A Warming Planet

Temperature varies over different time scales and geographic areas. The following provides a brief review of temperature patterns over the past 800,000 years.

Temperature Over 800,000 Years

Scientists have reconstructed temperature anomalies by analyzing the ice cores drilled in Antarctica and Greenland. The Dome C ice core in Antarctica provided the data scientists needed to reconstruct temperature anomalies as far back as 800,000 years. From an analysis of these ice cores, scientists discovered that there were approximately nine glacial-interglacial cycles over the past 800,000 years and that we are currently in an interglacial period.

Scientists also analyzed the Summit Greenland Ice Sheet Project Two data of the Northern Hemisphere’s climate between 50,000 and 10,000 years ago. Theses ice cores revealed rapid warming events, known as Dansgaard-Oeschger (D-O) cycles. Scientists think that these cycles were not global, but more localized, and caused by changes to deep-sea currents. These deep-sea currents redistribute heat between the equatorial and polar regions of the planet. From the Greenland data, scientists concluded that climate can abruptly change over a period of several decades.

Proxy data from tree rings, coral, and other temperature data is used to reconstruct the temperature anomalies over the past 2,000 years. The evidence from the climate proxy data indicated that Europe experienced a 300-year warm period (known as the Medieval Warm Period) from 950 to 1250, explained by more solar activity (increased sunspots) and less volcanic aerosols (decreased albedo). Europe then experienced a cold period (known as the Little Ice Age) from 1300 to 1850, explained by lower solar activity, increased volcanic eruptions, and a slowing thermohaline current in the North Atlantic Ocean.

Interglacial Periods over the last 800,000 years
Interglacial Periods Over the Last 800,000 Years
 

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Temperature Since the Industrial Revolution

The past 125-year instrumental record is derived from three different climate centers that collect monthly temperature data from the land and oceans using instruments (such as thermometers at thousands of weather stations and on ships, satellites, and weather balloons). These three centers are NASA’s Goddard Institute for Space Studies (GISS), NOAA’s National Climate Data Center (NCDC), and the United Kingdom’s Met Office in conjunction with the Climate Research Unit (CRU) at the University of East Anglia (HadCRUT3). These centers compute the global mean temperatures (GMT) and is published by several government and university research centers.

Since 1880, global mean temperature has risen by 0.8°C (1.4°F). However, this increase in temperature was not linear. Between the 1910s and the 1940s, temperature increased and then decreased until the 1970s. Scientists think that aerosol (airborne-particle) pollution may have caused the cooling effect shown in the temperature anomaly data between the 1940s and 1970s. Aerosols scatter visible light from the sun, thus cooling the earth. In the early 1970s, aerosol pollution decreased because of the sulfur filters placed on power station smoke stacks, and temperatures began to rise. Warming has accelerated over the past few decades. In the “Temperature Over Time Module,” you compared the rate of global mean temperature increase over recent decades (1.6 C or 2.9F/100 years) to that since the Industrial Revolution (0.6C or 1.1F/100 years). 

Since the Industrial Revolution, the surface temperature over land has increased more rapidly than that of the oceans. This is due to the higher heat capacity of oceans as compared to the land. The polar regions have warmed more than the tropical regions. This is happening because, as the brighter ice (with higher albedo and reflectivity) melts, the darker water (with a lower albedo) can absorb more heat and intensify the warming.

Five-Year Global Temperature Anomalies from 1880 to 2013
Video Source: NASA

A warming of almost 1°C (1.8°F) over the last century may seem small, but 4°C (7.2°F) is the approximate difference between a glacial and interglacial climate and just 1°F causes a fever in humans.


Can You Trust the Temperature Record?

Decadal Land Average Temperatures and Climate Model

Image Source: Berkeley Earth

In 2010, University of California Berkeley physicist, Richard Muller, along with other Berkeley physicists and statisticians initiated a study, known as the BEST (Berkeley Earth Surface Temperature) temperature study. The purpose of the study was to challenge previous global temperature studies. The consensus among scientists was that the data for land and sea surface temperature showed increased warming, but Muller wondered if these findings were exaggerated. His team set out to conduct a new analysis, using data from five times as many temperature stations as the historic record.

The figure shows a comparison between Berkeley Earth Surface Air Temperature and a collection of Global Climate Model (GCM) results from 2007 IPCC Report (AR4). The GCM results were created by sampling the entire GCM field at the same locations and times as the Berkeley Earth average. The GCM average is shown in red and the Berkeley average is in black.

The research shows a warming trend similar to what had been shown in previous studies. Read more about the Berkeley Earth Surface Temperature (BEST) at http://berkeleyearth.org/.