Science Basics
Global Warming: The Science
The science of global warming is not new. In 1859, British scientist John Tyndall discovered that carbon dioxide (CO2) can trap heat and in 1896 Swedish scientist Svante Arrenhenius calculated that doubling a the amount of CO2 in the atmosphere could raise Earth’s temperature as much as 10°F. Since Charles Keeling began measuring atmospheric CO2 in the late 1950s, scientists have accumulated a wealth of evidence documenting the increase in CO2 and other global warming gases in the atmosphere, the consequent rise in global average temperature and the influence of human activities on the Earth’s climate.
The Keeling Curve
What is Global Warming?
Global warming refers to the global temperature rise and subsequent impacts from the increase of heat-trapping gases in the atmosphere from human activities, primarily the combustion of fossil fuels. This additional pollution enhances the so-called “Greenhouse Effect” and warms the Earth. The latest report from the Intergovernmental Panel on Climate Change (IPCC) declares that the evidence for warming is “unequivocal” and that most of the observed warming is very likely – greater than 90 percent certainty – due to the increase of global warming pollution from human activities. Over the last century, the global average temperature has increased 1.3°F, with almost 90 percent of the warming occurring over the last fifty years.
What is the Greenhouse Effect?
Just like the glass of a greenhouse traps warm air inside, certain gases in the atmosphere trap heat that would otherwise escape into space. The effect is not limited to Earth. Much of what we know about the greenhouse effect comes from observation of our planetary neighbors, Venus and Mars. The atmosphere of Mars has such a low concentration of heat-trapping gases that it is in a perpetual deep freeze, while Venus has so much, its surface temperatures is hot enough to melt lead! As we rapidly enhance the greenhouse effect on Earth by adding heat-trapping gases to the atmosphere, we are turning up Earth’s thermostat to dangerous levels. 
USGCRP
What causes it?
There are a number of gases that occur naturally and are the major contributors to the greenhouse effect – water vapor, carbon dioxide, and ozone (at high altitudes, not smog at ground level). Methane and nitrous oxide play a lesser role. Some man-made industrial gases, like chlorofluorocarbons (more famous for their creation of the ozone hole), also contribute to the greenhouse effect.
The impact of each gas on global warming is a combination of their abilities to trap heat, their concentrations in the atmosphere, and how long they stay in the atmosphere. For example, while one molecule of methane traps more heat than one molecule of CO2, the higher concentration and longer atmospheric lifetime of CO2 means it has contributed more to global warming than methane has. Most efforts to control global warming pollution have focused on the CO2 emissions from human activities because they have the most affect and we have the greatest control over them.
Since the Industrial Revolution, the concentration of CO2 in the atmosphere has increased from 280 ppm to over 380 ppm. This 100 ppm change is the same increase as from the last ice age about 20,000 years ago until just before the 1800’s. Human activities have changed the atmosphere as much in 200 years as natural variations changed it over 20,000 years.
Water vapor is different from the other gases produced from human activities primarily because of the much shorter time it stays in the atmosphere – days rather than years, decades or centuries. The amount of water vapor in the atmosphere is dependent on the temperature and is not a direct result of human activities. The warmer the world, the more evaporation occurs and the more water vapor the atmosphere can hold, thus increasing the amount of heat-trapping gases in the atmosphere and increasing the temperature further.
How do we know the warming is caused by human activities?
For the heat-trapping gases with no natural source, the contribution from human activities is obvious. The other gases, especially CO2, also carry a human fingerprint. The analysis of carbon isotopes demonstrates that the majority of the increase in carbon dioxide comes from the combustion of fossil fuels. The geographic differences in concentrations of heat-trapping gases – higher concentrations over land in the more heavily populated Northern Hemisphere – also points to the combustion of fossil fuels and other human activities as the source of the globe’s rising temperatures.
Scientists can model the temperature effects of natural and human-induced, or anthropogenic, changes in the global temperature. The results show that natural variations alone can not explain the observed temperature rise of the last decades. The changes from human activities are necessary to fully explain the observed warming.
The most recent report IPCC has estimated that of the processes that can change global temperature, what they call “radiative forcings”, the components from human activities are cumulatively 10 times larger than the best estimates of the changes from solar activity. A recent paper shows that all the trends in the Sun’s activity that could influence the temperature of the Earth have been in the opposite direction needed to explain the rise in temperature over the last 20 years.
What are the impacts?
Scientists have now observed impacts from global warming on all continents and most oceans. For a closer look at impacts in specific areas, check out the "Impact Zones" section of this website. The IPCC has also identified the likely impacts on crucial areas if global temperatures continue to rise.
Can we stop it?
Because of the complexities of the climate system, more warming is inevitable, but we still have time to enact policies that will avoid the most catastrophic impacts. The IPCC has identified a variety of policies and technologies that could be used to cost-effectively reduce global warming pollution and avoid the most catastrophic impacts.
The challenge of reducing emissions may seem daunting, but there are reasons to be hopeful. Policies designed to reduce the release of methane from gas pipelines, rice paddy fields and landfill sites have succeeded in stabilizing the concentrations of methane in the atmosphere over the last few years.
The U.S. Senate has passed legislation to reduce emissions from transportation by imposing new energy efficiency standards on the manufacture of automobiles and light trucks, and the U.S. House of Representatives has passed legislation to reduce emissions from electric utilities by requiring higher use of renewable sources of electricity. If these initiatives are combined and passed, it could mean that as much as 25 percent of what we must do can be accomplished in this single piece of legislation.
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