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Lecture 2

ENVIRON 157 Lecture Notes - Lecture 2: Global Warming Potential, Radiative Forcing, Hexafluoride

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Anupom Ganguli

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PUB POL 461 Week 1 lect 2
Earth receives an average of 342 watts of incoming solar energy per square meter (yellow paths,
showing shortwave energy). Of that amount, about 30 percent (107 watts per square meter) is reflected
back to space, about 20 percent (67 watts per square meter) is absorbed within the atmosphere, and
about half (168 watts per square meter) is absorbed by Earth’s surface.
Earth, like all bodies, seeks to be in a temperature equilibrium with its environment. So, much of the
solar energy that Earth absorbs is emitted back to space in the form of heat (red paths, showing
longwave energy). Much of the heat energy emitted from the surface is absorbed and then re-emitted
by gases in the atmosphere so that, effectively, these so-called “greenhouse gases” slow the rate of
Earth’s cooling.
Over the course of a year Earth would typically reflect and emit about the same amount of energy that it
receives from the Sun. Thus, our world’s energy budget would be in balance. However, if any of the
mechanisms influencing Earth’s energy budget should change significantly, it would tip the balance in
either a warming or a cooling direction (depending upon the nature and the magnitude of the change).
Temp. will rise and cool depending on which is higher
Natural and Anthropogenic* (caused by humans):
Carbon Dioxide(CO2)
Methane (CH4)
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Nitrous Oxide (N2O),
Ozone (O3), and
Water Vapor (H2O)
Sulfur Hexafluoride (SF6)
Global Warming Potential Source: USEPA
The Global Warming Potential (GWP) for a gas is a measure of the total energy that a gas
absorbs over a particular period of time (usually 100 years), compared to carbon dioxide (CO2).
The larger the GWP, the more warming the gas causes.
For example, methane’s (CH4) 100-year GWP is 21, which means that CH4 will cause 21 times as
much warming as an equivalent mass of CO2 over a 100-year time period.
1 pound of methane= 21 pounds of CO2
Climate forcing is a change in the status quo. Basis for the change is the pre industrial time
The smaller the part, the smaller the forcing.
This graph shows climate scientists’ current estimates of key variables in Earth’s climate system that
have changed since 1750 to exert a warming or cooling influence on Earth’s climate — called “climate
forcings.” Red bars extending to the right indicate the relative magnitudes of warming influences; blue
bars extending to the left indicate the relative magnitudes of cooling influences. The smaller, lightly
shaded bars represent the margin of uncertainty in scientists’ estimates.
Climate scientists estimate with very high confidence that the total net forcing of human activities on
Earth’s climate since the year 1750 to be +1.6 watts per square meter — a global climate forcing in the
warming direction. The subsequent slides in this presentation explain in more detail each of the
individual items in this list of climate forcings.
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