DeLucia, Evan (2015): How Biofuels Can Cool Our Climate and Strengthen
Our Ecosystems. In Eos 96 (4), pp. 14–19. DOI 10.1029/2015EO041583.
Critics of biofuels like ethanol argue they are an unsustainable use of
land. But with careful management, next-generation grass-based biofuels
can net climate savings and improve their ecosystems.
Miscanthus and switchgrass, two perennial grasses that could be used for
biofuels, grow in front of a corn crib. Credit: Evan DeLucia
By Evan H. DeLucia <mailto:[email protected]> 22 December 2015
As the world seeks strategies to reduce emissions of carbon dioxide
(CO_2 ) into the atmosphere, bioenergy is one promising substitute for
fossil fuels [/Somerville et al./, 2010]. Currently, the United States
uses the starch component from roughly 40% of its corn harvest to
produce ethanol for the transportation sector (see the National
Agricultural Statistics Service website <http://www.nass.usda.gov/>).
Currently, the United States uses the starch component from roughly 40%
of its corn harvest to produce ethanol for the transportation sector.
Cornstarch production is technologically simple—a so-called
first-generation bioenergy technology. However, growing corn requires a
lot of fertilizer and field preparation that ultimately depend on fossil
fuels, tempering the net carbon savings.
Because of this, researchers have focused on developing fuel production
using more advanced methods and second-generation bioenergy crops. These
methods make liquid fuel, primarily ethanol, from lignocellulose, which
composes the structural elements of plants, leaves, stalks, and stems.
Because many of these crops are perennials (they grow back year after
year), they often require less fertilizer and tillage, avoiding many of
the negatives associated with corn and other annual crops that need
intensive management.
A challenge with second-generation energy crops, however, is that they
yield much less energy—lignocellulose produces less than one third of
the energy per unit mass compared to fossil fuels. Because of this low
energy density, the United States would need to invest a considerable
amount of land to meet a significant part of national demand, a land
area almost the size of Wisconsin to meet the Renewable Fuel Standard
mandate for 32 billion gallons of biofuel [/Hudiburg et al.,/ 2015].
Planting second-generation grass-based biofuel crops on marginal or
degraded land can reduce our carbon footprint and provide other
beneficial ecosystem services.
The conversion of current land uses and management practices to the
cultivation of bioenergy crops directly affects the climate system and
is a fundamental process underlying the ecological sustainability of
bioenergy production, as well as the ability of bioenergy crops to
mitigate climate change. Where conversion of native prairie to corn
negatively affects climate by releasing CO_2 and other greenhouse gases
to the atmosphere, planting second-generation grass-based biofuel crops
on marginal or degraded land can reduce our carbon footprint and provide
other beneficial ecosystem services.
*Changing the Landscape Changes Our Climate*
Fig. 1. Terrestrial ecosystems affect the climate system by influencing
the exchange of greenhouse gases between the land surface and the
atmosphere (biogeochemical regulation) and by influencing the exchange
of energy (evapotranspiration and albedo) with the atmosphere
(biophysical regulation). Biogeochemical processes affect the
concentration of greenhouse gases in the atmosphere, influencing global
climate on decadal time scales or longer, whereas biophysical processes
cause local cooling or warming over days and months. Credit: Evan DeLucia
Public discussion of climate change often focuses on the atmosphere. As
greenhouse gases (GHGs) like carbon dioxide, nitrous oxide, and methane
accumulate in the atmosphere, they warm the planet by absorbing infrared
radiation. But that’s not the whole picture: those greenhouse gases are
also constantly cycling between the atmosphere and the land (Figure 1).
Therefore, changes in land use, vegetation, and how we manage it affect
climate by altering that exchange.
The metabolisms of plants and soil microbes help to regulate the
exchange of GHGs with the atmosphere, as these molecules or their
precursors are stored in biomass and soil. Clearing a native forest, for
example, releases large quantities of carbon stored in biomass and soil
to the atmosphere (“storage”; Figure 2). Indeed, creating and managing
farmland contribute more than 14% of the world’s GHG emissions (U.S.
Environmental Protection Agency
<http://www.epa.gov/climatechange/ghgemissions/global.html>, global
greenhouse gas emissions data, 2015).
Fig. 2. (a) Biogeochemical climate services reflect the greenhouse gases
that would be released from land clearing and the change in ongoing
exchange with the atmosphere. (b) Similarly, land clearing affects
biophysical climate services, albedo, related to net radiation (R_net ),
and latent heat flux from evaporation (LE), related to changes in
evapotranspiration. Positive values represent a net cooling effect on
the atmosphere. (c) The sum of greenhouse gases and biophysical factors
is the climate regulating value (CRV) of an ecosystem. Values are
normalized to the warming potential of carbon dioxide (CO_2 ) and are
expressed relative to bare ground over a 50-year time frame. Replacing
an ecosystem with a high CRV with that having a low value would have a
net warming effect on the atmosphere and vice versa. Reproduced from
/Anderson-Teixiera et al./ [2012].
Terrestrial ecosystems also affect climate on local scales by regulating
the exchange of energy between the land and atmosphere (Figure 1)
[/Davin et al./, 2014; /Zhao and Jackson/, 2014]. Land covered with
vegetation generally absorbs more sunlight than bare soil, contributing
to local warming. Working against this, when water evaporates and passes
into the atmosphere from soil and vegetation (known collectively as
evapotranspiration), it carries heat away from the land, causing local
cooling. Thus, forest clearing would reduce the cooling effect of
evapotranspiration but would increase the reflectivity of the surface
(or its albedo), allowing it to reflect more radiation.
However, global effects are complicated because evapotranspiration means
more moisture in the atmosphere, which can increase cloud cover,
affecting the global radiation balance. Also, when the moisture
condenses into clouds elsewhere, it releases its latent heat, which can
cancel the local cooling effect [/Pielke et al./, 2002; /Snyder et al./,
2004]. Unlike GHGs that have a locally weak but globally strong effect
on the atmosphere, albedo and evapotranspiration affect climate locally
[/Bright/, 2015].
By normalizing the warming (or cooling) potential of nitrous oxide (N_2
O), methane, albedo, and evapotranspiration to the warming potential of
CO_2 , the contribution of different ecosystems to climate regulation
can be expressed as a single metric: the climate regulating value (CRV)
[/Anderson-Teixeira et al., /2012]; this value provides an integrated
index of the direct effects of land clearing on the surface energy
budget, where the greater the CRV is, the greater the cooling effect is
(Figure 2).
*Long-Term Strategic Planting*
In native forests and other ecosystems with large carbon stocks,
biogeochemical processes—those processes that store and exchange GHGs
with the atmosphere—can play a larger role in regulating the climate
than biophysical processes such as changes in evapotranspiration and
albedo (Figure 2). The opposite can be true for perennial grasses that
don’t store much biomass but have high evapotranspiration and albedo.
Therefore, displacing native forests, particularly tropical forests,
with annual or perennial crops for energy production will, in most
cases, have a net warming effect on the atmosphere, releasing large
quantities of carbon stored in biomass and soil (Figure 2). But
replacing annual crops or placing high-yield bioenergy crops on marginal
land (that is, land that cannot produce high-value crops) has a very
different effect.
Replacing annual crops with perennial grasses would pull carbon out of
the atmosphere and return it to the ground.
Replacing annual crops with perennial grasses such as miscanthus
<https://en.wikipedia.org/wiki/Miscanthus> and switchgrass
<https://en.wikipedia.org/wiki/Panicum_virgatum> would pull carbon out
of the atmosphere and return it to the ground (Figure 2). These crops
allocate a large fraction of their biomass below ground in their root
systems, and they can rapidly build up carbon stores in soil, reversing
losses associated with frequent tillage, particularly on degraded or
heavily tilled soils [/Anderson-Teixeira et al./, 2009, 2013; /Powlson
et al./, 2011].
In terms of climate effects, this is doubly helpful. In addition to
displacing fossil fuels by providing a renewable biofuel, replacing
low-CRV annual crops with high-CRV perennial grasses would have a net
cooling effect on the atmosphere because of the changes in
biogeochemical and biophysical properties. In particular, the increased
albedo from perennial grass and the heat carried away by
evapotranspiration can amount to a considerable cooling effect compared
to annual row crops [/Georgescu et al./, 2011].
*The U.S. Midwest: From Carbon Source to Sink*
Of the approximately 40 million hectares in the United States that are
planted with corn, mostly in the Midwest, only 8% of them directly feeds
humans; most of the rest (73%) is for feeding livestock and producing
ethanol (National Agricultural Statistics Service website, 2015).
Displacing corn currently grown for ethanol with high-yielding perennial
grasses would have enormous environmental benefits, without displacing
land used for food production. /Davis et al./ [2012] predict that
replacing ethanol-bound corn with perennial grasses would reduce
emissions of GHGs to the atmosphere while increasing soil carbon. The
emissions of N_2 O in particular would be reduced because perennial
grasses require so much less nitrogen than corn [/Smith et al.,/ 2013].
Over the entire region, this transition would convert soils in the
Midwest from a net source to a net sink for GHGs while simultaneously
increasing fuel production and reducing the contamination of groundwater
by fertilizer-derived nitrate.
Prime corn land is expensive. Restricting most bioenergy grasses to more
affordable marginal land would also drive a reduction in U.S. GHG
emissions, albeit a smaller one than replacing corn ethanol, and would
still meet the Renewal Fuel Standard’s mandate for 32 billion gallons of
renewable biofuel, with negligible effects on food crop production
[/Hudiburg et al/., 2015].
The biophysical processes will also help to regulate climate.
Evapotranspiration would increase slightly—less than 10% [/VanLoocke et
al./, 2010]—but combined with an increased albedo, that’s enough to
provide an additional local cooling effect.
*The Promise and Challenges of a Bioenergy Landscape*
In addition to displacing CO_2 emitted from fossil fuels, the expansion
of perennial bioenergy crops in the U.S. Midwest will likely have
positive effects on the climate system. This, however, is not
necessarily the case elsewhere. In much of the United States west of the
100th meridian (a line that roughly bisects the Dakotas and Texas), the
ability of the atmosphere to remove water (potential evapotranspiration)
exceeds precipitation. There, the irrigation necessary for energy crops
would pose severe environmental challenges.
In Southeast Asia, displacing native forest with palm oil plantations,
in part for biodiesel, has increased the amount of atmospheric GHGs and
hurt biodiversity [/Danielsen et al./, 2009]. Furthermore, by increasing
grain prices, displacing food crops with bioenergy crops may encourage
deforestation in the tropics for expanding agriculture [/Searchinger et
al./, 2008].
We must take care to avoid the unintended negative consequences of
expanding lignocellulosic bioenergy production.
We must take care to avoid these unintended negative consequences of
expanding lignocellulosic bioenergy production. But with appropriate
financial incentives [/Dwivedi et al./, 2015], there are many strategies
to use land sustainably to contribute to the U.S. demand for
transportation fuel: the use of marginal or underproductive lands
[/Gelfand et al./, 2013] or replacing intensively managed corn for
ethanol with high-yielding, low-input perennials. When annual crops that
require intensive management are replaced with high-yielding perennial
plants, bioenergy crops can simultaneously reduce the emission of GHGs
to the atmosphere and improve the health of agricultural landscapes
[/Werling et al.,/ 2014].
------------------------------------------------------------------------
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*Author Information*
Evan H. DeLucia, Department of Plant Biology, Institute for
Sustainability, Energy, and Environment, Energy Biosciences Institute
and Carl R. Woese Institute for Genomic Biology, University of Illinois
at Urbana–Champaign; email: [email protected]
*Citation:* DeLucia, E. H. (2015), How biofuels can cool our climate and
strengthen our ecosystems, /Eos, 96,/ doi:10.1029/2015EO041583.
Published on 22 December 2015.
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