issue of the
journal Climatic Change. The scenario is one of four that scientists
will use worldwide to independently study how the climate might respond
to different increases of greenhouse gases and how much of the sun’s
energy they trap in the atmosphere. It can also be used to study
possible ways to slow climate change and adapt to it.
The
team used the PNNL Global Change Assessment Model, or GCAM, to generate
the scenario. GCAM uses market forces to reach a specified target by
allowing global economics to put a price on carbon. And unlike similar
models, it includes carbon stored in forests, causing forest acreage to
increase — even as energy systems change to include fuels generated from
bioenergy crops and crop waste.
“The
RCP 4.5 scenario assumes that action will be taken to limit emissions.
Without any action, the emissions, and the heat trapped in the
atmosphere, would be much higher, leading to more severe climate
change,” said lead author Allison Thomson, a scientist at JGCRI, a
collaboration between the Department of Energy’s Pacific Northwest
National Laboratory in Richland, Wash., and the University of Maryland.
“This
scenario and the other three produced in this project will provide a
common thread for climate change research across many different science
communities,” Thomson said.
The forested future
Five
years ago, the Intergovernmental Panel on Climate Change asked the
climate science community to provide scenarios of greenhouse gas
emissions and land use change to guide computer models that simulate
potential changes to the Earth’s climate.
Researchers
decided on four possible targets that span a wide range of possible
levels of man-made greenhouse gas emissions over the next century. These
future scenarios are currently being used by climate modeling groups
worldwide in a coordinated effort to compare models and advance the
science of climate projections.
The
researchers assigned each of the scenarios a specific target amount of
the sun’s energy that gets trapped in the atmosphere, a property called
radiative forcing. Because of differences between the scenarios, each
one will produce slightly different degrees of warming.
The
RCP 4.5 scenario shoots for 4.5 Watts per square meters radiative
forcing in 2100 and lets economics reveal how to achieve that goal the
cheapest way possible. The scenario’s 4.5 W/m2 means roughly 525 parts
per million carbon dioxide in the atmosphere (currently, it hovers
around 390 parts per million). It also means approximately 650 parts per
million carbon dioxide-equivalents, which includes greenhouse gases
besides carbon dioxide.
Unlike
the other three scenarios, RCP 4.5 includes carbon in forests in the
carbon market. This affects how people use land, as cutting down forests
emits carbon dioxide but expanding forests stores it. An earlier
modeling study showed that without placing such a value, forests could
get cut down for use as biofuels and the land on which they stood used
for crops.
The greenhouse race
Starting
with the world as it looked in 2005 and setting the endpoint at 2100,
the team let the model simulate the greenhouse gas emissions and land
use change over the next century. They also ran the model without any
explicit greenhouse gas control policy or carbon price to compare how
such a future might turn out.
Without
any emission controls, carbon dioxide concentrations in the atmosphere
doubled by 2100. By design, RCP 4.5 limits them to about 35 percent
higher than 2005 levels.
The
conditions to limit emissions did not specify how to go about doing
that, only that carbon from all sources had economic value. Under
limiting conditions, carbon dioxide prices rose steadily until they
reached $85 per ton of carbon dioxide by 2100, in 2005 dollars.
In
the scenario, the price of carbon stimulated a rise in nuclear power
and renewable energy use. Also, it became cheaper to implement
technologies that capture and store emissions from fossil- and bio-fuel
based electricity than to emit carbon dioxide. Buildings and industry
became more energy efficient and used cleaner electricity for their
energy needs.
Additionally,
carbon dioxide emissions from man-made sources peaked around 2040 at 42
gigatons per year (currently, emissions are at 30 gigatons per year),
decreased with about the same speed as they rose, then levelled out
after 2080 at around 15 gigatons per year.
Resolving power
Also,
the team converted the results of the scenario to match the resolution
of the climate models that are using the results. That way, scientists
can more easily integrate RCP 4.5 with climate models. Economies, for
example, occur on national scales, but chemical reactions of gas in the
air occur in much smaller spaces.
This
change in scale to accommodate climate models reveals important
regional details. For example, although globally methane emissions
change little over the century, their geographic origins shift around.
As the century wears on, South America and Africa put out more methane
and the industrialized nations less.
In
addition, the percentage of people’s income that they’re spending on
food goes down even though food prices rise. The researchers attribute
this result to a shift from agricultural practices with high carbon
footprints to lower ones, as shown in previous work.
While
introducing this scenario to climate researchers, the PNNL researchers
provide comparisons to other scenarios with similar emissions limits, as
well as to scenarios of the other three radiative forcing targets
covered by this community activity. The special issue of Climatic Change
features papers documenting those other three scenarios as well as
several papers reviewing specific parts of the entire scenario exercise.
“It’s
very important that the climate community has this resource so that
they all work from the same data. This common thread will help
researchers and policymakers address the problems that climate change
will bring us,” said Thomson.
Data and results from RCP 4.5 studies are Open Access and are available from JGCRI’s website.
A Pathway for Stabilization of Radiative Forcing by 2100
Earlier modeling study from PNNL
