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Human Influence on Climate Change

Evidence that human activities influence the global climate system continues to accumulate. Data indicate that Earth's surface temperature is rising. This increase can be attributed, in part, to human-caused increases in greenhouse gases such as carbon dioxide. It is becoming apparent that these climatic changes are negatively affecting physical and biological systems worldwide.

Carbon dioxide and other pollutants result from the burning of coal to produce electricity.
Warren Gretz, NREL
Electricity generated at power plants is carried by power lines to users, sometimes hundreds of miles away.
Warren Gretz, NREL
The burning of gasoline by automobiles releases carbon dioxide and other types of air pollution that contribute to global climate change.
Dr. Edwin P. Ewing, Jr.,CDC
Pollution created by human activities is particularly evident around the world's largest cities.

Twentieth Century Warmest of the Past Ten Centuries

Understanding climate requires long-term measurements of Earth’s atmosphere. Direct measurements of global temperatures have been recorded for only the past 140 years or so. To extend these records back in time, scientists have learned that certain natural processes preserve indirect evidence of past atmospheric conditions. Using data from glacial ice cores, tree rings, lake-bottom sediments, and ocean corals, they can estimate global temperatures going back thousands of years.

The graph below shows reconstructed temperature data for the Northern Hemisphere for the past 1000 years. Instead of actual temperatures, the graph shows annual temperature anomalies—differences from the average temperature for each year. The gray lines are error bars, showing the possible degree of error in each measurement.

Examine the graph to interpret how the temperature has changed over the last 1000 years.

Reconstructed Northern Hemisphere temperature anomalies for the past 1000 years.

1. Why do you think the error bars become smaller in the more recent part of the graph?

Major Findings about Climate Change

Scientists have noted several significant trends in these data.

  • 20th century surface temperatures were the warmest of any century in the past 1,000 years.
  • The rate and duration of warming in the 20th century is greater than in any of the previous nine centuries.
  • The 1990s were the warmest decade in the past 1,000 years.
  • 1998 was the warmest year in the past 1,000 years.
  • The 11 warmest years of the past 140 have occurred since 1983, with the warmest years being 1998, 1997, and 1995.
  • Average global surface temperatures have increased 0.4 to 0.8 degrees Celsius over the past 140 years.

Another type of data that scientists can reconstruct is the amount of carbon dioxide (CO2) in the atmosphere.

Major Findings about Greenhouse Gases

  • The concentration of CO2 in the atmosphere has increased by 31% since 1750.
  • The current CO2 concentration is at the highest level in the last 420,000 years.
  • Atmospheric CO2 is increasing at a faster rate today than at any time over the past 20,000 years.
  • The current concentration of methane (CH4) is at its highest level in the last 420,000 years.

The graphs below show the changes in concentration of three different greenhouse gases over the past 1,000 years.

2. When did concentrations of these greenhouse gases start rapidly increasing? What might have caused these increases?

Each of these greenhouse gases affects climate differently. Some gases can trap the sun’s energy better than others.
The degree to which a gas traps solar energy is called its radiative forcing value. Radiative forcing values are indicated
on the right axis of the graphs. The higher the radiative forcing value, the stronger its effect on global warming.

3. Which gas affects global climate most? Which has the least effect?

Major Findings about Climate Change

  • Reconstructions of the past 1,000 years of climate data indicate that the warming which has occurred over the past 100 years was unlikely to be entirely natural in origin.
  • Most scientific studies estimate that the rate of global warming over the past 50 years coincides with increasing concentrations of greenhouse gases.

A report issued in 2001 by the Intergovernmental Panel on Climate Change (IPCC) stated that it is "virtually certain" that emissions of carbon dioxide due to fossil fuel burning are the main cause of increasing atmospheric carbon dioxide during the 21st century.

The graph below shows temperature records and atmospheric carbon dioxide concentrations over the past 1,000 years.

Adapted from USGCRP
Composite graph showing temperature anomaly and carbon dioxide level over the past 1,000 years.



4. Predict the effect that continued increases in atmospheric carbon dioxide will have on global temperatures.

Scientists develop mathematically-based climate models to help predict future climate changes. Each model uses different assumptions about the future to predict how atmospheric CO2 levels and temperatures will change.

The variables in each model include:

  • Population growth rate
  • Economic development
  • Energy use
  • Efficiency of energy use
  • Mix of energy technologies

The graph below shows the results from three climate models used by the IPCC, with predictions starting in 1990 and ending in the year 2100. In all three, the global population rate rises during the first half of the century, then declines.

  • The A1B model assumes rapid economic growth and increased equity—the reduction of regional differences in per-person income. New and more efficient technologies are introduced, without relying heavily on a single energy source.
  • The A1F1 model is the same as A1B, but assumes the continued use of fossil fuel-intensive technologies.
  • In the B1 model, the world moves rapidly from a producer-consumer economy toward a service and information economy. There is a reduction in the use of raw materials, and an emphasis on clean and efficient technologies and improved equity.

Other models have been developed, each based upon a different set of assumptions.

Adapted from IPCC, Third Assessment Report on Climate Change, 2001.
Global temperature increases predicted by three different IPCC climate models.

Although differing in degree, these three climate prediction models show similar trends:

  • The projected rate of global warming in the future is much larger than the rate of global warming during the 20th century.
  • Predicted rates of global warming are greater than any seen in the past 10,000 years.

5. Why do scientists develop numerous models rather than rely on just one?

6. Based upon all the models shown, what range of temperature increase is expected to occur by 2100? 

Atmospheric warming is not evenly spread around the world. Analysis of historical records suggests that the temperature of land areas will increase more rapidly than the global average. The greatest warming has occurred in the high northern latitudes, especially in northern Canada and Alaska. The map below shows differences between surface temperatures in 2001 and average global temperatures calculated for the period from 1951 to 1980.

Global temperature anomalies for 2001.

7. Which hemisphere (north or south) appears to have experienced the most significant temperature changes? Suggest why this is so. 

Complete the Climate Change Impact Assessment Table on your answer sheet using information from the following Web pages. You can find additional information by following the links on Step 8 of this investigation.

  !   Click the images below to access pages with additional information about each topic.

California Dept. of Fish & Game
Coastal Areas
Martos Hoffman
Agricultural Research Service

Water Resources

Martos Hoffman

Species and Natural Areas

Use information you have already collected as well as additional information from the links below to answer the following questions.

8. Which global climate change impact do you think poses the greatest risk to humanity? Cite evidence for your answer.

9. Suggest two actions that humanity might take to decrease its influence on global climate change.

Areas of the southeastern United States projected to be below water (red) if sea level rises one, two, four, and eight meters.