When Dr. Roy Spencer looked up summer temperature data for the U.S. Corn Belt, it showed no warming trend for over a century. But that was before temperatures were “adjusted” by National Oceanic and Atmospheric Administration climate scientists. Now the same data shows a significant warming trend.

Spencer, a climate scientist at the University of Alabama in Huntsville, said that the National Climatic Data Center made large adjustments to past summer temperatures for the U.S. Corn Belt, lowering past temperatures to make them cooler. Adjusting past temperatures downward creates a significant warming trend in the data that didn’t exist before.

“I was updating a U.S. Corn Belt summer temperature and precipitation dataset from the NCDC website, and all of a sudden the no-warming-trend-since-1900 turned into a significant warming trend,” Spencer wrote on his blog, adding that NCDC’s “adjustments” made the warming trend for the region increase from just 0.2 degrees Fahrenheit per century to 0.6 degrees per century.

NCDC temperature data downloaded by Spencer in March 2014 looked quite different from data he downloaded this month. That’s because NCDC constantly adjusts its data to correct for errors, but critics have said these adjustments seem to always increase the warming trend for the U.S. or globally.

“Being the co-developer of a climate dataset (UAH satellite temperatures) I understand the need to make adjustments for known errors in the data … when you can quantitatively demonstrate an error exists,” Spencer wrote.

“But a variety of errors in data measurement and collection would typically have both positive and negative signs,” Spencer noted, adding that he corrects for such errors when calculating satellite temperature data even if they tend to cancel each other out.

“In contrast, the thermometer data apparently need to be adjusted in such a way that almost always leads to greater and greater warming trends,” he added.

Spencer is not the first to criticize NCDC’s adjustments to temperature data. Science blogger Steven Goddard (his pen name) has been arguing for years that government climate agencies have been tampering with temperature data to create a significant warming trend where none actually exists.

“NCDC pulls every trick in the book to turn the US cooling trend into warming. The raw data shows cooling since the 1920s,” Goddard told The Daily Caller News Foundation in a previous interview.

Goddard says that NCDC has been cooling past temperatures in the data to make the present look much warmer by comparison, bolstering the case that global warming is being caused by human activity and is rapidly getting worse.

“NCDC does a hockey stick of adjustments to reverse the trend,” Goddard said. “This includes cooling the past for ‘time of observation bias’ infilling missing rural data with urban temperatures, and doing almost nothing to compensate for urban heat island effects.”

Meteorologist Anthony Watts has also caught NOAA changing the temperature record. For two years, NOAA claimed that July 2012 was the hottest month on record — that is, until it quietly adjusted the data so that July 1936 was the hottest month on record.

“Two years ago during the scorching summer of 2012, July 1936 lost its place on the leaderboard and July 2012 became the hottest month on record in the United States,” Watts wrote. “Now, as if by magic, and according to NOAA’s own data, July 1936 is now the hottest month on record again. The past, present, and future all seems to be ‘adjustable’ in NOAA’s world.”

“You can’t get any clearer proof of NOAA adjusting past temperatures,” Watts wrote. “This isn’t just some issue with gridding, or anomalies, or method, it is about NOAA not being able to present historical climate information of the United States accurately.”

But what does NOAA have to say about its temperature adjustments?

NOAA says it makes these adjustments to eliminate “artificial biases” in surface temperature data. For example, a major adjustment made by scientists to climate data was to take into account the big shift from taking temperature readings in the afternoon to the morning.

Changing the time temperatures were taken from the afternoon, when temperatures are warmer, to the morning, when temperatures are cooler, created a cooling bias in the record. NCDC also takes temperature from nearby stations to help create a baseline temperature for the region.

There are legitimate concerns with land and ocean temperature data. For example, surface temperature readings rely on a few thousand weather stations, boats and buoys spread throughout the planet.

Outside the U.S., weather stations are less reliable, according to NCDC climate scientists, especially stations that are located in the middle of nowhere — like the station on St. Helena Island in the middle of the Atlantic Ocean.

NOAA did not respond to The Daily Caller News Foundation’s request for comment.

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