About half of the sun’s energy (yellow arrows) is absorbed by Earth’s surface. However, when this absorbed energy is emitted back to the atmosphere (red arrows), heat-trapping gases prevent most of it from escaping toward space, resulting in higher temperatures. Note: the molecules depicted in the inset box represent heat-trapping gases that are well-mixed throughout the atmosphere, and the number of yellow and red arrows are proportional to the actual balance between incoming and outgoing energy. (click here to enlarge image)
As the reality and urgency of global warming have become a topic of national and international discussion, various solutions for minimizing the potentially harmful consequences of climate change have been put forward. To understand how—and whether—these solutions would work, it is first necessary to understand the mechanisms causing our planet to warm.
Earth receives warmth in the form of energy that travels from the sun in a variety of wavelengths, some of which we see as sunlight and others that are invisible to the naked eye, such as shorter-wavelength ultraviolet radiation and longer-wavelength infrared radiation. As this energy passes through Earth’s atmosphere, some is reflected back into space by clouds and small particles such as sulfates; some is reflected by Earth’s surface; and some is absorbed into the atmosphere by substances such as soot, stratospheric ozone, and water vapor (see the diagram above). The remaining solar energy is absorbed by the earth itself, warming the planet’s surface.
Too Much of a Good Thing
If all of the energy emitted from the Earth’s surface escaped into space, the planet would be too cold to sustain human life. Fortunately, as depicted in the diagram, some of this energy does indeed stay in the atmosphere, where it is sent back toward Earth by clouds, released by clouds as they condense to form rain or snow, or absorbed by atmospheric gases composed of three or more atoms, such as water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4).
Long-wave radiation absorbed by these gases is re-emitted in all directions, including back toward Earth, and some of this re-emitted energy is absorbed again by these gases and re-emitted in all directions. The net effect is that most of the outgoing radiation is kept within the atmosphere instead of escaping into space.
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Global Warming's Public Enemy #1
Climate scientists monitor the impact of more than 60 heat-trapping gases. So why does CO2 get all the attention?
Other gases may be more abundant in the atmosphere (i.e., water vapor), or have more potent heat-trapping ability molecule per molecule, but CO2 has made the most significant contribution to climate change since the dawn of the industrial age, and puts us at the greatest risk of irreversible changes if it continues to accumulate unabated in the atmosphere. There are two key reasons why.
The Intergovernmental Panel on Climate Change (IPCC) issued a new global climate assessment in 2007 that compared the relative influence exerted by key heat-trapping gases on our climate between 1750 and 2005. By measuring the abundance of these gases in ice cores, the atmosphere, and other sources, the IPCC calculated the “radiative forcing” (RF) of each gas—in other words, the net increase (or decrease) in the amount of energy reaching Earth’s surface attributable to that gas.
Positive RF values represent average surface warming and negative values represent average surface cooling. CO2 has the highest positive RF (1.65 watts per square meter) of all heat-trapping gases compared by the IPCC. While this may seem like a small number, it is in fact a significant amount of energy when averaged over Earth’s entire surface area.
The other major factor in determining the relative importance of specific heat-trapping gases is how long each lingers in the atmosphere. It takes about a decade for methane emissions to leave the atmosphere, and about a century for nitrous oxide. In the case of CO2, about half of today’s emissions will be gone in 30 years, but 20 percent will still exist around 800 years from now. That distinction provides the clearest possible rationale for reducing our CO2 emissions without delay.
Climate Influence 1750-2005 |
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Heat-trapping gases, in balanced
proportions, keep Earth at an optimal
temperature, but these gases are accumulating
in the atmosphere at increasing
concentrations due to human activities
such as the burning of fossil fuels, the
clearing of forests for agriculture or development,
and agricultural practices.
As a result, the insulating blanket these
gases form around Earth is getting thicker,
allowing less energy to escape into space
and causing temperatures to rise.
Engineering Solutions that Aren’t
Keeping these mechanisms in mind, we
must turn a critical eye toward some
proposed engineering solutions that would take an ambitious but ultimately flawed approach to slowing global warming. For example, one proposal would populate the upper atmosphere with tiny particles mimicking volcanic ash, under the premise that reflecting incoming sunlight with such particles would cool Earth’s surface. However, incoming solar radiation is not the core cause of global warming. The problem is that concentrations of heat-trapping gases are increasing in the atmosphere, so even if there were less incoming energy as a result of more sunlight being reflected, more of that energy that does reach the surface would be trapped over time.
In addition, reducing the amount of incoming sunlight would hinder plant photosynthesis and subsequent growth, reducing crop yields for farmers. It is easy to see, then, that engineering proposals like this would very likely create more problems than they fix. Their limitations become even more apparent when viewed in light of the fact that the proposed reflective substances would only last a matter of days in the lower atmosphere and a year or two in the upper atmosphere, while heat-trapping gases can last for decades or even centuries (see the sidebar).
A Smarter Approach
Instead of waiting for a future “pie in the sky” fix, we should continue to focus on the more practical goal of reducing heat-trapping emissions in the first place. If we fail to prevent CO2 emissions from continuing to increase at their current pace, we not only risk accelerated global warming but other environmental impacts as well. For example, CO2 interacts with ocean water to make it more acidic, threatening shell-forming
marine organisms.
Many cost-effective strategies are already available to reduce our heat-trapping emissions, such as increasing our use of renewable energy; improving the efficiency of our cars, power plants; and appliances; and developing sustainable forestry and farming practices. Combining these existing solutions
with more promising technologies (e.g., carbon capture and storage, advanced vehicles, green building design, low-emission power plants) can deliver the necessary reduction in atmospheric
concentrations of heat-trapping gases and ensure a safer climate for future generations. ■
Brenda Ekwurzel is a climate scientist at UCS.
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