The Twentieth-Century Warming

We will now take a look at the temperature record since 1860. This record is constructed from actual measurements and thus can be considered accurate. According to the first figure below, the global average surface temperatures on Earth have risen by about 0.8°C (1.4°F) since 1860. The red bars indicate the global average temperature each year relative to the average temperature over the period 1961-1990. The blue line is a smoothed version of the plot drawn to highlight trends in temperature. There is some uncertainty in determining global average temperatures from the available measurements due to instrumental errors and the fact that measurements are quite sparse over some areas of the world, but even the most conservative estimates indicate that the surface temperature has risen at least 0.5°C (0.9°F) since 1860.

The second figure below shows the temperature differences for the year 2000 compared to the 1961-1990 average temperature. Even though the average global temperature for 2000 was about 0.25°C higher than the 1961-1990 average, note that some regions of the world showed much larger warmings (over 2°C in some areas), while other regions actually had lower temperatures (over 1°C in some areas) for the year 2000. This again shows that even if the global average temperature goes up, there will be quite a bit of variability on regional scales.

Temperature Trends Over the Past Five Centuries Reconstructed from Subsurface Temperatures

Before discussing the details of the recent, measured temperature trends, we will attempt to place the twentieth century in a more historical perspective. Temperature changes that occur at the Earth's surface propagate slowly downward into the rocks beneath the surface. Thus, rock temperatures at shallow depths provide evidence of changes that have occurred at the surface in the recent past.

Analyses of underground temperature measurements from more than six hundred boreholes from all continents except Antarctica show that:

• The global average ground surface temperature has increased by at least 0.9 degrees F (0.5 degrees C) in the 20th century. This is a conservative estimate of the century-long rate of warming because many boreholes used in this study were drilled and logged 15 to 20 years ago, prior to the extraordinary warming of the final decades of the 20th century.
• The 20th century has been the warmest century of the last five centuries.
• The present-day mean temperature is at least 1.8 degree F (1.0 degree C) warmer than five centuries ago; of this change over half has occurred in the 20th century alone.

The five-century change can be thought of as a time- and space-averaged overall measure of climate sensitivity (the response of the global mean surface temperature to changes in climate forcing factors over this time interval. This includes natural forcing factors, e.g., volcanic erruptions and anthropogenic forcing factors, e.g., greenhouse gas emissions and deforestation.) These interpretations provide an historical perspective that indicates that the 20th century has not been just another century in terms of temperature change. In the context of the five-century interval investigated, the 20th century is clearly unusual.

However, if we go back another 500 years to the Medieval warm period (or Little Climatic Optimum), current global average temperatures are only slightly warmer than that period. The biggest difference today is that the rate of warming (0.8°C in 100 years) is much faster than the rate of warming that lead up to the Medieval Warm Period. And perhaps more significantly, the rate of increase from 1980 through today is much faster than that and shows no signs of slowing.

Observed Temperature Changes in the 20th Century

We need to answer the question: Is the recently measured warming of global average surface temperatures caused by the increasing amounts of greenhouse gases in the atmospere, or is it part of a natural cycle?

According to climate models the warming is largely due to the additional greenhouse gases. If greenhouse gas concentrations are left fixed at pre-industrial levels, the models show little if any warming during the twentieth century, while the same models do predict warming when they include the observed increases in greenhouse gases. However, most models are unable to reconstruct the irregular pattern of the observed warming since 1860.

The irregular warming has been used by global warming opponents as evidence that natural climate fluctuations dominate over the effects caused by increased greenhouse gases. In particular, they point to two periods:

1. The rather large warming that occurred from 1910-1940. This warming is too rapid and too large to be fully explained by the rather small increases in greenhouse gas concentrations that occurred during this period.
2. The cooling trend that occurred from about 1940 to the late 1970s. This cooling occurred even though greenhouse gas concentrations continued to increase during this period.

Proponents of global warming contend that even during the general warming trend expected as greenhouse gases increases, there will be fluctuations along the way. The cooling period from 1940-1970s can possibly be explained by high aerosol concentrations in the atmosphere during this period related to "dirty" coal burning. "Dirty" coal burning has since been curtailed due to concerns about acid rain. Once the aerosols cleared, we started to again see global warming caused by increasing greenhouse gases.

Based on the historical temperature record, the warming since 1980 appears to be the fastest rate of increase over the last 10,000 years. Including 2004, the warmest 11 years in the measured temperature record (since 1860) have been recorded since 1990. For many, this is enough evidence that the recent temperature increases are not part of some natural cycle, and must be due to human activity, caused by increasing greenhouse gas concentrations in the atmosphere.

What to do?

The global warming issue has been labeled the "Science of Uncertainty". This makes it difficult for all people to come together and agree on what, if anything, should be done. Scientists are working on reducing the uncertainties, but due to the complex nature of the climate system, we should not expect certain answers to all questions. We have to make decisions which weigh uncertain risks against the costs of taking action.

If nothing else, the human race is in the process of performing a huge experiment on global climate by increasing greenhouse gas concentrations. The concentration of carbon dioxide in the atmosphere is probably greater now than at any time over the last one million years. The outcome of this experiment is uncertain. Are we ready and willing to take the risks? So far the answer is yes, because in spite of all the talk about global warming, the rate of greenhouse gas emissions continue to increase. This is especially relevant for the people of the United States who by far emit more greenhouse gases per person than any other nation on Earth.

There are many diverse opinions on how to deal with the global warming issue. Each of us needs to make up our own mind. Are you willing to make sacrifices now to reduce the potential (and uncertain) consequences of global warming?