STANFORD SOLAR CENTRE

GLOBAL WARMING

What is it?

Graphics from Intergovernmental Panel on Climate Change Report Summary
Global warming -- a gradual increase in planet-wide temperatures -- is now well documented and accepted by scientists as fact. A panel convened by the U.S National Research Council, the nation's premier science policy body, in June 2006 voiced a "high level of confidence" that Earth is the hottest it has been in at least 400 years, and possibly even the last 2,000 years. Studies indicate that the average global surface temperature has increased by approximately 0.5-1.0°F (0.3-0.6°C) over the last century. This is the largest increase in surface temperature in the last 1,000 years and scientists are predicting an even greater increase over this century. This warming is largely attributed to the increase of greenhouse gases (primarily carbon dioxide and methane) in the Earth's upper atmosphere caused by human burning of fossil fuels, industrial, farming, and deforestation activities.
Average global temperatures may increase by 1.4-5.8ºC (that's 2.5 - 10.4º F) by the end of the 21st century. Although the numbers sound small, they can trigger significant changes in climate. (The difference between global temperatures during an Ice Age and an ice-free period is only about 5ºC.) Besides resulting in more hot days, many scientists believe an increase in temperatures may lead to changes in precipitation and weather patterns. Warmer ocean water may result in more intense and frequent tropical storms and hurricanes. Sea levels are also expected to increase by 0.09 - 0.88 m. in the next century, mainly from melting glaciers and expanding seawater . Global warming may also affect wildlife and species that cannot survive in warmer environments may become extinct. Finally, human health is also at stake, as global warming may result in the spreading of certain diseases such as malaria, the flooding of major cities, a greater risk of heat stroke for individuals, and poor air quality.

Climate change is very likely having an impact now on our planet and its life, according to the latest installment of a report published by the Intergovernmental Panel on Climate Change (IPCC). And the future problems caused by rising seas, growing deserts, and more frequent droughts all look set to affect the developing world more than rich countries, they add. The report is the second chapter of the IPCC's Fourth Assessment -- the most comprehensive summary yet of research into the causes and effects of climate change. To read more, visit Effects of climate change tallied up.


--------------------------------------------------------------------------------
Factors
Greenhouse Gases
The increase in greenhouse gases caused by human activity is often cited as one of the major causes of global warming. These greenhouse gases reabsorb heat reflected from the Earth's surface, thus trapping the heat in our atmosphere. This natural process is essential for life on Earth because it plays an important role in regulating the Earth's temperature. However, over the last several hundred years, humans have been artificially increasing the concentration of these gases, mainly carbon dioxide and methane in the Earth's atmosphere. These gases build up and prevent additional thermal radiation from leaving the Earth, thereby trapping excess heat.

Solar Variability & Global Warming

Some uncertainty remains about the role of natural variations in causing climate

change. Solar variability certainly plays a minor role, but it looks like only a quarter of the recent variations can be attributed to the Sun. At most. During the initial discovery period of global warming, the magnitude of the influence of increased activity on the Sun was not well determined.

Solar irradiance changes have been measured reliably by satellites for only 30

years. These precise observations show changes of a few tenths of a percent that depend on the level of activity in the 11-year solar cycle. Changes over longer periods must be inferred from other sources. Estimates of earlier variations are important for calibrating the climate models. While a component of recent global warming may have been caused by the increased solar activity of the last solar cycle, that component was very small compared to the effects of additional greenhouse gases. According to a NASA Goddard Institute for Space Studies (GISS) press release, "...the solar increases do not have the ability to cause large global temperature increases...greenhouse gases are indeed playing the dominant role..." The Sun is once again less bright as we approach solar minimum, yet global warming continues.

--------------------------------------------------------------------------------
Trends & Effects; Scientific Studies

The United Nations Intergovernmental Panel on Climate Change has been studying global warming for years. Their most recent report, issed in February 2007, (see Climate Change 2007: The Physical Science Basis (summary for policymakers), U.N. Report Confirms Human Activity to Blame for Earth's Warming Climate (from Voice of America), and Intergovernmental Panel on Climate Change), concludes that "The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous exide are primarily due to agriculture." The report goes on to note that these findings come with a "very high confidence rate [words emphasized in italics in the report summary] that the globally averaged net effect of human activities since 1750 has been one of warming."
The primary place where scientific studies related to global warming are reported is the American Geophysical Union (AGU). Based on the outcome from a considerable number of studies in various fields related to global warming, the AGU has issued a statement: Human Impacts on Climate.
The American Meteorological Society, which promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences, has also just issued a statement on global changes.

--------------------------------------------------------------------------------
Frequently Asked Questions
Where is the data that show global warming is largely attributed to the increase of greenhouse gases (primarily carbon dioxide and methane) in the Earth's upper atmosphere caused by human burning of fossil fuels?
To what extent does the Sun's variability affect and/or cause global climate change?
My spiritual leaders disagree with the scientists, how do I determine whom to believe?