Global average surface temperatures pushed 2005 into a virtual tie with 1998 as the hottest year on record.[1] For people living in the Northern Hemisphere—most of the world's population—2005 was the hottest year on record since 1880, the earliest year for which reliable instrumental records were available worldwide.

Because most global warming emissions remain in the atmosphere for decades or centuries, the energy choices we make today greatly influence the climate our children and grandchildren inherit. We have the technology to increase energy efficiency, significantly reduce these emissions from our energy and land use, and secure a high quality of life for future generations. We must act now to avoid dangerous consequences.

The year 2005 exceeded previous global annual average temperatures despite having weak El Niño conditions at the beginning of the year and normal conditions for the rest of the year. (El Niño is a period of warmer-than-average sea surface temperatures in the east-central Pacific Ocean that influences weather conditions across much of the globe.) In contrast, the record-breaking temperatures of 1998 were boosted by a particularly strong El Niño.

The record heat of 2005 is part of a longer-term warming trend exacerbated by the rise of heat-trapping gases in our atmosphere that is due primarily to our burning fossil fuels and clearing forests. Nineteen of the hottest 20 years on record have occurred since 1980 (see table).

The record surface temperatures of the past 20 years reinforce other indications that global warming is under way. For example, the observed rise in average surface temperatures has been accompanied by warming of the atmosphere and oceans, and increased melting of ice and snow. These observations, summarized briefly below, paint a consistent picture of widespread and significant changes in global climate over the past several decades.

Evidence of Twentieth Century Global Warming

Warming of the Troposphere

A 2005 re-analysis of satellite observations of temperature trends in the troposphere—the layer of atmosphere extending about five miles up from Earth's surface—uncovered errors in previous studies. The updated studies show that air temperatures have increased in the past 20 years or so, consistent with the fundamental understanding that increases in surface temperatures are accompanied by increases in air temperatures above the surface. The new results are also consistent with recent increases in tropospheric water vapor, which would be expected when rising temperatures accelerate ocean evaporation.

By comparing several sets of data from satellites and weather balloons, these new atmospheric analyses account for drifts in satellite orbits and changes in instrumentation over the measurement period. While the corrected results represent only one of several pieces of global warming evidence, they are important in part because the earlier flawed analysis has often been cited.

Melting of Snow and Ice

Further evidence of widespread warming comes from observations of seasonal snow and frozen ground coverage.

The extent and duration of frozen ground have declined in most locations. Snow cover in the Northern Hemisphere has declined about five percent over the past 30 years, particularly in late winter and spring, and the freezing altitude has risen in every major mountain chain. Alpine and polar glaciers have retreated since 1961, and the amount of ice melting in Greenland has increased since 1979. Over the past 25 years, the average annual Arctic sea ice area has decreased by almost five percent and summer sea ice area has decreased by almost 15 percent. The collapse of the Larsen Ice Shelf off the Antarctic Peninsula appears to have no precedent in the last 11,000 years.

Melting of the Greenland Ice Sheet

Warming of the Oceans

Oceans comprise 97 percent of Earth's water. They have an average depth of approximately 13,000 feet (4 kilometers). It takes a great deal of heat to raise the temperature of this huge body of water, and the oceans have absorbed the bulk of Earth's excess heat over the past several decades. (See figure, "Estimates of Earth's Heat Balance.") From 1955 to 1998, the upper ~9,800 feet (3,000 meters) of the ocean have warmed by an average 0.067 degrees Fahrenheit (0.037 degrees Celsius).

Estimates of Earth's Heat Balance

If only a small fraction of the heat currently stored in the oceans were released, it would significantly warm the atmosphere and melt the world's glaciers. For a hypothetical example, if the average temperature of the world's oceans increased by 0.18 degree Fahrenheit (0.1 degree Celsius) and this heat was transferred instantly to the atmosphere, the air temperature would increase by about 180 degrees Fahrenheit (100 degrees Celsius). In reality, the circulation and redistribution of this excess heat is a slow process. Even if we could maintain atmospheric CO2 concentrations at today's level, stored heat released by the oceans will cause Earth's average surface temperature to continue rising approximately one degree Fahrenheit (half a degree Celsius) in the coming decades. To put this into perspective, this is the same as the global average temperature rise that occurred over the last century. The warming of the oceans and the melting of glaciers worldwide have already caused sea levels to rise during the twentieth century, and most of this rise has come in the past few decades.

The Role of Natural Variability

Human-induced warming is superimposed on natural processes to produce the observed climate. Because these natural fluctuations (which are always present) play a role in determining the precise magnitude and distribution of temperature in a particular year, record warmth in any one year is not in itself highly significant. What is noteworthy, however, is that global average temperatures experienced a net rise over the twentieth century, and the average rate of this rise has been increasing. When scientists attempt to reproduce these twentieth century trends in their climate models, they are only able to do so when including human-produced heat-trapping emissions in addition to natural causes.

[1] The years 1998 and 2005 are so similar (i.e., within the error range of the different analysis methods or a few hundredths of a degree Celsius) that independent groups (e.g., NOAA, NASA, and the United Kingdom Meteorological Office) calculating these rankings based on reports from the same data-collecting stations around the world disagree on which year should be ranked first. Annual global rankings are based on combined land-air surface temperature and sea surface temperature since 1880.

Dr. Marcia Baker (professor emeritus in Earth and Space Sciences and Atmospheric Sciences at the University of Washington) prepared this summary with input from Dr. Brenda Ekwurzel (climate scientist at the Union of Concerned Scientists).

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