Healthy coral reefs support many
species, but they now are dying
off at unprecedented rates, largely
due to rising ocean temperatures.
What Is the
and What Can
We Do About It?
www.solartoday.org SOLAR TODAY
By Chuck Kutscher
Energy efficiency and renewable energy
technologies will play major roles in
addressing global warming.
While the media finally appear to be sounding
the alarm for global warming, climate
scientists who publish in the peer-reviewed
literature have agreed for years that
mankind is changing the climate. Yet polls
show that many Americans believe it is
simply a natural variation. What evidence has scientists so
convinced that we are to blame, and what can we do about it?
Examining the Science
Since the early 1800s, we have known that various atmospheric
gases, acting like the glass in a greenhouse, transmit incoming
sunlight but absorb outgoing infrared radiation, raising the average
air temperature at the Earth’s surface. Although these socalled
greenhouse gases are present in very small amounts,
without them the average temperature would be about 33°C
(60°F) colder than it is today. Other atmospheric constituents
like aerosols — suspended particles in the air — released by fossil-fueled
power plants, tend to lower the temperature by
blocking sunlight. Climate scientists compare the effects of
these gases and constituents in terms of radiative “forcings,”
or how much they change the net heat flux (the difference
between incoming solar radiation and the outgoing infrared
radiation) on the Earth’s surface. Figure 1 shows the radiative
forcings in watts per square meter as determined by the Intergovernmental
Panel on Climate Change (IPCC), an international
collaborative of scientists and government representatives
established to study global warming.
Carbon dioxide (CO2), a major byproduct of fossil fuel
combustion, is the most influential greenhouse gas. Although
methane (CH4) is about 20 times as powerful a greenhouse gas
on an equal volume basis, it is present in lesser amounts, is
shorter-lived when added to the atmosphere, and thus is less
important than carbon dioxide.
The most compelling evidence for climate change lies
in the so-called paleoclimatic data. In the 1980s scientists
began drilling for ancient ice-core samples in Greenland and
Antarctica. Seasonal depositions of snow leave distinct lines
in the ice, which, much like tree rings, serve as a time scale.
By analyzing air bubbles trapped in the ice when it formed,
scientists can determine the content of greenhouse gases and
even the average temperature at each point in time. The data
(figure 2 on page 30) show that over the past 420,000 years, the
temperature and greenhouse gas content of the atmosphere
has varied in 100,000-year cycles between about 180 parts per million
(ppm) by volume to a maximum of about 290 ppm. (This
is due to changes in the distribution of solar radiation associated
with natural variations in the Earth’s orbit.) And the
Earth’s temperature has closely tracked the greenhouse gas
concentration. Other techniques, such as the study of ocean fossils,
reinforce the ice-core data.
Around 1850, when the CO2 level was still stable at about 280
ppm, the level began to spike. It has now reached the unprecedented
value of 380 ppm — a 36 percent increase over the preindustrial
value — and is rising at about 2 ppm per year. In figure
2, the time scale from 1850 to the present has been expanded
to reveal the shape of the trend, but if plotted on the time scale
of the rest of the chart, the rises in greenhouse gases and temperature
would appear as vertical lines. Scientists now know that an
increase in temperature can release CO2 from the ground and seawater,
and, conversely, a rise in greenhouse gases will cause a rise
in temperature, so the two effects reinforce each other.
Not only has fossil-fuel burning released greenhouse gases, but
it has created air pollution in the form of aerosols like sulfur
dioxide. To some extent, these have counterbalanced greenhouse
heating by reflecting some sunlight away, and models show that
this explains a slight decline in the Earth’s temperature between
1940 and about 1970. Air pollution continues to block some
sunlight. However, with tighter air-quality standards and rapidly
increasing amounts of greenhouse gases, the greenhouse effect
July/August 2006 29
Confronting the Climate Change Crisis | What Is the Evidence?
is now the dominant net effect of fossil fuel combustion. In the
last 30 years, the average temperature at the Earth’s surface has
risen at the rate of 0.2°C (0.36°F) per decade.
If one considers all the heat flux that human activities have
added to the planet since 1850, it amounts to about 1.6 watts per
square meter over the Earth’s surface. The ice-core data reveal that
this amount of heating will increase the Earth’s temperature by
1.2°C (2.2°F). The temperature thus far has risen by about 0.7°C
(1.3°F) since the Industrial Revolution. So even if we stop adding
any more greenhouse gases, another 0.5°C (0.9°F) temperature rise
is already built in.
In actuality we are emitting even more greenhouse gases,
meaning that the radiation imbalance will grow and the temperature
will continue to rise at an increasing pace. The year 2005 was
the warmest year ever recorded — slightly higher than the previous
record year of 1998 (see figure 3). The high temperature in
2005 is especially significant because, unlike 1998, 2005 had no
El Niño to boost the temperature.
Weighing the Consequences
Since the last ice age, the Earth has been in a warm period
for about 8,000 years, which is relatively rare in our planet’s history.
It is only during this extended warm period that civilization
has blossomed. Clearly, however, an exploding human
population burning more and more fossil fuels now has a greater
effect on the climate than do natural mechanisms. The atmosphere
can no longer be viewed as an infinite sink into which we
can dump our wastes.
What are the consequences of not addressing our carbon emissions?
The IPCC has identified the potential impacts, and many
can already be observed. They include sea-level rise and earlier
spring runoffs in many areas, resulting in increased summer
drought in some regions. Drought conditions are expected to
become worse in Africa, where millions already face famine.
Storm severity will increase due to the additional energy in the
atmosphere. A new study indicates that the high intensity of
recent hurricanes cannot be explained by the normal 75-year
cycle of hurricane activity. Low-lying coastal areas like the Florida
and Gulf coasts will be more prone to storm surge.
Ninety-eight percent of mountain glaciers are shrinking, and
their disappearance will result in severe water shortages for millions
worldwide. Global warming is also expected to increase the
strength of El Niño events that warm the Pacific, resulting in more
so-called “super El Niños” like those that occurred in 1983 and
1997-98. These extreme El Niños are associated with severe weather-related
events, including floods, heat waves, mudslides,
drought, wildfires and famine.
Plants, animals and humankind will find it difficult to adapt
because the changes are occurring so quickly. Because different
species will react to the changing conditions differently, the food
chain will be interrupted. The projected additional 3°C rise in temperature
this century would place the Earth’s temperature very close
to levels that existed during mass extinctions when more than 50
percent of species were lost. One study has indicated that up to onethird
of the Earth’s species could become extinct by the year 2050
as a result of climate change. Already global warming has been
blamed for outbreaks of a fungus that has resulted in the extinction
of dozens of colorful frog species in Central and South America.
In many cases, insects and germs will spread beyond their current
boundaries; we are now seeing insect-borne diseases of the
tropics, like West Nile Virus, appearing in northern climates.
Coral reefs, which provide sea life critical to the economies of
island nations and which offer a promising source of life-saving
drugs, can survive only in a narrow temperature band. They show
unprecedented die-off largely due to warming oceans. Reports
from the U.S. Virgin Islands indicate that during a recent fourmonth
period of elevated sea temperatures, as much as one-third
of the coral has died.
We now know that the climate has many “positive feedback”
mechanisms, which reinforce changes and can result in “tipping
points” beyond which irreversible changes occur. It is because of
positive feedback that the Arctic is the region hardest hit by climate
change. As the ice melts, the resulting darker water and
ground absorb more sunlight, exacerbating the warming. The average
air temperature in Alaska has risen 2.8°C (5°F) in just the last
50 years. This warming has caused permafrost to melt, undermining
building foundations and even forcing the relocation of
Unlike an ice cube melting slowly on a countertop, the destruction
of ice sheets is a highly dynamic, nonlinear (i.e., with positive
feedback) process. The melt water flows like a river, causing
rapid heat transfer and erosion. The melt water also lubricates the
www.solartoday.org SOLAR TODAY
How much do we need to reduce carbon emissions? The key is to decide
what additional temperature rise can be tolerated.
base of ice sheets, causing them to move faster. Scientists in
Greenland have found that these positive feedback mechanisms
have greatly accelerated the melting of the ice sheets. To make
matters worse, methane and carbon dioxide are released from
newly exposed soil as it heats up, promoting still more greenhouse
Tackling the Problem
According to the Energy Information Administration, 82 percent
of U.S. greenhouse gas emissions (about 1.6 billion metric
tons of carbon, or GtC, per year) are in the form of carbon dioxide
resulting from the burning of fossil fuels. (One metric ton is
1,000 kilograms.) That represents 23 percent of the world’s total
CO2 emissions — a large proportion considering that we have only
5 percent of the world’s population. Electricity production
accounts for 42 percent of our total carbon emissions, and the
burning of transportation fuels accounts for 32 percent. So targeting
electricity generation and transportation fuels will address
about three-quarters of our CO2 emissions.
How much do we need to reduce carbon emissions? The key is
to decide what additional temperature rise can be tolerated. Studies
show that if no action is taken, the most probable rise in the
average air temperature at the Earth’s surface by the end of this century
is about 3°C (5.4°F) above the year 2000 value; much higher
increases are possible. Sea level will rise due to both the thermal
expansion of the oceans and the melting of land-based ice sheets.
The last time the Earth’s temperature was that high was 3 million
years ago, when sea level ultimately reached 25 meters (82 ft)
higher than today. Estimates of how quickly sea level may reach
such a height vary. The latest measurements of rapid melting in
Greenland suggest that the IPCC’s model-based projection for a
maximum rise of 0.9 m (3 ft) by 2100 may be too conservative.
NASA climate scientist Jim Hansen has suggested that once the
temperature reaches 2°C to 3°C above the year 2000 value, the disintegration
of ice sheets would be irreversible and sea level could
rise 6 m (20 ft) within a century. That could reshape the world’s
coastlines, with dire consequences for the world’s large coastal
populations. Hansen has argued that we must aim to keep the
additional temperature rise to less than 1°C (1.8°F) above the 2000
value to ensure that the sea-level rise will be less than 1 m and to
minimize the loss of species. He has argued that a target CO2 level
of 475 ppm could be sufficient to limit the temperature rise to 1°C
if we simultaneously reduce methane emissions by a factor of two.
Stephen Pacala and Robert Socolow of Princeton have
described a simplified scenario that would allow the CO2 to level
out at 500 ppm. It involves limiting world CO2 emissions to the
current rate of 7 GtC per year for 50 years, followed by substantial
emissions reductions. The amount of carbon emissions that
would be displaced over the next 50 years can be represented
roughly by the difference between the rising business-as-usual
level of emissions and the current level, which Pacala and
Socolow approximate with a triangle on a graph of emissions versus
time (see figure 4 on page 32). The triangle has an area of 175
GtC. Because that is an immense amount of carbon emissions,
Pacala and Socolow divide the triangle into seven smaller
“wedges,” each having an area of 25 GtC. They then hypothesize
various possible mechanisms that can each displace 25 GtC.
The mechanisms fall into five categories: energy conservation/efficiency,
renewable energy, enhancing natural sinks (such as
smarter land use), nuclear energy, and management of fossil
energy (capturing and storing carbon and switching from coal to
What does this mean for the United States? World carbon
emissions are split roughly evenly between developed and developing
countries. If the developing countries limit their increase
in emissions to 60 percent between now and 2050, we in the
industrialized countries will need to reduce our emissions at the
same rate to keep world emissions constant. Accounting for a projected
business-as-usual 1.2 percent U.S. carbon-growth rate, this
will require the United States to displace about 55 GtC, or about
two wedges, of carbon emissions over the next 50 years. This
means an average carbon displacement rate of about 1 GtC per
July/August 2006 31
Confronting the Climate Change Crisis | What Is the Evidence?
year, which is two-thirds of our current emission rate. To put this
amount of carbon-free energy in perspective, it is approximately
equivalent to displacing a typical 500-megawatt (MW) U.S. coal
plant every week for the next 50 years. (Even with such large carbon
reductions, our per capita emissions, now at five times the
world average, would still be twice the world average.)
So how can we achieve such large decreases in emissions?
The first priority is to address opportunities to improve energy efficiency,
especially at the point of use. In the buildings sector,
energy-efficient appliances, windows, lighting, air conditioners
and insulation have already made a big impact, and these measures
hold great promise to further reduce our electricity consumption.
In the transportation sector, expanded deployment of
public transportation and the use of lightweight materials and
improved aerodynamics can significantly reduce carbon emissions.
In the industrial sector, more efficient motors and piping systems
and the use of heat recovery offer significant opportunities. The
Rocky Mountain Institute has estimated that a combination of
these measures could save 0.7 GtC per year by 2025 at costs of less
than 6 cents per kilowatt-hour of saved energy. This represents
more than half of the U.S. carbon reduction needed.
But we must also address energy supply. Consider first electricity.
Coal- and natural gas-burning plants are responsible for most
emissions from electricity generation. Coal is the bigger problem
because it is more widely used, contains more carbon and is
burned in plants with lower overall efficiencies. It is also likely to
remain less expensive than imported liquid natural gas. To produce
electricity, we have three alternatives to coal- and gas-burning:
1) capturing the carbon emissions and storing them, 2)
expanding our usage of nuclear power, and 3) switching to renewable
sources (wind, solar, biomass and geothermal).
Carbon capture and sequestration offers promise. By gasifying
coal, it is possible to create hydrogen fuel and capture the carbon
dioxide. This carbon dioxide can then be pumped at high pressure
into geologically stable reservoirs. However, even tiny leakage
rates of CO2 into the atmosphere could defeat the whole
purpose (and be deadly to nearby populations). So sequestration
must first be demonstrated to work safely on a large scale, and several
demonstration projects are under way. The environmental
issues associated with mining the coal would remain, however.
Nuclear power is essentially carbon-free. Nuclear fusion offers hope
for the distant future, but presents an enormous technical challenge.
Fission is the only viable nuclear option in the near term. However,
the electricity from new nuclear power plants would be relatively
expensive, and nuclear faces significant obstacles. The biggest
challenges are interrelated: the disposal of radioactive waste and, for
international development, the threat of nuclear proliferation (as
underscored by current concerns with Iran’s program). New U.S.
plants also require long licensing times, so it would be at least a decade
before any additional nuclear power could be brought to bear on the
climate-change problem, even if the obstacles can be overcome.
Some have proposed limiting U.S. nuclear power to simply building
new plants on current sites as the existing plants are retired.
Of the three electricity-generation alternatives, only renewable
energy causes no additional environmental problems, can be
applied immediately to mitigating the crisis and is completely sustainable.
The major challenges are cost, intermittency of supply
(necessitating some form of storage), and the transmission distance
between the resources and the end use.
A recent study by the Western Governors’ Association demonstrated
how much electricity could be produced in the West by
solar, wind and geothermal sources. For example, in the case of
solar energy, prime sites for concentrating (parabolic trough)
solar power plants could provide 200 gigawatts of electricity in California,
Nevada, Arizona and New Mexico at estimated costs
ranging from 7 to 16 cents per kilowatt-hour. That is approximately
twice the current electric-generating capacity of the region
(20 percent of national demand) and would provide about 10 percent
of the U.S. carbon reduction needed (assuming the plants
have six hours of thermal storage).
www.solartoday.org SOLAR TODAY
To reach targeted carbon-emissions reductions,
the United States will need to displace the equivalent of
a typical 500-megawatt U.S. coal plant every week for the next 50 years
While centralized concentrating solar power and geothermal
electric plants are primarily limited to the Southwest, every part
of the United States has access to some form of renewable energy
(see figure 5). Based on the latest estimates by researchers at the
National Renewable Energy Laboratory, a combination of wind,
concentrating solar power, biomass and rooftop photovoltaics
could de-carbonize the U.S. electric grid, assuming dispatchability
and transmission issues can be adequately addressed. How fast
this may occur depends on many factors, including manufacturing
scale-up rate, materials-supply issues and policy measures.
What about transportation fuels? They represent the most rapidly
growing source of carbon emissions. Burning a gallon of gasoline
in a vehicle results in the emission
of about 3 kilograms (6.6 lb) of
carbon (accounting for both fuel
production and combustion). Thus
an average car emits more than a
ton of carbon per year.
Hybrid vehicles represent an
important advance. Interest in the
development of flexible-fuel, plugin
hybrids is growing, largely
because of the recognized need to
reduce our oil imports. A hybrid
electric vehicle with enough battery
storage to cover a distance of
10 to 20 miles in electric-only
mode and the ability to be plugged
into the grid to be recharged
overnight would result in a gas
mileage of greater than 100 mpg.
At today’s gasoline and electricity
prices, such a vehicle would also be
much cheaper to operate. The batteries
provide distributed storage,
which makes the electric grid more
secure and helps address the dispatchability
issue associated with wind power. The use of biofuel
blends with plug-in hybrids would result in further reductions
in net carbon emissions.
The Next Step
We face a daunting challenge. We will have to adapt to a certain
amount of environmental damage resulting from our carbon
emissions to date, while aggressively reducing our emissions to
avoid the worst consequences. Unwilling to wait for a federal commitment,
leaders in state and city governments have forged ahead
to address climate change. Regional carbon cap-and-trade initiatives,
a national coalition of mayors and renewable portfolio
standards that now exist in 22 states, all will have an impact.
However, only a comprehensive, bipartisan federal effort
that is initiated very soon can help us avoid a dangerous tipping
point in the climate. The longer we wait to act, the more
difficult the problem becomes. As was the case with the ozone
problem, a well-informed public will likely play a key role in
History shows that intelligent regulation works faster than
volunteer programs. For example, a legislated cap on sulfur dioxide
emissions with provision for tradable allowances has greatly
reduced air pollution and acid rain in the United States
through market forces. A similar national carbon cap-and-trade
policy could stimulate carbon reduction. Other measures such as
increased CAFE standards for vehicles, more rigorous appliance
standards and various federal policy incentives can also have a
Although some business groups have expressed concern
about the potential impact on our economy, corporations
such as DuPont and IBM have reduced their carbon emissions
and improved their profitability in the process. We
must consider the enormous costs of environmental damage
if we do not begin to address the problem and focus on the
economic opportunities that carbon mitigation offers. If we
have learned anything from Hurricane Katrina, it is that the
cost of preventing environmental disaster pales in comparison
to the cost of recovering from it. As Pacala and Socolow
note, it will require a combination of approaches to solve
this problem. Energy efficiency and renewable energy will
play major roles. ●
Dr. Chuck Kutscher (firstname.lastname@example.org) is general chair of the
SOLAR 2006 conference, taking place July 7-13 in Denver with the
theme, “Renewable Energy: Key to Climate Recovery.” Top climate
change scientists including NASA’s Jim Hansen, NCAR’s Warren
Washington, Princeton Professor Robert Socolow and NYU Professor
Emeritus Marty Hoffert will describe the latest science on this global
threat. Experts in renewable energy and energy efficiency will
discuss how these technologies can help reduce U.S. carbon emissions.
The various theme-related presentations will be published by the
American Solar Energy Society in a special postconference report.
July/August 2006 33