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 ofClimatic
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.
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DOE/Pacific Northwest National Laboratory