Climate Through Human History

The Twentieth-Century Warming

Sustained warming began at the end of the 19th century. Since that time temperatures have risen by about 0.7°C.

Temperature Trends Over the Past Five Centuries Reconstructed from Subsurface Temperatures

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 about half has occurred in the 20th century alone, and 80% has occurred since the year 1800.

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).

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.

Observed Temperature Changes in the 20th Century

Changes in Temperature Extremes

One of several pieces of evidence used to gauge climate change is an increase in extreme climate events.

Evidence from paleoclimatic data suggests that current temperatures are the warmest in the past 1000 years, and more recent observations of global temperatures indicate that temperatures have warmed approximately 0.6 degrees C (1.1 degrees F) over the past 100 years.

However, an important piece of information related to understanding the sensitivity of the climate system to increases in carbon dioxide and other greenhouse gases, is the rate of warming.

Since 1990, society has witnessed some of the warmest years on record. Nine of the 10 warmest years have occurred since 1990, including 1999 and 2000, when the cooling influence of the tropical Pacific La Niņa weather pattern contributed to a somewhat lower global average temperature. 1998 was the warmest year on record. 2004 was the 4th warmest year on record.

Furthermore, embedded within the temperature records of 1997 and 1998, was a string of sixteen consecutive months where the monthly global temperature broke the previous record for that month. In fact, during much of 1998, monthly records were broken that had just been set the previous year.

Changes in Daily and Yearly Temperatures in the U.S.

The average climate warming observed within the continental United States is about 1 degrees F (0.5 degrees C) over the past 100 years. It has been shown that most of the warming represented by the global average temperature is associated more with warming in minimum temperatures (nighttime lows) than in maximum temperatures (daytime highs).

Analysis of changes in the number of days where the minimum temperature dips below freezing indicates that, for the U.S. as a whole, there has been a decline of two fewer days per year where temperatures fall below 0 degrees C (32 degrees F).

However, since the southeastern U.S. is one of the few placeqs in the world that has exhibited a cooling, there has been an increase in this region in the number of days below freezing. In contrast, the western U.S. has witnessed significant decreases in the number of days below freezing.

Global, annual-mean radiative forcings (Wm-2) due to a number of agents for the period from pre-industrial (1750) to present (late 1990s; about 2000) The height of the rectangular bar denotes a central or best estimate value, while its absence denotes no best estimate is possible. The vertical line about the rectangular bar with "X" delimiters indicates an estimate of the uncertainty range, for the most part guided by the spread in the published values of the forcing. A vertical line without a rectangular bar and with "O" delimiters denotes a forcing for which no central estimate can be given owing to large uncertainties. A "level of scientific understanding" index is accorded to each forcing, with high, medium, low and very low levels, respectively. This represents the subjective judgment about the realibitlity of the forcing estimate, involving factors such as the assumptions necessary to evaluate the forcing, the degree of knowledge of the physical/chemical mechanisms determining the forcing, and the uncertainties surrounding the quantitative estimate of the forcing. The well-mixed greenhouse gases are grouped together into a single rectangular bar with the individual mean contributions due to CO2, CH4, N2O and halocarbons shown. Fossil fuel burning is separated into the "black carbon" and "organic carbon" components with its separate best estimate and range. The sign of the effects due to mineral dust is itself an uncertainty. The indirect forcing due to tropospheric aerosols is poorly understood. The same is true for the forcing due to aviation via its effects on contrails and cirrus clouds. Only the "first" type of indirect effect due to aerosols as applicable in the context of liquid clouds is considered here. The "second" type of effect is conceptually important, but there exists very little confidence in the simulated quantitative estimates. The forcing associated with stratospheric aerosols from volcanic eruptions is highly variable over the period and is not considered for this plot. All forcings shown have distinct spatial and seasonal features intended to give, in a relative sense, a first -order perspective on a global, annual mean scale and cannot be readily employed to obtain the climate response to the total natural and/or anthropogenic forcings.