Animation of sea surface temperatures
Source: NASA/Goddard Space
Flight Center Scientific Visualization Studio
What are the basic causes of climate change?
What is the difference between weather and climate?
How is the climate changing? What is global warming and what are scientists’ predictions for the planet in our lifetimes and beyond? Where will the impact be greatest? How does water play a role in heating and cooling our climate? What is the greenhouse effect and what are fossil fuels? How fast have greenhouse gas emissions grown in recent decades? And what forces actually cool the atmosphere? This section on Climate 101 provides a quick overview of the factors causing temperatures to rise and the climate to change.
The world’s climate is changing, and the changes will have an enormous impact on our planet’s people, ecosystems, cities, and energy use. Average global air temperatures are already 1.4 degrees higher than they were at the start of the 20th century, and have risen about 1.1 degree F over just the last 30 years.
According to the latest report from the Intergovernmental Panel on Climate Change (IPCC), average global temperatures are likely to rise by another 2 to 11.5 degrees F by 2100. If we take aggressive action to reduce emissions, the temperature change could be modest. If we continue on our present course, however, the amount of change will be substantial. Most experts agree that the changes are anthropogenic — caused by humans — largely from human emissions of greenhouse gases that are the result of burning fossil fuels. The chief greenhouse gas is carbon dioxide and CO2 levels are at their highest in 650,000 years.
What is the difference between weather and climate?
Climate describes the average or typical conditions of temperature, relative humidity, cloudiness, precipitation, wind speed and direction, and other meteorological factors that prevail globally or regionally for extended periods. Weather describes the hourly or daily conditions that people experience each day. Which is why it’s often said that “Climate is what you expect; weather is what you get.”
People expect the weather to change, but have a harder time agreeing exactly how climate is changing. This is because climate change manifests itself differently in different places; because people tend to remember extreme events — such as very hot or cold days or hurricanes and droughts — more than subtle changes in trends; and because there have been disagreements between scientists about the magnitude, causes and effects of climate change.
Chart of two billion
years of climate change
(Click image to enlarge)
How is the climate changing?
There have been long-term and short-term climate cycles for hundreds of millions of years, as far back as these changes can be measured. Many of those climate changes were dramatic and rapid, some the result of impacts from comets or asteroids.
During 2006 and 2007, several major reports from the IPCC, founded on wide scientific agreement, catalyzed broad acceptance that Earth’s climate is changing rapidly and that human activities are the primary factors causing this change. Many of the factors that caused earlier changes continue to influence today’s climate, but human activities have now become the predominant force causing our climate to change very rapidly. There is now strong consensus throughout the scientific community on this point.
What is global warming?
Humans are creating climate change by burning large amounts of fossil fuels — coal, oil, natural gas. Another factor is deforestation; when forests are cut down or burned, they can no longer store carbon, and the carbon is released to the atmosphere. The gases, especially CO2, act like a blanket and restrict the rate at which Earth’s surface can radiate heat to space. The result is global warming. Today, current levels of carbon dioxide in the atmosphere are higher than at any time during the last 650,000 years.
Earth has warmed at an unprecedented rate over the last hundred years and particularly over the last two decades. Since 1992, each year has been one of the 20 warmest years on record. 2010 was the hottest year on record, worldwide. Exactly how much warmer the atmosphere gets will depend on how quickly and effectively people can substantially reduce the activities that are causing rising temperatures.
Variations of the Earth’s surface
temperature from 1000 to 2100 plus the seven
principal scenarios from the IPCC
(Click image to enlarge)
What do scientists predict in terms of climate change?
In 2007, the IPCC published a series of scenarios ranging from “business as usual/no actions taken” to “aggressive actions taken” to reduce climate change. Models based on these scenarios from the IPCC 2007 report predict that average global surface temperatures will likely rise by an additional 2 to 11.5 degrees F (1.1 to 6.4 degrees C) above the 1980s-1990s average by 2100. This temperature increase will be accompanied by other environmental changes such as an increase in global sea level up to two feet or more.
Where will the impact be greatest?
During the remainder of this century, different locations will experience greater or lesser increases in temperature, with the greatest impact toward the North Pole and the least increase toward the South Pole and in the tropics. As an example of what may be in store, New England’s temperature is projected to increase by 6 to 10 degrees F by 2100, in which case Boston’s climate would resemble that of Charlotte, North Carolina (a 6 degree increase) or Atlanta, Georgia (a 10 degree increase).
Multi-year Arctic sea ice
How does water heat and cool our climate?
Water is one of the key reasons that life has flourished on Earth, but not on the other planets in our solar system. Our planet is neither too near the sun, nor too far from it, but instead lies within a narrow habitable zone whose central feature is water in the form of liquid, vapor or ice. Water in its liquid form is a basis for all life and ecological function, and its transition to a gas (as water vapor) and a solid (as ice) also has important ecological and climatologic functions.
Water molecules also heat the atmosphere directly by absorbing sunlight. Even though the amount of water in the atmosphere at any one time is relatively small (equivalent to only 0.001% of Earth’s total water volume), air contains enough water molecules to absorb about 70% of incoming sunlight, thereby warming the atmosphere.
What is the greenhouse effect?
The greenhouse effect is a naturally occurring phenomenon that blankets the earth and warms it, maintaining the temperature that living things need to survive. Surprisingly, the atmosphere’s most abundant gases — nitrogen, oxygen, and argon — do not influence climate. Instead, it’s the molecules of trace gases, especially water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3) that strongly absorb infra-red radiation contained in sunlight, or emitted by land and water as they cool.
Just as greenhouses keep the air inside them warm, so water vapor and trace gases keep Earth about 54º F warmer than it would be without them. This retention of heat is called the greenhouse effect and the gases that cause it are known as greenhouse gases.
What are fossil fuels?
Fossil fuels are the carbon-rich remains of terrestrial plants (coal), marine phytoplankton and zooplankton (oil), or natural gas (methane) that has been buried and compressed under sediments for millions of years. Burning fossil fuels that have been mined from deep in the earth or seabed returns ancient fossil carbon as new CO2 to the atmosphere from the earth where they had been out of circulation. In addition, large amounts of methane have been generated by anaerobic digestion of wastes at sewage treatment plants, dumps and stockyards as well as by cattle.
This graph of atmospheric CO2 concentration
is known as the Keeling Curve,
after Charles Keeling, a climate scientist from
Scripps Institute of Oceanography, who was the
first to make precise annual measurements.
(Click image to enlarge)
How fast has greenhouse gas emissions grown in recent years?
Human emissions of greenhouse gases have grown at an accelerating rate in recent decades, increasing 70% between 1970 and 2004, according to the IPCC 2007 report. Consequently, the amounts of CO2 in the atmosphere have increased by 31%.
There is a strong correlation between the rise in global temperature and the increasing concentration of carbon dioxide in the atmosphere. As CO2 increased from 1850 to 2000, average atmospheric temperature increased by about 1 degree C. Atmospheric CO2 continues to rise each year.
Once released, greenhouse gases remain in the atmosphere until they are either absorbed by plants or animals or degraded by sunlight or by chemical reactions with other molecules. But molecules of CO2 remain in the atmosphere for approximately 100 years, which is why it is so hard to reverse global warming once it gets started.
China is now the largest emitter of greenhouse gases. The U.S. is number two, although per capita emission in the U.S. are substantially higher than China. It is also important to note that the U.S., Japan and European nations have contributed about two thirds of the greenhouse gases that have accumulated on the atmosphere since the advent of the Industrial Revolution.
What factors cool rather than heat the atmosphere?
Climate “forcings” are factors in the climate system that either increase or decrease the effects to the system. Positive forcings such as excess greenhouse gases warm the earth while negative forcings, such as the effects of most aerosols and volcanic eruptions, actually cool the earth.
Atmospheric aerosols include volcanic dust, soot from the combustion of fossil fuels, particles from burning forests and mineral dust. Dark carbon-rich particles such as soot from diesel engines absorb sunlight and warm the atmosphere. Conversely, exhaust from high-sulphur coal or oil produce light aerosols that reflect sunlight back to space, producing a cooling effect. Aerosols that form naturally during volcanic eruptions cool the atmosphere. Large volcanic eruptions can eject enough ash into the atmosphere to lower temperature for a year or more until the sulfate particles settle out of the atmosphere.