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Animation of sea surface temperatures

  Source: NASA/Goddard Space

Flight Center Scientific Visualization Studio
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What are the basic causes of climate change?

What is the difference between weather and climate?

Is there a connection between human activities and climate change? 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.

This section, Climate Change 101, provides a quick overview of the factors causing temperatures to rise and the climate to change. Find more resources with these climate links and learn about the Greening of the Aquarium as well as our national leadership in global climate change education.

Climate basics

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 1.5 degrees higher than they were at the start of the 20th century, and have risen about 1 degree 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 8.6 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 emissions of heat-trapping gases released to the atmosphere when fossil fuels are burned. Carbon dioxide is the most significant of these gases; CO2 levels are at their highest in 650,000 years.

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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. This is why it’s often said that “Climate is what you expect; weather is what you get.”

People expect the weather to change and experience those changes daily. It’s harder to see how climate is changing because climate is measured over many years rather than as single events. Like weather, climate may change differently in different places. Unlike weather, climate represents trends made up of all the weather variables in a region. Changes in the trends can be subtle, but over time they indicate that what is “normal” is shifting.

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2 Billion Years of Climate Change

Chart of two billion

years of climate change


(Click image to enlarge)


Is there a connection between human activities and climate change?

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.

In its most recent five-year assessment, the IPCC confirmed again that Earth’s climate is changing 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 strong consensus throughout the scientific community on this point.

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What is global warming?

Burning large amounts of fossil fuels — coal, oil, natural gas – is a root cause of global warming. Burning releases heat-trapping gases to the atmosphere. The gases, especially CO2, act like a blanket that retains heat and restricts the rate at which Earth’s surface can radiate heat to space. 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 result is global warming. Today, current levels of carbon dioxide in the atmosphere are higher than at any time during the last 800,000 years.

“Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.” — Quotation from IPCC 5th Assessment Report Summary for Policymakers

Earth has warmed at an unprecedented rate over the last hundred years and particularly over the last two decades. All of the top 10 warmest years on record have occurred since 1998. For nearly 300 consecutive months, each month has been warmer than its historical average. 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.

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Maps of the projected late 21st century annual
mean surface temperature change, annual

mean precipitation change, Northern Hemisphere
September sea ice extent and change in ocean
surface pH from the IPCC
(Click image to enlarge)


What do scientists predict in terms of climate change?

In 2014, 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 2014 report predict that by 2100, average global surface temperatures will likely rise by an additional 2 to 8.6 degrees F (1.1 to 4.8 degrees C) above the 1986-2005 average. This temperature increase will be accompanied by other environmental changes such as an increase in global sea level by up to 1–2 feet.

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Where will the impact be greatest?

During the remainder of this century, different locations will experience different levels of 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 average temperature would resemble that of Charlotte, North Carolina (a 6 degree increase) or Atlanta, Georgia (a 10 degree increase).

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On September 13, 2013, Arctic sea ice reached its
likely minimum extent for the year. It was the sixth
lowest* in the satellite record, and reinforces
the long-term  downward trend in Arctic ice extent.

Source: National Snow and Ice Data Center

(Click image to enlarge)

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 necessary 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 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 percent of Earth’s total water volume), air contains enough water molecules to absorb about 70 percent of incoming sunlight, thereby warming the atmosphere.

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Chart from the White House Initiative on

Global Climate Change

(Click image to enlarge)

What is the greenhouse effect?

Water vapor and trace gases keep Earth about 54 degrees F warmer than it would be without them. This function is often called the greenhouse effect, and the gases that cause it are known as greenhouse gases.

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.

When we burn fossil fuels, we release additional CO2 that builds up and traps heat that would otherwise escape. This human-caused blanket effect leads to warming of the planet, disrupting the atmospheric balance that keeps the climate stable.

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What are fossil fuels?

Fossil fuels are the carbon-rich remains of terrestrial plants, marine phytoplankton and zooplankton that have been buried and compressed under sediments for millions of years. Under certain conditions, the remnants of these organisms turn into coal, oil or natural gas (methane). Burning fossil fuels that have been mined from deep in the earth or seabed returns ancient fossil carbon, which has been out of circulation, to the atmosphere. In addition, large amounts of methane are generated by anaerobic digestion of wastes at sewage treatment plants, dumps and stockyards as well as by cattle.

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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 have emissions grown in recent years?

Emissions of greenhouse gases have grown at an accelerating rate in recent decades. In 2011 the annual rate of emissions was about 54 percent higher than the rate in 1990. Consequently, the amount of CO2 in the atmosphere has increased by 10% in the past two decades.

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 2010, the average temperature on the earth’s surface increased by about 0.8 degree C (1.4 degrees F). Atmospheric CO2 continues to rise each year.

Once released, greenhouse gases remain in the atmosphere until they are either absorbed by plants or animals, dissolved into the ocean, or degraded by sunlight or by chemical reactions with other molecules. 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.

Although China is now the largest emitter of greenhouse gases, per capita emissions in the U.S. are substantially higher than in China. It is also important to note that the majority of cumulative emission to date have come from developed countries.

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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, pale-colored 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.

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