Measurements from tree rings, boreholes and retreating glaciers provide sufficient evidence that the surface temperature of the Earth in the last few decades of the 20th century was higher than any comparable period for the past 400 years, according to a report released Thursday by the NRC that both reinforces and curtails conclusions in an earlier study.
The data also indicates that many parts of the Earth in the past 25 years were hotter than anywhere else on the globe than any other 25-year period since 900 A.D.
The cause? A lot of it is from human activity. "Surface temperature reconstructions for periods before the Industrial Revolution--when levels of atmospheric greenhouse gases were much lower--are only one of multiple lines of evidence supporting the conclusion that current warming is occurring in response to human activities, and they are not the primary evidence," stated a press release that accompanied the study.
The report was requested by Congress after a controversy arose last year overpublished by climatologist Michael Mann and his colleagues in the late 1990s. Mann concluded that the warming of the Northern Hemisphere in the last decades of the 20th century was unprecedented in the past thousand years. The 1990s, Mann added, was the warmest decade ever and 1998 was the warmest year ever.
The researchers behind the new report said they had "high confidence" in the conclusion that global temperatures were at a peak over the last 400 years. However, because of the spotty nature of data earlier, particularly for the Southern Hemisphere, it had less confidence in Mann's conclusions for the period from 900 to 1600 and "very little confidence" in conclusions about previous years. As a result, the researchers had little confidence in the conclusion that the previous decade was the hottest ever and 1998 was the hottest year.
Still, the researchers found Mann's conclusion that temperatures in the Northern Hemisphere in the last few decades represented a high for the last 1,000 years to be plausible. None of the climate reconstructions in the new study indicate that temperatures were warmer during medieval times than during the past few decades. (The Spanish Armada took place in 1588.)
Scientists do not have temperature records going back hundreds of years. Temperature records actually go back only 150 years. To determine surface temperature, researchers examine corals, ocean and lake sediments, ice cores, cave deposits and documentary sources such as historic drawings of glaciers.
Climactically speaking, the last 400 years have been sort of a roller coaster ride. From 1500 to 1850, the Earth was wrapped in what climatologists call the little ice age, where temperatures dipped. Since 1850, they have been rising. This last period also coincides with the Industrial Revolution.
In the past century, the temperature of the Earth has risen about 1 degree Farenheit. Scientists, however, believe the rate of temperature increase will accelerate. Ice reflects sunlight, so as glaciers retreat, more heat gets absorbed by the Earth. The greenhouse gas layer will also grow from burning fossil fuels, thereby trapping more heat.
It doesn't sound like much, but relatively small fluctuations like that can have a significant impact over a wide surface. A 5 degree to 8 degree rise in global temperature in the next 100 years, expected by many scientists, could cause sea levels to rise half a meter and put some island nations underwater.
"Six to 10 degrees is the difference between the temperature today and the temperature of the deepest ice age,", director of Lawrence Livermore Labs, said recently.