The paper found that 92% of these stations are positioned in sites that can cause errors of 1.8ºF (1ºC) or more.  For example, some stations are located over asphalt (making them hotter at certain times), and others are located in partial shade (making them cooler at certain times).
By comparing data from poorly positioned stations with other stations that are properly positioned, the study determined that the temperature irregularities in the poorly positioned stations cancel one another so that their average temperature trends are “statistically indistinguishable” from the properly positioned stations.
as “an increase in the average temperature of the Earth’s atmosphere,” either by “human industry and agriculture” or by natural causes like the Earth has “experienced numerous” times “through its history.” * Some writers use the phrases “global warming” and “climate change” to mean temperature changes strictly caused by human activity.   Other writers use adjectives such as “man-made” and “anthropogenic” to distinguish between human and non-human causes.  (“Anthropogenic” means “of human origin,” and “AGW” stands for “anthropogenic global warming.”) * Just Facts’ Standards of Credibility require the use of “language that is precise and unambiguous.” Hence, when human causes are stated or implied, this research uses terms like “man-made” and “human-induced.” * The greenhouse effect is a warming effect caused by certain gases that retain heat from sunlight. Without such gases, the average surface temperature of the Earth would be below freezing, and as explained by the , “life, as we know it, would not exist.” The global warming debate is centered upon whether added greenhouse gases released by human activity will overheat the Earth and cause harmful effects. * Human activities currently release about 37 billion metric tons of CO2 per year, which equates to about 5% of natural CO2 emissions.
Natural processes absorb the equivalent of all natural emissions plus about 57% of man-made emissions, leaving an additional 16 billion metric tons of CO2 in the atmosphere each year. † In permafrost regions, perennial snow accumulations trap air bubbles that leave records of past airborne CO2 concentrations,   and because regional CO2 concentrations vary by less than 10 parts per million over the Earth, these local records are globally representative.  * Instruments located on satellites can measure certain properties of oxygen that vary with temperature.
The report states that: some of the global warming since 1850 could be a recovery from the Little Ice Age rather than a direct result of human activities.
So it is important to recognize that natural variations of climate are appreciable and will modulate any future changes induced by man.
* This graph is called the “hockey stick graph” because the curve looks like a hockey stick laid on its side (click on the footnote for a graphic illustration). The red part of the curve represents modern instrument-measured surface temperatures, the blue represents proxy data, the black line is a smoothed average of the proxy data, and the gray represents the margin of error with 95% confidence.  * This graph has been the subject of disputes in scientific journals,  congressional hearings,  and legal proceedings including a Freedom of Information Act lawsuit.  Just Facts presently does not have the resources to conclusively assess all the competing claims on this issue, but the facts we have verified are as follows: medieval warmth,” and shows the following graph of temperature changes for the Northern Hemisphere over the past 1,300 years.